Judging exposure length

[vorkus]

I recently purchased a 383L+ mono.  I have a lot of experience using Canon DSLRs.  I know how to judge proper exposure with a DSLR but not with the 383.  The Artemis software doesn't seem to have a histogram to look at.  I've tried judging using the values for black/white with autostretch turned on as well as playing with the sliders.  With my 1000D, a typical target at home (orange zone), I can't go past 2 minute exposures (1600 ISO).  I took 5 minute exposures last night with the 383, but didn't seem to over saturate the image.  What is the best way to assess adequate exposure?  Thanks for your help.

John

[timwebster]

So many factors (temperature,binning,f ratio of scope, brightness of object, guiding accuracy) involved it is difficult to judge, however you can achieve much longer exposures with a cooled CCD than a DSLR.
In general I image at -20c, use 2x2 binning (especially with my C8 telescope) and for fainter objects  use 5min subs.
Could probably do longer subs but if an image is ruined by satellites, planes, clouds or bad tracking at least you have only wasted 5mins imaging time rather than more.
With brighter objects (globular clusters, M42 etc) I find 1 or 2 min subs are adequate.
I think there are debates whether using fewer longer subs is better or if using more shorter subs  result in better signal to noise ratio.
Hope that helps, Tim

[vorkus]

Isn't there a more quantitative approach?  Am I up above the read noise?  Is the light pollution becoming too dominant?  I was able to judge this with a histogram of my RAW DSLR files.  Should I be using a 3rd party program to do the same with this camera?  The dynamic range seems higher so perhaps I don't need to get the histogram as far to the right?  I was hoping for some measure to use.

John

[bwa]

Isn't there a more quantitative approach?

John,

I base my exposure on the histogram of the image.  If it is off (right of) the left axis of the histogram, you're not losing anything...  Make sure you don't expose to push the curve off the right side of the histogram and you're great.

Yes, one thing I would really like in Artemis is a histogram display but you can work with the Black/White sliders and the Auto Stretch checkbox to get a pretty good feel for where you're at.

bwa

[vorkus]

I pulled the tif I created (5 min exposure of M96) into PaintshopPro to check out the histogram.  I also examined the background with the eye dropper tool.  The background sky was about 18 on average (0 to 255) scale which seems low.  I have a RAW file from my 1100D from the same night and its background was more like 60 (a bit high).  I probably over exposed it.  For the Atik shot however, I think a background of 18 with a 5 minute exposure from an orange zone is probably underexposed.  Would you agree?  Or is this camera just much cleaner noise wise than a DSLR?

[rfdesigner]

Optimum exposure length is always as short as you can get away with without impacting other things.

Normally that means not letting the readout noise start to impact.  To avoid that you need enough background glow so as to just swamp the readout noise.

You get this with a background level in the region of 2000 counts on the 383.

(2000 counts +~1000e, this gives an uncertainly or noise of 31.6e, if you add in the readout noise it comes to 33.1e, only 5% more.)

If features you want to image are getting washed out at this exposure you can always take some short frames as well and merge them together in processing.

hope this helps

Derek