AP 130mm F8.35 again

Does anyone know the backfocus of this scope?

I want to set it up in the following configuration (with 2.7" focuser)

AP 0.75 reducer (2.25")
Optec TCF-S focuser (3.5 - 4.1")
ST-10 XME CFW8a 2"nosepiece (1.75")

Two questions:

1. Will I be able to reach focus with the stock focuser in place? (on my NP-101 without the AP 0.75 reducer I had to remove the stock focuser and use extension tubes to reach focus)

2. The focal reducer FL is 700 mm and a distance of 6.9" inches gives the stated 0.75" compression. My imaging train will actually be ~0.9" longer than this. What impact will this have on the effective compression from the reducer?

I don't have the scope or the focal reducer yet to actually try it out.

Thanks in advance for any help,

cheers,

Jeff

Jeff,

My AP130EDT F/8.35 had enough backfocus to use a binoviewer with no OCS. Where I see a problem is with the Optec focuser.
If the focuser has AP threads, this will all work, if not, the extra spacing on the reducer could create a problem. If the
distance is less, all you lose is compression. As you add spacing, the compression goes up, and the illumination goes down.
Might not be an issue with the smaller chip on the ST-10MXE, though. What I would worry about with that is how much it would
impact where you come to focus. My experience with other reducers (I had all the right adapters for the DSLR, so I did not
attempt to vary from recommended spacing), has been the focus point can change significantly with a difference as large as you
are discussing (close to an inch). I used the AP focuser with Feathertouch Micro adapter, and had zero issues nailing focus,
so if you found you needed to remove the Optec focuser from the mix, I don't think you will find it a real problem.

Tom

>>AP 0.75 reducer (2.25")
Optec TCF-S focuser (3.5 - 4.1")
ST-10 XME CFW8a 2"nosepiece (1.75") >>

The AP0.75 reducer screws into the back of the focuser in lieu of the 2.7"-2" visual adapter, so you lose no back focus at all from this device. Normally the unreduced beam comes to focus approximately 7 inches behind the fully retracted drawtube. With the reducer in place, this becomes 7" x 0.75 = 5.25 inches. You do have to screw the 2.7"-2" adapter back into the rear of the reducer, so you lose a small amount there.

Therefore you have the following distances:

2.7"-2" visual adapter = 0.75"
Optec focuser = ?
ST10 with 2" nosepiece = 1.75"

So, for the ? distance you have approximately 2.5" distance left over. This may not be enough. If you want to automate your focuser, you can add a motor to the focuser pinion quite easily. We sell a mounting bracket (Robobracket) for the Robofocus system, and it works quite well.

Rolando

Picked it up on Wednesday, absolutely mint condition other than some felt coming loose in the dewcap (fixed - thanks Roland for the advice). Set it up in my dome last night during the rain (holy crap the thing barely fits!!). It's clear right now and I'm impatiently waiting for darkness - I feel like a kid on Christmas eve....

Jeff