Can you get rid of dark smudges in APP?

What images did you use to create the one you posted? Lights, Flats, Darks, Bias?

Posted by: @wvreeven

What images did you use to create the one you posted? Lights, Flats, Darks, Bias?

All of the above.  Unfortunately this will not calibrate out.

Yesterday I believe that I found the issue on the glass of the flattener.  Ironically I noticed this smudge on the previous shoot, I thought it had to be on the telescope glass which I cleaned but I was wrong.  There is a small mark on the flattener which I will clean up.

If I could get the light pollution tool to make the background "really" black I could get rid of the issue. Just seeing if somehow this could be salvageable.

If it is caused by a smudge on the flattener then I'd say it should calibrate out by using proper flats. The fact that at first it showed up as one dark smudge and then as two seems to indicate that either the smudge has moved or the camera has been rotated in between the first and second set of images. Either way is means that in my humble opinion there is room for improvement in the way you take and apply your flats. Did you take flats for both sessions or are you reusing flats from a previous session? I am inclined to guess the latter but I'd prefer you give us the answer yourself.

Yes, there was only 1 blob and I rotated the setup in between sessions which created the 2nd blob.

I shot my darks on the same evening as the posted picture.  So it has all of the same shooting conditions applied as the lights have.

The bias were shot from a session prior to the blob (I believe).  It had a similar temperature as the last shoot so I borrowed them.

The flats were shot prior to any of my shooting sessions with my new telescope.

Are you thinking that if I applied another set of bias or flats with the speck on the telescope that I could remove the dark blob?

Darks and bias are taken with the cover on the telescope and don’t involve light. So you can take them at any time as long as the temperature of the camera sensor is (nearly) the same as when the lights were taken.

Flats are taken to compensate for the non-homogeneous light distribution cause by the optical train, like vignetting and dust specs. If you take lights with the camera oriented in different ways then you need to take flats for every orientation since the location of the dust specs in the light path will change with the changing camera rotation.

In other words, if you manage to take flats now with the camera oriented such that the spec aligns with the first set of images and more flats with the camera rotated for the second set of images then you indeed may be able to get rid of the dark blobs.

You should use multi-session processing with the lights and the flats for the first set in one session and the lights and flats for the second set in another session. You can reuse the darks and bias for both sessions.

Posted by: @wvreeven

Darks and bias are taken with the cover on the telescope and don’t involve light. So you can take them at any time as long as the temperature of the camera sensor is (nearly) the same as when the lights were taken.

Flats are taken to compensate for the non-homogeneous light distribution cause by the optical train, like vignetting and dust specs. If you take lights with the camera oriented in different ways then you need to take flats for every orientation since the location of the dust specs in the light path will change with the changing camera rotation.

In other words, if you manage to take flats now with the camera oriented such that the spec aligns with the first set of images and more flats with the camera rotated for the second set of images then you indeed may be able to get rid of the dark blobs.

You should use multi-session processing with the lights and the flats for the first set in one session and the lights and flats for the second set in another session. You can reuse the darks and bias for both sessions.

This sounds like a very promising procedure but the only problem would be in trying to line up the defective area of the flats with the 2 shooting session's defective areas.

Also unfortunately I have rotated the telescope orientation since the last shooting session.

Since this evidently would take a lot of lucky precision I'm thinking that it likely wouldn't work.

This would be a lot more convenient if the speck was on the camera sensor.

Thanks a lot for your idea!  I appreciate your help in trying to solve this problem.

Thanks a lot for this clear explanation Wouter!

Flats should be taken after or just before each session, just to avoid having the situation where dust is suddenly introduced in between sessions and you can't use previous flats. Can you post one of the masterflats that you tried to use in this sessions?

Posted by: @vincent-mod

Thanks a lot for this clear explanation Wouter!

Flats should be taken after or just before each session, just to avoid having the situation where dust is suddenly introduced in between sessions and you can't use previous flats. Can you post one of the masterflats that you tried to use in this sessions?

Vincent.  I am new with many of the concepts of astrophotography.  When I looked at your post I thought to myself "You can't possibly be right about needing flats with every shooting session."  So the natural thing to do is back research a little to either confirm or doubt the statement. 

My perception was that I only needed 1 set of flats to cover all shooting conditions with a specific lens; in my case being a 60mm telescope.  I thought that the only pictures that adhered to a shooting session were the darks.  Further I wasn't %100 sure what you were referring to when you were looking for the master flat.  (Actually you said master flats and multiple sessions and I only compiled this in 1 session).  Perhaps this is a lack of perception from the beginning but I have been under the perception that the term flats could also refer to any or all of the calibration frames.  (Such as bad pixel map, darks, bias as well as flats.). I think my perception was wrong in that flats are only flats.

Upon further review I see that you hit on another area that I didn't comprehend correctly.  There were some elements that are flat out incorrect with my flats.  For instance I did not do any kind of a focus prior to establishing my flats.  So this alone would leave me somewhat out of sync with my lights as well as darks.  I was also surprised to find that my flats were of a different ISO than my darks and lights.  This again is a problem.  

I still might contend (and I believe others have done this) that I theoretically wouldn't need a set of flats shot after every session.  This however in most cases would be possible (especially now) because every shooting session I will likely adjust my focus or rotate my view.

Long story short: Thank you for pointing out a flaw with my shooting; I'll look at shooting flats as a part of my regular process.

I will attach my master flat that was being used.  As discussed earlier in this thread even though the flat shows the flaw it doesn't mean that it would be possible to line it up with the light frames because it would be rotationally different.  (Correct me if I'm wrong)

There is another anomaly that I noticed within the APP folder.  I don't see any of the master calibration files saved within that project folder.  Perhaps I can understand the fact that the bias, BPM, flats were taken from another project, but the dark master also was not in the folder.  I have a feeling that this may not have applied correctly.  Perhaps I didn't sequence things correctly as the dark frames were the only non-masters.

Let me know if there is anything possible to get rid of the dark blob in APP.

Thanks for your help!

Vincent.  I am new with many of the concepts of astrophotography.  When I looked at your post I thought to myself "You can't possibly be right about needing flats with every shooting session."  So the natural thing to do is back research a little to either confirm or doubt the statement. 

My perception was that I only needed 1 set of flats to cover all shooting conditions with a specific lens; in my case being a 60mm telescope.  I thought that the only pictures that adhered to a shooting session were the darks.  Further I wasn't %100 sure what you were referring to when you were looking for the master flat.  (Actually you said master flats and multiple sessions and I only compiled this in 1 session).  Perhaps this is a lack of perception from the beginning but I have been under the perception that the term flats could also refer to any or all of the calibration frames.  (Such as bad pixel map, darks, bias as well as flats.). I think my perception was wrong in that flats are only flats.

So, darks can be taken at any time. I usually do that when it's cloudy and just make sure the temperature is somewhat similar (or exactlt the same when you have active cooling) as my lights. You can create a masterdark from that and use it for a long time. Also, together with a good flat, you can create a Bad Pixel Map, which can be used for all situations and also for a long time. Flats are the only ones I'd recommend to make after each session. Why? Because dust can show up and if you're able to rotate your camera and use a filter-wheel, you need to take flats for those rotations as well. Having older flats is possible ofcourse, but you'll be very frustrated when you discover that 1 dust particle decided to settle on your filter and not having fresh flats.

Flats are just that, 1 of the calibration frames. They are only meant for correcting dust and light falloff in the corners. Darks are for hot pixels and amp-glow, bias is for shot-noise and the bad pixel map combines hot- and cold pixels and is almost a replacement for darks, unless you have amp-glow, then you still need darks as well.

Upon further review I see that you hit on another area that I didn't comprehend correctly.  There were some elements that are flat out incorrect with my flats.  For instance I did not do any kind of a focus prior to establishing my flats.  So this alone would leave me somewhat out of sync with my lights as well as darks.  I was also surprised to find that my flats were of a different ISO than my darks and lights.  This again is a problem.  

Yes, those should be the same and focus should be the same as when you took the lights. This is because you need to have the exact same picture of the dust in your flats, as you have in the lights.

I still might contend (and I believe others have done this) that I theoretically wouldn't need a set of flats shot after every session.  This however in most cases would be possible (especially now) because every shooting session I will likely adjust my focus or rotate my view.

The only time you don't always need to take new flats every time, is when you have a permanent scope setup, with camera and filters always in place and not taken off in between sessions. Still, I'm always more safe then sorry.

Long story short: Thank you for pointing out a flaw with my shooting; I'll look at shooting flats as a part of my regular process.

I will attach my master flat that was being used.  As discussed earlier in this thread even though the flat shows the flaw it doesn't mean that it would be possible to line it up with the light frames because it would be rotationally different.  (Correct me if I'm wrong)

If you rotated the camera and don't have the same rotation for the flats, then it won't work indeed.

There is another anomaly that I noticed within the APP folder.  I don't see any of the master calibration files saved within that project folder.  Perhaps I can understand the fact that the bias, BPM, flats were taken from another project, but the dark master also was not in the folder.  I have a feeling that this may not have applied correctly.  Perhaps I didn't sequence things correctly as the dark frames were the only non-masters.

You set your working directory when starting APP, this is where all the things get saved when you use your current session of APP. Maybe you forgot to change the directory when you switched to another photo-session?

Let me know if there is anything possible to get rid of the dark blob in APP.

Thanks for your help!

Well, not when you don't have the correct flats I'm afraid. You would need to get into software that can clone it out for instance (photoshop is pretty good at that).

Vincent,

Thank you for a good primer on the calibration frames.  We've had a bad stretch of weather otherwise I'd be putting this into effect.

I have been able to clone out the smudges using the Gimp software.

I may at some point want to re process this project to see if I can improve any.