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Troubleshooting FAQ | Problems? PM a Red or a Green Gu.... uh, User
Samir Kharusi
Pooh-Bah
Reged: 06/14/05
Posts: 1073
Loc: Oman
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Re: Guiding guidance?
11/03/09 11:04 PM
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I think your conclusions are fine. These days I do not even bother to re-think all this stuff each time. For white light I use 1 minute subs at f2.8, 2min at f4, 4min at f5.6, etc. Triple length for narrowband. ISO 1600 at a dark site, lower ISO if there is appreciable light pollution. I suspect too many people have not yet caught onto the fantastic ease of everything when using a fast focal ratio OTA. The mentioned Orion 8" f4 Newt sounds intriguing! If one can find a suitable flattener for it (Parracor?).
Another thing that changes all the parameters fundamentally is that the new Canon DSLRs have such tiny pixels; eg the 500d (T1i) has a 4.7 micron pixel pitch. People used to be very happy to have a large, super-priced APO ($10,000+) and a $10,000 35mm format astroCCD to image at 1.5 to 2 arc-sec per pixel, to end up with a decent A3 sized print. Well, with the 500D you can do similarly with a focal length of only 500mm to 650mm. Basically, for average seeing (around 3 arc-sec) you never need a focal length longer than 650mm to shoot DSOs with the 500D. This reduces the cost of the OTAs tremendously, and you often end up with a faster focal ratio. The OTA being much lighter also enables the use of less expensive mounts. Of course, because of the fine image scale, guiding may still be necessary. You cannot have high resolution without paying a price somewhere  But the total package price is much, much lower. The prices in these things go up exponentially, so they have now also come down exponentially. If only people took the time to comprehend what impacts their budgets and how.
Camera? Tiny pixels, modded. OTA? Fast focal ratio, max focal length of 650mm. Mount: best you can afford for the load. Autoguiding? Depends on previous selections.
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