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Old 15-12-2008, 10:06 PM
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Best ISO for Canon 450D DSLR

Apologies if this has been done to death before, but I'm curious to hear people's recommendations on preferred ISO for imaging DSOs. I'm usually working from city skies with a light pollution filter (Hutech IDAS) and auto-guiding.

I've wanted to get out and experiment, but time and weather haven't allowed of late. From my very limited experience to date 1600 always seems too noisey for darks/flats to tame. 800 with max subs of 5-10 mins seems more realistic for my skies?

From the bit of reading around I've done higher ISOs are basically doubling the amp gain on the imaging chip. Supposedly there is some risk of losing dynamic range at 1600, but unlikely to be a problem for most objects.

Any comments/experience appreciated....
(but no prompts to go and buy a CCD please!)
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Old 15-12-2008, 10:24 PM
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Octane (Humayun)
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Rob,

I've always used ISO-400.

Make sure you take dark frames at the same ISO for optimal results.

Regards,
Humayun
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Old 15-12-2008, 10:31 PM
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I found 800 got noisey up around 10minutes with the 450D I had. Short shorts on clusters, like 2-3 minutes where ok at 1600. All you can do is suck it and see. With this 40D I tend to leave it on 400 for everything although I have a HA filter in it tonight shooting 20minutes @ 1600. I'm sure I'm wasting these clear skys we have
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Old 15-12-2008, 10:33 PM
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Hi Rob,

I think the optimum ISO setting for DSLR's is between ISO 400 and ISO 800. That being said, I always use ISO 800 with my 450D and only use a lower setting on brighter objects such as the Orion nebula.

Like Humayun has said, always take dark frames. I like to use ICNR darks as this is the best possible way to match the darks to the lights.

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Old 16-12-2008, 09:40 AM
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I've a 450D and still experimenting. Have had some success with 1600 as long as I take quite a few subs and cancel out much of the increased noise. 800 ISO seems the best compromise. Trouble is you double the exposure time and hence increase the dark current and dark noise. But sometimes you need a 10 min exposure at 1600 just to record faint objects. If you use ISO 800, you need a 20 min exposure. I think 10 mins is about the limit for a DSLR unless its cooled.
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Old 23-12-2008, 02:07 AM
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Thanks guys. There's no substitute for experience, but can't hurt to try and borrow some...!

I've shot a whole series of frames at various ISOs and overlapping exposure times tonight, both with and without IDAS LP filter, so will be interesting to see what those reveal.
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Old 23-12-2008, 02:41 PM
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The ISO almost doesn't matter. If you take a 10 min exposure, you get the same total amount of light hitting the camera sensor, no matter what the ISO setting is. However, if the total light on a pixel is very low (<0.5 on the 0-256 scale), then quantization will round it off to zero, so doubling the ISO would help to actually record a signal. The same sort of thing happens if you have big faint areas that only differ by a fraction; you won't distinguish the detail. If on the other hand you have something very bright which gives lots of readings from 128 to 256, then doubling the ISO will blow all these out to the same value of 256.
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Old 23-12-2008, 06:45 PM
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Quote:
Originally Posted by ghsmith45 View Post
The ISO almost doesn't matter. If you take a 10 min exposure, you get the same total amount of light hitting the camera sensor, no matter what the ISO setting is. However, if the total light on a pixel is very low (<0.5 on the 0-256 scale), then quantization will round it off to zero, so doubling the ISO would help to actually record a signal. The same sort of thing happens if you have big faint areas that only differ by a fraction; you won't distinguish the detail. If on the other hand you have something very bright which gives lots of readings from 128 to 256, then doubling the ISO will blow all these out to the same value of 256.

Thanks Geoff. Hadn't thought of it that way before. Very helpful to understand what can happen at both ends of the scale.

Rob
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