Jump to content

NLC-Banner.thumb.jpg.acb5ba835b9e8bf0718b90539633017d.jpg

Flats problem


Recommended Posts

46 minutes ago, michael8554 said:

Yes, in what way aren't they working ?

RGB values seem okay, about 50%.

Camera is well off centre.

Michael

 

When integrated into the lights, the centre looks darker than the four corners.

What would cause the off centre issue?

I think the focuser thumbscrew lock is pushing the focuser drawtube off axis. Would that cause it? What implecations would this have on the lights, and should I continue to calibrate with it like this as the flats should match the lights orientation?

Link to comment
Share on other sites

Are you able to try calibrating the flats with flat darks or bias frames? Other than that the other thing I'd try is varying the adu of the flats and see if that changes anything.

Edited by scotty38
  • Thanks 1
Link to comment
Share on other sites

38 minutes ago, Pitch Black Skies said:

All settings the same. No other calibration frames.

From what you've said it sounds like you're not using Darks and Flat Darks to calibrate your lights and flats, which would account for the bright corners dark middle, and which you need to use to calibrate correctly

Edited by Laurin Dave
  • Thanks 1
Link to comment
Share on other sites

1 hour ago, scotty38 said:

Are you able to try calibrating the flats with flat darks or bias frames? Other than that the other thing I'd try is varying the adu of the flats and see if that changes anything.

I used dark flats, no difference.

Link to comment
Share on other sites

2 hours ago, Laurin Dave said:

From what you've said it sounds like you're not using Darks and Flat Darks to calibrate your lights and flats, which would account for the bright corners dark middle, and which you need to use to calibrate correctly

I don't use Darks, they aren't needed with the 533.

I'm using Dark Flats.

How would not using them account for the bright corners? They are only removing noise correct?

Link to comment
Share on other sites

I always use around 23,000 ADU but definitely no higher than 30,000.  40,000 sound too high and I wonder if the flats are over compensating.  If the flats are not showing any dust when you take them, the chances are it's too high.   I am not familiar with your camera (still iusing CCD), but 2 - 4secs also sounds too long, unless you have dimmed the light right down.  Mine are usually less than a sec.  

Carole 

Edited by carastro
  • Thanks 1
Link to comment
Share on other sites

6 minutes ago, Pitch Black Skies said:

I don't use Darks, they aren't needed with the 533.

Why do you say that?

7 minutes ago, Pitch Black Skies said:

How would not using them account for the bright corners? They are only removing noise correct?

That is precisely why you have bright corners.

Dark frames don't remove noise. No calibration frames remove noise. They all remove / correct some sort of signal.

Dark frames remove dark current signal.

Imagine following scenario.

You have illumination of 80% in corners due vignetting and 100% in center of frame.

Your light sub gathers 100 electrons over whole field. Your dark current is 10e.

In center you will therefore have 100 electrons (no vignetting) and 10e from dark current - so that is 110e

In corner you will have 80 electrons from light (80% illumination) and 10e from dark current so that is 90e total

You divide that with flat frame which is 1 for center and 0.8 for corners

110 / 1 = 110

90 / 0.8 = 112.5

What just happened? How come that corner is brighter than center? All we did was to apply correct flat frame?

Look now what happens when you remove dark:

(110 - 10) / 1 = 100

(90 -10) / 0.8 = 100

No more brightness in the corners! After removing dark signal we have proper flattening of illumination.

 

  • Like 1
  • Thanks 1
Link to comment
Share on other sites

You need to subtract the Bias plus appropriate Dark current signal from both Lights and Flats  in order for calibration to work properly ..  Darks may not be needed with the ASI553 (because its Dark current is so low) but Bias certainly is ...   

  • Thanks 1
Link to comment
Share on other sites

4 hours ago, carastro said:

I always use around 23,000 ADU but definitely no higher than 30,000.  40,000 sound too high and I wonder if the flats are over compensating.  If the flats are not showing any dust when you take them, the chances are it's too high.   I am not familiar with your camera (still iusing CCD), but 2 - 4secs also sounds too long, unless you have dimmed the light right down.  Mine are usually less than a sec.  

Carole 

I can see one dust mote. They were at around 23, 000 and exactly 2 secs.

Link to comment
Share on other sites

5 hours ago, vlaiv said:

Why do you say that?

That is precisely why you have bright corners.

Dark frames don't remove noise. No calibration frames remove noise. They all remove / correct some sort of signal.

Dark frames remove dark current signal.

Imagine following scenario.

You have illumination of 80% in corners due vignetting and 100% in center of frame.

Your light sub gathers 100 electrons over whole field. Your dark current is 10e.

In center you will therefore have 100 electrons (no vignetting) and 10e from dark current - so that is 110e

In corner you will have 80 electrons from light (80% illumination) and 10e from dark current so that is 90e total

You divide that with flat frame which is 1 for center and 0.8 for corners

110 / 1 = 110

90 / 0.8 = 112.5

What just happened? How come that corner is brighter than center? All we did was to apply correct flat frame?

Look now what happens when you remove dark:

(110 - 10) / 1 = 100

(90 -10) / 0.8 = 100

No more brightness in the corners! After removing dark signal we have proper flattening of illumination.

 

Awesome! I have got to try this soon!

I was under the illusion that Darks weren't needed as the camera doesn't suffer from amp glow, and it's read noise is supposedly very low.

Maybe it's bias that take care of read noise/signal? And that's why bias aren't needed?

So is it incorrect to say dark noise then? It should be dark current signal?

I have the recommended beginners book 'Every Photon Counts', but would like something to help me progress a bit further. Is there anything you could recommend?

Link to comment
Share on other sites

5 hours ago, Laurin Dave said:

Darks may not be needed with the ASI553 (because its Dark current is so low) but Bias certainly is ...   

I'm not sure, I've heard of people saying Bias are calibration frames that should be skipped. Some have said they can actually make things worse.

Link to comment
Share on other sites

2 minutes ago, Pitch Black Skies said:

I'm not sure, I've heard of people saying Bias are calibration frames that should be skipped. Some have said they can actually make things worse.

So long as you're applying them properly, calibration frames done right rarely make matters worse. Almost all cameras benefit from dark frames. Many benefit from bias. All optical systems benefit from flats. Dark flats and variations like that get a bit more application specific but honestly capture bias+dark+flat to start and go from there. Even if your camera has "low" dark currents/amp glow etc, it won't be zero.

  • Thanks 1
Link to comment
Share on other sites

49 minutes ago, Pitch Black Skies said:

I was under the illusion that Darks weren't needed as the camera doesn't suffer from amp glow, and it's read noise is supposedly very low.

Maybe it's bias that take care of read noise/signal? And that's why bias aren't needed?

So is it incorrect to say dark noise then? It should be dark current signal?

Ok, so here is a quick break down of terms and what they represent and how to calibrate in different circumstances.

We have dark signal and dark noise and bias signal and bias noise (which is always referred to as read noise).

Bias signal and read noise are not related in any obvious way. Signal is the bit you want to remove by calibration, while noise bit is reduced by stacking (Signal to noise ratio is improved). In order for stacking to work noise needs to be truly random. Bias signal is just "offset" added to pixel values - which is not the same for every pixel but in general it is pretty uniform as far as value goes - in modern CMOS cameras you can set this overall level by using offset parameter.

Dark signal and dark signal noise are completely related. Dark signal is buildup of electrons due to thermal fluctuations in electronics. Dark signal noise is just randomness in this build up - similar to shot noise associated with light signal.

There is strong relationship where dark signal noise has magnitude that is exactly square root of dark signal (expressed in electrons - in ADUs this does not hold).

When you shoot bias exposure - it only contains that bias signal (and read noise, but we don't care about the noise bit here).

When you shoot dark exposure - it contains both bias signal and dark current signal

When you shoot your regular light exposure - it contains bias, dark and light signal.

Point of calibration is to remove all signals and only leave light signal (light gathered by telescope - you don't care about thermal properties of camera or offset added - and you don't want them as they mess things up).

You can remove dark from your lights and that removes both dark current signal and bias signal as darks contain both.

You can use both bias and darks when calibrating your lights - but there is really no point in doing so - as darks remove both (using bias in addition to darks won't mess things up as algorithm produces correct results). There is one special case where you can and need to use bias and that is in the case of dark scaling - which in general you should not do unless you know what you are doing (both knowing what you are doing and being sure your camera is capable of that). That is when you use different exposure time for darks and lights and you want to compensate by scaling darks.

You can use bias only for calibrating lights - but that is not proper way to do things. It will work under two distinct cases:

- using DSLR that internally subtracts dark current for you - and all that is left is bias. This is actually a good thing - newer DSLR cameras have some clever ways to measure dark current while exposing so dark is taken at the same temperature as light.

- your camera has exceptionally low dark current at temperature used and your exposure is short enough that dark current is virtually 0 for duration of exposure. This is something that you really need to check for your setup as two same cameras can behave differently depending on scope and light pollution levels.

For example ASI533 has 0.00013e/s/px - which is very low dark current. But if you expose for say 5 minutes, total value of dark current will be 0.039e. Now that might seem very low dark current - and in principle it is. It is only very small percent of background signal in most cases, but what if you use high resolution and you shoot in very dark skies and your background signal in exposure is something like 0.1e?

Then this dark current is no longer negligible small compared to background signal and you will still see over correction in the corners (if you have strong vignetting).

 

 

 

  • Like 2
  • Thanks 1
Link to comment
Share on other sites

1 hour ago, vlaiv said:

Ok, so here is a quick break down of terms and what they represent and how to calibrate in different circumstances.

We have dark signal and dark noise and bias signal and bias noise (which is always referred to as read noise).

Bias signal and read noise are not related in any obvious way. Signal is the bit you want to remove by calibration, while noise bit is reduced by stacking (Signal to noise ratio is improved). In order for stacking to work noise needs to be truly random. Bias signal is just "offset" added to pixel values - which is not the same for every pixel but in general it is pretty uniform as far as value goes - in modern CMOS cameras you can set this overall level by using offset parameter.

Dark signal and dark signal noise are completely related. Dark signal is buildup of electrons due to thermal fluctuations in electronics. Dark signal noise is just randomness in this build up - similar to shot noise associated with light signal.

There is strong relationship where dark signal noise has magnitude that is exactly square root of dark signal (expressed in electrons - in ADUs this does not hold).

When you shoot bias exposure - it only contains that bias signal (and read noise, but we don't care about the noise bit here).

When you shoot dark exposure - it contains both bias signal and dark current signal

When you shoot your regular light exposure - it contains bias, dark and light signal.

Point of calibration is to remove all signals and only leave light signal (light gathered by telescope - you don't care about thermal properties of camera or offset added - and you don't want them as they mess things up).

You can remove dark from your lights and that removes both dark current signal and bias signal as darks contain both.

You can use both bias and darks when calibrating your lights - but there is really no point in doing so - as darks remove both (using bias in addition to darks won't mess things up as algorithm produces correct results). There is one special case where you can and need to use bias and that is in the case of dark scaling - which in general you should not do unless you know what you are doing (both knowing what you are doing and being sure your camera is capable of that). That is when you use different exposure time for darks and lights and you want to compensate by scaling darks.

You can use bias only for calibrating lights - but that is not proper way to do things. It will work under two distinct cases:

- using DSLR that internally subtracts dark current for you - and all that is left is bias. This is actually a good thing - newer DSLR cameras have some clever ways to measure dark current while exposing so dark is taken at the same temperature as light.

- your camera has exceptionally low dark current at temperature used and your exposure is short enough that dark current is virtually 0 for duration of exposure. This is something that you really need to check for your setup as two same cameras can behave differently depending on scope and light pollution levels.

For example ASI533 has 0.00013e/s/px - which is very low dark current. But if you expose for say 5 minutes, total value of dark current will be 0.039e. Now that might seem very low dark current - and in principle it is. It is only very small percent of background signal in most cases, but what if you use high resolution and you shoot in very dark skies and your background signal in exposure is something like 0.1e?

Then this dark current is no longer negligible small compared to background signal and you will still see over correction in the corners (if you have strong vignetting).

 

 

 

Thank you so much for that detailed response. I will definitely be taking some notes from this for going forward.

Link to comment
Share on other sites

23 hours ago, vlaiv said:

No more brightness in the corners! After removing dark signal we have proper flattening of illumination.

You're right, this is exactly what was happening!

Foolish me for thinking I didn't need to use darks, lesson learned.

How you explained it with the maths made it logical and very easy to grasp, thanks again.

I'll upload an image calibrated with just flats and dark flats, and then the same image with darks included to show the difference it made 

Link to comment
Share on other sites

I am going to set the cat amongst the pigeons here.  

I use an Atik460EX and the only calibration I do is flats, I don't bother with dark flats, I don't bother with darks (well I do sometimes if I have them available), and I definitely don't use Bias they completely mess up the image with streaks.  Possibly it's the stacking software I use (Astroart), but my images don't seem to suffer any problems as a result.  Of course I do have the Atik EX chip which does have very low noise.  But darks themselves are easy to do and you can keep a library of them so long as they are the same length and temperature.  

This is an example comparing the same data with and without bias. (You may need to open another window to enlarge in order to see the streak problem). 

spacer.png

Sorry Vlaiv this will probably do your head in.

Carole     

Edited by carastro
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.