17 replies to this topic

[UP4014Fan]

I think I'm making process progress (edit - there's a Freudian slip if I've ever seen one!).  I've been using M101 as sort of a control, so I can gauge the improvement in my techniques.  I'd appreciate some thoughts from you.  

Before I go further, I do want to thank everyone here for being so generous in your advice to a newbie.  As I've mentioned, I've been a serious amateur photographer for nearly 50 years.  This is a whole new challenge with a very special learning curve.

This is how I processed M101, based on the suggestions you've given me:

 

First, the equipment:  Celestron NexStar8SE mounted on a polar wedge.  I used the Celestron .63 flattener/corrector in the optic chain.  Guiding was provided by a Celestron StarSense Auto Guider, with Precise Goto disabled and Guiding enabled in CPWI.  The camera was a Canon 200D (a/k/a EOS SL2).  Celestron Autofocuser is also on the telescope.  The scope has a dew ring and dew shield.

 

Software:  CPWI driving the mount, with NINA driving it.  Guiding was set as "Direct."  Polar alignment and alignment were done in CPWI.  NINA's framing wizard using plate solving (the fast one, I can't remember the name as I type this) was used to aim the scope after getting the coordinates from Stellarium.

 

Imaging:  585 thirty-second subs at ISO 1600 over two nights.  I used NINA's autofocus to ensure sharp images.  I had NINA dither 10 pixels every 10 frames.  I used NINA's flat wizard to generate 45 light flats and 25 dark flats each night, using a large tracing tablet.  I also shot 40 thirty second darks each night.  Finally, I shot 40 bias frames at ISO 1600 and .001 second each night.

Processing was in PixInsight, using a keyword to combine the two nights' effort and 1x drizzle in WBPP.  I stretched the galaxy and the stars separately after using PI's gradient correction process, SPCC, BlurXterminator, and NoiseXterminator.  

So here's the result. 

First, the original image had a huge circular gradient.  Interestingly, it was a smoother gradient now that I've added the dew shield, so, as someone pointed out to me in another topic related to this, I was probably getting stray light.  PI's gradient correction process seemed to clear up a lot of this, but there's still an interesting artifact.  If you look closely at the edges of the image, you'll see a faint rectangular gradient.  When stretched, this shows up all the way around the frame.  Is this light bouncing around the optic chain, particularly the rectangular opening in the camera ahead of the mirror?

Second, if you look closely at the galaxy itself, you'll see some faint evidence of walking noise.  Am I not dithering enough, or could this be something else?

Third and finally, are the above still due to a low SNR?  I know M101 is 7.86 magnitude and even on a good night, I'm fighting the light pollution in my town west of St. Louis.  How much integration time should I be considering?  I know about improvement being a function of the square of increasing the processing time, so I'm trying to get a sense of reasonableness.

Thanks again.  I appreciate your help!

 

Bob

Edited by UP4014Fan, 12 June 2024 - 09:54 PM.

[rj144]

You shot bias and dark flats?  You need one or the other, not both.  Also, shouldn't the bias be a faster shutter speed?  Isn't the fastest speed 1/4000th of a second?

 

What is your Bortle zone?

Edited by rj144, 12 June 2024 - 11:06 PM.

[jml79]

A few thoughts. You are crushing the blacks to much IMHO. There should be a hint of something in the background. It can really help to make sure you aren't crushing some details. You are off to a good start.

 

Without seeing the unedited stack, all I can really comment on is the processing. Many beginners can find it useful to share the unedited stack on google drive or something and let some experienced people do a processing run. It can generate more input on your capture and processing because we can see a lot more. It can also provide a base line on what is actually in the image. You have just under 5 hours of time on M101 and unless your light pollution is horrible, I expect that there is a lot more detail in there than you are able to pull out yet.

[PirateMike]

Many beginners can find it useful to share the unedited stack on google drive or something and let some experienced people do a processing run.

Ya, I would love to see what this acquisition could look like with some better processing, and the OP can see how excellently his capturing process is working.

 

 

Miguel   8-)

 

 

.

Edited by PirateMike, 12 June 2024 - 11:54 PM.

[UP4014Fan]

A few thoughts. You are crushing the blacks to much IMHO. There should be a hint of something in the background. It can really help to make sure you aren't crushing some details. You are off to a good start.

Without seeing the unedited stack, all I can really comment on is the processing. Many beginners can find it useful to share the unedited stack on google drive or something and let some experienced people do a processing run. It can generate more input on your capture and processing because we can see a lot more. It can also provide a base line on what is actually in the image. You have just under 5 hours of time on M101 and unless your light pollution is horrible, I expect that there is a lot more detail in there than you are able to pull out yet.

I agree that I’m clipping the blacks. However, if I don’t, the remnants of gradient show up.

Someone asked for the Bortle number in my area. At my stage of the game, that’s like me asking a student pilot if the turbulence was moderate. I’ll guess five or six, based on the definitions I’ve seen.

I can upload the files to a shared drive later today.

[UP4014Fan]

You shot bias and dark flats? You need one or the other, not both. Also, shouldn't the bias be a faster shutter speed? Isn't the fastest speed 1/4000th of a second?

What is your Bortle zone?

As for shutter speed for the bias images, I assumed that 1/1000 was sufficient. Is there a significant difference in the sensor noise at that point?

I didn’t know that it was one or the other for darks or biases. I assumed both would be helpful. Was there a downside to include both?

As for my Bortle Zone? Well, I’m pretty new to all this, so this is something of a wag, but I’ll say five or six.

[imtl]

I agree that I’m clipping the blacks. However, if I don’t, the remnants of gradient show up.

Someone asked for the Bortle number in my area. At my stage of the game, that’s like me asking a student pilot if the turbulence was moderate. I’ll guess five or six, based on the definitions I’ve seen.

I can upload the files to a shared drive later today.

 

 

As for shutter speed for the bias images, I assumed that 1/1000 was sufficient. Is there a significant difference in the sensor noise at that point?

I didn’t know that it was one or the other for darks or biases. I assumed both would be helpful. Was there a downside to include both?

As for my Bortle Zone? Well, I’m pretty new to all this, so this is something of a wag, but I’ll say five or six.

Bob,

 

If you have remenants of gradients, the solution is not to clip your blacks, but to improve your gradient removal techniques. When you're clipping blacks you are removing data.

 

According to your description, this whole game is brand new to you, yet you already gone to use AI tools like the Xterminators. Maybe it is better to master simple processes first? That would be my suggestion.

 

The shutter speed for bias should be the fastest your camera is capabale of. 

 

The comment above was about bias and DARK FLATS. It should be one or the other. Not both. Do you understand the purpose of each type of calibration frame? If not, just ask.

[TelescopeGreg]

As above.  If you have taken Dark-flats, you don't need Bias.  Bias can be an optimization that can apply for some situations, and I've not found their use to be helpful mostly because they're different.  Especially for folks getting started, for calibration think pair of pairs...  Lights and Darks, Flats and Dark-flats.  No need for Bias.  After 7 years in the hobby, I still don't use them.

 

What sorts of pixel values are you getting with your exposure setting?  I'm guessing they're pretty low.  When I was using a DSLR for imaging, I had the ISO cranked up pretty high in an attempt to compensate for the short (20 second) exposures I was using, and had some pretty noisy images with a lot of walking noise as a result.  Increasing the exposure by 50% to 30 seconds and reducing the ISO helped tame the noise.  That was with a relatively fast f/5 telescope; yours is slower.  I'm guessing your 30 second exposures were set by the camera as a maximum limit.  If you can increase the exposure and not saturate too many stars as a result, you might try a longer exposure by getting an external "Intervalometer" to drive the camera.  M101 isn't a super bright target, so you should have a bunch of headroom there.

[Spaceman 56]

 Lights and Darks, Flats and Dark-flats.  No need for Bias.

If you use Siril and the One Shot Color Script, you have to include biases, or the script won't work. 

 

So I am still using biases, and not Dark-flats. 

 

Spaceman. 

[Oort Cloud]

The only reason to use dark flats is because of non-linearity or amp glow. Modern sensors (IMX5xx) do not require them. It is just adding extra complexity with no benefit.

[TelescopeGreg]

The only reason to use dark flats is because of non-linearity or amp glow. Modern sensors (IMX5xx) do not require them. It is just adding extra complexity with no benefit.

So I understand what you mean, but I disagree when I look at it from a different perspective.  The reason I suggest not using Bias (excepting where software requires it) is that they're "different".  From a teaching perspective, Lights and Darks (aka Dark-lights) are a pair of image sets, taken the same way, same exposure, same temperature, same gain, same everything but with one set having the lens cap on.  Similarly, Flats and Dark-flats.  The two sets are taken the same way, with the same rules, handled the same way in processing, etc.  "Pair of pairs".  Yes, there's a cost, but Dark-flats always work regardless of the camera. 

 

That's not to say that Bias doesn't have its place; it does as an optimization for some circumstances, with some software, and some cameras (if not most), for folks who know what they're doing.  But as a beginner trying to learn stuff about this complicated hobby, it's important to be taught a simple structure that works.  "Same" is good when you're trying to learn.  Bias is different.  The moment you start explaining the various contributions to a pixel's ADU, and what part Bias represents, you've lost them.  On the other hand, "Pair of Pairs" is easy to explain. 

 

For me they ended up causing more grief than any potential optimization that might have been had.  I expect I had done something wrong, but as I recall I ended up with a really bad couple of columns of dead-ish pixels right through the heart of M33.  After a lot of wasted time I tried leaving out the Bias frames during stacking, and the column cleared up enough for a reasonable image.  I've never used them since, never spent the time to try to understand what happened, and never regretted it.  I also have evidence that my new camera (new since the M33 image) actually needs Dark-flats in order to properly calibrate a couple of hot pixels that aren't represented by Bias.

Edited by TelescopeGreg, 13 June 2024 - 10:04 PM.

[jml79]

If you use Siril and the One Shot Color Script, you have to include biases, or the script won't work. 

 

So I am still using biases, and not Dark-flats. 

 

Spaceman. 

Process wise, Dark flats replace Biases and the correct thing to do with Siril is put your Dark Flats in the Biases folder (and only the dark flats, no biases) and then it will work just fine. I have the infamous 294 camera and must use dark flats because it is non-linear below 3s and has huge amp glow. It works just fine in Siril putting the dark flats in place of the biases. But your camera is a 571(2600) so just use biases anyway. No need for dark flats unless you experience issues like Greg.

Edited by jml79, 13 June 2024 - 11:01 PM.

[UP4014Fan]

I reprocessed the stack last night without the bias frames.  The linear image looks the same, with a strong gradient towards the center (dark on the edges).  I then looked at my master flats and they are, well, dead flat, with just random noise.  Is it possible that I have something wrong in NINA where I’m overwhelming the sensor?  

 

Or, I’ve noticed that the new version of WPBB doesn’t differentiate between light and dark flats (only one entry for flats) - is WPBB getting confused?

[rj144]

I reprocessed the stack last night without the bias frames.  The linear image looks the same, with a strong gradient towards the center (dark on the edges).  I then looked at my master flats and they are, well, dead flat, with just random noise.  Is it possible that I have something wrong in NINA where I’m overwhelming the sensor?  

 

Or, I’ve noticed that the new version of WPBB doesn’t differentiate between light and dark flats (only one entry for flats) - is WPBB getting confused?

What are the ADU of your flats?

[UP4014Fan]

I seem to have found the problem.  When looking at my flats, they are dead white, with no gradient whatsoever.  Even taking them into photo shop and backing them down five full stops only yields gray card images.  

 

With that, I found a procedure to generate a synthetic flat in PI published on the Trapped Photons website.  It's not perfect, as you'll see in the image below.  I still had to run the image through PI's gradient corrector.

So what did I do wrong in NINA to blow out the flats?  Obviously, since I've moved the camera, etc., I can't reproduce the optics chain to fix this one.  But the next time I set up, I want to set NINA's Flat Wizard correctly.

 

Thanks in advance.

 

Bob

 

Edited by UP4014Fan, 16 June 2024 - 05:57 PM.

[UP4014Fan]

What I think is a final go around for this one.

I separated the stars from the image, then used PI's gradient correction and clone stamper tools to flatten the gradients as much as possible. I then brought up the saturation of the galaxy a bit to bring out the dust clouds and star lanes.

 

[Oort Cloud]

Synthetic flats is a nice tool to have in your belt. Obviously it's best to not need them, but I had to do it as well when I was starting out.

For NINA, I have the flat wizard working pretty well, and I can't honestly think of any settings that I needed to set in order for it to work properly, but I am using an astrocamera. For a DSLR, it very well probably depends on your camera settings, and how you have the camera working (firing it with NINA or using an intervalometer, for example), so I'll let the DSLR gurus handle that part.

But I'm fairly certain (again, I have no real need for this) that once you start the flat wizard, you should be able to switch to the imaging tab to view the flats as they're taken so you can see if there are issues with them in near real-time.

[UP4014Fan]

Synthetic flats is a nice tool to have in your belt. Obviously it's best to not need them, but I had to do it as well when I was starting out.

For NINA, I have the flat wizard working pretty well, and I can't honestly think of any settings that I needed to set in order for it to work properly, but I am using an astrocamera. For a DSLR, it very well probably depends on your camera settings, and how you have the camera working (firing it with NINA or using an intervalometer, for example), so I'll let the DSLR gurus handle that part.

But I'm fairly certain (again, I have no real need for this) that once you start the flat wizard, you should be able to switch to the imaging tab to view the flats as they're taken so you can see if there are issues with them in near real-time.

Yep - you can see them in near real time.  However, as they were firing off, in my lack of experience in this type of photography, I thought there was something more subtle happening that I couldn't see. Nope - just blown-out images.  Live and learn.  

It's going to be hot and humid with bad seeing the next few days here in St. Louis.  I'll set up everything in the basement and play with the flat wizard settings so I get a good ballpark figure.