Hi Paul,
I can give you some info based on my experience with a Canon 450D and the three different scopes that I image with (as shown in my signature line). Responses below your questions:
Quote:
Originally Posted by pfitzgerald
Hi Folks
1. Have I chosen the best ISO setting for my chosen subjects?
2. What should be the length of my exposures? (This will be my first time imaging from a good dark sky location so I'm not sure about this wrt SNR)
3. What would be the optimum total number of light subs required for 1. and 2. for my chosen subjects to maximise bringing out the nebulosity of these DSOs in the processing process?
4. Should I also include some shorter exposures so that the brighter stars in the images aren't burnt out?
Any advice/suggestions would be much appreciated.
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1. Yes I believe ISO 800 is the best setting for many Canon cameras. It represents a good trade off between senstivity and noise. Craig Stark has an excellent paper online that looks at the optimum ISO level for DSLRs (Canon's I believe). My own experience at my dark site shows pushing the ISO just gives you more headaches than gains when it comes to post-processing. Here is a link to Craig Stark's paper on ISO verse gain, it's worth a read before you go:
http://www.stark-labs.com/craig/reso...nLinearity.pdf
2. The length of your exposures are going to come down to your mount and your guiding (the camera and scope don't really come into this other than a consideration for the speed of your scope f6 in your case). So first of all (you did not mention your mount did you?) make sure that your tripod is firmly setup and I mean on solid ground that will not give when weight transfer during a slew. I sometimes take three concrete pavers (about 200mm x 200mm) that I use as a tripod leg base in the field. In my case with the NEQ6, I have drilled dimples into the pavers to locate the point of the tripod leg securely.
I usually start off running subs at 180 secs and then see how they look. My last dark site outing I was running 300 sec subs with no problem. Make sure your mount counter weights are setup to leave alittle more weight on the counter weight side so that you keep the gears engaged and things don't slop around under tracking. I would not suggest going above 300 sec subs but it depends on conditions and the graph being thrown up by your guide software. If your RA and Dec lines are very flat then try for longer, but be aware that any sort of wind will impact guiding and sub times.
I should add that at home I now have a pier and observatory so longer subs can easily be done on that setup. I don't push sub lengths too far on a tripod. Something as simple as a guy walking by can influence the quality of a sub. Also watch out for red light pollution, and lasers, I have had subs ruined by fellow IISers walking towards me with their red headlight on - shining into my scope, or waving lasers around the sky to point out something to someone (this tends to be visual observers who are unfamiliar with the damage that this can cause to subs or your whole run).
I always check my subs (using the Liveview screen) after taking two or three so make sure I do not have any setup problems, then I page back through the whole run to make sure I don't need to shoot some more.
3. This will depend on your target, but I like to cover as much of the sky as I can when I am out at the dark site (there is just so much available without LP) so I tend to stick to ten subs per target object. So that can eat up 50 minutes a night on each target. Allowing for setup time on the next etc your looking at at least an hour of time spent on each target. Seeing as how most people struggle to get more than about five hours a night of productive imaging (due to waiting for total darkness, alignment time, equipment issues, power problems, and DEW!), you maybe doing well to cover five target objects per night. If your focusing on three only you might get up to two hours (or 20 subs) but don't expect more.
4. For targets like M42 for example, yes do some short subs (say 30 seconds onward) as you can use these for layering later in processing.
NGC372 is a popular target, have a look at images posted here on the forum and check the sub details if they are provided, that's a good guide. Honestly there is not much in there that will burn out your image at say 180 seconds but check your test subs.
General comment on that area of the sky. you will need to keep an eye on the movement of these targets towards the Meridian. I don't image across the Meridian (usually stop before it crosses and then reset on the other side if continuing, but some systems can image across, I just don't do it).
Make sure you have enough power for your mount, guide PC, and camera for the whole night; there is nothing worse than having great conditions and finding that your power is gone by 1am. If your going to run un-attended (and get some sleep) then make sure you have adequate power for the whole load.
These are my personal opinions and certainly there will be people that approach it differently.
Hope that helps in some way.
