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Old 26-10-2010, 07:39 PM
astrospotter (Mark)
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astrospotter is offline
Join Date: Jan 2010
Location: San Jose, CA, USA
Posts: 146
Canon EF 200mm f/2.8 L II Prime focus lens

So more options are given I'll pipe in and say I am having a ton of fun and very happy with the Canon 200mm L series prime focus lens that I had already very much enjoyed for years for daytime shots. It yields 5.3 degree wide shots in front of a Qsi583 camera with filter wheel so this lens can do narrowband or LRGB with the wheel.

They say a picture is worth a thousand words so I'll post a link after a word more. The focus is extremely touchy but once obtained on a bright star and home made bahtinov mask the lens holds rock solid for hours.

The pictures are with this lens on a Qsi583 but that has just about same sensor size as Canon 40D so you would end up with about 5+ degree field wide open at f/2.8 so tighter stars can be had by stopping down 1 stop but I have been happy so far so as not to double my exposure time for the added stop (life is a trade off ... always).

Here is a shot on my site but basically all shots there are this lens and camera. (site in in north so hope link works) I don't know how to post pics to Ice In Space.

Good luck in whatever you end up using.

For stopping down the lens I use step-down rings which are cheap and very accurate. They are real cheap so you can easily pick up a few different sizes and do the stop down that way so that your stars remain perfectly round without the Canon 8-point stars from the 8-blade stop in canon lenses. With a step down to 37mm I can obtain very tight focus to do RGB shots without re-focus (seen in the M31 image on the same website). This too is tradeoff but not bad looking IMHO. See below
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