Simple/Dumb prime focus problem

[danmtslatter]

Hi there guys,

I'm just starting out so am just getting up to speed with equipment and all that so my starting point was the moon.

I have my Canon DSLR attached directly to the viewfinder of my Skywatcher 1145PM (via T-Ring and 1.25" adaptor) without an eyepiece (prime focus) but the length of focal range I have from the manual focuser on the tube will not let me pull focus on the moon at all.

Am I missing something? Do I need a reducing lens? or do I need the ability to attach one my eyepieces in between the camera and tube (thus not being prime focus anymore)...

I'm a bit confused to be honest as I've followed the books to the letter so far.

Can anyone help this novice astro-tard?

Thanks all!

Dan.

[Merlin66]

Dan,

Bad news.... sounds like you don't have enough backfocus distance available. This usually means you either need to move the main mirror further up the tube or use a low profile focuser which will give you the necessary distance for the camera body....sorry:(

Ken

[danmtslatter]

Thanks Ken,

Is this not something that any reducer will help?

And does this also mean that the camera will not focus on deeper sky objects?

Thanks, Dan.

[themos]

I had this problem too with my 114mm f/8 Newt. I "solved" it by taking the tube apart, slicing off about 30mm of tube and putting it back together.

Alternatively, you could put a Barlow lens in the optical path. That pushes the focus point further out but gives you a longer focal length and extra aberrations to worry about.

You can test your scope at daytime: Just put the DSLR in (without Barlow) and see what is the farthest object you can focus on when the focuser is racked all the way in. Then you can calculate how far the focus is:

http://stargazerslounge.com/diy-astronomer/86762-should-i-take-hacksaw-my-focuser-housing.html#post1282568

[danmtslatter]

Thanks Themos, I'll try my 2x Barlow later on the moon and see if I can get a glimpse at Saturn too. I shall report back anon. Thanks!

P.S. Is a reducer worth getting?

[themos]

I think that a reducer won't work for a Newtonian.

[Merlin66]

I'll leave the reducer until later.... if it won't focus without a barlow you may have problems getting the reducer into position....

[danmtslatter]

Thank you both. I'll try out your suggestions this evening (except sawing my tube just yet).

[themos]

It's a rite of passage.

[copperknickers]

Hi

I had a similar problem and discovered that the adapter is in two pieces. A cylinder and a smaller diameter cylinder end piece attached by a screw thread. I had started off by attaching camera via the smaller diameter cylinder and anchoring with a thumb screw but that set up would not focus.

But unscrewing the end of the camera adapter took a good inch off it (no saw needed) and I could then screw the adapter directly into my refractor scope - now I can focus.