does paracorr require in or outfocus?

Hi.

Given the same eyepiece is at focus in a scope at the middle of the focusser's travel (let's just say we're talking about a pentax 10.5mm XL in my f4.5 dob).

What would happen if I add a paracorr? Would I need to rack the focuser in (toward the secondary) or out (toward my eye) when I put that same eyepiece back in the chain?

do you need to rack 1/2" in? out? 1"? parfocal? approximations are all i'm looking for.

Thanks.
TomL

(the reason I'm asking is i am building a f4.5 dob right now and just measured the distance between cage and mirror box to 35.75" apart in order to bring my EPs to focus in the middle of the focusers travel. knowing this is going to be a fast dob, i want to plan for a paracorr and if i need to round that 35.75 up or down that would be the deciding factor).

Tom Lacovik said:

What would happen if I add a paracorr? Would I need to rack the focuser in (toward the secondary) or out (toward my eye) when I put that same eyepiece back in the chain?
Thanks.
TomL

Hi Tom,
I'm new to the paracorr since I just got mine, but have used it in my Orion dob.

I think it needs about 1/4" infocus. I would check it to make sure, but I'm at work. I'll double check it tonight when I get home.

I am in the middle of a fast dob project, too, only f4.3. I have the benifit of having the paracorr before hand so I can just install it before I cut my poles. I plan on following K/B's advice and cut the poles a bit longer and then fine tuning balance with the mirror box before I cut the poles to their final length.

The Paracorr requires more back-focus (inward travel toward the secondary).

I found that I was unable to use one on my 12.5-inch Portaball, even with the low profile Feathertouch focuser. The Portaball simply does not have enough back focus for use with the Paracorr, and raising the primary mirror is not a trivial task because of the design of the ball and mirror support structure. I was not interested in shortening the truss poles, so I opted for the Baader coma corrector and adaptors.

Ron
Equipment Forum co-moderator