Trying to choose a mono camera

[mikey2000]

Loungers, once more I call upon your help and advice!

 

I've recently posted a thread 'trying to understand LRGB imaging' and I'm now persuaded to make the step to a 'proper' astro camera.

I'd like to do this for two main reasons...   I can't seem to get the faint red stuff very well with my non-modified Sony A6000 (24mpix APS-C)  I'd like to see 'deeper' and also get into this Ha super-long exposure stuff.  I have a mount that will cope now (EQ6-R).  Secondly, I'm fed up with my A6000's lack of remote control options.

 

So, the present kit-list is EQ6-R, A6000, Skywatcher 150PDS, Baader Mk3 MPCC coma corrector (2" wide), Baader Neodymium 2" light pollution filter (rather necessary where I live...) All happily guided by QHY guidecam and a miniguidescope.   All linked up by a powered USB2.0 hub to my windows 10 laptop.

 

I'm most interested in photographing the DSOs (galaxies, clusters, nebulae inc supernova remnants like M1, dumbell etc)    My imaging sessions tend to be 2-3 hours or so before I pack it all away for the night.

 

Now, I'm facing a long list of unknowns - I'm not 100% sure of what I need.    I'm sure I will need:  Camera, Electronic/USB filter wheel, set of filters, some sort of capture/control app for Windows10.

 

The vague bits for me are:

1) what camera?

2)What wheel and what size filters? 1.25 would be nice - compact and cheaper.  Would they be OK for a 4/3" or 1" sensor?

3) Will I be able to use my Baader MPCC corrector still? 

4) Which software to use to control these things?  (Will it need to interface with PHD2 for dithering?)

5) Will I still need to use by neodymium filter? (Maybe not for Ha capture, maybe yes for L,R, G and B )

 

For a camera, I'm thinking along the lines of a ZWO ASI1600M (4/3" sensor) as this is close to what I'm used to with an APS-C sensor.  I often end up cropping the APS-C image anyway.  Also, the ZWO ASI 183MM-PRO looks interesting - slightly smaller sensor but the specification looks very good  plus it includes some kind of ram buffer to help avoid dropped files.  As I'll be using USB2, is this important??

 

That 183MM is right at the top end of my budget (allowing a few hundred quid extra for filters and a wheel.)

 

I've only mentioned CMOS  - should I consider CCD cameras?  (Apparently CMOS can't bin - should I care?)  CCD seems more expensive and you end up with fewer megapixels.  Should I care?

 

Of course, I'm open to other ideas too - I'd definitely like to stay with the mid-size sensors and avoid one-shot colour cameras.  I'm persuaded that LRGB+ha+other filters offer more possibilities.

 

 

All advice happily received!

 

 

[mikey2000]

Of course, I've tried googling some of this stuff.  In the fast changing world of sensor tech, it's hard to tell what's up to date though.

[alexbb]

I use an ASI1600MMC. I cannot compare to other CCD cameras, but I also know a few people getting very good results with the same camera or same sensor.

Regarding some of your questions and some remarks: the ASI1600MMC has a new version, 1600MM Pro which uses the same memory buffer as you mentioned about the 183.

You will be able to use 1.25" filters with the ASI1600 and ZWO's slim filter wheel. The other cameras using the same sensor, made by other manufacturers have a longer sensor-to-T-thread distance so 1.25" filters will introduce significant vignetting.

The 150PDS has 750mm focal length and a F-ratio of 5. With the ASI183 and its 2.4u pixels, you would be oversampling a bit and you will aim for 0.66 arcsec/pixel. Do you have the seeing for this? Perhaps the 3.8u pixels of the ASI1600 are more appropriate at 1.04 arcsec/pixel, not to mention it will give you a larger FOV.

If you decide going with ZWO, I recommend buying the 8 position 1.25"/31mm unmounted filter wheel or the 7 position 36mm filter wheel (should you want faster optics/larger filters in the future) instead of the 5 position filter wheel. Just to avoid the later upgrade.

The info I found about the Baader MPCC: 55mm from the T2 thread and 58mm from the M48 thread. So you should even be able to fit a filter wheel and an OAG and still be in the CC working distance.

Software? SGP with more options or APT with a simpler interface.

You may want to use the neodymium filter instead of the L filter, but I'm not sure it will bring any real benefits. You don't need to stack it with any filter.

HTH,

Alex

PS. My opinion about purchasing astro stuff is: Don't make compromises for medium prices. Either buy very cheap so you don't care, either save for what you really want. You will eventually want to upgrade if you don't choose wisely and that will cost you more in the end.

[ollypenrice]

Alex's post makes good sense to me. I can't comment on CMOS because I've never used this kind of camera but having overly tiny pixels at this FL doesn't sound like a great idea if you can't do 'real' binning.

Olly

[carastro]

Quote

3) Will I be able to use my Baader MPCC corrector still? 

4) Which software to use to control these things?  (Will it need to interface with PHD2 for dithering?)

3) Yes

4) Depends on what camera you buy, APT and SGPro seem to work on a range of cameras and will interface for PHD2 for dithering.

I like Olly know nothing about the CMOS cameras, but I would recommend you get a an electronic filterwheel capable of taking all the filters you are planning to use, most likely LRGB + Ha, Oiii, Sii = 7 filter positions.  

The larger format cameras will take a nice wide FOV, but they can come with their own problems.  Such as needing large filters, are more prone to problems with taking flats and getting good shaped stars in the corners.

There are also the cameras to take into consideration which have their own built in filterwheels, but I am not sure whether they can take as many as 7 filters, some-one else will need to answer that question.

Carole 

[mikey2000]

Good info about the filters - thanks!  And I totally agree about the need to save'n'spend - "buy once, cry once" etc.

 

Hmm.  This oversampling/undersampling thing is a pretty new concept for me.  I shall have to go an read up.   My current A6000 camera has 3.9micron pixels.  That's quite close to the ASI1600mm.

 

 

Is there a good way of estimating the seeing I get?  PHD2 gives dec and RA RMS of 0.5" each (0.7" overall rms) when guiding the mount.  I put my guide camera in to the main scope just out of curiousity and got very similar results (my QHY guide camera has 3.75micron pixels, similar to my A6000)   Is that a good way of estimating the seeing?

 

I've just thrown the central 1/3 of every frame I ever kept from this new mount into the Pixinsight SubframeSelector script -  FHWM gives between around 4.0 per pixel for nearly all of the 70-ish raw unprocessed frames (all 5 or 10min exposures with my A6000)  Some are above, some are below.  The seeing has definitely not been great hear all January.  I looked back at some of last year's raw files and found some FHWM's down to 3.0.

 

I threw that data with my scope and proposed camera into https://astronomy.tools/calculators/ccd_suitability

It seems the ASI1600MM pixels are *just* maybe a tiny tiny bit too small for my "poor" expected seeing conditions. 

 

Is it fair to say that software binning 'doesn't count'?    

Is slight oversampling a big issue?

Is it better to over or undersample (just a little either way)?

 

 

[kens]

I don't get overly excited by oversampling. I think the aversion comes from cameras with higher read noise where small pixels mean quite long exposures are needed to collect enough electrons to bury the read noise. With CCD, hardware binning improves the SNR and makes the pixels bigger but in CMOS the architecture does not lend itself to that. But the low read noise means you don't need to collect as many electrons to bury the read noise so binning is not as necessary.

If you are oversampled you can "bin" in post processing which can give an improvement in SNR without loss of resolution.

With undersampling you are losing information.

 

[carastro]

I find binning very useful with my CCD cameras as I can get twice as much data in the same time.  I only use it for RGB and Oiii and Sii leaving the Luminance and Ha non binned as that is where the fine detail comes.

I have been imaging for some 8 years now and I don't even know what under and over sampling is, can some-one explain it to me please.

Carole 

[alexbb]

In addition to all above, as a general rule, you should aim for a resolution about 2x lower than your tracking RMS error. So for a 0.7" RMS tracking error, you should go for a 1.4"/pixel resolution. I expect the EQ-6 R tracking and guiding capabilities to be better than this so this means that the limit is the seeing.

For the same condition as yours, I chose the 130PDS with the SW CC which results in a ~585mm focal length and 1.33"/pixel with the ASI1600.

 

Carole, let's say your scope has a 1"/pixel Dawes limit (116mm aperture). If the pixel size/focal length combination aims for a resolution higher than this, you're oversampling the scope's resolution. With 3.8u pixels you reach this resolution with ~780mm focal length. If you're aiming for a lower imaging resolution, you're undersampling. Undersampling can lead to stars recorded on a single pixel (which will look rectangular when zoomed >100%) and loss of details the scope can really provide. Oversampling tries to see details that the scope can't provide. The smallest stars will cover more than a single pixel, but there's no real gain to oversample too much.
Same applies to seeing and tracking and pixel size/focal length ratio.
All of these 3 limits (scope's Dawes limit, seeing and tracking) combined will yield a lower reachable resolution limit than each limit itself.

The Dawes limit does not apply for lucky imaging (very short many exposures for bright targets).

[wimvb]

14 hours ago, mikey2000 said:

Loungers, once more I call upon your help and advice!

 

I've recently posted a thread 'trying to understand LRGB imaging' and I'm now persuaded to make the step to a 'proper' astro camera.

I'd like to do this for two main reasons...   I can't seem to get the faint red stuff very well with my non-modified Sony A6000 (24mpix APS-C)  I'd like to see 'deeper' and also get into this Ha super-long exposure stuff.  I have a mount that will cope now (EQ6-R).  Secondly, I'm fed up with my A6000's lack of remote control options.

 

So, the present kit-list is EQ6-R, A6000, Skywatcher 150PDS, Baader Mk3 MPCC coma corrector (2" wide), Baader Neodymium 2" light pollution filter (rather necessary where I live...) All happily guided by QHY guidecam and a miniguidescope.   All linked up by a powered USB2.0 hub to my windows 10 laptop.

 

I'm most interested in photographing the DSOs (galaxies, clusters, nebulae inc supernova remnants like M1, dumbell etc)    My imaging sessions tend to be 2-3 hours or so before I pack it all away for the night.

 

Now, I'm facing a long list of unknowns - I'm not 100% sure of what I need.    I'm sure I will need:  Camera, Electronic/USB filter wheel, set of filters, some sort of capture/control app for Windows10.

 

The vague bits for me are:

1) what camera?

2)What wheel and what size filters? 1.25 would be nice - compact and cheaper.  Would they be OK for a 4/3" or 1" sensor?

3) Will I be able to use my Baader MPCC corrector still? 

4) Which software to use to control these things?  (Will it need to interface with PHD2 for dithering?)

5) Will I still need to use by neodymium filter? (Maybe not for Ha capture, maybe yes for L,R, G and B )

 

For a camera, I'm thinking along the lines of a ZWO ASI1600M (4/3" sensor) as this is close to what I'm used to with an APS-C sensor.  I often end up cropping the APS-C image anyway.  Also, the ZWO ASI 183MM-PRO looks interesting - slightly smaller sensor but the specification looks very good  plus it includes some kind of ram buffer to help avoid dropped files.  As I'll be using USB2, is this important??

 

That 183MM is right at the top end of my budget (allowing a few hundred quid extra for filters and a wheel.)

 

I've only mentioned CMOS  - should I consider CCD cameras?  (Apparently CMOS can't bin - should I care?)  CCD seems more expensive and you end up with fewer megapixels.  Should I care?

 

Of course, I'm open to other ideas too - I'd definitely like to stay with the mid-size sensors and avoid one-shot colour cameras.  I'm persuaded that LRGB+ha+other filters offer more possibilities.

 

 

All advice happily received!

 

 

You setup is close to what I have. (See signature). I'm very happy with my ASI174, but wouldn't have minded a larger sensor. Otoh, the larger pixel size and smaller sensor keeps filesize down. Great if you need to process 100+ subs & calibration frames.

ASI cameras work great with the baader cc. Usb2 isn't a problem, but you will get some amp glow (will be less in a camera with buffer). Amp glow is readily calibrated out with matching darks. The QHY equivalents of the models you mention have less amp glow.

Ccd is more expensive in £/mm of sensor. Variable gain of cmos makes imaging less dependent on guiding.

I wouldn't combine a light pollution filter with other filters, unless absolutely necessary. The more glass you have, the higher the risk for reflections. You could swap the clear glass L filter for a lp filter. The ZWO red/green filters are designed to block sodium and mercury light.

ASI cameras support both ascom and indi, and computer control isn't an issue.

Hope this helps.

[carastro]

Quote

Carole, let's say your scope has a 1"/pixel Dawes limit (116mm aperture). If the pixel size/focal length combination aims for a resolution higher than this, you're oversampling the scope's resolution. With 3.8u pixels you reach this resolution with ~780mm focal length. If you're aiming for a lower imaging resolution, you're undersampling. Undersampling can lead to stars recorded on a single pixel (which will look rectangular when zoomed >100%) and loss of details the scope can really provide. Oversampling tries to see details that the scope can't provide. The smallest stars will cover more than a single pixel, but there's no real gain to oversample too much.
Same applies to seeing and tracking and pixel size/focal length ratio.
All of these 3 limits (scope's Dawes limit, seeing and tracking) combined will yield a lower reachable resolution limit than each limit itself.

The Dawes limit does not apply for lucky imaging (very short many exposures for bright targets).

Thanks Alex, so how do I work out my Pixel size/resolution?

I have 3 scopes, and two cameras. 

SW ED80, SWED120, WOZS71 & Atik314L and Atik460EX

Am I over/undersampling, I have no idea? 

I am rubbish at all this technical stuff, I just image and hope for the best. 

Carole 

[swag72]

I'm not a slave to theory I'm afraid and the under sampling / over sampling stuff turns me off. I prefer to look at examples in the real world... One rig works at 3.38" per pixel and that is fine. My other works at 0.63" per pixel and that works just fine too Both of these are not ideal according to theory, but they seem to produce OK results.

If I was looking at a new camera, my starting point would be with the FOV calculator  and looking specifically at framing and Field of view......... That would be my first priority. From there I'd be looking at manufacturers that make the camera with the sensor that I wanted and whether I can get specifically what I wanted...... for example, all of my camera's have an integrated filter wheel and 2 have an integrated Off Axis guider..... for me this is very important and means that the filters are close enough to the sensor to be able to use 1.25" filters....... they are cheaper is the advantage and the disadvantage is that these filters have now affected the camera choices I can make..... If I want the nice new larger chipped cameras for example I'd have to change my filters... On account of them costing well over £1k at the small size this is NOT happening!

I hope that these thoughts help  

[alexbb]

Fill your details in http://astronomy.tools/calculators/field_of_view/ in imaging mode and check if the resolution is above or below the Dawes limit.

[swag72]

14 minutes ago, moise212 said:

Fill your details in http://astronomy.tools/calculators/field_of_view/ in imaging mode and check if the resolution is above or below the Dawes limit.

And if it above or below the Dawes limit? ................

I have one rig above it and one rig below it.... So? 

[AKB]

1 hour ago, swag72 said:

Both of these are not ideal according to theory, but they seem to produce OK results.

Sara, I think that has to be one of the biggest understatements I’ve ever heard, judging from your images. 

[alexbb]

35 minutes ago, swag72 said:

And if it above or below the Dawes limit? ................

If the aimed imaging resolution is higher (covering less arcsec/pixel than the limit. ie. 0.9"/unit(pixel) with a Dawes limit of 1"), you oversample and you're sure that you record all the details the aperture can provide.

If above the limit (lower aimed resolution than the limit), you sacrifice the maximum theoretical resolution the scope can provide in favour of an increased SNR.

Nothing wrong with either.

You will stop seeing the benefits of oversampling when exceeding the limit very much.

You will have less details and bigger stars when shooting a target through a ~67mm lens and 135mm focal length (135 F/2) compared to a 66mm scope and 330mm focal length (66 F/5) and doing a mosaic covering the same area. Same camera, of course and high quality optics.

[swag72]

Thanks for the explanation......... interesting. ??

But in the real world....... does it actually make a difference? Of course I have often complained about  my rigs under sampling and over sampling when I'm gathering data and subsequently processing an image. How often I can be heard moaning that if only I was at the Dawes limit everything would be so much better ??

???

[ollypenrice]

Because theory and practice seem so reluctant to hop into bed together in AP I now treat sampling rates as interesting but not always of much use. I do think that using a DSLR in an SCT is going too far against theory, for instance, and the system will simply give a smaller FOV, need more time and produce no more detail than could be had at half that focal length. But, like Sara, I've imaged happily between 0.66 and 3.5"PP. I wouldn't go beyond 3.5 but there is no stellar pixelation whatever at 3.5. This really does need emphasis.

Kens makes a good point about CMOS oversampling being less of an issue with the low read noise.

And then there's that data lent to me by Julian Shaw on the Sombrero. About 0.4"PP (at about F15). The result is to die for.

Theory makes you weary!!!

Data makes you greater!!!!!!!

lly

 

[ollypenrice]

2 hours ago, swag72 said:

And if it above or below the Dawes limit? ................

I have one rig above it and one rig below it.... So? 

So you're a very naughty girl!  Let's have no more of it, you hear?

Olly

[wimvb]

2 hours ago, swag72 said:

And if it above or below the Dawes limit? ................

I have one rig above it and one rig below it.... So? 

I'd say that on average, you're perfectly sampled.

When Göran and I processed images from the Liverpool telescope, I found it very easy to process those oversampled datasets. On astrobin, I generally prefer (galaxy) images with a smaller "/pixel value. For nebulae, I don't see much difference between small or larger sample scales. There's so much more that makes a great image.

But between Dawes limit, diffraction patterns, seeing, and guiding errors, I don't think pixel scale is that critical. That being said, I think I'd still prefer to be on the low side, just because I feel that "processing seems somewhat easier".

And as for theory: all the above factors (Dawes ... guiding) and pixel scale, determine the final "sharpness" of an unprocessed image. The question is, which of these factors would you want to be the limiting one?

[alexbb]

4 minutes ago, wimvb said:

The question is, which of these factors would you want to be the limiting one?

I think we can conclude that the EQ6-R mount should be able to track&guide better than 0.7" RMS with a not so heavy scope and the limiting factor then is the seeing. 0.66"/pixel seems rather too ambitious for the given conditions and will give no real benefits compared to using the larger pixels.

[swag72]

26 minutes ago, wimvb said:

And as for theory: all the above factors (Dawes ... guiding) and pixel scale, determine the final "sharpness" of an unprocessed image. The question is, which of these factors would you want to be the limiting one?

I would think that for most of us mere mortals, the sky conditions would be the biggest limit ..... am I right?  

[mikey2000]

Interesting discussion!  I have been illuminated.  I think I'll be near some theortical limits but that also these limits are just guidlines (unless you miss the mark by a *very* wide margin...)

 

I've also had some advice on the various spacers and fittings to get a 7x wheel to work with the 1600MM-Cool Pro and also with my Coma Corrector.   I'll just need to go and tot up the final price...

[wimvb]

54 minutes ago, swag72 said:

I would think that for most of us mere mortals, the sky conditions would be the biggest limit ..... am I right?  

In my case it's a close call between sky conditions and guiding. I've had less than satisfactory guiding results lately. Otoh, that may also be caused by sky conditions. But yes, unless you're high on a mountain top, I'd say the sky is the limit. 

[alexbb]

42 minutes ago, mikey2000 said:

the 1600MM-Cool Pro

Just to stress: neither of the the MM-C variants has a memory buffer. MM Pro is the one with a memory buffer. The ZWO ASI cameras ending with a -C have cooling, but no buffer, whereas those marked as Pro have cooling and buffer.