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Focusing with a refractor


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I have just purchased a barska magnus 80mm Triplet.  I recently tried to take images with it where it looked like the focus was on when I started imaging, but after I put the image together It was obviously out of focus.  Not by a lot, but enough to see that stars looked funny.  Can anyone show me an image of a focus mask on a refractor or the best way to achieve correct focus in a refractor telescope?

When I did the focusing I was using the focus/framing function on backyard eos, and found the std dev number to be the smallest I could get it. 

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Most here seem to hang the appropriate Bahtinov mask over the front, aim at a bright star and get focus with that, lock everything up then carefully slew to the object to be imaged.

The mask is made  for the scope as the dimensions depend on the scope diameter and focal length (not necessarily I think the f number) - have a search.

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When imaging with my refractors I use an STI Starlight Instruments Stilleto focuser. This uses a ronchi grating and is very accurate and fast. Before I got that I was struggling with various focus softwares and screens. Workflow is... choose a brightish star, fit sti, adjust focus till no bars visible,r emove sti, fit camera, done.

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I have a Mask that I use on my 6" sct.  Can I point this at a bright star and use this size of mask or do I need to create a smaller mask that fits onto the 80mm.  The Barska focuser unit has a fine adjustment so turning to get the adjustment isn't too bad, its just finding which spot I am actually at perfect focus.  Sometimes locking it in place it moves focus so I need to be able to see fairly easy if I can still in focus.  It works great on my NEWT, however when I pointed my refractor it at Capella there weren't the diffraction spikes to align like with the newt.

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The mask is matched to the scope so one for an f/10 SCT will not work on a 80mm refractor.

By rights the 6" SCT would not be accurate on a 5" Mak.

Cannot recall what the parameters are that generate the mask, but in strict sense they are not interchangeable. You need to get the diameter right and the seperation between the mask blades/arms or whatever you want to call them. Possibly the blade/arm width also.

I suspect therefore that the diameter of the mask has to match the diameter at the front and the spacing/width of the blades is set by the focal length.

There are sites that will generate one and if you are good can print and cut out.

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The only other one I have seen on the net is the mask where there are three holes placed exactly spacing apart from each other, and they have a solid line in the middle of the holes cut out.  When the three holes converge into one in the image the focus is spot on.  Ill have a mess with both of em and I will see what I come up with. 

I went to a site just a few mins ago and printed two masks out.  A few cuts with a razor blade and this should work well.  Just gotta find a piece of nice poster board paper. 

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