bahtinov mask

[sinbad40]

So i have a bahtinov mask for my 80mm refractor.  I have managed to use it on the 50mm refractor as well by balancing a flat rod across the outside of the top of the scope and putting the 2 pins from the mask on either side so it is sort of in the middle of the scope, obviously it is to big, but it works fine for getting everything focused.

Now I know you can download masks for different size scopes, but if I had something like a 120mm refractor, could I use a cut card to cover the scope, cut a circular hole in the centre that would then allow my 80mm mask to fit over the hole and let the light through, or does it need a particular size slot size for the light to go through?

[Gfamily]

You could do that.

I think the biggest issue would be the reduction in light coming through, so you wouldn't benefit from shorter exposures for your 'focusing', but it should certainly be able to use it.  

I don't know for sure, but I get the impression there you would ideally have different thickness slits for different f/ ratios, but you could confirm this by using one of the online mask generator tools and putting different focal lengths for the same aperture, to see if the suggested masks are any different.  

[Gfamily]

Just to follow up on this - it seems that (according to the Mask Generator at https://satakagi.github.io/tribahtinovWebApps/Bahtinov.html ) the higher the f/ ratio, the larger the slits.

f/10 mask on the left

f/20 mask on the right