Bahtinov mask - quick question

[Martin-Devon]

I've just purchased a StarSharp Bahtinov mask from FLO. Can I just check on how exactly to use this - I know that I need to focus on a bright star and get the diffraction spikes symmetrical on my imaging screen - but once focused in this way, does it remain “in focus” for the rest night, no matter what target I choose? In other words, when I swing round to my DSO target, where there may be no bright stars, can I rely on the earlier focussing, or do I need to try and re-focus again in the precise area of my target, even if this means I have to use a dim star?

Just to mention the excellent service from FLO - my Ikharus refractor is 102mm/F5.6, and FLO stocked standard masks of F7.0, or F5.0 for a 102mm. I just dropped them a note and they arranged for a custom made Bahtinov for my exact scope of F5.6, and did not charge any extra either - fantastic service.

[Psychobilly]

With a frac you should be ok but it a quick exposure with the mask on will confirm if you need to re-focus...just remeber to rermove it before the imagign run.. I guess most of us who use them will have left them on at some time or other.. I know I did for a 4 hour run on M42 a few years back...

Focus can change with temperature so its soemthing to keep an eye on...

Peter...

[ollypenrice]

As above. Ideally, if you can't use a star in the image, use one at a similar altitude in the sky because the atmosphere affects focus as well. A quick slew to the focus star from time to time is a good idea. Faster sopes are more sensitive to change, slower ones a little more tolerant.

Olly

[JimStan]

I read elsewhere on this forum, that you had to have the EXACT mask for your telescope's FL . Is this true ? I would think that the important thing would be having a mask that matched the diameter of the front of the scope so the pattern would cover the whole opening. Isn't focus - focus, and the diffraction pattern that is caused by the mask would be centered on a star, whether the scope was an 8" F:5 or an 8" F:10, for instance. Can someone clarify this for me?

Jim S.

[Bizibilder]

It does depenf on focal length and you should have a unique mask for each but you can often get away with it with a "near enough" mask.

[davew]

I'm unsure where the information about these masks needing to be manufactured for every FL comes from.

For an idea of how they work see here -

http://www.brayebrookobservatory.org/BrayObsWebSite/HOMEPAGE/forum/Fraunhofer-primer.pdf

Y masks work very effectively. The problems start to arise, not because they don't work over a certain diameter but, because they are more delicate. I'd draw the line at about 8".

Dave.

[ollypenrice]

I read elsewhere on this forum, that you had to have the EXACT mask for your telescope's FL . Is this true ? I would think that the important thing would be having a mask that matched the diameter of the front of the scope so the pattern would cover the whole opening. Isn't focus - focus, and the diffraction pattern that is caused by the mask would be centered on a star, whether the scope was an 8" F:5 or an 8" F:10, for instance. Can someone clarify this for me?

Jim S.

I doubt that this is true. I used a mask made for my 140/980 TEC with the Altair Astro 115/805 and it seemed to give a perfectly convincing result. I must say that I always do final focus using FWHM but that's because I'm obsessive! The 'wrong'mask was as close as the 'right' one tends to be, which is very, very close.

Olly

[fwm891]

Martin - try this: astrojargon - Bahtinov Focusing Mask Generator: Overview

You enter the details of your scope and it generates a mask file for you - you just need to print it out...

[JamesF]

Perhaps you always get diffraction patterns, but get better/clearer patterns if the mask is of the right dimensions?

On the other hand, if to get a perfect mask you need to cut it to very fine tolerances then I doubt many people have the facilities to do that and yet their masks still work, so perhaps it just doesn't matter.

James