Boom Boom
Good on you Gary.
Yes its a double/multiple star problem. When you stop down the aperture with the mask your reduce your telescopes ability to clearly resolve many double stars. Its not such a problem with VERY close multiples, but with stars like Castor and Alpha Centauri for example, the separation is such that when you stop down the telescope and try to focus with the mask you can not get a single star image.
There are a number of factors that contribute to the problem, resolving power of telescope with mask (the smaller the holes in relation to the telescope aperture, the less resolving power) seeing conditions, quality of optics, turbulance in the OTA for reflectors, internal/external temp dif in refractors/sct.
So how do we deal with the problem? Easy make sure that the star being used is not a multiple (in the case of Castor a sextuple, 3 pairs).
And if you are having trouble focusing with the mask, just check your star chart or atlas, maybe your using a double star and don't know it. But make sure you come back later after imaging and check it out. It could be well worth looking at in its own right.
Quick work Gary
