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Flat field exposures and well depth?


SteveA
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Just need some advice regarding the exposure needed for taking flat field frames.

I’ve bought an EL panel and have been “playing” around with my Atik 314L. The currently accepted wisdom talks about taking a flat field, with an exposure that gives you between 60% & 75% of the full well capacity. Question is how the heck do I do that?

I think I’ve found out that the full well depth on the Atik 314L is about 17,5000 e

http://www.dangl.at/ausruest/atik_314/atik_314_e.htm

I’m using MaximDL and using the information window I can get a pixel value, but is this the same thing ? I suspect not as the pixel values can far exceed the value above.

Steve

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Hi Steve assuming that this is the Sony 285 chip, the figure of most interest to you for this purpose is the saturation value which is about 60,000 for your CCD so ideally I would say that you require a flat field with a maximum value of around 26,000. Using MaximDL you can measure this value using the information dialogue box and the cursor set to 'aperture'. Personally, I think 60-70% is way to high but there is much debate on this!

Edited by steppenwolf
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Thanks Steve, I'll give that a go. The flats I've taken so far were with values of around 14,000 and suprisingly short exposures of between .007 seconds through a luminance filter and 0.01 through RGB filters. I'm not sure whether I need to calm the ELP down a bit? I'm already using a sheet of frosted perspex, which I assumed would do the job, I did try introducing a sheet of heavy A3 paper between the ELP and the perspex but the illumination then didn't look quite so even. I was also quite suprised that the ELP had a distinct pinkish hue when turned on...not sure if that's an issue or not??

I'll try with your values..though I guess at the end of the day the proof that these are any go is if they make a difference to the images.

Thanks (again..) Steve

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My EL panel also has a pinkish hue but with your mono CCD, this is not an issue. With a one shot colour CCD it is an issue but it can be resolved by applying a 'boxcar filter' to that FLATS to destroy the Bayer Matrix at the time you combine them in median mode to produce the Master Flat.

I am not surprised at the short exposures with this sensitive CCD sensor - I had the same result. If you get 'banding' then you may need to increase the exposure time to ensure that you exceed the 'refresh rate' of the panel. To compensate for this you may well need to insert one or more pieces of paper in between the panel and the 'scope front

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I use a Gerd Neumann panel which appears quite blue (it is actually very white) and with five sheets of paper and a plastic diffuser I get exposure times of between 1 and 4 seconds. I use an ST10 and an STL both of which have mechanical shutters so exposure times must be longish.

I would read the average value in Maxim's info window but beware of any non-linearity in the chip. The sensitivity will tail off at the top end due to the anti-blooming gate on the chip. Go too close to this value and your flats will not correct properly.

Dennis

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Thanks guys..

As its the weekend and the weather looks good tonight is going to be my first attempt to take some "real" flats whilst imaging...so fingers crossed!...just hoping the ash cloud doesn't mess with the seeing:(:D:(

Steve

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