Telescope Reviews: ISO Settings

Just because one can autoguide one-hour exposures does not make one-hour subs a good idea. Very long subs are not terribly practical unless you have a permanent set-up, otherwise you may find that your 2-hour drift alignment torture session during set-up was simply not good enough to get rid of field rotation, despite the autoguiding. When you have only 3 subs totalling 3 hours and one has a plane in it, the next has a satellite... Personally I do not believe in "exposing to the right" for astro. Seeing seems to impact the longer exposures more than the shorter ones, and getting rid of noise artifacts (be it thermal, or cosmic rays or banding or...) is easier the more subs there are in a stack. The sharpest images I have ever shot personally have always been the shortest subs I could get away with while keeping away from Read Noise and unguided. So there are many facets one has to compromise against. Most people's mounts simply cannot track precisely for long enough even just to keep away from Read Noise, so ETTR is often no more than a theoretical objective.

Now to the CGE. The CGE should be able to track unguided for one-minute subs at a focal length of 1200mm, and in inverse proportion, 2min subs at 600mm, 4min at 300mm, etc. The MINIMAL length subs you can get away with at a dark site are 1-minute at f2.8, 2-minutes at f4, 4-minutes at f5.6, etc. These will give you a skyfog histogram in all recent Canon DSLRs that is detatched from the Read Noise at the origin enough that your subs will stack beautifully. I suspect that you are using an SCT? At f10 you need subs that are 13minutes each at a dark site, and even at f6.3 you need 5-minute subs. Even autoguiding with a separate guidescope is not the answer. You will eventually need an off-axis guider. That's the bad news. The good news is that if your SCT is a C11 or C14 just purchase a Hyperstar and enjoy. No autoguiding, ever. A C8 at f6.3 is still too much magnification (image scale in arc-sec per pixel is too fine) for pleasant DSO imaging on a recent DSLR. You need to keep the image scale coarser than 1 arc-sec per pixel since you are unlikely to have seeing at better than 2 arc-sec FWHM anyway. So heroic efforts are very unlikely to satisfy fully. I have not found much difference between ISO 1600 and ISO 800 at very dark sites, so again IMHO the choice between the two is at best a minor quibble when using Minimal Length subs, as opposed to ETTR. ETTR at a pristine site at f6.3 or f10 is enormously difficult. Minimal length at f10 being 13 minutes, ETTR ends up 60+?Pristine site, use 1600 (makes the frames look brighter), some skyfog use 800, more skyfog drop to 400, etc. So, with the CGE focal-length rule-of-thumb and the f-stop rule-of-thumb you may be able to do all your DSO imaging with no autoguiding if you have a suitable OTA or camera lens. In addition, train your PEC and remember to initiate it every time you switch on the mount. The CGE does NOT initiate the PEC by default. Also note that 1st generation handpaddles for the CGE had non-functioning PEC. You need to replace it with a 2nd generation handpaddle. You can do the training at a long focal length visually and the focal length rule-of-thumb quoted above may improve dramatically. Consequently I think this should be your first step. Establish your own rule-of-thumb for subs-length vs focal length for unguided subs. You do not need a 100% yield. As long as you can achieve a 70+% yield you will find it much more pleasant to do unguided imaging than autoguiding. The best use I have found for my autoguider was to do the PEC training. After that I have put the autoguider away for 2 or 3 years now. I am currently using an unguided AP1200 at home so I cannot tell you what focal length vs sub-length rule of thumb is achievable with a CGE+PEC, but you may find that establishing that for your own mount maximises your facility for DSO imaging enormously. First essential IMHO is to understand your mount's behaviour and how it impacts your choice of OTA and sub-length. It takes us all many years of fumbling to find which rules-of-thumb work best with our own equipment or in our purchasing decisions.

I would put the order of importance for DSO imaging as follows:
1. Sub-exposures SHORT enough that your tracking is pixel-perfect in 70+% of your frames
2. Sub-exposures LONG enough that your skyfog is clear of Read Noise on the histogram. Points 1 and 2 really do put a squeeze on and ought to determine which OTA you purchase.
3. Ample integration time for your skyfog level
4. Full calibration of the frames; darks, bias, flats.
5. All the peripheral stuff that actually have little impact compared to the above 4, like choice of which Canon DSLR, ISO, ETTR, etc, etc
But that's just IMHO...

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