Telecentric lens (powermate or explore scientific focal extender) for binoviewer

[esculapius1975]

Hi all,

I am trying to choose the right piece of equipment to buy to get in focus with my zeiss binoviewer on a 12" dob.
Barlows have the issue of increasing magnification proportionally to the distance between barlow and eyepiece.

Telecentric lenses like the televue powermate and the equivalent from explore scientific are not affected by this issue.

Will they provide, like barlows, the focal plane shift(backfocus) necessary to use the binoviewer? Or do they keep the focusing distance as it is?

[John]

Hello and welcome to the forum.

In general I don't think a telecentric extender is the right tool for this job because they do not move the focal point in the same way that barlow lenses do.

 

[esculapius1975]

Thank you!

[esculapius1975]

Will an antares 1.6x 2" do the job? Is the focal plane extraction (namely back focus) ,proportional to the power of the barlow?

[John]

I don't know I'm afraid. I don't use a binoviewer.

 

[Louis D]

I use the nose piece of a 1990s Meade 140 2x barlow screwed onto the front of my binoviewer for about 3x or so gain in power.  It reaches focus in my Dob just fine despite having less than 20mm of in focus.  It is sharp across the field.  Just use 16mm to 25mm eyepieces to counter the 3x boost.  You'll never get down to low powers without an expensive OCA.  I made a cheap one with a 0.5x focal reducer and some spacer rings between it and the Meade 2x nose piece, with the nose piece closest to the mirror.  It results in some field curvature, but it is serviceable for quick bino looks.

I've toyed with the idea of putting the optics from a 5" long Japanese barlow I own into a short nosepiece to see if I can minimize growth in focal length.  If I was a machinist, I'd have already tried it.

[Floater]

Issues with focusing while using binoviewers have been exercising me for some while now. ??? And I’m still ‘playing’ with different combos and trying to learn how they work together.

So this may be simplistic/ignorant, but I have found that the higher the Barlow effect the easier I could achieve focus. The 1.6x nosepiece provided with my WO binos were, basically, of no use to me. A more recently purchased 2x WO nosepiece has helped to solve any problems. But the Powermate 2.5x always helps me achieve focus, even using a 2”diagonal.

I try to compensate for the sometimes gross magnification by using higher focal length EPs.

Hope that helps in some way.