Re: SV: [MaxImDL] Proper dark correction of flatfields ?


Apr 14, 2003

 


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#6271 Apr 14, 2003

Please - one question concerning flatfields.



When entering the flatfields into Maxim (setup calibration command),

should the flatfield be bias and dark corrected in advance ?



If not -imagine, a flatfield is taken at 10 sec exposure, while the sky

image has been taken at 200 sec.



Is it reasonable to apply a scaled down version of my 200 sec dark to my

10 sec flat ?



Or is there any way by which I may correct my short duration flat fields

with the related short duration dark ?





Best greetings



Mogens W.





6.3 hour student image of M51 :

www.amtsgym-sdbg.dk/as/m51apr2003/



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#6274 Apr 14, 2003

Mogens Winther wrote: > Please - one question concerning flatfields.

>

> When entering the flatfields into Maxim (setup calibration command),

> should the flatfield be bias and dark corrected in advance ?



No. Flat fields are automatically bias subtracted. If you want to dark

subtract them as well, then use the Auto Scale and Apply to Flat options.

> If not -imagine, a flatfield is taken at 10 sec exposure, while the sky

> image has been taken at 200 sec.

>

> Is it reasonable to apply a scaled down version of my 200 sec dark to my

> 10 sec flat ?



In that case, I would not bother with the Auto Scale / Apply to Flats

option and let it subtract just the bias frame. The dark current in 10

seconds is negligible. Unnecessary calibration steps just add more read

noise into the result.



Doug



---------------



Doug George

dgeorge@...



Diffraction Limited

Makers of Cyanogen Imaging Products

www.cyanogen.com



100 Craig Henry Drive, Unit 106,

Ottawa, Ontario, Canada, K2G 5W3



Phone: (613) 225-2732

Fax: (613) 225-9688



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#6275 Apr 14, 2003

Thanks for your reply , but there is still one question.



Usually we prefer 200 sec exposures, which have to be corrected with of

course 200 sec darks.



However - if our flat is 100 sec - and I do want to correct that one

with a 100 sec dark, please, what should I do ?



The problem does arise if you have a large grade 2 chip with a few but

significant hotspots.



These will easily saturate in 100 sec exposure.



However - correcting them with half a 200 sec exposure does not give the

correct result.



So, please, I do look for a method which will allow me to correct flats,

which are not necessarily of the same duration as the main images.



Best greetings



Mogens











-----Oprindelig meddelelse-----

Fra: Douglas B. George [mailto:dgeorge@...]

Sendt: 14. april 2003 18:28

Til: MaxImDL@yahoogroups.com

Emne: Re: [MaxImDL] Proper dark correction of flatfields ?



Mogens Winther wrote: > Please - one question concerning flatfields.

>

> When entering the flatfields into Maxim (setup calibration command),

> should the flatfield be bias and dark corrected in advance ?



No. Flat fields are automatically bias subtracted. If you want to dark



subtract them as well, then use the Auto Scale and Apply to Flat

options.

> If not -imagine, a flatfield is taken at 10 sec exposure, while the

sky > image has been taken at 200 sec.

>

> Is it reasonable to apply a scaled down version of my 200 sec dark to

my > 10 sec flat ?



In that case, I would not bother with the Auto Scale / Apply to Flats

option and let it subtract just the bias frame. The dark current in 10

seconds is negligible. Unnecessary calibration steps just add more read



noise into the result.



Doug



---------------



Doug George

dgeorge@...



Diffraction Limited

Makers of Cyanogen Imaging Products

www.cyanogen.com



100 Craig Henry Drive, Unit 106,

Ottawa, Ontario, Canada, K2G 5W3



Phone: (613) 225-2732

Fax: (613) 225-9688



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----------------------------

#6276 Apr 14, 2003

Mogens Winther wrote: > Thanks for your reply , but there is still one question.

>

> Usually we prefer 200 sec exposures, which have to be corrected with of

> course 200 sec darks.

>

> However - if our flat is 100 sec - and I do want to correct that one

> with a 100 sec dark, please, what should I do ?



We're going to make this a bit more flexible in a future release. In

the meantime, here is a procedure that will work:



1. Open your 100 second dark frame, and a bias frame. (If you wish,

you can average multiple frames for each using Combine Images in Average

mode with no alignment.)



2. Run Process / Pixel Math, and subtract the bias from the dark, both

scaled 100%, and add a constant of 100.



3. Open your flat-field image, and using Pixel Math subtract the result

of Step 2 from it. Once again add a constant of 100.



4. Now calibrate your images as per normal, with unaltered bias and

dark frames, and the altered flat from step 3. Make sure "Apply to

Flats" is off.



Doug



---------------



Doug George

dgeorge@...



Diffraction Limited

Makers of Cyanogen Imaging Products

www.cyanogen.com



100 Craig Henry Drive, Unit 106,

Ottawa, Ontario, Canada, K2G 5W3



Phone: (613) 225-2732

Fax: (613) 225-9688



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