Jump to content


Bahtinov confusion


Recommended Posts

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;


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;



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?

Link to comment
Share on other sites

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!

Link to comment
Share on other sites

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:


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


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

Edited by Stub Mandrel
  • Like 1
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.