Flats not correcting on the red channel

[emyliano2000]

Hello guys,

I finally managed to have first light with the Altair Astro 6" RC that I bought quite a while ago from a member and I'm very happy with it but I have encountered a problem that I can't find a solution for.

I shot NGC206 with the R, G and B filters, with the corresponding flats but for some reason the flats are not fully correcting the red subs. The Green and Blue seem to be ok.

I used the telescope at native focal length, 1370mm, f9 with my recently purchased QHY163M binned 2x2 to have a pixel scale of 1.14"/px

My first thought was that the filters might be a bit too far from the sensor as I'm using 1.25" filters in a SX USB filterwheel but why are the flats correcting the blue and green and not the red?

Here is a single red filter shot uncalibrated, stretched in pixinsight and saved as png

You can see a part of the circle in the bottom left of the photo

Now, here's how the master flat looks like

And here's the calibrated red lights stack

I tried all kinds of ADU values for the flats and nothig works. A friend had the same problem with the Ha filter and he fixed it by putting the flats panels further away from the tube. I tried it too with no luck.

If you have a look at the Green

and the blue stacks, they look OK

Another friend told me it's a light leak and the Red filter might be more sensitive to it than the green and blue.

Any suggestions please? I'm really excited abot this combination and I feel down in the dumps because I can't correct the red subs .

Thanks

Emil

Edited September 18, 2020 by emyliano2000

[Uranium235]

How clean are your filters? Because theoretically, you can use any flat from another filter (try L) to correct the red channel (as long as all your filters were clean when you took the images) - try it

[emyliano2000]

6 minutes ago, Uranium235 said:

How clean are your filters? Because theoretically, you can use any flat from another filter (try L) to correct the red channel (as long as all your filters were clean when you took the images) - try it

thank you for the suggestion but it looks like the filters have a few of their own dust bunnies and the circle looks different in each of the flats so I don't think it will work.

Emil

[Laurin Dave]

How are you calibrating your flats and lights. Are you using matched flat darks and darks with no dark optimisation ?  If not this may well be the issue ..  as suggested by @Uranium235 try using a different flat to calibrate the red.. other than dust bunnies does it remove the vignetting?  Also try flattening the flats ie flatten the red flat with the green and Blue flats etc..  again other than the dust do they produce a flat image ..  

[Uranium235]

this is why my filters are kept fanatically clean   the only motes I cant get to are on the sensor window itself (which is usually hidden behind a shutter).

[emyliano2000]

29 minutes ago, Laurin Dave said:

How are you calibrating your flats and lights. Are you using matched flat darks and darks with no dark optimisation ?  If not this may well be the issue ..  as suggested by @Uranium235 try using a different flat to calibrate the red.. other than dust bunnies does it remove the vignetting?  Also try flattening the flats ie flatten the red flat with the green and Blue flats etc..  again other than the dust do they produce a flat image ..  

I'll give it a try but the flats look quite different.

[emyliano2000]

I tried with the blue flats but it made it worse

[vlaiv]

Use corresponding flat for each filter. Use flat dark with corresponding flats.

Use dark that match your  lights for each filter.

Turn off all "enhancements" and "optimizations" and you should be fine.

[emyliano2000]

3 minutes ago, vlaiv said:

Use corresponding flat for each filter. Use flat dark with corresponding flats.

Use dark that match your  lights for each filter.

Turn off all "enhancements" and "optimizations" and you should be fine.

I did, I always do. Never had problems before with any of my other scopes. 

[emyliano2000]

I triex stacking them in APP, pixinsight and DSS but the red lights are not corrected properly in any of them. 

[vlaiv]

3 minutes ago, emyliano2000 said:

I did, I always do. Never had problems before with any of my other scopes. 

You have red lights, you shot red flat field and corresponding flat darks and you are using matching darks for your red lights and you are having issues with calibration?

Could you post one of each:

1. red channel light fits

2. red channel dark fits

3. red channel flat

4. red channel flat dark

(darks don't need to be specifically for red channel of course as long as they match corresponding lights).

[emyliano2000]

Yes, I'll upload one of each and put a link here 

[emyliano2000]

5 minutes ago, vlaiv said:

You have red lights, you shot red flat field and corresponding flat darks and you are using matching darks for your red lights and you are having issues with calibration?

Could you post one of each:

1. red channel light fits

2. red channel dark fits

3. red channel flat

4. red channel flat dark

(darks don't need to be specifically for red channel of course as long as they match corresponding lights).

Here's the link

https://we.tl/t-UXgwT6An08

 

[vlaiv]

Yes, even in single frame, there is issue with calibration:

There is arc visible in bottom left corner that is due to flats

There is same arc visible in uncalibrated sub - it is feature common to both light sub and corresponding flat:

Which means that flat is under correcting.

calibrated = light / flat and calibrated has higher value than it should have - means either light signal is stronger than it should be or master flat is "weaker" than it ought to be.

Two possible explanations:

1. Light leak - any chance of that? Light leak might have happened when red lights were taken but not when flats were taken nor when blue and green channels were taken

2. Weaker master flat usually means stronger flat darks.

Now, flat darks were taken at same settings as flats, correct?

I have found rather interesting discrepancy. It is something that I would otherwise expect but under different circumstances - so it might be important or it might not be important.

This shows that 4.24 second flat darks have higher mean adu value than 120s regular darks. Such thing should not happen, but it sometimes happens on CMOS sensors. It usually happens when one has very short exposure - like less than one second because these exposures are timed by sensor it self (to enable high fps) - longer exposures are sort of bulb mode - drivers or external electronics signals start and stop of exposure.

It also means that internal calibration of sensor is different and different bias level is used - hence stronger bias in such short exposure - one of the reasons we don't use bias subs with CMOS - they are unreliable.

Here, I believe 4.24 seconds is too long exposure to have this effect. Could there have been a light leak while you took red flat darks?

How long were your blue and green flats? Could you examine blue and green flat darks to see their mean ADU value and how it compares to this value?

If you find that both green and blue flat darks have smaller mean value - smaller even than regular 120s darks (as they should have because we have less time for dark current build up) then you should retake red flat darks to eliminate possibility of light leak there.

[emyliano2000]

3 minutes ago, vlaiv said:

Yes, even in single frame, there is issue with calibration:

There is arc visible in bottom left corner that is due to flats

There is same arc visible in uncalibrated sub - it is feature common to both light sub and corresponding flat:

Which means that flat is under correcting.

calibrated = light / flat and calibrated has higher value than it should have - means either light signal is stronger than it should be or master flat is "weaker" than it ought to be.

Two possible explanations:

1. Light leak - any chance of that? Light leak might have happened when red lights were taken but not when flats were taken nor when blue and green channels were taken

2. Weaker master flat usually means stronger flat darks.

Now, flat darks were taken at same settings as flats, correct?

I have found rather interesting discrepancy. It is something that I would otherwise expect but under different circumstances - so it might be important or it might not be important.

This shows that 4.24 second flat darks have higher mean adu value than 120s regular darks. Such thing should not happen, but it sometimes happens on CMOS sensors. It usually happens when one has very short exposure - like less than one second because these exposures are timed by sensor it self (to enable high fps) - longer exposures are sort of bulb mode - drivers or external electronics signals start and stop of exposure.

It also means that internal calibration of sensor is different and different bias level is used - hence stronger bias in such short exposure - one of the reasons we don't use bias subs with CMOS - they are unreliable.

Here, I believe 4.24 seconds is too long exposure to have this effect. Could there have been a light leak while you took red flat darks?

How long were your blue and green flats? Could you examine blue and green flat darks to see their mean ADU value and how it compares to this value?

If you find that both green and blue flat darks have smaller mean value - smaller even than regular 120s darks (as they should have because we have less time for dark current build up) then you should retake red flat darks to eliminate possibility of light leak there.

thank you for looking into it.

so far I tried 3 ADU values for the flats and none of them worked. I tried 1500 ADU @ 2.73sec with corresponding flat darks, I tried 25000 ADU @ 4.24sec and 30000 ADU @ 2.73sec and none of them worked. The best result that I got was from the 25000 ADU @4.24sec

 

[vlaiv]

2 minutes ago, emyliano2000 said:

thank you for looking into it.

so far I tried 3 ADU values for the flats and none of them worked. I tried 1500 ADU @ 2.73sec with corresponding flat darks, I tried 25000 ADU @ 4.24sec and 30000 ADU @ 2.73sec and none of them worked. The best result that I got was from the 25000 ADU @4.24sec

 

How long were your blue and green flats?

Could you measure mean ADU values for their flat darks?

[emyliano2000]

Just now, emyliano2000 said:

Light leak might have happened when red lights were taken but not when flats were taken

I tried with the lights and flats from 3 separate nights with the same result, green and blue, ok, red, not

 

2 minutes ago, emyliano2000 said:

flat darks were taken at same settings as flats, correct?

correct

 

3 minutes ago, emyliano2000 said:

Could there have been a light leak while you took red flat darks?

I took all the red, green and blue at the same time

[emyliano2000]

1 minute ago, vlaiv said:

How long were your blue and green flats?

2.77 for green and 3.3 for blue

[vlaiv]

3 minutes ago, emyliano2000 said:

2.77 for green and 3.3 for blue

And their mean ADU values?

[emyliano2000]

1 minute ago, vlaiv said:

And their mean ADU values?

29k for green and 28k for blue

[vlaiv]

Just now, emyliano2000 said:

29k for green and 28k for blue

No, not flats - flat darks, can you load a single green flat dark and calculate stats on it and see what is mean ADU value and one blue flat dark - do the same.

Regular dark that is 120s long has mean ADU value of ~1548ADU while 4.24s long flat dark has mean ADU value of ~1574

This is wrong - shorter dark must have lower mean ADU value because both darks should have same bias / offset level and only difference should be dark current build up - longer the dark - more dark current there is - higher mean ADU value.

I want to check what sort of ADU value you are getting for 2.4s and 3.3s exposures.

If you are having trouble measuring this for some reason (no software support or whatever) - upload one green and one blue flat dark sub and I'll measure it.

[emyliano2000]

15 minutes ago, emyliano2000 said:

Could you examine blue and green flat darks to see their mean ADU value and how it compares to this value?

they are all the same, 1574 ADU

[emyliano2000]

2 minutes ago, vlaiv said:

This is wrong - shorter dark must have lower mean ADU value because both darks should have same bias / offset level and only difference should be dark current build up - longer the dark - more dark current there is - higher mean ADU value.

it does seem wrong indeed. culd it be that I shot the darks with the camera in the fridge, in complete darkness and the flat darls with camera on the scope covered, outside in the garden?

 

[vlaiv]

3 minutes ago, emyliano2000 said:

it does seem wrong indeed. culd it be that I shot the darks with the camera in the fridge, in complete darkness and the flat darls with camera on the scope covered, outside in the garden?

 

It could be if there was a slight light leak or something - like IR light.

It is strange that all three flat darks have the same mean ADU value however - I would expect some variation if there was source of the light (or even IR radiation) present.

In any case - I don't think this is causing you issues with red channel - as it is no causing issues with green nor blue and all three have same mean ADU for their respective flat darks.

[emyliano2000]

24 minutes ago, emyliano2000 said:

Light leak might have happened when red lights were taken but not when flats were taken nor when blue and green channels were taken

I have SGP set to take 10 of each throughout the night to be sure I have enough of each at the end of the night so if there was a leak when I shot the red, it should've been there when I shot the other ones too