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Bahtinov Masks


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I had my first try at using a bahtinov mask last night, and whilst I think it was reasonably successful I wanted to check some basics to make sure I’m doing it right.

 

Firstly, size. The mask I used was one I bought many years ago for an 8” SCT but never used, and subsequently upgraded to a 9.25” SCT. Apart from the obvious issue of getting it in place, will the 8” mask work ok or should I just pass it on and get a 9.25”?

 

Secondly, installation. I’d already set up my dew shield on the scope before I decided to try it, so I put it on the end of the shield, which is about 14” long. It later occurred to me that maybe it needs to be close to the glass to work effectively, i.e. before fitting the dew shield, or removing the shield before using the mask. Is this necessary?

 

Otherwise it seemed to do what I expected, refraction spikes that were picked up by the APT Bahtinov Aid and Bahtinov Grabber, that moved appropriately as I adjusted focus. It was a bit of a struggle to get the numbers to stay down.

 

I guess a third question: collimation will affect focusing using a mask? I would guess that an imperfect start shape will alter the spikes. Collimation is on my list of things I need to learn ……

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Hi Dave,

 

It should do the job with a few caveats 🙂 The smaller mask will work even if it is offset to one side and whatever distance  from the OTA end - the light rays are parallel. Because the OTA central obstruction will block some of the slits the spike pattern may not be perfect but generally its position is not critical.   The spikes will be fainter compared to the full sized one. If you let stray light in because it doesn't completely cover the aperture then the contrast will be lowered a bit too.

 

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I made a mask for my 200mm reflector which has a dew shield of similar length to yours. The mask works fine fitted onto the shield.

All of my refractors have retractable dew shields too and the respective masks work fine when the shields are extended.

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