Flats help.

[alan potts]

This is something I didn't have much of an issue with before when using the Canon, just set 400iso and banged away 25 or so shots at 1-2 seconds with a clean pair of Sloggies over the dew shield and light box. Then covering up and shooting Dark Flats the same values. Never had a problem.

Now for whatever reason I am seeing fairly large and not nice dust blotches on stacked results. I use APT and normally set about 1 or 2 seconds and cover with either a double layer T shirt or similar, then use my light box. I do not understand what APT's Flats CCD assist is meant to do or how it works.

I have now taken 3 sets and the problem still appears there even though I have not moved the camera at all.

I have stretched this a bit more from yesterday so people can see what I mean, can anyone give advice to stop this as I am losing faint detail hiding the problem.

Thank you in advance,

Alan

 

[carastro]

I have a friend who uses APT and when he was starting out he was using the APT flats facility, but it wasn't producing correct flats.  In the end I sat down with him and we worked it out. 

We had to use a "shoot" method in APT and open up the histogram,  and keep adjusting the length of the exposure until the histogram was approx 1/3 to just under half on the left hand side.  I can't remember now whether he was doing sky flats or using a light box but the length of the exposure was considerably less than 1 sec.

Moving from a DSLR to dedicated camera takes a bit of a re-think.

When I had a CCD camera with a shutter that also worked completely differently as I had to have a long enough exposure to avoid shutter shadow, which meant I had to dim the light source down a fair bit.  

Just get the histogram in the right place and you should be fine. 

HTH

Carole 

Edited October 20, 2019 by carastro

[vlaiv]

There might be an issue with APT and the way it shoots flats - but I can't help there as I'm not using it (or have ever).

What I can do is offer you list of reasons why this can happen so you can check if anything on that list is causing issue for you.

You have under correction of the flats. Corrected value is equal to base value divided by flat value. For that number to be lower than it should, we have two options:

1. base value is lower than it should be

2. flat value is higher than it should be

Usually this happens with wrong calibration or if there is some sort of light leak in the system.

Case one can happen if you have:

mismatching darks for your lights - longer duration darks, shot at higher gain, shot at higher temperature, or there were light leak while you shot your darks (but not with lights) - i.e. You did darks on scope during the day, or you took your camera off scope and did darks and there was either IR leak, or regular light leak (cap was not good enough at blocking the light).

Case two can happen if you have:

mismatching flat darks - shorter than flats, at lower gain or at lower temperature. Light leak while shooting flats can also be a problem. People sometimes calibrate flats with bias only - again that can be a problem.

If you can post one of each: light, dark, flat and flat dark subs, we could possibly be able to tell what happened by examining fits header for information and doing measurements on each.

 

 

[tooth_dr]

APT flats assist is pretty essential! I use it for my CCD flats.

1) choose a ADU you want

2) choose an acceptable range outside of that ADU eg 5%

3) select starting exposure time and max exposure

4) APT will take a number of exposures until it finds an exposure that delivers the correct ADU.

5) It then sets up an image run in the flats section

HTH

Adam 

 

[alan potts]

1 hour ago, vlaiv said:

There might be an issue with APT and the way it shoots flats - but I can't help there as I'm not using it (or have ever).

What I can do is offer you list of reasons why this can happen so you can check if anything on that list is causing issue for you.

You have under correction of the flats. Corrected value is equal to base value divided by flat value. For that number to be lower than it should, we have two options:

1. base value is lower than it should be

2. flat value is higher than it should be

Usually this happens with wrong calibration or if there is some sort of light leak in the system.

Case one can happen if you have:

mismatching darks for your lights - longer duration darks, shot at higher gain, shot at higher temperature, or there were light leak while you shot your darks (but not with lights) - i.e. You did darks on scope during the day, or you took your camera off scope and did darks and there was either IR leak, or regular light leak (cap was not good enough at blocking the light).

Case two can happen if you have:

mismatching flat darks - shorter than flats, at lower gain or at lower temperature. Light leak while shooting flats can also be a problem. People sometimes calibrate flats with bias only - again that can be a problem.

If you can post one of each: light, dark, flat and flat dark subs, we could possibly be able to tell what happened by examining fits header for information and doing measurements on each.

 

 

No light leak of this I am 100% sure. Same temperature which was about 23 degrees , does the camera need to be cooled to minus 5 like the lights.

Darks were shot cooled to minus 5.

I am wondeing if Gain here is the issue, I think the Darks could be at zero gain.

Am I correct in believing Gain has to be the same on all shots Light, darks etc etc. All frames are Fits I will try to post as I don't know how to change them apart from after stacking in DSS.

Dark

D_2019-10-12_10-38-13.fit

Flat

F_2019-10-19_18-43-38.fit

Dark Flat don't take notice of the F at the front

F_2019-10-19_18-48-51.fit

Light of M33 short last night gain 90 offset 65, I can't recall if the Darks were at this, I think they were. Feel I best not touch anything in future.

L_2019-10-19_20-39-35.fit

 

Thanks for the advice.

Alan

[alan potts]

44 minutes ago, tooth_dr said:

APT flats assist is pretty essential! I use it for my CCD flats.

1) choose a ADU you want

2) choose an acceptable range outside of that ADU eg 5%

3) select starting exposure time and max exposure

4) APT will take a number of exposures until it finds an exposure that delivers the correct ADU.

5) It then sets up an image run in the flats section

HTH

Adam 

 

Thanks Adam, does it now, I was wondering what that is, what do you do with it? I don't recall having problems in the first 3-4 images I took, this will be idle hands again I can feel it.

Alan

[vlaiv]

16 minutes ago, alan potts said:

No light leak of this I am 100% sure. Same temperature which was about 23 degrees , does the camera need to be cooled to minus 5 like the lights.

Darks were shot cooled to minus 5.

I am wondeing if Gain here is the issue, I think the Darks could be at zero gain.

Am I correct in believing Gain has to be the same on all shots Light, darks etc etc. All frames are Fits I will try to post as I don't know how to change them apart from after stacking in DSS.

Dark

D_2019-10-12_10-38-13.fit 30.97 MB · 0 downloads

Flat

F_2019-10-19_18-43-38.fit 30.97 MB · 0 downloads

Dark Flat don't take notice of the F at the front

F_2019-10-19_18-48-51.fit 30.97 MB · 0 downloads

Light of M33 short last night gain 90 offset 65, I can't recall if the Darks were at this, I think they were. Feel I best not touch anything in future.

L_2019-10-19_20-39-35.fit 30.97 MB · 0 downloads

 

Thanks for the advice.

Alan

Yes, gain is the issue.

Here is comparison of the information from fits header for darks and lights:

Darks were taken with gain 0 and lights were taken with with gain 90.

There is slight mismatch between flats and flat darks in temperature. In all likelihood it will not make much difference, but I would recommend you following:

- Take new set of darks at Gain 90 and try with those to calibrate your image

- In future make sure that you take flats / flat darks at the same temperature

- it would be best to have some settings that you will not change - offset, gain and temperature and always work on those. Offset of 65 seems fine - so keep it there, gain 90 is unity gain so again fine - keep it there and -5C looks like reasonable temperature that you can reach most of the time, so that is also something you should keep. Take all of your subs with these settings.

 

[alan potts]

Just now, vlaiv said:

Yes, gain is the issue.

Here is comparison of the information from fits header for darks and lights:

Darks were taken with gain 0 and lights were taken with with gain 90.

There is slight mismatch between flats and flat darks in temperature. In all likelihood it will not make much difference, but I would recommend you following:

- Take new set of darks at Gain 90 and try with those to calibrate your image

- In future make sure that you take flats / flat darks at the same temperature

- it would be best to have some settings that you will not change - offset, gain and temperature and always work on those. Offset of 65 seems fine - so keep it there, gain 90 is unity gain so again fine - keep it there and -5C looks like reasonable temperature that you can reach most of the time, so that is also something you should keep. Take all of your subs with these settings.

 

Idle hands have done the devils work yet again, look many thanks for sorting this out for me, I was clueless as to this affecting anything, I must learn to do thing at exactly the same gain etc. So in view of the fact Darks can be done now then I must take another set at 90 Gain and label them as such and stop messing. As the camera is not moved I can keep the others for zero gain subs, I am sure I will take some, grouping is the key.

Alan

[kirkster501]

http://www.skyandtelescope.com/wp-content/uploads/documents/Flatfields+Mar11.pdf

Well worth a read.