exposure?

[speach (Simon)]

I've been thinking about exposure length. What is the best to do, say 100 subs @60 sec or 10 subs @600 sec? Now as I've found out when a pixel is filled there is no spilling into the adjoining pixel, unlike photographic film were the burnt out image will bleed into the adjoining emulsion. So that isn't a problem with digital. Of course light pollution will come in to the length of exposure, but let us pretend that L/P doesn't exist!

[Karlzburg (Karl)]

As far as I know the longer the shutter is open the more light comes in, light hits the sensor at different times as well. I've seen it in the subs I have done when I got through and look for star tails. I can see certain pixels have lit up when in a previous sub it hadn't.
If my current set up could do 600 sec subs I'd be doing that. Alas, it's 60 sec subs for me.

[RickS (Rick)]

Originally Posted by speach

I've been thinking about exposure length. What is the best to do, say 100 subs @60 sec or 10 subs @600 sec?

Hi Simon,

If it wasn't for sensor read noise, you'd get about the same result either way. To make best use of limited imaging time and get the best SNR you can, you should try to do subs that are long enough that the read noise is swamped by the shot noise of the captured data. A sub like this is called "sky limited." A shorter sub is called "read noise limited." There's no benefit in doing subs any longer than is needed to become sky limited and there are some downsides.

Here's a nice thread which describes a clever way to figure out how long is long enough.

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

If you have a camera with high read noise then you'll generally need to take longer subs. With a low read noise camera, like some of the latest ones that use CMOS sensors, you may be able to take quite short subs.

Cheers,
Rick.

[Atmos (Colin)]

Last night in Heathcote I was taking 300s exposures at ISO400 because I know that is long enough to sufficiently swamp the read noise but not so long as to saturate anything but the brightest of things (M42 core).

If Melbourne though I’d be limiting myself to 30s exposures because of light pollution and ISO100.
It is basically a balance between long enough to move the bottom of the histogram far enough to the right but short enough to not burn out the stars.

[silv (Annette)]

@Atmos "If Melbourne though I’d be limiting myself to 30s exposures because of light pollution and ISO100."
ISO100? Really? Please say it's a typo or my world view is kaputt.

[Atmos (Colin)]

I’m sorry Annette, welcome to one of the most light polluted cities in the Southern Hemisphere

With the QHY163/ASI1600 I could do 2-3s exposures with a luminance filter at Gain 76 and be sky limited

[xelasnave]

Hi Colin
I think my mount and tracking has me pegged to two minutes which I expect is rather short for NB but thats where I am at the moment.
I am up North and its raining so I dont know that I will get to do anything this trip but if I do what gain do you think is suitable.

I am reading all I can and have Rays spreed sheet but I am not close to being able to use it yet.

But last time out two minutes seemed the limit for me.
Alex

[Atmos (Colin)]

I'd suggest running it at Gain 200 Alex. Going further than that doesn't appear to have any real benefit and it should allow you to get the most out of your two minute exposures.

If two minutes is the limit on what your mount and tracking can reliably do then that should be your maximum
That is something else that sets limits, there is no point in going for that 5 minute exposure to get a nice background if all your stars are eggs

[xelasnave]

Thank you Colin I very much appreciate your help.
Alex