WO binoviewers on a 90mm refractor

sailorbebop (Nick) · #1 05-01-2013, 12:12 AM

Hi,
i recently bought a 90mm refractor as a beginner telescope. it's on an EQ mount and has been a great learning experience. Being in the city I mostly view planets. I'm still in awe of saturn and today jupiter came into my field of view! I can't explain how exciting it is to see a planet with my own eyes.

Recently I bought a pair of WO binos - I like stargazing with binoculars and i figured that this would be similar only I'd get awesome views of saturn. Looking through an eyepiece all night with one eye closed makes me afraid of going to the shops the next day in case people think i'm winking at them.

*anyway* my problem is that I can't focus with the WO binos. it's like my focusser doesn't travel inward enough to achieve focus. Am I not using them right? I've tried with an without a diagonal.

Any ideas?

Nick

ZeroID (Brent) · #2 05-01-2013, 06:21 AM

Hi Nick, sounds like the added distance of the binoviewer puts it outside the focal point of the scope. You could try using a Barlow but I'm not sure how that would effect the FOV and viewing. I'd suggest you borrow one first to see if it works
The other rather drastic action is to cut the OTA and shorten it. With a Newt I would adjust the main mirror height to acheive it. I had to do it to get my camera into the right place.

MattT · #3 05-01-2013, 06:47 AM

Nick are you using the 1.6X corrector (barlow) that comes with WO bino's? If you are take it off and put a 2X barlow on the nose piece and see if that works. Cutting the tube is the last thing you would want to do. I use a 1.25" prisim diagonal on my WO's. Prisims have a shorter light path than mirror diagonals. Hope this helps.
Matt

sailorbebop (Nick) · #4 05-01-2013, 01:15 PM

Thanks guys,
I have tried with and without the 1.6x barlow, so i'll try with only a 2x barlow. For planetary viewing this would be fine.

I have a mirror diagonal only, but i guess i can try without that as well. Thanks for the tip that a prism diagonal has a shorter light path.

I think i'll just get a bigger scope before i go around cutting the OTA.

Hopefully I get clear skies tonight. Last night jupiter was fantastic.

Larryp (Laurie) · #5 05-01-2013, 03:24 PM

Hi Nick
Some refractor manufacturers make their optical tubes with removeable sections to accomodate bino-viewers. My Stellarview Nitehawk is made that way. The expensive version of the TMB 90MM has the same set-up

Steffen · #6 05-01-2013, 03:58 PM

Quote:

Originally Posted by sailorbebop

I have a mirror diagonal only, but i guess i can try without that as well. Thanks for the tip that a prism diagonal has a shorter light path.

The is no difference in the length of the light path with prism vs mirror diagonals, but there is one between 2" and 1.25" diagonals.

Cheers
Steffen.

MikeyB (Michael) · #7 05-01-2013, 04:13 PM

Quote:

Originally Posted by Steffen

The is no difference in the length of the light path with prism vs mirror diagonals...

Really? Mirror vs Prism Diagonal (Cloudy Nights)

Steffen · #8 05-01-2013, 07:07 PM

You're right of course, I was only thinking geometry. The prism will act like an optical flat and move the focal point further back.

Cheers
Steffen.

sailorbebop (Nick) · #9 05-01-2013, 07:09 PM

laurie: Mine is really an inexpensive celestron refractor. No removable sections unfortunately.

Mikey: thanks, that's a really informative link. I never would have imagined the effect that the refractive index of the prism has on the optical path length.

I think i'll end up getting a prism diagonal regardless if it helps this situation or not.

Steffen · #10 05-01-2013, 07:14 PM

Quote:

Originally Posted by sailorbebop

I never would have imagined the effect that the refractive index of the prism has on the optical path length.

I know, right? It should be a hard to miss fact since it is the very same refractive index that gives us the total internal reflection on the back surface…

Cheers
Steffen.

dannat (Daniel) · #11 05-01-2013, 08:22 PM

most b/viewers add about 10cm to the optical train

sailorbebop (Nick) · #12 06-01-2013, 09:25 AM

wow 10cm is a lot.

So I tried it again last night with a 2x barlow without a diagonal and i got it! Although the viewing was pretty uncomfortable, and the weight was a little too much for my poor mount at least it confirms to me how amazing binoviewers can be.

I really like the immersion.

thanks for everyones help.

ZeroID (Brent) · #13 07-01-2013, 02:38 PM

Uh Oh ! Upgrade time by the sounds of it. Hide your credit card !!

Well done ...

brian nordstrom (As avatar) · #14 09-01-2013, 10:15 AM

Hi Nick , what I would do it rack the focuser right out and measure the length of draw tube sticking out , if its over 100mm focus travel , remove it and the finder and carefully mark 100mm around the tube ( use an A4 sheet to keep it square ) with a hack saw , re-drill the holes , re-fit the finder and focuser and you are away . Just be sure to bung a tea towel in the tube past the cut line so no swarf falls in the lense .
If you are not sure a local engineering firm will do it quite cheeply , dont forget the teatowel after the cutting and drilling .
That way you can use the bino viewers and single eyepieces .
I got a WO bino viewer for christmas and its awsome ! focuses in both my Takahashi M210 and 127mm Istar/Saxon refractor w/o the barlow , but not in my Takahashi SKY90 , but that dont matter as its only 500mm f/l and use my Takahashi ExtenderQ 1.6x ( a fancy Takahashi barlow ) that brings it to only 800mm f/l .
Brian.