What is wrong with my flats?

[DanielF]

Hello.
My latest image (attempted image at least) was a shot at M45.
I only got 30 min on it and then collected bias, flats and darks.

The problem I'm having is that the flat seems to overcompensate the uneven illumination during calibration.
This image shows an uncalibrated light and flat frame and the resulting master light straight out of the batch preprocessing script.
All with AutoSTF.



Anybody has any idea what could be wrong and is there anything that I can do to fix it in PixInsight. If I have done something wrong acquiring the flats then what could done to correct it the next time?

I use a home built light-box to shoot my flats. Could it be that the lit surface is unevenly illuminated? (I have two diffusers inside the box). Is the exposure wrong?



Here are links to one light and one flat fit file if anybody wants to take a look.
https://dl.dropboxusercontent.com/u/8802413/M45%20problem/M45_2m-1_010.fit
https://dl.dropboxusercontent.com/u/8802413/M45%20problem/Flat_L_001.fit

Thanks!
/Daniel

[Phil Leigh]

This is normally because the vignetting is so strong that the flat cannot flatten the field and over-compensates. Check out the dark areas of the flat; if the pixels values in there are zero or very low then that is probably what is happening. I'll download the flat and take a look.

What camera and scope are you using and how EXACTLY is the camera attached to the scope? - ah OK it's an 8300 chip Starlight Xpress? - shouldn't really have this much vignetting unless you are using 1.25 inch filters or something like that?

[DanielF]

This is normally because the vignetting is so strong that the flat cannot flatten the field and over-compensates. Check out the dark areas of the flat; if the pixels values in there are zero or very low then that is probably what is happening. I'll download the flat and take a look.

What camera and scope are you using and how EXACTLY is the camera attached to the scope?

Hi.
The dark areas do not seem to be very low and are not zero.
The camera is a Starlight Xpress SXVR-H18 (KAF8300) and is connected to a filterwheel and a Baader MPCC and the scope is a SkyWatcher Explorer 200 8" f/5 Newton.
And it all looks like this:

[Phil Leigh]

Sorry, got it the wrong way round... I downloaded your light and flat and did a linear fit of the flat to the light (so they have similar "exposures") and the flat worked MUCH better.

Your flat is too bright. You need to dial the brightness of your light box back a bit or use a shorter exposure for the flats. Try taking flats at various lower exposures and see what works.

Personally I'd still take a long hard look at that vignetting because in my opinion it is asking a lot of the flats to deals with all of it. I can see from your setup that the scope and focuser are not an issue - my guess is the filter wheel or the filters within it. Which wheel/filters are you using?

[pfile]

the flat seems okay to me. maybe just a tad bright and the histogram looks a little weird but it's not grossly overexposed.

but what is wrong with it is that it was apparently created in nebulosity... calibration frames usually do not work well across programs.

usually when you get overcorrection from a flat it's because something went wrong in the flat calibration.

if the flat really came out of neb, i'd re-make the flat master (and dark and bias) in PI and try again.

rob

[DanielF]

Sorry, got it the wrong way round... I downloaded your light and flat and did a linear fit of the flat to the light (so they have similar "exposures") and the flat worked MUCH better.

Your flat is too bright. You need to dial the brightness of your light box back a bit or use a shorter exposure for the flats. Try taking flats at various lower exposures and see what works.

Personally I'd still take a long hard look at that vignetting because in my opinion it is asking a lot of the flats to deals with all of it. I can see from your setup that the scope and focuser are not an issue - my guess is the filter wheel or the filters within it. Which wheel/filters are you using?

Hi and thanks for helping me.
So do you think I could salvage the flats by running linear fit in this case?

The exposure times of the flats where 10 seconds, so I can definitely dial that down a bit. I never really know exactly for how long I should expose my flats…

The filter wheel is also from Starlight Xpress and the filters are unmounted 36mm LRGB.

/Daniel

[DanielF]

the flat seems okay to me. maybe just a tad bright and the histogram looks a little weird but it's not grossly overexposed.

but what is wrong with it is that it was apparently created in nebulosity... calibration frames usually do not work well across programs.

usually when you get overcorrection from a flat it's because something went wrong in the flat calibration.

if the flat really came out of neb, i'd re-make the flat master (and dark and bias) in PI and try again.

rob

Hi.
All raw frames are from Nebulosity but all the calibration frames was created in PixInsight with the Batch Preprocessing script.

[pfile]

whoops, sorry, i thought that it was a master flat but i can see that it's a flat sub.

still nothing seems to be terribly wrong with that flat, the histogram is right in the middle of the display. i suspect that something has gone wrong in the calibration of the flat. that's usually the cause of overcorrection.

can you post a bias frame as well?

rob

[DanielF]

Sure thing, here's a bias sub.

https://dl.dropboxusercontent.com/u/8802413/M45%20problem/Bias_001.fit

/Daniel

[pfile]

just some observations.

the bias has about 4x the signal level of my camera, which is also based on a KAF-8300M. the bias also has some very interesting "moire" type patterns in it. i don't know if this is significant at all as i've only ever looked at bias frames from my camera.

the flat does not seem to match the light. the dust motes are in different places. there can be a lot of reasons for this, from as simple as the dust having moved to something as bad as the filter wheel not being able to return to a precise position. was the camera moved or rotated between the lights and the flats? did you take the flats before moving the filter wheel?

what's strange is the dust motes seem to have moved outward radially. that could have something to do with the focusing point. was focus changed between lights and flats?

i don't see vignetting as bad as yours in a single sub calibrated with one bias frame and one bias-subtracted flat. but that's probably because it's a single image.

rob

[Phil Leigh]

That bias frame also has 4x the signal and 22x the noise of my cooled DSLR bias frames...
Those horizontal patterns look like electrical interference to me...

Were the bias frames taken with the camera on the scope? - what power supply was used?

[DanielF]

just some observations.

the bias has about 4x the signal level of my camera, which is also based on a KAF-8300M. the bias also has some very interesting "moire" type patterns in it. i don't know if this is significant at all as i've only ever looked at bias frames from my camera.

the flat does not seem to match the light. the dust motes are in different places. there can be a lot of reasons for this, from as simple as the dust having moved to something as bad as the filter wheel not being able to return to a precise position. was the camera moved or rotated between the lights and the flats? did you take the flats before moving the filter wheel?

what's strange is the dust motes seem to have moved outward radially. that could have something to do with the focusing point. was focus changed between lights and flats?

i don't see vignetting as bad as yours in a single sub calibrated with one bias frame and one bias-subtracted flat. but that's probably because it's a single image.

rob

Hi.
I have no idea why the bias has more signal than your camera. Maybe it's just how this camera performs? Same with the pattern you described.

It's strange what you say about the flat that it doesn't match the light, the really bad specs are actually on the ccd and not the filters so they shouldn't move during filter changes. The camera wasn't moved or rotated but I did a very small adjustment to the focus one time but can that really move the dust motes that much? How do people do that refocus after filter changes for example?


If I calibrate just one light with the posted bias and flat I get the same overcorrected vignetting as the master shown in the first post.

/Daniel

[DanielF]

That bias frame also has 4x the signal and 22x the noise of my cooled DSLR bias frames...
Those horizontal patterns look like electrical interference to me...

Were the bias frames taken with the camera on the scope? - what power supply was used?

I had removed the camera from the scope when I took the bias and dark frames.
I used the original 220V power supply that came with the camera.

/Daniel

[Phil Leigh]

OK - not sure about the Bias frame - personally I would discuss with the manufacturer and/or other users of that camera.

I tend to agree with Rob - if you subtract your bias frame from your flat (to get a calibrated flat)  the flat ends up with the Bias pattern noise superimposed on it...

I'm not seeing the same thing as Rob regarding the flats moving vs the lights... they seem to line up pretty well to me.

I would try running the script again without supplying any bias frames and see what happens.

[pfile]

here's the dust in the lower left corner. to me it looks like it just does not match right. obviously part of it is overcorrected but i don't see how part can be overcorrected and part of it not corrected at all…