Flat Field Problem Over Correcting

[tdgm]

Hello Everyone

Recently I have been having a problem with over-correcting with my flat field. My equipment is Takahashi FSQ 85 mm refractor Monochrome camera and a flat man panel for my flat field images. I have recently change the dew shield to be not extended ( bringing the FlatMan panel closer to the glass of the refractor).

What is happening is the flat fields of the red filter (I have not changed how I obtain the flats from the past when they worked properly) except the dew shield on the front of the scope. Luminance images and flat field are calibrating correctly and no over-correcting is noted. On the red images wit the flat field one dust mote is changed from dark to light and shows up in the calibrated images but all the other dust motes are flat fielded out.

I have tried changing the ADU count of the red filter and no chnage is noted still the over-correcting. I have tried on a different object and still the same problem is noted.

So can the distance the flat panel of the FlatMan change the effect of the flat fielding  and why is it in the red filter and not the luminance???[/size]

If someone wants the files let me know

Thanks for any help you guys/gals can provide.

[sharkmelley]

Are your light frames taken with the same dew shield as the flat frames?  They need to be.

Mark

[stevek]

Are your light frames taken with the same dew shield as the flat frames?  They need to be.

Mark

Yep, I have also learned this one the hard way too.  The flat field illumination will not match the lights accurately if the dew shield is not present or positioned very differently from when the lights were taken.  It should not effect the dust bunnies that badly but will effect the field illumination and vignetting.

It took me months to work this out through trial and error and I was pulling my hair out trying to work it out.

[tdgm]

Hey Guys Thanks for The Reply

Yes the FlatMan illumination and the lights (Luminance and Red) were taken with the dew shield in  the same position.So I still need why in ony 1 filter and not the other.   Maybe the flat image is wrong somehow even though the ADU  is the same maybe the the brightness of the panel has to be the same for all filters because when i created the flat the brightness the same for all filters except red which was twice as bright.

Will test that today.

Steve   

[rbotero]

Steve

I had an issue like this some years ago and it was an optical issue, nothing to do with the way my flats were being acquired.  You can try this method to "correct" your flats:

1 - Calibrate - dark and bias only - your best raw frame (lowest background or FWHM for example)
2 - LinearFit that calibrated raw to your master flat (obviously raw filter to coincide with flat)
3 - Use the new master flat to calibrate all the raws for that filter

I learned this method here (search my posts) and it helped me somehow.  The only way to fully solve the issue was to replace my field flattener which was causing the difference in brightness for my R and Ha filters.

Roberto

[stevek]

Steve

I had an issue like this some years ago and it was an optical issue, nothing to do with the way my flats were being acquired.  You can try this method to "correct" your flats:

1 - Calibrate - dark and bias only - your best raw frame (lowest background or FWHM for example)
2 - LinearFit that calibrated raw to your master flat (obviously raw filter to coincide with flat)
3 - Use the new master flat to calibrate all the raws for that filter

I learned this method here (search my posts) and it helped me somehow.  The only way to fully solve the issue was to replace my field flattener which was causing the difference in brightness for my R and Ha filters.

Roberto

Roberto,
Thanks, that sounds interesting.
Regarding step 2.  You mean use the best raw light frame as the reference and linear fit that to the master flat?  Thereby adjusting the master flat?  Just want to be sure I have this the right way!

[rbotero]

Steve

Yes, exactly.  Make sure you calibrate that light frame with dark and bias - no flat.

Roberto

[niteman1946]

tdgm,

FWIW, I had outfitted my rather antique 12”LX200 Classic (F10) with the Mead 6.3 focal reducer.  The overcorrection was evident when using the reducer vs. not.  I replaced the Meade with a Starizona f7.5 SCT Corrector, and most of the problem disappeared.  Actually it was a significant improvement.  But there still was a bit of problem that cropped up when (I think) seeing was not great.
I then generated some evening flats with no diffuser.  These added another level of improvement.

Good luck,
Mark