Bahtinov Hyperstar

I know what you're thinking.  Some of you old timers read "Bahtinov Hyperstar" and you're sure it's some Romanian phrase that would get my mouth washed out with soap if it were translated to English.  You would be wrong my friends.  The younger ones in the crowd are thinking..."Isn't that the unholy offspring of David Bowie's Ziggy Stardust persona and Siouxsie and the Banshees from the early 80's?  Has it finally returned to prosecute its foul terror on humanity?"  Don't go diving for your still stocked Y2K cellar just yet.  No, it's not that either.  You're thinking of Lady Gaga.  It's a simple focusing mask my friends.  A simple focusing mask.  No need to panic.  Nothing to see here.  Let's move along people.  Let's move along.

http://www.spike-a.com/

If you want to know all that there is to know about a Bahtinov mask, you can find it here.  I'm going to make the assumption that if you're already here, you already know what it is.  The mask as it's been traditionally created does present some problems for those of us that are strung out on Hyperstar imaging.  The problem with the Hyperstar of course is the large central obstruction which is further complicated if you shoot with a DSLR.  This came up as a matter of topic on Cloudy Nights just a few days ago.  At the time I was away on business and unable to provide any pictures or truly useful information on how I'd solved the manual focusing problem with Hyperstar using a Bahtinov mask. 

Pretty simply, I took the normal design for a Bahtinov mask and added a big cutout in the center for the central obstruction of the Hyperstar.  For an 11" telescope, this is about a 4.25" (108 mm) hole in the center.  Second, I cut it in half down the middle so that I could get it onto the telescope without having to remove the camera.  This is especially handy if you shoot with the decidedly non-cylindrical DSLR.  Finally, I attached some cheap adhesive velcro tabs I had laying around to each end so that it would stay in place while focusing.  The mask is pretty crude but it works very, very well.  It cost me about $3 and less than 2 hours of my time to make it.  I got the template for my mask from http://www.focus-mask.com/ but it appears that the site is down or they are out of business.  I had to modify the template for the central obstruction which took the bulk of the time.  The mask is made out of a 20" x 30" sheet of black foam board and I cut it with a razor knife.  I live in the desert where dew is rarely an issue so the foam board has held up pretty well.  If you live in a humid environment, I'd recommend a different material as the foam board will absorb the dew and fall apart pretty much straight away.  So below are a handful of photos and a link to a video at the end that show the 'features' of my mask and how to attach it to the telescope.  Feel free to take any of this and use it for your own mask.  If you find a way to improve it or a better material, drop me a line and tell me about it.  I'd love to hear.



15 Second video showing how simple it is to attach even with the Hyperstar & Camera in place.

The Mask is really nothing more than a normal Bahtinov Mask that I've split down the middle and then cut out the central obstruction.

  

I cut the hole out to be 4.5 inches ( 114 mm) to fit around the Hyperstar lens on a C11.  If you're making your own...mine fits pretty loose.  If I had to do it again, I'd cut it at 4.25" (108 mm).  This is larger than the central obstruction of the mirror alone.





The two pieces are held together by nothing more than a velcro tab that has double sided adhesive.



I cut a sort of bumper for each corner and grooved a vaguely accurate radius on the inner edge of each one.  It does a serviceable job of holding the mask in place while it sits on the telescope.