Celestron 9.25 focus

[jaygpoo]

Hi all you out there. I wanted to be able to fine focus this big scope and took the route of buying a 2 speed craford focuser to attach to the scope so I could rough focus with the usual knob and fine focus using the secondry unit. Used it to look at the moon the other night but tonight I set it up and put my focal reducer infront of the craford?  when I attached a diagonal to align the scope there was no way I could focus it. When I removed the diagonal and inserted the eyepiece straight in I could focus.  So the diagonal is the problem but why and can I solve it? I am placing my QHY8L straight into the focuser. Jay

[Dave In Vermont]

Did you adjust the scope's built-in focuser before when it worked for you on the Moon? Each time you use it, you need to get the scope focused with it's built-in focus - with the Crayford set to around the middle of it's travel-range - just roughly is fine. Then you fine-tune the focus using the Crayford. This is especially necessary when you placed something new into the optical-path, such as a diagonal & a focal-reducer.

Get focused with the built-in on the scope, then fine-tune with the Crayford.

Let us know? Thanks,

Dave

[ronin]

Is the new 2 speed item any "longer" then the original.

The other thing is that a reducer will (I would expect) reduce the focal length of the scope a little and so move it back in towards the scope, so if there is a bit less travel or the assembly is longer then you may not reach the focal plane.

It could of course be a bit of both, the new crayford is say 1cm longer and the focal plane has moved back 1cm.

[jaygpoo]

Yes I did use the inbuilt focuser to get rough focus on the moon and was very excited that it was all working. Then did exactly the same when aligning but all I got was a big doughnut that I could not focus out until I removed the diagonal. I will try it again tonight but remove the reducer as that was not in the focal path before. Annoying as you need the reducer for imaging with this scope. The other thing is that I am using a 2in diagonal. Would a 1.25 make any difference?   jay. Thanks for your time

[ronin]

Would have thought that a 2" diagonal had a greater optical path then a 1.25" item. Certainly the difference in the ones I have appear significant.

I suspect that it is the combination of the reducer and the 2" focuser. Not sure there is a great deal that can be done unless there is a sort of "dumpy" 2" diagonal that has in effect a slightly shorter optical path.

Any idea if a prism diagonal would help ?? Just a thought.

Odd question: Should the diagonal be on for imaging ? Half think the idea would have been to remove it, however if using a DSLR there may be no option.

[Stu]

The reducer will bring the focal point in, and fitting the Crayford focuser adds length to the optical path so that explains the problem. What I don't understand is why you would need to use a diagonal when imaging? Best to just remove the diagonal and use the camera straight through.

For visual you can remove the reducer, add the diagonal and use a longer focal length eyepiece.

Stu 

[Dave In Vermont]

I have a 'feeling' the focal-reducer should be suspected. Are we talking a 6.3 FR? If it were a 3.3 FR, that won't work visually. As for the 90° 2" diagonal, I'd doubt that could be the cuprit, but do try a 1.25" just to be certain.

Please keep us informed -

Thanks!

Dave

[jaygpoo]

Please note that I only used the  diagonal to insert an eyepiece to centre the first star in alignment. Then I removed it and inserted the ccd. The reducer appears to have been the problem but will test it out as soon as the clouds take a holiday.  Reducer is a 6.3.  Jay

[rgm]

For visual use, a C9.25 plus 6.3FR plus secondary focuser plus 2" diagonal will not reach focus.  The length of the light path is too long for the optical design to adjust  for.  For visual use of the C9.25, the 2" SCT diagonal is the best option when paired with the 6.3FR.