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Old 24-10-2011, 01:15 PM
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troypiggo (Troy)
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QSI583ws fast shutter speeds for flats

I have a lightbox for flats, and while it does have a brightness adjustment, even at low settings I only need pretty short exposures. Read somewhere that it is good to get flat exposures a few seconds long though, to do with the shutter affecting exposure and the longer the exposure, the less it will come into play? That makes sense to me, but I don't know how fast this shutter is.

On a DSLR you can take shots 1/1000s and the shutter isn't seen. Wondering how fast these CCDs can be?

I did some test flats .1s and I think faster, couldn't notice any visible issues. Am I missing something?
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Old 24-10-2011, 02:18 PM
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pvelez (Pete)
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Troy

I had issues with my SBIG ST8300 - unless my flats were longer than 1 second, I had shutter artifact which screwed with my flats.

I don't have that issue with the QSI - that said, as a matter of course I use some plain white cotton (about 4 layers of it) between the OTA and diffuser to step down the brightness. I aim for exposures of 2-3 seconds.

Pete
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Old 24-10-2011, 02:40 PM
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Quote:
Originally Posted by troypiggo View Post
Wondering how fast these CCDs can be?
The maximum speed of the QSI-500 series mechanical shutter is 0.03 second.
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Old 24-10-2011, 02:57 PM
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Thanks guys. Kind of confirms what I was seeing and finding out myself, but one can't be too careful
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Old 24-10-2011, 03:21 PM
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Kevin Nelson mentioned to aim for about 33000ADU from memory.
There was a lengthy thread over on the QSI Yahoo group posts about optimum flats.
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Old 24-10-2011, 03:28 PM
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Thanks mate. I think I'm ok on what the ADU and histogram is supposed to be, it was more about how I achieve that. I can either use the current lightbox and faster shutter speeds, or try to cut the light down and go for longer shutter speeds.
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Old 24-10-2011, 06:28 PM
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The shutter speed is only relevant in that with some cameras if the exposure is under about 3 seconds you can see the leaves of the shutter in the image.

Over about 3 seconds you can't.

With KAI series chips with the right controlling software you can use their electronic shutter in video mode for flats. Apparently it works well and is very fast for daytime flats with a white T-shirt.

SBIG cameras have a different shaped shutter which looks like a bar with a flare at each end and it spins.

This does not seem to show up like leaf type shutters seem to.

I can't comment on a QSI as I don't know what type of shutter it uses.

I find between 20 and 30,000ADU is a good level for flats and take darks same temp and subtract them from the flat when making the master flat.

What I do is take them during the day in my observatory which is painted matt black inside and a bit of light filters in through the gaps and I put a white T-shirt over the lens. You get used to what a flat should look like.

An accurate flat should be taken with the scope in focus, in the same orientation as your light exposures and same setup and I usually take 3 each of flats and their darks for Luminance, Ha and RGB. I used to do separate RGB filter flats now I just use one, usually the red filter. It seems to work fine.

If you darks don't match you light exposures precisely for both temperature and exposure you will get bum flat reduction on a sensitive image. So if your flats aren't "working" then look to how good your darks are first.

Greg.
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Old 24-10-2011, 06:54 PM
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Quote:
Originally Posted by troypiggo View Post
I have a lightbox for flats, and while it does have a brightness adjustment, even at low settings I only need pretty short exposures. Read somewhere that it is good to get flat exposures a few seconds long though, to do with the shutter affecting exposure and the longer the exposure, the less it will come into play? That makes sense to me, but I don't know how fast this shutter is.

On a DSLR you can take shots 1/1000s and the shutter isn't seen. Wondering how fast these CCDs can be?

I did some test flats .1s and I think faster, couldn't notice any visible issues. Am I missing something?
I have a bright EL sheet that really dims if I feed it 6V instead of 12V. Have you tried limiting the voltage on your lightbox? Then the exposure time is a moot point as you can reach the target ADU easily.
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Old 26-11-2011, 08:36 AM
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Dimming the electroluminescent panel

I found the combination of EL panels and sensitive Kodak CCD meant it needed a very fast shutter speed. To overcome this I added a sheet of Foamcor from a local art shop in front and this dimmed it enough to allow a luminance speed of 3 to 4 seconds.
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Old 26-11-2011, 01:14 PM
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When I remade my light box I added white paper diffusers over the internal white LEDs to drop down the light and found I now get good flats at 30-40000 ADU in 4-6 seconds exposure
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Old 27-11-2011, 08:29 PM
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Quote:
Originally Posted by frolinmod View Post
The maximum speed of the QSI-500 series mechanical shutter is 0.03 second.
I can get 0.02 on my camera.

Typically Troy I use the light box Pete made for me (which is prior to the dimmers being added) and this is usually around 0.07 for colours. 0.06 for Lum and 0.7 for Ha. I don't have problems with my flats, but mine are pretty high up too. Still it works for me. Experiment a little to see how things work best for you.
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