A question re. exposure

[speach (Simon)]

Would you get the same or very similar results with 600 exposures @ 6 sec as opposed to 6 exposure @ 600 sec? The total exposure time is the same and after stacking and all the other prepossessing would the resultant frame be the same or significantly different? If it's more or less the same why do we spend hours on getting an exact polar alignment?

[RickS (Rick)]

That's very topical, Simon. There's another discussion about whether there's any benefit in taking more than 30 subs.

The answer is that you'll collect the same number of photons with 600 x 6 seconds or 6 x 600 seconds but you have to consider the effect of read noise. If you have a very low read noise camera and 6 second subs are sky limited (i.e. the effect of read noise is negligible) then you'll get the same final SNR either way. If you have higher read noise and the short subs are read noise limited, then you'll get lower SNR from the many short subs.

One of the best things you can do when imaging is figure out what exposure time is needed to get sky limited. There's a thread by Shiraz which describes a clever method for doing this:

http://www.iceinspace.com.au/forum/s...d.php?t=117010

The attached graph shows the effect of read noise on overall SNR.

Cheers,
Rick.

[speach (Simon)]

Originally Posted by RickS

That's very topical, Simon. There's another discussion about whether there's any benefit in taking more than 30 subs.

The answer is that you'll collect the same number of photons with 600 x 6 seconds or 6 x 600 seconds but you have to consider the effect of read noise. If you have a very low read noise camera and 6 second subs are sky limited (i.e. the effect of read noise is negligible) then you'll get the same final SNR either way. If you have higher read noise and the short subs are read noise limited, then you'll get lower SNR from the many short subs.

One of the best things you can do when imaging is figure out what exposure time is needed to get sky limited. There's a thread by Shiraz which describes a clever method for doing this:

http://www.iceinspace.com.au/forum/s...d.php?t=117010

The attached graph shows the effect of read noise on overall SNR.

Cheers,
Rick.

Thanks Rick for that answer. So if I've got a very quite camera I can get away with short exposures and conversely a noisy camera needs longer exposures as the pixels that are noisy get filled and stay filled, with a longer exposure they can only be filled once, but if you use a short exposure on a noisy camera when you combine the frames there will be noise on noise on noise at that pixel and it will blow out? Now this comes to mind if the bad pixel gets filled will it bleed over to a good pixel? Also what is considered a long exposure? I can only reasonably get 300 sec before some trailing appears is that a 'long' exposure time? To me 1200 sec and above start to be 'long'.
So the options is to get an expensive quite camera and use it on a mediocre mount or a dslr on and expensive mount of course the ultimate is a quite camera on an expensive mount!

[glend (Glen)]

In your example, you need to consider the effect of sky glow (sky limited). As Shiraz has shown, paying attention to background ADU is important. Ray has also produced a set of exposure charts for the ASI1600 cameras, based on the sky quality in your location. Basically, the more sky glow the shorter the exposures, you simply shoot more, alot more, short subs to build the Signal. Of course as Rick has pointed out, the read noise of your camera is important in being able pull off this strategy. Thankfully the latest ultra low read noise CMOS cameras can work in this way.
As far as "pixels filling" is concerned, there are usually optimal well-depth settings for CMOS cameras. Pixels do not bleed.

[RickS (Rick)]

Hi Simon,

Yes, low read noise cameras produce good results with short subs. If you have a high read noise camera then, all other things being equal, you'll need to take longer subs to get the same result in the same total time.

As Glen mentioned, there are low noise cameras CMOS like the ASI1600 that work well for short subs.

Noise doesn't generally fill up pixels. Most forms of noise we deal with are a statistical uncertainty, not unwanted photons. It's sometimes helpful to think of unwanted signal as something separate to noise. For example, sky glow is unwanted signal. It's not actually noise, though it does have shot noise associated with it.

Most of the cameras we use these days are anti-blooming. That means that overflow from a pixel is gated away and doesn't overflow into neighbouring pixels.

As to what is a long exposure that's somewhat subjective. For me, 1800 seconds is as long as I normally go. I have done 1 hour subs a few times.

It is possible to determine the ideal sub length for your specific set up and skies. Shiraz wrote a nice post about one way to do it:

http://www.iceinspace.com.au/forum/s...d.php?t=117010

Cheers,
Rick.