Overcorrecting flats confusion

[spaceman_spiff]

Hi all,

I have been battling with this problem for a few weeks now and following a few ideas in a previous post...see below, I found a work around.

I just want to voice my concern and confusion on this problem and see if anyone has better fixes to it.

The problem came out of the blue, suddenly my flats (which used to work perfectly) started over-correcting the field illumination causing inverse vignetting and light dust bunnies. In the post above I read that Olly tried dimming the flats (reducing the their dynamic range which in turn reduces the amount they correct the lights). This worked pretty well but I still have some residual brightness on the left of my images (see below).

I don't know what started the problem and as far as I can tell the solution is very dependent on the brightness of the lights since the brighter the lights the more contrast will be thrown onto the uneven field illumination meaning the flats will become useless again.

Can anyone help?

I have added some images below to illustrate the issue.

This is a final stacked image with no flats.

 

This is the same stack with my normal flats (taken at 1/40s) with a laptop monitor screen. Notice the over-correction.

Here is the same stack using dimmed flats (1/250s) on a laptop screen. I have really stretched the image to show that there is still some residual field unevenness. After a lot of trial and error this is the best I could do.

[sharkmelley]

I assume you included bias frames in your processing sequence?  Missing bias frames will generally lead to the flats overcorrecting.

Mark

[spaceman_spiff]

Hi Mark,

Yes I have included bias frames, but they were taken at the lowest ISO (100) when they should be the same ISO as the lights (800). I will redo the bias frames tomorrow and see if the problem persists.

Dan

[sharkmelley]

Normally taking bias frames at the wrong ISO would not cause overcorrection problems because the average pixel value will not be any different.  However, it can't be totally ruled out that a camera might have different average levels at different ISO values.  It would be interesting to know.

Mark

[ollypenrice]

In the end I found the solution to my overcorrecting flats problem. The problem clearly arose from using Nebulosity to capture them from the SXVH36. Harry Page put me onto the idea that inadequate flushing between frames might be the cause. I made a set of flats in AstroArt instead and, although I still capture the lights in Nebulosity, the AstroArt-captured flats work correctly. I did need to do a vertical flip in the AA flats to make them compatible with the Nebulosity lights.

Whether or not this has any relevance to your situation I'm afraid I don't know.

Olly

[spaceman_spiff]

Ok that's interesting Olly,

When you say inadequate flushing, do you mean that I could solve the issue by altering the interval between flat exposures? At the moment, I have an interval of 15s between my lights and 1s between flats. I will try taking flats with the same interval. 

I'll let you know what happens.

D

[ollypenrice]

3 hours ago, spaceman_spiff said:

Ok that's interesting Olly,

When you say inadequate flushing, do you mean that I could solve the issue by altering the interval between flat exposures? At the moment, I have an interval of 15s between my lights and 1s between flats. I will try taking flats with the same interval. 

I'll let you know what happens.

D

I'd certainly give that a go. Each read out adds thermal noise so a good delay ought to help. If I don't give a few seconds delay on my set point cooled CCDs I see the chip temperature rise.

Olly

[spaceman_spiff]

Problem solved:

I would just Like to say that adding a delay of 15s between flat exposures solves the problem of overcorrecting flats. Perhaps the problem was temperature related or residual charge in the pixel wells. I guess the moral of the story is that the time between exposures is as important as the exposures themselves!

Many thanks for the suggestions! I will post some images later.

Dan 

[spaceman_spiff]

Well as promised, here is a jpg of M33 corrected with the new flats (massively stretched) . There is only the slightest trace of under-correction I may try larger intervals between flats and see what happens. Anyway, thanks very much guys!

D 

[Astroscot2]

Glad you resolved the issue,  im currently trying to re work through my own over correcting flats, i will leave an interval  when re-shooting.

 

Mark

 

[spaceman_spiff]

So am I! I was literally stripping the telescope down looking for internal reflections or light scattering trying to solve this problem. I'm now going back and re-processing old stacks.

I made a new library of flats with different exposures (about 1/3, 1/2 and 2/3 on the pixel brightness histogram), each flat exposure was separated by 30s. All seem to work well.

Dan.

[sharkmelley]

23 hours ago, spaceman_spiff said:

Well as promised, here is a jpg of M33 corrected with the new flats (massively stretched) . There is only the slightest trace of under-correction I may try larger intervals between flats and see what happens. Anyway, thanks very much guys!

D 

 

That's definitely fixed your flats problem!  Now you can concentrate on the cause of the "walking noise" in the background - probably caused by mismatched darks.

Mark

[spaceman_spiff]

42 minutes ago, sharkmelley said:

That's definitely fixed your flats problem!  Now you can concentrate on the cause of the "walking noise" in the background - probably caused by mismatched darks.

Mark

Interesting...I was thinking that noise was something to do with the bias correction.

The darks I use are taken from a library and are pretty old. Perhaps I should redo them and reprocess the image.