Thursday, April 12, 2012

Focusing; imaging at a dark site

I recently posted that I was going to test the parfocality (if that's a word) of my Baader LRGB and narrowband Ha/SII/OIII filters. The test has been done, and it was less than conclusive. If you use a Bahtinov mask to do focusing, all the filters are essentially parfocal. That is after focusing for one filter, they're all in focus. They all have nicely centered spikes in the test images.

However, the centered appearance of the central spike when estimated by eye is only approximate, and the range of error is about the same as the range over which final focusing is done. I think you're still better off handling focus with the assistance of software that quantifies the sharpness of focus. Likewise, I think every time you change filters, it's time to refocus. At least that's what I'm going to do for the foreseeable future.

Last night I gathered another bunch of photons for my M101 project, this time from the dark-sky site of Cherry Grove. It's really nice to have a dark place to image from only an hour's drive away. The site has a warming house, too, which was nice. Imaging was largely automatice, so I could while away the hours reading and listening to the Twins game in comfort, as the temperature outside fell to around 0C.

Added note, 5/22/12: Further imaging with the Baader filters suggests that the LRGB filters are essentially parfocal when used with an apochromat. Likewise the Ha-SII-OIII filters.

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