jmi focuser

[alan meehan]

Hi guys I have a problem someone might be able to help me with ,I have acquired a 12 inch dob for our club orion it had a old type screw in and out focuser so I thought I might change this for a jmi event horizon that I had on my 10inch s/ newtonion as I have just replaced it with a lovely moonlite but the problem is on the dob I cannot get enough inward travel to focus a eyepiece this is only for visual use ,would moving the mirror upwards in the tube make any difference not sur how to approach this without selling the jmi focuser and buying just a manual one Crayford style ,the jmi is very smooth and focusers really well

[Merlin66]

Alan,
Moving the main mirror up the tube would seem to be the easiest way of resolving the problem.....

[alan meehan]

Thanks Ken yes I think that might be the way to go ,I wonder how far up the tube I should move it ,if iam approx. 10mm short of infocus I wonder if that is the distance I should move the mirror any idears

[Merlin66]

Alan,
Yes, move it by the extra distance you need at the eyepiece.

[Rainmaker]

Depends on which of your eyepieces needs the most inward focus, then add a couple more mm ....

[Startrek]

My GSO 6” f6 imaging newt (900mm focal length) out of the box just reached focus using my DSLR so obviously I wanted a bit more free travel focus in and out
Bought some 4mm thick x 20mm dia round cork spacers from Bunnings and packed out the primary mirror using two ( 8mm thick )
Now I have plenty of free travel in and out for focus ( I would estimate between 1 to 1.5 full additional turns of the main focus wheel )
Problem solved !!

[alan meehan]

Thanks Martin I think I will go that way better than drilling new holes in the scope tube

[Merlin66]

Alan,
I don’t know the type of mirror cell is used, but there may be enough adjustment in the collimating bolts/ springs??

[rrussell1962]

If you move the primary up the tube will your secondary still be large enough? Although I can see that it would have to be very marginally sized for 10mm to be an issue.

[Merlin66]

Russ,
I don't think moving the main mirror 10mm or so will make any significant difference to the secondary mirror coverage......