Bahtinov Masks

[ultramol]

Can anyone tell me if fitting a bahtinov mask in front of my scope has to be accurately centred, centred or give or take a mm or two. I only ask as I have downloaded one for my scope and need to know when setting it up how accurate it needs to be. I bought 5 self adhesive tiles from the pound shop and these are great to stick the template on before cutting, makes a rigid almost unbreakable mask.

Regards.....Geoff

[ronin]

I think just reasonably centered is OK.

Otherwise they probably wouldn't be a great deal of use if you had to get then to a fraction of a mm.

[ultramol]

Thanks Ronin, get my sharp knives out now....( and alert the A&E )......

[glowjet]

It does not matter whether the mask is on centre or not, so long as it is reasonably positioned across the telescope aperture. Chris Lord wrote an in depth paper on the subject some years ago. Basically any obstruction in the form of a strip or slot placed anywhere across the aperture opening of a telescope produces a diffraction pattern on the telescopes central optical axis, in addition it seems this diffraction appears at 90° to the orientation of the original obstruction. This is why many members have had success using a simple Y formation made out of black plastic strip or heavy duty garden wire. I even recall one field Astronomer, having left his mask at home, used twigs from a handy tree to create diffraction spikes to focus his camera correctly

[ultramol]

Thanks for the info John, It's good to know these things, much appreciated

Regards geoff