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Bahtinov confusion


jnb
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I understand the principle of a Bahtinov mask and I've tried using one to focus my old setup. Now I'm changing scope (4" achro to 150p-ds) I'll get a new mask to go with the scope. But with the old setup I gave up using the mask because I found the images it produced confusingand not much better than focussing without. With a Bahtinov mask I would expect to see something like this;

bahtinov_focusing_mask.jpg

That should produce three clear diffraction spikes and they can be balanced comparitively easy. What I got when I bought a mask for my old scope was something more like the bottom example in this;

 

$_57.JPG

Rather than three clear diffraction spikes that produced four (the top grating is not one set of parallel apertures but two sets slightly offset) which gave an image which was much harder to focus.

I'll get a mask for the 6" scope and expect something that will give three spikes but is there any reason why someone would make a Bahtinov mask with that pattern?

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It's all a bit of a mystery!

I have a 200PDS with a specific Bahtinov which works a treat, but the mask doesnt workl so well with my F2.8 300mm EFL camera lens - mostly cos the spikes are too small/short. Apparently a Y mask is the way to go, but I havent had time to make one yet.  Have a look here.

Hope that helps!

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The ones with four alignments of slits seem to be Carey masks They will produce two crosses that need to be superimposed rather than the classic cross and line pattern of the Bahtinov mask.

The article on Wikipedia shows the Carey mask as producing a much more subtle pattern which must be harder to use:

https://en.wikipedia.org/wiki/Carey_mask

<edit> The inventor says it's easier to use - 'orses for courses!

http://www.geoastro.co.uk/careymask.htm

The pattern of any mask does need to be matched to focal length for best results.

Edited by Stub Mandrel
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