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Wednesday, September 3, 2008

The LCD "zoom/review" focus method

A simple way to focus your DSLR for astrophotography without a lot of other expense like a costly knife-edge device or laptop with cabling and focus control software, involves zooming in and reviewing diffraction lines from a bright star near the deep sky target. This method works best with a Newtonian telescope, where the secondary mirror is suspended by 3 or 4 straight flat bars 90 degrees apart within the optical tube assembly.

With diffraction lines, the zoom-review focus method can be accomplished very quickly. Three focus frames taken during the prior night's efforts are shown here.

The metal bars commonly referred to as "spider" vanes, cause the attractive (or distracting) star-like diffraction lines observed in my photos. I find it a pleasing photographic effect. For non-Newtonian scopes, one could fabricate some vanes to attach in front of a tube assembly to create diffraction lines just during focusing. I've seen some photos on other web sites where amateurs have used Photoshop to add the diffraction lines artificially (honestly, it looks as fake as it sounds).

You always want to try and focus on something nearby, as slewing the telescope clear across the sky can disrupt the mirror collimation on your touchy Newtonian.

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