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masjstovel

ZWO ASI 1600MM Pro (mono) or not?

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Posted (edited)

Hi,

I bought my first telescope, SW 150pds about 6 months ago with the purpose of astroimaging "when i feel ready".  So far ive used my Nikon D810 for that, and I'm now  planning on taking the step buying my first AP camera.
My targets would be DSO's, and not planetary. I want a mono-camera, not color. 

I want to get away with a very good camera to a reasonable price (wouldn't we all...) and in this regard I've been drooling on the ZWO ASI 1600MM Mono for some time. The price for it is in the upper part of my budget, but I'm willing to if its worth it.  I've seen from other treads that sensor-size isn't everything, and dynamic range and gain and all is just as important, but i have trouble understanding it all 100% when it's all new to me, but in my experience i am a practical person who learns things much better and faster with the gear in my hand. So without getting to technical, and staying as objective as possible - please help me with; 

1. Is this a good camera to go for?

2. It's sold with options of filters 1.25", 31mm or 36mm - Why these options, and what determines what i would choose?

3. Would you go for another camera in this price range, and why? - Or to rephrase it a bit; If you were in my shoes, which camera would you og for?

I'd appreciate any help:)

I might add, that i understand that with my lack of experience, buying a mono-camera with filters and all might seem premature, but for some strange reason. I enjoy these "way over my head"-projects and figuring  out things as time goes - I just need some guiding in the right direction. 

Edited by masjstovel
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6 hours ago, masjstovel said:

1. Is this a good camera to go for?

Yes. Many people have made very good images with it

2. It's sold with options of filters 1.25", 31mm or 36mm - Why these options, and what determines what i would choose?
Depending on sensor size, F-ratio and distance from sensor to filter, vignetting may occur. The ASI1600MM enables the filters to be placed close to the sensor so even with 1.25" filters, vignetting with F-ratio down to F/5 is minimal and can be easily calibrated out. Larger filters cost more. You may want to use higher quality filters especially for narrowband. The lower cost of 1.25" filters may make this more affordable for you. I use Astronomik filters.

If you intend to go with a larger sensor down the track you may want to get larger filters to suit. Filters are a significant part of the cost in mono imaging.

3. Would you go for another camera in this price range, and why? - Or to rephrase it a bit; If you were in my shoes, which camera would you og for?
I went for the ASI1600 and have not regretted it.

 

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Posted (edited)
48 minutes ago, kens said:

 

Thank you @kens, just the kind of answer I was looking for! 
That clarifies it for me regarding question #2. If i understand you correctly - the bigger the filter-diameter - the better, or at least the more ready i will be for other equipment further down the line?
Are you familiar with the ZWO-filters? I've read that the old ones are bad, but the new ones are good alternatives to Astrodon and Baader- except some haloing on the OIII. 

 

( https://www.cloudynights.com/topic/627187-review-of-the-new-zwo-narrowband-filters-comparison-to-the-old-zwo-and-astrodon-filters/ )

Edited by masjstovel

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To answer this you really need to know what type of targets you want to image. Because it matters for sensor selection. The asi1600mm pro is great but with your scopes focal length it's not really going to give wide field views of emission nebula. A friend uses his 150pds for detailed looks at smaller targets like the bubble nebula or Cresent or galaxy imaging. The other thing you will want to be aware of is that the focuser will not support a asi1600mm pro and filter wheel without some droop / flex and so it's hard to get good corners, from my personal experience with the 130pds. A smaller higher resolution sensor will be easier and more suited to imaging smaller targets in detail. For example you will get very good results on small galaxys with a asi183mm pro or even better a qhy178m and they will be good right to the corner, but I sence you want the bigger sensor in the expectation of trying to go for larger targets. That being the case consider dropping down to a 130pds as it's just that bit wider. I. Terms of filters I went with 1.25 mounted, they are good down to F4. 

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Posted (edited)

I am sure, you will be very happy with ASI1600 as it has quite good sensor (much better than DSLR) and a very small (if not the smallest in the market) Back Focus and it will enable you to use 31mm filters without any frustration.

I went the same,  ASI1600MM way, with my 130PDS, - had some problems with focuser's tilt, but sorted it out with DIY autofocuser and tightening draw-tube quite strongly.

Just to make your life a bit more difficult, -  a main competitor of ASI1600 is Altair Hypercam 1600M PRO TEC, which has the same sensor, but larger -  4GB ram (not sure if there is any use of it),

keep in mind, - it has 17.5mm back focus, which will probably  still be OK with 31mm filters at F5. 

P.S.

 I am not sure if Hypercam  has it's own matching EFW and etc.

Edited by RolandKol

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2 hours ago, Adam J said:

To answer this you really need to know what type of targets you want to image. Because it matters for sensor selection. The asi1600mm pro is great but with your scopes focal length it's not really going to give wide field views of emission nebula. A friend uses his 150pds for detailed looks at smaller targets like the bubble nebula or Cresent or galaxy imaging. The other thing you will want to be aware of is that the focuser will not support a asi1600mm pro and filter wheel without some droop / flex and so it's hard to get good corners, from my personal experience with the 130pds. A smaller higher resolution sensor will be easier and more suited to imaging smaller targets in detail. For example you will get very good results on small galaxys with a asi183mm pro or even better a qhy178m and they will be good right to the corner, but I sence you want the bigger sensor in the expectation of trying to go for larger targets. That being the case consider dropping down to a 130pds as it's just that bit wider. I. Terms of filters I went with 1.25 mounted, they are good down to F4. 

Thanks! I see what you mean. I'm planning on buying something like i.e an 80mm f/6 in addition, which i think will be a good choice when the 150PDS makes it too close (?). Maybe down the line i will add something like the 130.


I don't quite understand what you mean about the focuser? It will not stay in place because of the weight or?

 

1 hour ago, RolandKol said:

I am sure, you will be very happy with ASI1600 as it has quite good sensor (much better than DSLR) and a very small (if not the smallest in the market) Back Focus and it will enable you to use 31mm filters without any frustration.

I went the same,  ASI1600MM way, with my 130PDS, - had some problems with focuser's tilt, but sorted it out with DIY autofocuser and tightening draw-tube quite strongly.

Just to make your life a bit more difficult, -  a main competitor of ASI1600 is Altair Hypercam 1600M PRO TEC, which has the same sensor, but larger -  4GB ram (not sure if there is any use of it),

keep in mind, - it has 17.5mm back focus, which will probably  still be OK with 31mm filters at F5. 

P.S.

 I am not sure if Hypercam  has it's own matching EFW and etc.

To @RolandKol and @AdamJ: Will this sort the problem? https://www.astroshop.eu/motors-controls/skywatcher-auto-focus-system-for-telescopes/p,11469#tab_bar_0_select

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Posted (edited)
2 hours ago, masjstovel said:

Thanks! I see what you mean. I'm planning on buying something like i.e an 80mm f/6 in addition, which i think will be a good choice when the 150PDS makes it too close (?). Maybe down the line i will add something like the 130.
I don't quite understand what you mean about the focuser? It will not stay in place because of the weight or?

To @RolandKol and @AdamJ: Will this sort the problem? https://www.astroshop.eu/motors-controls/skywatcher-auto-focus-system-for-telescopes/p,11469#tab_bar_0_select

Shortly No, that motor will not help.

A bit more about my 130PDS focuser:

No, don't get it wrong, camera and EFW did not slip due to the weight, but it had a slight wobble/tilt.

I am almost sure yours has it also. Simply test it, once you in focus, try moving camera to sides and it will most likely tilt the focuser's draw-tube by a fracture of the millimeter.

That tilt, usually is enough to cause some prolonged stars in one of even few corners of the images even if you use coma corrector.

That tilt is also noticeable once you try to focus using any Focusing aid software, - once you lock focuser, you will notice figures will "run away" from your "perfect" value. These issues, do not cause any Major problems, just a bit annoying.

You will most likely have it with any other camera & EFW together anyway.

PDS are good for the price sold, but are not perfect, so some modding or tinkering is needed.

 

As per autofocuser: it must be strong enough to keep the rig without locking the dawtube (in this case it eliminates the draw-tube tilt during the lock, plus if it is not locked, it can refocus without your interference).

if you go mono, you will use minimum 3 filters if you go narrow band only (LRGB, + four more filters), - you will need to refocus on each of them once filter is changed.
So Autofocuser MUST  be computer controlled and with a Stepper Motor, which has strong Holding power, not like simple motors, which you can easily rotate the shaft with your fingers.

So moving forward, - think of autofocuser also...

You can buy one, - starting price from something around £200 or do it yourself.

Just keep in mind, if you will go autofocuser way, - always get one with thermometer. I did my autofocuser for around £65 (I had only some electric and software related experience in my teenage times, but managed to do it in  few months).

My tests showed, focus leaves critical focus area once temperature changes by 1C on 130PDS. Yours is larger, - probably more sensitive. So my autofucser does it's job not only once filter is changed by SGP, but also, once temp changes.

VERY VERY HANDY TOOL!

Allows me to sleep all night while imaging! :)

 

Edited by RolandKol

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1. Yes

2. 1.25" filters work down to F/4.8 if mounted very close. Here is flat from my ASI1600 and 80mm F/6 with x0.79 FF/FR (effectively somewhere around F/4.75?):

image.png.b5cc99b513e3cfc61b14979f111b7464.png

Vignetting in upper part looks like straight edge - and it is because it has to do with OAG prism being mounted on that side - just a bit too close to cast some shadow on sensor (and very small diffraction spike on stars in that part of image, but I'm more careful now of it's positioning).

Corners are at about 90% (88% in upper left corner) in this combination, but I was using filter drawer which can be mounted even closer than filter wheel.

Difference between filter sizes can be summarized as:

- bigger - more expensive

- bigger - likely a bit better optically (same surface imperfection will be spread over more light cone because you can mount them further away from sensor, so in percentage it will be smaller). Not something that you will ever notice if filters are decent quality

- Bigger - more suitable for larger sensors, faster optics and can be mounted further away.

ASI1600 has diagonal of about 22mm, 1.25" sensor has clear aperture of about 27-28mm, so it has plenty of aperture to match that sensor. Problem comes from mounting distance, here is a quick diagram describing what can happen:

image.png.676a781fb5cb86f7db7217050a7c8628.png

3. At the time I was looking to purchase mono camera - that was only sensible option in that price range that had set point temperature cooling. I believe it the same now, except there is maybe 183 sensor based camera (and other vendors with same sensor as 1600), but I would still go for ASI1600 - it is the largest sensor at that price.

On other comments:

- I would not recommend 183 over 1600 for 150 F/5 scope - it will give you sampling rate that you won't be able to utilize, it has smaller surface area. For 750mm ASI1600 is about right (maybe a tad oversampling at 1"/px - but that will depend on your skies and mount / guide performance).

- You don't need to get smaller scope to do wide field - you can do wide field with 150mm F/5 as well. It is not as straight forward, but since you like challenging projects, I think you will like it. You can do mosaics - that means shooting multiple panels, stacking each and stitching them to get wider FOV.

There is a tiny overhead in imaging time to do that - panels need to overlap a little and you need to center each panel in correct place (imaging software often has mosaic planning feature that you can use). It involves a bit more processing (and some changes to way you process your data unless you have it automated by software like APP).

Other than that - F/5 scope with be "as fast" as smaller F/5 scope with same camera, although you need to take multiple panels, if you aim for same resolution (which would mean binning wide field panels to match resolution of scope with shorter FL).

Since you have 750mm FL scope, binning x2 in software will produce same sampling rate as 375mm scope. You will need to take x4 panels to cover same area, which means x4 less time per each panel if you want to match total exposure time, but binning x2 will improve SNR by factor of x2 - same as imaging for x4 more time, so it works out the same (except for framing and slight overlap of panels - giving slightly smaller FOV than it would have with shorter FL scope).

- Don't have experience with focuser on 150mm newtonian, but I can tell you that focuser tilt is real issue and I ended up upgrading my focuser so I can have threaded connection and everything now stays in place.

 

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11 minutes ago, vlaiv said:

- Don't have experience with focuser on 150mm newtonian, but I can tell you that focuser tilt is real issue and I ended up upgrading my focuser so I can have threaded connection and everything now stays in place.

Threaded draw-tube connector is probably the best option, - the plastic end with bolts on the focuser can be removed and connector attached, however, it will not allow to use Collimation tools properly.

So I went "TS-Optics 2" Adapter compression Ring" way, - it is placed instead of the plastic part at the end of the draw-tube and centers the imaging "toys" in the focuser quite nicely, plus it allows to use collimation tools also.
There is a similar Baader Click Clock which is more comfy, - but as my web explorations showed, 130 PDS+ClickClock+ASI1600=No focus... :(
Baader is a bit too long/thick for my PDS.... not sure about 150PDS, I guess, - also.

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1 hour ago, RolandKol said:

Threaded draw-tube connector is probably the best option, - the plastic end with bolts on the focuser can be removed and connector attached, however, it will not allow to use Collimation tools properly.

So I went "TS-Optics 2" Adapter compression Ring" way, - it is placed instead of the plastic part at the end of the draw-tube and centers the imaging "toys" in the focuser quite nicely, plus it allows to use collimation tools also.
There is a similar Baader Click Clock which is more comfy, - but as my web explorations showed, 130 PDS+ClickClock+ASI1600=No focus... :(
Baader is a bit too long/thick for my PDS.... not sure about 150PDS, I guess, - also.

No the click lock will work on the 150pds but not the 130pds. 

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4 hours ago, vlaiv said:

1. Yes

2. 1.25" filters work down to F/4.8 if mounted very close. Here is flat from my ASI1600 and 80mm F/6 with x0.79 FF/FR (effectively somewhere around F/4.75?):

….

3. At the time I was looking to purchase mono camera - that was only sensible option in that price range that had set point temperature cooling. I believe it the same now, except there is maybe 183 sensor based camera (and other vendors with same sensor as 1600), but I would still go for ASI1600 - it is the largest sensor at that price.

……….…...

Thanks alot for that excellent explanation! I understood the model. Wouldn't  a ZWO filter-wheel be close enough to an ZWO camerasensor when its from the same manufacturer? I mean, without vignetting? 

Stitching would be great to try, but i imagine there has to be careful work with calibrated frames if the stitching is to look Natural. I mean no contrast between "core" and corners and so on - the frames must be totally even all over?

 

4 hours ago, vlaiv said:

- I would not recommend 183 over 1600 for 150 F/5 scope - it will give you sampling rate that you won't be able to utilize, it has smaller surface area. For 750mm ASI1600 is about right (maybe a tad oversampling at 1"/px - but that will depend on your skies and mount / guide performance).

This i am not sure i understand? what is oversampling?

And about the binning. I may have misunderstood, but i thought binning was moving the frame some pixels back and forth to sort out different kind of distortions/errors in the frames? Wouldnt those still be there if i used binning for making bigger frames?

5 hours ago, RolandKol said:

Shortly No, that motor will not help.

A bit more about my 130PDS focuser:

…...

Thanks @RolandKol! You habe an URL/name for a useful autofocuser in my case?
It's almost funny - this "It would be cool to take a photo through a Telescope"-project never stops surprising me with must-have-gadgets :D.

@RolandKol , @vlaiv and @AdamJ: Yes the inner adaptor-ring With the locking-screws is slightly moveable (<1mm). This is the issue you're addressing? Is it not fixable with just a toothpic pushed in between or something? Maybe im cursing in the church now, but… :)

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2 minutes ago, masjstovel said:

Stitching would be great to try, but i imagine there has to be careful work with calibrated frames if the stitching is to look Natural. I mean no contrast between "core" and corners and so on - the frames must be totally even all over?

Yes indeed - you need to do proper calibration of your subs, and even then, you will still need to contend with different gradients and different signal strengths because of selecting reference frames shot at different times for your panel stacks (signal strength depends on target position in the sky - higher it is, stronger the light because less atmosphere to block some of the light).

On the other hand - proper calibration is something that you best adopt from the beginning if your camera supports it - and one prerequisite for that is set point cooling - ASI1600 has that, and it calibrates well so don't worry about that aspect. Flats will probably be hardest part, not because they are hard to record but because you will need good flat source - I would recommend flat panel (that is additional cost, but in my view worth it).

All other "problems" should be handled by software.

6 minutes ago, masjstovel said:

This i am not sure i understand? what is oversampling?

And about the binning. I may have misunderstood, but i thought binning was moving the frame some pixels back and forth to sort out different kind of distortions/errors in the frames? Wouldnt those still be there if i used binning for making bigger frames?

Recording signal to certain "fidelity" requires certain number of samples. For example - CD sound is recorded at 44100 samples a second, and that is because most humans hear sound frequencies up to about 20Khz. Number of samples needs to be twice that of maximum frequency (for CD quality it is 22050hz). Similar thing happens when you take the image. Telescope can't "magnify" image past certain point due to laws of physics. Atmosphere also plays a part and on most nights even this theoretical "magnification" is too much - atmosphere blurs details. With long exposure AP there is another aspect - how well the mount tracks the stars (and how well your guiding works to correct for mount errors and other problems) - that also reduces level of detail that can be recorded. It just removes higher frequencies from the image (yes there is such thing as frequencies in the image, and it is not simply related to size of features - it is more complicated than that, but you don't need to understand that fully for this discussion) and if you sample image too much - and that means "zooming in" too much, or using too many pixels to cover certain object (which will make it larger on screen) - you will simply record bigger blurry image without additional detail. In it self - there is nothing wrong with that if it were not for noise.

When you do that - record image with too much zoom - you spread the light over many pixels and there is not enough signal per pixel and your SNR is poor - your image will look noisier and you might not even be able to distinguish object from the noise if SNR is too low. That is why you don't want to oversample.

There is opposite of oversampling and that is under sampling - you don't zoom image enough to capture all possible detail (that scope, guiding and atmosphere can provide on a given night) - things in this case just look smaller on screen, but you get better SNR by doing that.

This is story of sampling very oversimplified, luckily you don't need to know all the details - you need to know how to calculate sampling rate - in is expressed in arc seconds per pixel, depends on pixel size in micrometers and focal length of telescope in millimeters. Divide the two and multiply with 206.3 to get sampling rate in "/px. In your case that would be (3.8 / 750) * 206.3 =  ~1.045"/px.

For wide field imaging with short focal length scopes - this figure is in general larger than 3"/px (there is no upper limit really - one is limited by focal length of lens used for milky way landscape shots for example). "Regular imaging" that amateurs use is in range of 3"/px - 1.5"/px. Sampling in range 1.5"/px to 1"/px is considered advanced (you need good mount, steady skies and aperture that is capable of it) and you can easily slip into oversampling if some of that is not playing ball. Going below  1"/px for most people with most gear in most conditions will lead to oversampling, you need large aperture, very good mount and guiding and very stable atmosphere to go below 1"/px.

Now on to binning. Binning is process of adding adjacent pixels to form larger pixel (usually groups of 2x2 or 3x3). It affects signal collected (summing 4 values gives bigger value, hence better SNR) and also reduces number of pixels covering target (2x2 pixels is replaced with single value - single pixel), or brings up sampling rate (binning x2 on 1"/px image will make it 2"/px). It should be done if you oversample - in that case you don't loose any detail, as there is no additional detail in the image in the first place, and you recover some of SNR lost due to oversampling (you would recover it 100% if you had sensor without read noise - or in case of CCD where you have hardware binning, and there is one read noise per binned pixel).

FOV is not affected by binning, but pixel count / resolution is. So image viewed in "screen size" (fit to screen, or posted here of forum) will not change, but image looked at 1:1 (100% zoom, or one image pixel - one screen pixel) will change object scale when binning.

30 minutes ago, masjstovel said:

Thanks @RolandKol! You habe an URL/name for a useful autofocuser in my case?
It's almost funny - this "It would be cool to take a photo through a Telescope"-project never stops surprising me with must-have-gadgets :D.

@RolandKol , @vlaiv and @AdamJ: Yes the inner adaptor-ring With the locking-screws is slightly moveable (<1mm). This is the issue you're addressing? Is it not fixable with just a toothpic pushed in between or something? Maybe im cursing in the church now, but…

You don't need autofocuser - but it is handy to have. I'm yet to make two of them for my two scopes - they will be DIY projects.

Using toothpick will stop focuser from being loose and shifting left/right, but it might not solve the problem you have because there is slack in the focuser - tilt. If focuser tube is loose it will shift left/right and in doing so it will change angle, and important thing with focuser is that it is squared to optical axis - that ensures that sensor lies in focal plane and is not tilted to it. If sensor is tilted with respect to focal plane - some parts of it will be out of focus and that produces poor stars (usually corners since they are furthest of center so most out of focus). Using toothpick will stop focuser from tilting but it will lock it in angled position - which will still leave sensor at an angle to focal plane.

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13 hours ago, vlaiv said:

Yes indeed - you need to do proper calibration of your subs, and even then, you will still need to contend with different gradients and different signal strengths because of selecting reference frames shot at different times for your panel stacks (signal strength depends on target position in the sky - higher it is, stronger the light because less atmosphere to block some of the light).

On the other hand - proper calibration is something that you best adopt from the beginning if your camera supports it - and one prerequisite for that is set point cooling - ASI1600 has that, and it calibrates well so don't worry about that aspect. Flats will probably be hardest part, not because they are hard to record but because you will need good flat source - I would recommend flat panel (that is additional cost, but in my view worth it).

All other "problems" should be handled by software.

Recording signal to certain "fidelity" requires certain number of samples. For example - CD sound is recorded at 44100 samples a second, and that is because most humans hear sound frequencies up to about 20Khz. Number of samples needs to be twice that of maximum frequency (for CD quality it is 22050hz). Similar thing happens when you take the image. Telescope can't "magnify" image past certain point due to laws of physics. Atmosphere also plays a part and on most nights even this theoretical "magnification" is too much - atmosphere blurs details. With long exposure AP there is another aspect - how well the mount tracks the stars (and how well your guiding works to correct for mount errors and other problems) - that also reduces level of detail that can be recorded. It just removes higher frequencies from the image (yes there is such thing as frequencies in the image, and it is not simply related to size of features - it is more complicated than that, but you don't need to understand that fully for this discussion) and if you sample image too much - and that means "zooming in" too much, or using too many pixels to cover certain object (which will make it larger on screen) - you will simply record bigger blurry image without additional detail. In it self - there is nothing wrong with that if it were not for noise.

When you do that - record image with too much zoom - you spread the light over many pixels and there is not enough signal per pixel and your SNR is poor - your image will look noisier and you might not even be able to distinguish object from the noise if SNR is too low. That is why you don't want to oversample.

There is opposite of oversampling and that is under sampling - you don't zoom image enough to capture all possible detail (that scope, guiding and atmosphere can provide on a given night) - things in this case just look smaller on screen, but you get better SNR by doing that.

This is story of sampling very oversimplified, luckily you don't need to know all the details - you need to know how to calculate sampling rate - in is expressed in arc seconds per pixel, depends on pixel size in micrometers and focal length of telescope in millimeters. Divide the two and multiply with 206.3 to get sampling rate in "/px. In your case that would be (3.8 / 750) * 206.3 =  ~1.045"/px.

For wide field imaging with short focal length scopes - this figure is in general larger than 3"/px (there is no upper limit really - one is limited by focal length of lens used for milky way landscape shots for example). "Regular imaging" that amateurs use is in range of 3"/px - 1.5"/px. Sampling in range 1.5"/px to 1"/px is considered advanced (you need good mount, steady skies and aperture that is capable of it) and you can easily slip into oversampling if some of that is not playing ball. Going below  1"/px for most people with most gear in most conditions will lead to oversampling, you need large aperture, very good mount and guiding and very stable atmosphere to go below 1"/px.

Now on to binning. Binning is process of adding adjacent pixels to form larger pixel (usually groups of 2x2 or 3x3). It affects signal collected (summing 4 values gives bigger value, hence better SNR) and also reduces number of pixels covering target (2x2 pixels is replaced with single value - single pixel), or brings up sampling rate (binning x2 on 1"/px image will make it 2"/px). It should be done if you oversample - in that case you don't loose any detail, as there is no additional detail in the image in the first place, and you recover some of SNR lost due to oversampling (you would recover it 100% if you had sensor without read noise - or in case of CCD where you have hardware binning, and there is one read noise per binned pixel).

FOV is not affected by binning, but pixel count / resolution is. So image viewed in "screen size" (fit to screen, or posted here of forum) will not change, but image looked at 1:1 (100% zoom, or one image pixel - one screen pixel) will change object scale when binning.

You don't need autofocuser - but it is handy to have. I'm yet to make two of them for my two scopes - they will be DIY projects.

Using toothpick will stop focuser from being loose and shifting left/right, but it might not solve the problem you have because there is slack in the focuser - tilt. If focuser tube is loose it will shift left/right and in doing so it will change angle, and important thing with focuser is that it is squared to optical axis - that ensures that sensor lies in focal plane and is not tilted to it. If sensor is tilted with respect to focal plane - some parts of it will be out of focus and that produces poor stars (usually corners since they are furthest of center so most out of focus). Using toothpick will stop focuser from tilting but it will lock it in angled position - which will still leave sensor at an angle to focal plane.

Regarding over/undersampling. I think i get it in form of: "picture looks best with between 1-2"/px". The result says what is "covered" of target area per pixel?
I tried transforming this rule to general photography just for fun. Nikon D810 sensor pixel size (or pixel pitch= distance from center of pixel to center of next pixel which is the same(?)) = 4,87μm. I also have a 14-24mm f/2.8 lens which i've used on s star adventurer mini for Milkyway photos. This gives me: (4,87 / 14mm) * 206.3 = 73,76"/px - waay undersampled? I understand widefield photography is a different ballgame though. Is this because the level of star-detail is irrelevant at these focal lengths? At what focal length spectre is this rule handy? Is the 206.3 number made as an average factor for DSO-imaging?

Apologize for the question-bombing, but i found this interesting. 

Edited by masjstovel

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15 hours ago, masjstovel said:

Thanks @RolandKol! You habe an URL/name for a useful autofocuser in my case?
It's almost funny - this "It would be cool to take a photo through a Telescope"-project never stops surprising me with must-have-gadgets :D.

@RolandKol , @vlaiv and @AdamJ: Yes the inner adaptor-ring With the locking-screws is slightly moveable (<1mm). This is the issue you're addressing? Is it not fixable with just a toothpic pushed in between or something? Maybe im cursing in the church now, but… :)

Hi again,

Unfortunately I cannot recommend any brand of autofocuser, as I never used them... As I mentioned before, - mine is DYI. The cheaper brands are Pegasus and ZWO (not joking :) ZWO has it all), but ...

To be honest, it is 4 or even 5 times cheaper to go DYI way, just check https://www.youtube.com/watch?v=G8jLOAR2ZHU, - it is almost simple like this (the software he uses is no more accessible, but he shows the basics, currently the same software, but a bit more advanced is accessible, just a bit more difficult than the basic one. There are some threads in this forums about it and etc.). The main headache is to attach the motor to the focuser, - it can be a challenge, the rest is just some fun :)

As per focuser drawtube tilt/wobble:

You simply need slightly tighten the drawtube bolts which are based under the focuser, - toothpick will not help.
There are several threads on this forum about it.

P.S.

Have you decided which camera you will go for? :) 

P.P.S.

"It would be cool to take a photo through a Telescope"-project never stops surprising me with must-have-gadgets :D.

I placed my smartphone to the eyepeace to find Andromeda as my skies are very light poluted....

and ended up with this one :) 
https://www.flickr.com/photos/154881882@N05/40483860193

Welcome to the dark side! :)

 

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57 minutes ago, masjstovel said:

Regarding over/undersampling. I think i get it in form of: "picture looks best with between 1-2"/px". The result says what is "covered" of target area per pixel?
I tried transforming this rule to general photography just for fun. Nikon D810 sensor pixel size (or pixel pitch= distance from center of pixel to center of next pixel which is the same(?)) = 4,87μm. I also have a 14-24mm f/2.8 lens which i've used on s star adventurer mini for Milkyway photos. This gives me: (4,87 / 14mm) * 206.3 = 73,76"/px - waay undersampled? I understand widefield photography is a different ballgame though. Is this because the level of star-detail is irrelevant at these focal lengths? At what focal length spectre is this rule handy? Is the 206.3 number made as an average factor for DSO-imaging?

Apologize for the question-bombing, but i found this interesting. 

206.3 number can be easily derived with a bit of trigonometry. You are right - it represents mapping between angles in the sky and surface of the pixel on the sensor. 1"/px means that single pixel will "cover" 1 arcsec x 1 arcsec of the sky. Here is simple derivation of that relationship:

image.png.31211563a22d0de151116625c9215da3.png

It is obvious that distance on the sensor equals FL * tan(angle). When we want to calculate angle from focal length and "distance of one pixel" (pixel pitch), we will write:

tan(angle) = pixel pitch / focal length (in same units)

Now we can use small angle approximation which says sin(alpha) ~= tan(alpha) ~= alpha if alpha is small (and in our case it is, since it is only arcseconds in magnitude), so we have

angle in radians = pixel pitch (in mm) / local length (in mm)

We want angle in arc seconds, so we must do conversion. One degree = PI / 180 radians, One arc minute = PI / (180*60) radians, One arc second = PI / (180*60*60) radians

Pixel pitch in millimeters is pixel pitch in micrometers divided with 1000

After all that unit conversion stuff we get:

angle * ( PI / (180 * 60 * 60) ) * 1000 = pixel pitch (in um) / FL (in mm)

( PI / (180 * 60 * 60) ) * 1000 = ~ 0.0048481368111 and reciprocal of that (if we take it to the other side of equation) is ~206.26481 = ~ 206.3

In order to further understand proper sampling rate, you need to understand resolution of imaging system. Stars are never pinpoints in the image - they are more Gaussian shaped curves. This is because there are couple of factors that "smear" point light. First is aperture of telescope - it produces airy pattern from point source. Second is seeing and third is guiding precision. All of these combine to form star profile that is well approximated by Gaussian shape.

Based on average seeing conditions, amateur scope aperture sizes (from 60mm to 300mm or so) and usual mount performance - that gaussian shape will have FWHM of couple of arc seconds. Ideal sampling rate for such image - blurred with gaussian blur of certain FWHM is close to FWHM / 1.6. That is why we say that ideal sampling rate is 1-3"/px, depending on conditions, because produced star image FWHM is in range of 1.6" to 5". To see how gaussian profile impacts image detail, examine these images:

image.png.6ff2235418e7f6bc8f3c587c67f65384.png

Left is image composed out of plain sine functions in x direction, but each growing wavelength in y direction (higher frequencies on top, lower on bottom). Next two images are gaussian blurred first image with different size of gaussian blur. Middle image uses gaussian blur with radius of 2 while right image gaussian blur with radius 4.

Larger blur radius (larger star FWHM) - more frequencies get cut off (no detail in above images - just gray uniform image), That means that resolution depends on star FWHM - larger it is, less detail there is in the image. This holds for any sort of imaging. It just happens that most telescopes, skies and mounts produce blur that is in certain range - hence recommendation for sampling rate in range 1-3"/px.

 

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21 hours ago, vlaiv said:

On other comments:

- I would not recommend 183 over 1600 for 150 F/5 scope - it will give you sampling rate that you won't be able to utilize, it has smaller surface area. For 750mm ASI1600 is about right (maybe a tad oversampling at 1"/px - but that will depend on your skies and mount / guide performance).

I was thinking more of the 178, I have seen amazing detailed galaxy images with it and IMX290 using short exposure method in both cases. 

Alot of it is to do with having very low read noise and a smaller sensor to allow vast numbers of subs to be collected at very short exposures. 

Adam 

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1 hour ago, vlaiv said:

206.3 number can be easily derived with a bit of trigonometry. You are right - it represents mapping between angles in the sky and surface of the pixel on the sensor. 1"/px means that single pixel will "cover" 1 arcsec x 1 arcsec of the sky. Here is simple derivation of that relationship:......

I must say I am impressed with your knowledge. I have to read through this a few times to understand it fully, but i think i understand the key points. Its the first time I've seen "derivation" and "simple" in the same sentence, btw:)

 

@RolandKol, @vlaiv and @Adam J I took a look at my scope now and see i messed up. I DO have threaded adaptor, it was just not tightened all the way (ever since i bought the scope). Thats why it was slightly movable. (pictures attached). This means the tilt-problem is out of the picture for me? The focus-tube is tight, and the adaptor is tight (when screwed in). And RolandKol, I'm pretty much decided on the ZWO ASI 1600MM mono.

20191010_114252.jpg

20191010_114259.jpg

20191010_114248.jpg

Edited by masjstovel
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3 minutes ago, masjstovel said:

I must say I am impressed with your knowledge. I have to read through this a few times to understand it fully, but i think i understand the key points. Its the first time I've seen "derivation" and "simple" in the same sentence, btw:)

 

@RolandKol, @vlaiv and @Adam J I took a look at my scope now and see i messed up. I DO have threaded adaptor, it was just not tightened all the way (ever since i bought the scope). Thats why it was slightly movable. (pictures attached). This means the tilt-problem is out of the picture for me? The focus-tube is tight, and the adaptor is tight (when screwed in).

20191010_114252.jpg

20191010_114259.jpg

20191010_114248.jpg

No you will still get tilt in my experience if using a ASI1600mm pro and filter wheel on that focuser, its too heavy, If nothing else the weight actually causes the focuser to flex slightly relative to the tube. A baader click lock will help remove any wobble in the actual connection to the coma corrector but all in all the focuser and the scope are not sufficiently stiff. This is why people pay lots of money for upgraded focusers. Its not going to stop the camera giving you nice results all I am saying is don't expect nice round stars all the way into the corners unless you are willing to make some modifications. 

 

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15 minutes ago, masjstovel said:

This means the tilt-problem is out of the picture for me?

Nope :D

Once you take image and confirm that stars are tight and round in each corner - then you don't have to deal with tilt. It might be the case, but it might not.

There are couple of reasons that can cause tilt. It can be focuser draw tube that moves left / right, has some slack in it. Even if there is no slack in focuser - it can still tilt / bend slightly under weight of the camera - and that will depend on how you mount your scope (you can rotate tube so camera can be in any position along the circle of tube rotation) and what part of the sky you are imaging. Gravity will pull on the camera, and if pull of gravity is such that it is acting perpendicular to focus travel - it can bend it a bit even if on inspection by hand - focuser seems solid.

It might even be the case that focuser is rock solid but the way it's attached to tube is not squared properly - there is a bit of tilt in focuser base.

Don't worry about tilt if your focuser is now working ok by touch until you can see what your subs look like - if there is actual tilt then you will decide how to deal with it (for example it might be so small that only one corner is mildly affected - cropping image in that part will remove slightly distorted stars).

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33 minutes ago, Adam J said:

I was thinking more of the 178

I agree that 178 is very nice little cam - I've got one - color version. It is very versatile camera, and I would recommend it to anyone wanting cheap camera for multiple use cases like doing a bit of long exposure DSO, a bit of EEVA and some planetary imaging. It will work in all of those roles well (matched with suitable optics).

I still prefer 1600 for DSO imaging though. It's a bit more expensive but it does offer much more in terms of sensor surface.

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4 minutes ago, vlaiv said:

I agree that 178 is very nice little cam - I've got one - color version. It is very versatile camera, and I would recommend it to anyone wanting cheap camera for multiple use cases like doing a bit of long exposure DSO, a bit of EEVA and some planetary imaging. It will work in all of those roles well (matched with suitable optics).

I still prefer 1600 for DSO imaging though. It's a bit more expensive but it does offer much more in terms of sensor surface.

Well as you know I own a ASI1600mm pro and I love it. Its just not easy to get a flat field on it with  my 130PDS.  The same will be true of a 150PDS. 

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I would agree with Vlaiv,

You may, or may Not suffer from a drawtube tilt problem.

And even if you will, - I almost sure, you will not really notice that for the first 3 months, till you get your eyes/brains adjusted.
and as I mentioned before, if needed, you can tighten the drawtube travel by the screws at the bottom of the focuser, it will reduce or completely eliminate the tilt.

Asi1600mm will be a good all rounder cam, it will fit almost any scope you decide to use later.
Once you get it, - do not put it on the scope. Power up, connect to the PC, put it in the dark cool place and collect Dark subs at gain 139 (unity gain) and in -20C.

not sure about your imaging sub length, but do not go further than 600sec for NB  (I stay in 300sec area) and probably up to 60sec with LRGB. In other words, collect library of Darks.

For Bias, do not use shorter exposures than 0.3sec, or even move to FlatDarks if you have enough patience! :) (I don't). 

P.S.

600sec is approximate value.

ASI1600 and other CMOS sensors do not really like very Long exposures and you may get AMP glow at some part of the image (usually, top, top right and etc).

Once you get your camera, you can test with Darks, stack several darks which are longer than 600sec and stretch them.  https://astronomy-imaging-camera.com/tutorials/what-is-amp-glow.html 

So you can learn your camera limits.

Edited by RolandKol
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4 minutes ago, RolandKol said:

I would agree with Vlaiv,

You may, or may Not suffer from a drawtube tilt problem.

And even if you will, - I almost sure, you will not really notice that for the first 3 months, till you get your eyes/brains adjusted.
and as I mentioned before, if needed, you can tighten the drawtube travel by the screws at the bottom of the focuser, it will reduce or completely eliminate the tilt.

Asi1600mm will be a good all rounder cam, it will fit almost any scope you decide to use later.
Once you get it, - do not put it on the scope. Power up, connect to the PC, put it in the dark cool place and collect Dark subs at gain 139 (unity gain) and in -20C.

not sure about your imaging sub length, but do not go further than 600sec for NB  (I stay in 300sec area) and probably up to 60sec with LRGB. In other words, collect library of Darks.

For Bias, do not use shorter exposures than 0.3sec, or even move to FlatDarks if you have enough patience! :) (I don't). 

P.S.

600sec is approximate value.

ASI1600 and other CMOS sensors do not really like very Long exposures and you may get AMP glow at some part of the image (usually, top, top right and etc).

Once you get your camera, you can test with Darks, stack several darks which are longer than 600sec and stretch them.  https://astronomy-imaging-camera.com/tutorials/what-is-amp-glow.html 

So you can learn your camera limits.

Thank you, that is very useful to me. I saw your setup. Do we have the same focuser (except yours is modified)? Also, why the DSLR on the lower part of the scope?

The normal procedure is to make a darks library which you always use, or do you take new darks every session?

I am very curious in "start - to finish"-explanation regarding setup of hardware, and software, and workflow in general. I've seen alot (ALOT) of videos, but I feel most of them presumes you have alot of background knowledge allready. Is there any good ones around? (There is a fair chance i've already seen them) 

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1) Do we have the same focuser (except yours is modified)? - yes, 130PDS and 150 have the same one. And no, my focuser is not modified, I just attached autofocuser to the focuser's shaft directly.

2) Why the DSLR on the lower part of the scope?  - balance... 130PDS is quite small, once I placed autofucser (500g) also  ASI + EFW, I was not able to balance... I wanted to move scope lower in the rings, but autofocuser's motor was in the way... So I needed something heavy at the bottom. My Canon fit the purpose! :)

3) The beauty of the Cooled Astro Camera, - you can use library of Darks and you do not need to collect them each night as you control the sensor's temperature.

4) I consider myself as a new starter with just a bit of experience :) (only around 2.5 - 3 years in hobby)... but my routine is as follows:

If camera was removed before session, I focus using Bahtinov Mask and SharpCap Bahtinov focusing aid on any Brighter but not the brightest star. (if camera was in, SGP autofofus is enough).

As my scope is permanent I do not do Polar Alignment each time, however, if mount suffers from bad guiding, - after focusing, I start from PA routine (usually using PHD2 drift alignment). That Means I delete all sync points on EQMOD invest another 30mins to PA.

Once PA is done, I move scope just a bit lower than 0 Alt, later lift/slew to the Star which is located just above 0 ALT and close to the Meridian to do PHD calibration (I move scope just like this as PHD calibration starts from moving scope North, and I want all gears in DEC to be intact, without any backslash).

Once done, I start my imaging session. SGP uses EQMOD in a "dialog mode" and manual sync points are not needed ... (At least in my case it works without them nicely).

I do flats at the end, in the morning...  I use a simple Stencil LED panel as this one  https://www.ebay.co.uk/itm/A4-LED-Art-Craft-Drawing-Board-Tracing-Light-Box-Table-Pad-Tattoo-Arts-Stencil/264220730411?hash=item3d84c8642b:m:mmdtuhOAM_wQQuyyu5iAR6Q

It came in a card box, so I just cut the hole in it to fit the scope, placed 3 peaces of white paper in and used SGP Flats routine to get exposure times for each filter. I use full light ON all the time and regulated the light reduction by adding or removing white paper, to keep the same brightness for all sessions and all filters.

I do not re-do Flats each session if I have not moved the camera. Can be a bit dangerous with Newtonians as they are wide open, but I am lucky so far.

What else...
If you check the image of my set up again, - keep your ASI camera on the scope in the same position, at 6 o'clock or at 12 (I cannot use 12 as I have cannon on the top and it would become useless).

ASI +EWF are quite heavy and such a position will give you the best balance. Also, try keeping counterweights as close to the mount as possible, if you use 1 which is at the end of the shaft, - place 2 and bring them closer to the mount, it will make mount RA movements easier which = to a better guiding performance. 

Hmm.... Some more novels can be written here :) 

but this is probably the basics I can drop out at the moment :)

 

Edited by RolandKol
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I bought the ZWO 1600MM Pro with 1.25" Filter-wheel, LRGB filters, and Narrowband filters now..


Please dont tell my girlfriend.

 

As a bandage on the wound: My friend is 3D-printing a Bahtinov-mask for free ($1800 used - $20 saved :)

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