Pitch Black Skies

Definitely a bit of an improvement here I think..

Before

After

24 minutes ago, symmetal said:

Here's my effort using Startools and ( PS to lift the background). Not so much noise reduction but that's a personal thing. The Spacially Variant PSF Decon has made spots in the bright stars but I used the default mask. I usually just create a second blurred layer in PS and soft edge mask in the blurred version over the star centres

Alan

That's pretty cool.

1 hour ago, Budgie1 said:

Not really helpful if you're using StarTools but, I ran your image through PixInsight to see that I could get with the data and here's the result.

 

That's really nice. Great detail in the core. Background light level looks good.

57 minutes ago, bottletopburly said:

my version skipped  a few modules just to see, so autodev ,wipe ,autodev,contrast ,colour ,noise reduction 4.0 pixels 

 

Yeah that wouldn't be my cup of tea. Definitely an improvement over mine though.

1 hour ago, alacant said:

For the second AutoDev, you are selecting neither a region of interest nor setting the gamma, fine detail or shadows, after which the stars and background will be taken care of.

HTH

Is it just a matter of playing around with those settings until it looks pleasing to the eye?

I've just tried cropping in with the ROI levels and it reduced the stretch considerably, so made a huge difference. I didn't touch gamma, shadows and fine detail.

TY

1 hour ago, alacant said:

Try AutoDev instead of Film. 

So I am trying AutoDev after Wipe now but it is redoing the global stretch and looks far too aggressive. Is there a way of dialing it back a little? What do think I am doing wrong?

45 minutes ago, vlaiv said:

I think it is down to processing.

Here is simple processing done in Gimp and ImageJ.

Color calibration is on a single star (B-V index of 0.16) and I did not pay much attention to color work flow, but data is good and sharp. Not sure what step in processing above resulted in "mushy" looking image.

 

Excellent.

It would be great to find out how I can produce something similar with just Startools.

3 hours ago, alacant said:

My guess is that you used Film Dev.

Yes, I start with AutoDev, Crop, Wipe, and then FilmDev. I typically stretch to between 96-97% in Digital Development.

49 minutes ago, ollypenrice said:

dependng on what software you use? 

DSS for stacking, Startools for post processing.

@vlaiv M101 19 hr 10 mn calibrated.zip

1 hour ago, vlaiv said:

That is already stretched version like one attached above as image, so not really usable for anyone wanting to see quality of the data itself.

Thanks, will attach un-stretched file soon.

@alacant

M101 19 hr 10 mn calibrated.tiff

Hi guys,

Could I get a bit of feedback please?

I feel my pictures look heavily processed and unrealistic. This is an example of M101.

It is 19hrs fully calibrated. I use DSS to stack and Startools to post process. I think Startools is the culprit, however I am only using each module once and never increase their parameters.

12 hours ago, Padraic M said:

Obviously the crop shows some 'artistic license' 😀

Don't know what that means. Are you referring to the name of the file being very long?

"in my opinion the background is too black - I suspect you may have clipped some of the darker detail."

When I make it brighter, the galaxies tend to look animated or CGI.

2 hours ago, vlaiv said:

As far as I know - yes, Sharpcap has the feature (not sure what it is called) - that can be used to determine necessary exposure length.

If you want to do it yourself - procedure is rather simple:

- take one of your raw subs and calibrate it properly.

- convert ADU values to electron values - you can read e/ADU value from fits header or from data published by manufacturer of the camera. This is simple multiplication of pixel values with constant.

- select patch of sky without any target / nebulosity and as empty as possible (odd faint star here or there should not be a problem). Measure median value in that patch of the sky. If patch is completely void of objects you can also use mean value, but median is better as it will remove impact of few stars if there are any.

- take 25 and multiply with squared value of read noise at given gain settings and divide with above value - this will give you factor needed to multiply current exposure length to get proper one.

Example:

You have sub that you debayered since camera in question is 2600mc, that you exposed for 3 minutes at gain of 100. e/ADU value for this gain is ~0.25, so you multiply pixel values with 0.25 to get electron count (use green channel).

Now you measure background value and you find that it is 200e, you also see that there is 1.5e of read noise at gain 100 from ZWO graphs.

Exposure length multiplier will be 25 * 1.5^2 / 200 = 56.25 / 200 = 0.2815

You exposed for 3 minutes or 180s when in fact it was enough to expose for 180 * 0.2815 = 50.625s or ~50s

(if you get number greater than 1 - that will mean longer exposure than you used, if you get smaller than 1 that will mean shorter exposure than you used).

Is there any benefit to the OP exposing for 180s over 50s? Is 50s the minimum amount of time to expose, and then anything longer increases SNR?

On 20/03/2022 at 11:25, Padraic M said:

I've just finished first light with the ASI533MC Pro. I've spent the weekend in Bortle 2 but with a full moon and a breeze so good conditions but not ideal. Bode was a good test subject as it was at a good remove from the moon.

The 533 is very easy to handle with the SW 150PDS and TS coma corrector - just using the spacers supplied with the camera to give the correct back focus. I could cool to -20c when outside, but couldn't match it the following day to shoot darks, so the image below is shot at -20c and calibrated with -15c darks. I'll shoot at -15c or -10c in future. 

Stacking RGB in APP is a lot more straightforward than mono too - one workflow rather than three! 

On the downside, I'm surprised that there wasn't more colour in the final image. The galaxy is a jaundiced yellow straight out of APP, and the stars are also yellow. APP star colour calibration (csc) washed out the galaxy and still didn't add much colour to the stars. I experimented with galaxy-only and star-only frames using Starnet++; also tried Startools; but in the end settled with a lpc-cbg (no star calibration) output from APP with some ASI1600MM Ha added from an earlier session and finished in Gimp.

Total exposure is 3hrs 20m RGB + 2hrs 45m Ha.

I've continued to struggle with flats for the 150PDS as it leaks light, but I think I have it sorted now - there is light ingress from the secondary end, and also around the focuser drawtube. There were some remaining circular bands of brightness but thanks to @mackiedlm's recent capture, I've realised that they are actually IFN!

Any comments on better star colour or size appreciated.

Really nice 👍

Mine seem to end up looking unrealistic.

Any ideas?

8 hours ago, vlaiv said:

While pixel size and guide RMS are often connected with some sort of rule of the thumb - I'd rather try to explain it a bit differently.

For the moment, forget pixel size. Forget pixels completely.

Let's just look what happens at focal plane of the telescope as the light comes in. There are three different major contributor to the blur that happens to the image.

- seeing

- guiding/tracking precision

- aperture size

Each one of these produces some level of blur on its own. In perfect conditions with perfect guiding - there is maximum magnification of the telescope that can be used. After that image just gets bigger without detail - it is blurred. Seeing of course blurs the detail and tracking precision does the same.

When all three are present - they combine and produce larger blur then each of individual components. Thing is - they don't combine in trivial way - just by simply adding some numbers. They combine a bit more complicated than that (in fact - quite a bit more complicated by process called convolution). However, we can simply things by using some approximations that will help us understand things.

Each of components can be represented by RMS value. Guide RMS comes as RMS value already. Seeing comes as FWHM, but can be converted to RMS by simply dividing it with 2.355. Telescope aperture or more precisely Airy disk it produces can also be represented as RMS (although somewhat more complicated calculation) - but for the time being, we won't pay much attention to it - other than to say that for most apertures - like 80mm+ - it is the smallest component.

You will notice that I emphasizes the smallest component in last sentence - that is for a reason.

Simplified formula for calculating total blur goes like this: square_root(first_rms_squared + second_rms_squared + third_rms_squared).

So we have square root of sum of squares. If this reminds you of Pythagorean theorem - then great, because we want to look at it in that way:

why? Well, because of this case:

If shorter leg is significantly smaller than longer leg then hypotenuse is almost the same length as longer leg. I'll reiterate in a bit different way - if one component of the sum is much smaller then the others - then it is contributing much less.

You will say - hold on, that is true for ordinary sum as well: 10 + 1 = 11, 10 and 11 are not that far away. Yes, but look what happens when we add 10 and 1 in quadrature - square_root(100 + 1) = ~10.05

Look how much smaller the difference gets when things are added this way.

What does it all mean - how big your guiding error is then? Well - that depends on largest factor, or what your guide RMS is compared to other big RMS out there that is seeing.

You mention seeing of 1.2" FWHM - well, year, that does not happen , or rather happens once a year on average site if at all.

Usual value is 2" FWHM or 3" FWHM if seeing is average to poor. 1.5" FWHM is excellent seeing for most sites.

Let's translate that into RMS:

1.5" FWHM = 0.637" RMS

2.0" FWHM = 0.85" RMS

3.0" FWHM = 1.274" RMS

4.0" FWHM = 1.7" RMS

In order not to contribute much - we need guide RMS to be quite a bit smaller than seeing RMS, and if your guide RMS is 1.2" - it is never significantly smaller than seeing RMS.

Guide RMS needs to be as small as you can make it. Simple as that. Only when you reach 0.2-0.3" RMS levels - you can say, ok, so I made it small enough compared to average seeing conditions (~x4 smaller) so I don't have to worry too much about it.

What about that - versus pixel size thing? Making your RMS half of your imaging scale is good rule of the thumb that works for most common cases. Here is an example:

Say you are imaging in 3" FWHM and you have 1.2" RMS guide error. Rule of the thumb says you should have 2.4"/pixel - as that is twice your guide RMS.

Let's see if that is true.

3" FWHM is 1.274" RMS and combined with 1.2" RMS that gives: sqrt(1.274^2 + 1.2^2) = 1.75" RMS or ~4.12" FWHM (when we multiply back with 2.355).

Optimum sampling rate for that level of blur is 4.12 / 1.6 = 2.575 - which is very close to 2.4

Even if you add aperture size in the mix - you still get very close results in common range - that is (1.5"/px - to 2.5"/px, 2" FWHM - 3" FWHM seeing and 0.7"-1.2" guide RMS, 4"-8" aperture).

However, if you want accurate results - there are complex formulae that will calculate effective resolution of your system and what pixel size to use (but these contain some approximation - like perfect optics, which is not always the case and so on ...)

Bottom line - make your guiding the best you can (lowest RMS value) always. Don't "settle" for it until you reach 0.2-0.3" RMS. Mind you - that low numbers are not always possible with mass produced mounts, so do research of what can be done and at what cost.

So I've read your post and really enjoyed it, and it's not the first time you've answered one of my questions with such clarity and detail. I really think you should write an astronomy/astrophotography book, seriously. I've been wanting to say it for a while.

Now here's an utterly stupid question.

How is someone supposed to know what FWHM they have at a given location?

6 hours ago, vlaiv said:

While pixel size and guide RMS are often connected with some sort of rule of the thumb - I'd rather try to explain it a bit differently.

For the moment, forget pixel size. Forget pixels completely.

Let's just look what happens at focal plane of the telescope as the light comes in. There are three different major contributor to the blur that happens to the image.

- seeing

- guiding/tracking precision

- aperture size

Each one of these produces some level of blur on its own. In perfect conditions with perfect guiding - there is maximum magnification of the telescope that can be used. After that image just gets bigger without detail - it is blurred. Seeing of course blurs the detail and tracking precision does the same.

When all three are present - they combine and produce larger blur then each of individual components. Thing is - they don't combine in trivial way - just by simply adding some numbers. They combine a bit more complicated than that (in fact - quite a bit more complicated by process called convolution). However, we can simply things by using some approximations that will help us understand things.

Each of components can be represented by RMS value. Guide RMS comes as RMS value already. Seeing comes as FWHM, but can be converted to RMS by simply dividing it with 2.355. Telescope aperture or more precisely Airy disk it produces can also be represented as RMS (although somewhat more complicated calculation) - but for the time being, we won't pay much attention to it - other than to say that for most apertures - like 80mm+ - it is the smallest component.

You will notice that I emphasizes the smallest component in last sentence - that is for a reason.

Simplified formula for calculating total blur goes like this: square_root(first_rms_squared + second_rms_squared + third_rms_squared).

So we have square root of sum of squares. If this reminds you of Pythagorean theorem - then great, because we want to look at it in that way:

why? Well, because of this case:

If shorter leg is significantly smaller than longer leg then hypotenuse is almost the same length as longer leg. I'll reiterate in a bit different way - if one component of the sum is much smaller then the others - then it is contributing much less.

You will say - hold on, that is true for ordinary sum as well: 10 + 1 = 11, 10 and 11 are not that far away. Yes, but look what happens when we add 10 and 1 in quadrature - square_root(100 + 1) = ~10.05

Look how much smaller the difference gets when things are added this way.

What does it all mean - how big your guiding error is then? Well - that depends on largest factor, or what your guide RMS is compared to other big RMS out there that is seeing.

You mention seeing of 1.2" FWHM - well, year, that does not happen , or rather happens once a year on average site if at all.

Usual value is 2" FWHM or 3" FWHM if seeing is average to poor. 1.5" FWHM is excellent seeing for most sites.

Let's translate that into RMS:

1.5" FWHM = 0.637" RMS

2.0" FWHM = 0.85" RMS

3.0" FWHM = 1.274" RMS

4.0" FWHM = 1.7" RMS

In order not to contribute much - we need guide RMS to be quite a bit smaller than seeing RMS, and if your guide RMS is 1.2" - it is never significantly smaller than seeing RMS.

Guide RMS needs to be as small as you can make it. Simple as that. Only when you reach 0.2-0.3" RMS levels - you can say, ok, so I made it small enough compared to average seeing conditions (~x4 smaller) so I don't have to worry too much about it.

What about that - versus pixel size thing? Making your RMS half of your imaging scale is good rule of the thumb that works for most common cases. Here is an example:

Say you are imaging in 3" FWHM and you have 1.2" RMS guide error. Rule of the thumb says you should have 2.4"/pixel - as that is twice your guide RMS.

Let's see if that is true.

3" FWHM is 1.274" RMS and combined with 1.2" RMS that gives: sqrt(1.274^2 + 1.2^2) = 1.75" RMS or ~4.12" FWHM (when we multiply back with 2.355).

Optimum sampling rate for that level of blur is 4.12 / 1.6 = 2.575 - which is very close to 2.4

Even if you add aperture size in the mix - you still get very close results in common range - that is (1.5"/px - to 2.5"/px, 2" FWHM - 3" FWHM seeing and 0.7"-1.2" guide RMS, 4"-8" aperture).

However, if you want accurate results - there are complex formulae that will calculate effective resolution of your system and what pixel size to use (but these contain some approximation - like perfect optics, which is not always the case and so on ...)

Bottom line - make your guiding the best you can (lowest RMS value) always. Don't "settle" for it until you reach 0.2-0.3" RMS. Mind you - that low numbers are not always possible with mass produced mounts, so do research of what can be done and at what cost.

Awesome, will have to give this a proper read later. Legend.

Hi @alacant

Following our chat earlier,

Is there anything obvious that you can see I should do to improve performance?

I'm thinking the next steps could be;

- a longer dovetail or Losmandy plate

- possibly flocking the interior although not too pushed on that

Cheers

1 hour ago, iantaylor2uk said:

There are some at the links below - note though that I didn't use a coma corrector or UV/IR filter for these photos, so they have been cropped a bit. I used an APS-C sized sensor (a ZWO 071 Pro camera). You can still see a bit of coma towards the edges. I now have a coma corrector for my reflectors! Also I collimated the reflector before imaging using a laser collimator.

M3 & M51: 10, 30 & 60 sec exposures: https://chesterastroblog.wordpress.com/2021/04/11/m3-and-m51/

M101: 60 sec exposures: https://photos.google.com/share/AF1QipOnmkqD1kWZYsvqMm8W8diHwSoXJn47oD7ZgiVB2iS3j6m_KeyJCI-lWtSR6Ss36g?pli=1&key=cnZtb3RGbHE3aWxNSGQ5OS1CN2dva1YyYzVnanpR

Hope this helps

 

Nice detail in them.

2 hours ago, alacant said:

If you had an eq6 it would be good.

Remember though that with all low end reflectors, there is still quite a bit of work to do to get them to astrograph standard. The difficulties with adjusting and retaining collimation are a result of not addressing the necessary upgrades; one collimates an f4 in EXACTLY the same way as an f5. The upgrades are outlined here. Apart from the dovetail plate, none are particularly expensive, but you will need time to pull the thing apart to replace the defective parts.

Even though it's still far from ideal, if you want to stay at 6" and comfortable on your mount, the TS UNC is the nearest sensibly designed f4 we've seen, but even then, the enormous secondary is blocking so much light that you may as well stay with your 130.

Cheers and HTH

Yeah it's probably too much for the HEQ5. I think I'll stick with the 130 for now. It's a great little scope when tuned up.

Would you recommend replacing the 130s dovetail with a Losmandy plate?

Actually, if I post a picture of my setup in the 130 thread, would you take a look and recommend some improvements or mods?

42 minutes ago, Adam J said:

My opinion in general on any scope is that you need to spend the big cash to get reliable performance below F5. So for a reflector that means going to something like a hyperbolic mirror Newtonian or a RASA etc. There are better F4 newts available but I dont know of any in the 6 inch class. 

Adam 

Do you think the Quattro 8s would be a more sensible upgrade?

40 minutes ago, Clarkey said:

The 130 is certainly capable. I only ever image one target per night and am able to leave my kit out so never have less than 6 hours integration time. Quite often for faint stuff it will be more than one night. Makes all the difference. F4 does help in this case.

In a similar situation myself but I try to get about 3 nights per target. I'm aiming for 16-24 hrs total integration time.

24 minutes ago, Clarkey said:

I think to add some context, here are a couple of images taken with the F4 6" TS Newtonian. This is with a new focuser and a number of mods to give OK results. Make your own judgement. There are also lots of images on line. It should be added that I am no expert and my processing skills still need work....

Really nice, although I wonder if the 130 could produce something similar? I'm guessing yes, but it would require more time.

9 hours ago, Clarkey said:

If you have an HEQ5 it is worth doing the belt modification to improve tracking. You should be able to get below 1” RMS with some fettling. I think mine is typically around 0.7. There is a predictive PEC algorithm within PHD2 in the advanced guiding settings. Change to this and you should see some improvement.

With regards to the 200pds this would be pushing an HEQ5. Although possible, any breeze will ruin your subs.

WRT OAG I am not sure it is needed on a 130. To be honest I have gone back to a guide scope on my RCI at 1600mm because of the issues I had with OAGing and difficulties finding good guide stars.

Ordered the belt kit yesterday, hopefully it will make a difference.

I use an ASIair Pro, I don't think there is any PEC training capability with it yet unfortunately.

I should probably try to spend more time with the 130. It's a very capable scope.