Hello,
I have been following a post on
Cloudy Nights about a revolutionary focusing mask designed by a Russian amateur astronomer, Pavel Bahtinov. The mask consists of a pattern of solid bars with equal width spaces between the bars. One side of the mask has horizontal bars, the other side has bars tilted at angles of ±20°. I decided to make one and give it a test drive.
Using Pavel’s formula: Focal Length/(
Range 150-200) = Bar Thickness + Inter-Bar Gap, I came up with the following design for my Tak Mewlon 180mm F12 with a focal length of 2160mm.
2160/180=12mm, giving a Bar Thickness of 6mm and an Inter-Bar Gap of 6mm.
NOTE: The divisor range of 150-200 allows for a Bar Thickness/Bar Gap dimension that doesn’t come out as an awkward value of, say 6.5735mm.
I drew up the mask in Corel Draw (phew!) and printed it off on a sheet of overhead transparency film as a prototype for testing. I then designed and built a foam collar which I glued to the transparency mask and finally, trimmed the rectangular transparency.
Last night I was able to conduct some testing with the following results.
- The Bahtinov Mask does work.
- It works very well indeed!
- As you approach focus, you can easily see a horizontal “moving diffraction bar” approaching the shallow “X” of the two “fixed diffraction bars”.
- At focus, the “moving diffraction bar” sits nicely centred between the shallow “X” of the two “fixed diffraction bars”.
- When past focus, the horizontal “moving diffraction bar” begins to leave the shallow “X” of the two “fixed diffraction bars”.
- Transparency film is not suited to making the mask as it does change the focus due to the thickness of the transparency film; visually, this was easy to see. Using Fomalhaut (1.2m), focusing without the mask was relatively precise by watching the 6 diffraction spikes produced by the Mewlon 3 vane spider snap into focus. When I fitted the mask, I had to adjust the focus by 3 or 4 short stabs on the Moonlite motor focus button – the difference definitely noticeable visually. Also, the transparency film has a sort of ground glass texture which dims the incident light and makes things a little smudgy.
So, it looks like I’ll have to make a mask with clear gaps! Many thanks to Pavel for sharing his design for this elegant, incredibly useful astro-accessory.
Cheers
Dennis