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flats and how to apply them in IP


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#1 shoste

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Posted 07 July 2009 - 12:41 PM

Dear All,
The calibration routine in the Autostar Image Processing never divides flats correctly for me. It always 'over-corrects', in fact usually by stressing the background inhomogeneity (e.g. due to vignetting) and enhancing dust rings rather than removing them. This has to do with the application of the wrong scaling factors. Can anyone give details about the algorithm implemented by meade to do the flat division? One could of course reduce the flat's gain manually by trial and error or by adjusting the illumination but that would really be tedious. I've tried it with ImageJ and it does do the job, but only after fiddling with it for a rather long time.
Any help is greatly appreciated.
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#2 Mark Sibole

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Posted 16 July 2009 - 06:14 PM

It is more than likely the flat being captured incorrectly.
When shooting flats watch the histogram and only let it fill half way.
This should give you the proper info on the flat.
I generally let it spike in the middle.
This can vary in exposure time depending on how bright your flat source is.

Regards

Mark
Mark Sibole
MTSO Observatory
Fife Lake, Mi.

http://astronomy.qteaser.com

#3 shoste

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Posted 17 July 2009 - 11:28 AM

Dear Mark and all,

Thanks for the tip. I usually allow 20K ADU, as estimated from the high countrate cutoff in the histogram. That is not the culprit, though.

My best results are obtained by dividing non-dark-substracted/normalized flats directly into the (dark corrected) image. This is counterintuitive, but substracting darks from flats results in an increase in the min-max count-ratio of the flat and therefore yields an over-compensation of detector inhomogeneity. A non calibrated flat exhibits about 3-5% detector inhomogeneity but after calibration, that increases to 10%!)

In my chase for the proper flat division using Envisage, I stumbled upon a software quirck when comparing count rates in darks as time increases (at contant temperature). As can be seen from the excel-graph attached here, a glitch occurs in the "sensitvity" above 10 seconds. This may be related to additional software based treatment (sharpening?) occuring behind the scenes. Can this feature be turned off?
Yours truly,
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#4 shoste

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Posted 07 August 2009 - 04:30 AM

Dear all,

After recording several hundred flats during twilight or by using a lightbox with four adjustable white leds and a milky plastic diffusor, I still struggle to obtain acceptable calibration with comlete removal of vignetting and dust trails. The problem remains exactly the same as when it was first diagnosed several weeks ago. When applying any dark substracted flat from a decent, dark substracted image, overcompensation always occurs, i.e. white or light grey dust dougnuts and an inverse background gradient remain, particularly around the edges of the frame. I can only obtain perfectly flat substracted images if I apply a brightness scaling to the flat (prior to division), in such a way that the brightness scaling is inversely proportional to the brightness range in the original picture (logical but artificial). Summarizing, the dynamic range of my flats is much larger than the background of my image frames.

I was suspecting deviation from linearity in the DSI pro II under low light conditions, but that seems acceptable, particularly at low ADU and up to 80% of the maximum. The best (but still poor) flats are obtained when twilight is very advanced and the background is close to the night sky brightness. In that case, rather long (20 sec) exposures are required to obtain these flats, which then obviously exhibit poor S/N ratios.

The DSI has te following characteristics, as determined from the AIP4win procedure: gain 2-3 (?)e/ADU, readout noise=13 e rms, dark current 10 e/pixel.sec at 29°C.

Any help is greatly appreciated.

Serge

#5 hargy

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Posted 02 October 2010 - 05:19 AM

can anyone explain this in english please.
LX200R 10 inch Semi Peir Mount, Willam Optics 2.5 inch Guide Scope. Orion Auto Guider CCD. Meade DSI II Pro CCD. Deepsky Stacker, PhotoShop 5.

#6 Mark Sibole

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Posted 12 October 2010 - 09:08 PM

For your flats to properly be subtracted you need to bias subtract the flatsas bais subtract the lights befor you fubtract the flats
This is what I have n=been told and am doing here


Mark
Mark Sibole
MTSO Observatory
Fife Lake, Mi.

http://astronomy.qteaser.com

#7 hargy

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Posted 13 October 2010 - 04:28 AM

Sarcasim is another service I offer. That being said, It's MY demented sense of humor. What I am trying to say is that I dont have a P.h.d in Photography. You guys are using terms that I don't understand. I get the basics of subtracting the flats and the dark to get a master flat or bias flat. I dont get the procedure on how to do that with the meade software. Is there an Idiot's guide to using envisiage and imaging processing somewhere?
                                                Hargy; Village Idiot
LX200R 10 inch Semi Peir Mount, Willam Optics 2.5 inch Guide Scope. Orion Auto Guider CCD. Meade DSI II Pro CCD. Deepsky Stacker, PhotoShop 5.




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