Bahtinov mask

[pastiche]

This may be a really stupid question, but what does a Bahtinov mask do? :)

[Stargazer Jack]

From what I have heard it helps getting perfect focus.

[hemihaggis]

yep well i think stargazer jack means perfect ,not prefect lol

[pastiche]

So how does it work?

[Snaggerman]

It's a focusing aid that you attach to the end of the telescope - it has a series of slits in it that result in a characteristic pattern in the viewfinder, which changes as you finely alter the focus. When the pattern lines up - the object is perfectly in focus.

Each telescope has it's own requirements for the mask, you can either buy one for your particular model, or do as I did and cut it out of a bit of card! To generate the pattern of the mask you can go to this website: astrojargon - Bahtinov Focusing Mask Generator: Overview - link provided by Steve Richards in his excellent book Making Every Photon Count.

[pastiche]

Hi Snaggerman,

Thank you. It's all starting to make sense now.

[pritc]

It is a great bit of kit...

Use mine all the time to get perfect focus.

Wayne

[Stargazer Jack]

yep well i think stargazer jack means perfect ,not prefect lol

Thanks for that lol

[JamesF]

If you're really interested in why they work, this is a place to start:

Fraunhofer Diffraction and It's Effects on Aperture Masks - A Primer - Article

It's probably not for the faint of heart though.

James

[JimStan]

I saw that page a few days back. It is a very fascinating and thorough study of diffraction patterns, and his conclusion that a simple "Y" device can do as well as the Bahtinov mask is a revalation !

As far as making such a device, I cannot see why one couldn't form such a thing with stiff copper, brass, or steel wire and a ring of strap that would fit the front of just about any telescope !

Good luck, and if you make a successful one, let us know!

Jim S.

[JamesF]

I saw that page a few days back. It is a very fascinating and thorough study of diffraction patterns, and his conclusion that a simple "Y" device can do as well as the Bahtinov mask is a revalation !

As far as making such a device, I cannot see why one couldn't form such a thing with stiff copper, brass, or steel wire and a ring of strap that would fit the front of just about any telescope !

Good luck, and if you make a successful one, let us know!

Jim S.

I made one earlier this year from card for my 127 Mak. It worked fine, but isn't really robust enough for long term use and it's now looking distinctly second-hand. I've ordered a Bahtinov mask to see how it compares, but may well have a go at making another Y mask from some scrap copper tube or steel sheet if I can find the appropriate dimensions again. For keeping it on the scope though I think all that's required is two pins through the arms of the Y at right angles so it hangs across the front of the objective. That way it can just be lifted off without disturbing the scope set-up in any way.

James

[JimStan]

As Lord says, the intersection of the "Y" does not even have to be in the center of the opening! As long as those arms are across the front of the scope, and radiating at a proper angle, the effect should be the same ! I would think that it could make a great metal shop project. I would even suggest the bisected "V" so "one size would fit all " or at least a fair range of scope tubes !