Friday, January 4, 2013

Are Baader filters Parfocal?

At a recent star party I made an offhand statement that I doubted that my narrowband filters were parfocal. That set me wondering if I was correct and how would I go about determining this for all my filters. So I did a little experimenting one night by imaging Betelgeuse through all my filters.

I decided to set focus with the luminosity filter in place, and then cycle through all my filters seeing if the focus changed. Sorry, but I don't have a focuser with a digital readout. To get some sense of any focal shift I put a Bahtinov mask on my scope and imaged the results. I used three stars of different spectral types (O, A, and M), just in case the results would be influenced by wavelength. (I was imaging through my TV-102, so I didn't expect to see any significant wavelength dependence.) Here is a test strip for Betelgeuse (click to see full scale image):

Betelgeuse imaged through various filter configurations with no adjustment of focus

I used 5 and 25 s exposures for the broadband and narrowband filters, respectively, and processed only using ImagesPlus digital development. It looks like the central spike produced by the mask moves little if at all as the filter is changed. As expected, the focus shifts significantly when changing to no filter in use. So how much shift is significant? Here is an image resulting from 1/12th turn of my fine-focus knob away from focus:

Appearance when defocused by about 0.3mm

This is approximately an adjustment of 0.3 mm in focus and the central Bahtinov spike has shifted quite a bit. From past imaging experience, I think my final focusing "tweaks" are about 1/5 of this in magnitude.

So are the filters (Baader LRGB and 7 to 8.5 nm Ha/S-II/O-III) parfocal? They look close to being so. To my surprise, the broadband and narrowband filter sets seem to be almost, but not quite, parfocal with each other. The results were the same for the other two stars, Sirius and Altinak. But I'll probably still refocus every time I change filters in the future, particularly when using anything less than an apochromatic telescope or there's been a change in temperature.

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