View Full Version here: : Exposure times & max values
24-06-2012, 11:29 PM
What is the recommended exposure % of the max value of the CCD.
For example CCDsoft reports a max level for my stl11000 of 65535.
So what max value should I be aiming for to have an acceptably exposed image ... or is anything close to 65535 OK ???
25-06-2012, 09:51 AM
Depends on what you are trying to image or achieve. Night sky images have a huge dynamic range. A bright star will saturate the detector with less than a 1 sec exposure. A dim nebula will never saturate the detector. If both are in the same field then you have a problem. You use exposures that will give you a reasonable S/N ratio of the component you want to image.
My interest is photometry so I will use an exposure that keeps the maximum count below about 45000 for the stars I'm interested in. This is the linear range for my CCD.
26-06-2012, 04:19 PM
STL11 will do well with 10 or 15 minute subs for most objects. For bright globs you go shorter like 3 minutes otherwise you burn out the bright core.
15 minutes is probably better if your tracking is up to it. But if you setup each night then 15 minutes is probably unrealistic. You'd get rounder stars with 10 minutes.
So whats ideal for the camera is not the only consideration. Round stars is your first milestone to achieve. You have to be able to get round stars almost every time with 10 minute subexposures otherwise you will have a lot of trouble getting bright enough images that you will be happy with.
You want to standardise this to some degree to minimise the number of darks you need.
For example 10 or 15 minutes at -20C should be a good achievable all year round subexposure for an STL11. I have been using 10 minutes for the last few years but now I tweaked my permanent setup a bit more 15 minutes is more common for me. 20 minutes for narrowband or even 30 minutes.
Now you only need a few darks in your library for all shots.
Say 10 minutes for LRGB and 20 minutes for Ha O111 and S11 all at -20C and then make sure your mount and autoguiding can provide that and tweak it until it does and you just completed step 1!
I used to run my STL at -30C in winter but in summer as I recall -20C was more practical. I think it does about 42C below ambient so
with summer temps of around 15C then -20C was achievable. I would go 90-95% power on cooling. Stronger cooling is better than warmer and the STL11 really sparkles at -35C on those really cold winter nights.
26-06-2012, 10:54 PM
Thanks Terry you are of course spot on and i have realised now how very general my question was.
Great info Greg. I have been running the STL at -20 and thats about 60%. Ive room to crank it harder eh. The luminence (clear) image i posted of ic4604 in the deep spce section was 10 min subs ..stars were ok i think. I was taking 5 min colour subs but that wasnt anywhere near enough ... They are pretty useless ... I have since collected about 1.25 hrs each of rgb .. 15 min subs but as yet not had the time to proces it. I have a very reasonable polar alignment and guiding is ok i think. Have nice round stars at 15 mins could go 20 i reckon. Keep an eye out for the results and let me know what you think.
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