Quote:
Originally Posted by jenchris
take a series of photos and check the bahtinov diffraction pattern to get your focus - it only need doing once per session.
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I used to use a Canon 30D, without live-view and never had a problem getting good focus. I placed the Bahtinov mask over the scope and would shoot a bright star. Then I'd use the display on the camera back to view the star. I'd zoom the image to see the diffraction pattern, make a change in focus and re-check. It only took a couple iterations to get the focus
exactly right.
However, since I shoot at f5, I usually focus a lot more than once a session, especially if there has been a large temperature drop. Depending on the construction of your telescope, and the f#, the temperature drop can cause the scope to contract and pull the scope out of focus. Scopes made from Carbon Fiber will shrink less than Aluminum ones. Also, scopes with larger f#'s will have a wider Critical Focus Zone (CFZ) and will require less focusing.
For your scope @f10, your CFZ is pretty wide at ~245 microns (for blue). As far as scopes are concerned, this is fairly large. Don't get caught-up in the details, just understand that large temperature changes can shift the focus.
Keep working and most importantly, have fun. That's what its all about.