Sunday, March 13, 2011

Focusing with a Bahtinov Mask

Polaris B asked in a recent post if I was using a Bahtinov mask for focusing. I'd never heard of such an animal until then, but after some research on the Internet I found that it is a very simple device to use. Basically, the mask is a pattern of slots that one places over the end of the scope. The slots produce a pattern of diffraction spikes around stars, one pair of which moves as the scope's focus is adjusted. The scope is in focus when this pair of spikes is centered in the pattern.

Bahtinov masks are available from various manufacturers and retailers. You may also generate your own mask using the astrojargon Bahtinov Focusing Mask Generator. One advantage to using the generator is that you can experiment with the parameters to tweak the performance of the mask.

I generated a mask for my ST80 using the default parameters, printed it on card stock, and cut out the slots with a hobby knife:

Bahtinov Mask printed on card stock.
I remove my scope's dew shield and fasten the mask in front of the objective lens using transparent tape. After polar aligning the mount, I swing the scope around to a bright star and view it through the live view mode of my DSLR camera.

The image below was taken while focusing on Sirius:

Sirius through the Bahtinov Mask.  At left, the image is out of focus.  At right, the star is in focus.
My new electronic focuser is capable of making fine adjustments, so centering the spike is relatively easy. When the star is in focus, I remove the mask, replace the dew shield, and everything is ready to go! The whole process takes only a couple of minutes, which includes the time that it takes to manually point the scope at the star. That's a big time saver over my old method, which would take up to half an hour and didn't always yield satisfactory results.

Despite the mask's ease of use, though, I still cannot see the monkey.

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