TheSkySearchers

hello folks! i just desided to upgrade an olde 6" F8 dob and need some advice on placing the focuser. i am going from a helical to a crayford focuser. and the new crayford's profile is a good bit higher than the old focuser. ( see pic) should i shorten the distance the same amount as the difference between the two? and if so, will this mean i need a larger secondary? thanks for any help i can get on this.



Rick

Since the eyepiece is riding higher, I think you will need to move the focuser back slightly to maintain the same focal length. About the distance from the height of the eyepiece in the old focuser to the height of the eyepiece in the Crayford. Looks like maybe an inch or so. On the other question, I don't think a larger diagonal is needed. If you did use a larger diagonal this would lead to more obstruction of the primary. I'm guessing the worst thing that could happen if you use the same diagonal is maybe some darkness on the edge of the field of view, but probably not.

I'm going back to my days building Dobs - both a 6" f/5 rich field reflector and an 8" f/6. But, let's see what other folks think.

right! thats what i was thinking but at 1.25 inch difference i thought i needed to ask. lol. thanks very much.

if you were to move the mirror up then i have no doubts you would need a larger secondary . is the old focuser at the position for your eyepeices ?

it's is. the difference would be 1.25 inches.

You will need to move the primary mirror forward the difference between the two focusers, if you decide to use the taller one. If the secondary is already marginal, you will need a larger secondary. I would have tried to find a low profile focuser, to avoid all the problems associated with a taller focuser.