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.

Another telescope, WOW, images

Yes, I've purchased another telescope, this time a used C9.25. This is something I've wanted for a long time; it fills the niches for long focal length deep sky imaging and planetary imaging/observing. I've been using it so far only for deep sky imaging, and it's very nice, even at f/10. Here are some samples:

Messier 16 (Eagle Nebula) LRGB

M27 (first light image) Ha+OIII full frame

NGC 6820 (full frame) Ha
Sort of looks like M 16, doesn't it?

NGC 6781 (full frame) Ha + OIII

At f/10 it has a nice flat field. These are imaged with an SBIG ST-8300M and Baader filters. I usually bin 3x3.

WOW stands for the Wisconsin Observers Weekend, an annual event that brings together amateurs from all over Wisconsin and nearby states. It's held at the lovely Hartman Creek state park near Waupaca, WI, and features very dark skies. If you're in the MN, IA, WI, IL area and looking for a nice group of people to observe with under great skies, you can't do much better. I had bad luck with a faulty dew preventer, but still managed one night of imaging that got me the M 16 image above.

Next for me is the backyard observatory, the resumption of the school year, and a weekend trip to Jeffers Petroglyphs. More images coming soon, too.

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.