Image of the day

Captured by
Eric Jeziorczak

Crescent Nebula

My Account

New to Astromart?

Register an account...

Need Help?

Stupid Question?

Started by cdculbertson, 09/24/2003 12:05PM
Posted 09/24/2003 12:05PM Opening Post
Ok, this may be a stupid question, but how do you know if a binoviewer needs collimating? I was out last night using my TeleVue BinoVue and various eyepieces and, for some reason, decided to star test with the binoviewer...Well, the defocused star showed what would be a classic miscollimation, with the secondary shadow offset from the middle of the Fresnel rings.

I could merge images without any trouble, and the funny thing is that stars all looked pretty good. I checked my collimation with the laser and then the star test using a moderately high power, and the collimation of the scope was spot on. Hmmmmm, put the binoviewer back in and got the offset secondary shadow again, but this time I did notice that in focus some of the brighter stars had a slight flare to them.

Anyway, I had always thought that a miscollimated caused problems with merging images, which didn't happen in this case...So, any of you binoviewer experts, does it sound like my bino needs a trip to TeleVue? Of course, I guess I could always call TeleVue too...

Midway, FL
Posted 09/24/2003 03:30PM #1
Perhaps there's some flexure/tilt of the Bino Vue, relative to the scope's optical axis?

The usual tilt/miscollimation checks apply to binocular viewers as they do to other accessories. See if the fresnel pattern changes orientation when (or after) you:
-rotate eyepiece(s)
-rotate viewer
-nudge/rock viewer (verify aligned/seated in focuser)

Also, can you detect any tilt in the focal plane, while using the viewer? Do stars on one side of the field reach focus just a hair "before" stars on the other side, or does the entire field (as best you can discern) reach focus at the same "point?"

One more question: is the Bino Vue being used "solo" -- without an auxilliary corrector/relay/Barlow?

Them be my wonderings, at this stage.

As for detecting misalignment of the binocular itself (i.e. split images/fields not aligned/identical), that's simply a matter of locking the system down while focused on a stationary target -- preferably a distant one (a mile or more away, if can) which allows good/thorough visual orientation of the image -- and comparing one eyepiece's view with the other. Rooftops, walls, fences, a line of trees -- these are some examples. Blinking back/forth while comparing the images (left, right, top, bottom) will show you (quickly) what's what. Most folks' eyes can accomodate minor left/right (divergence). Top/bottom misalignment (dipvergence) poses more trouble for the eyes; even minor/subtle dipvergence can prevent binocular merging &/or cause significant eye strain. If you know the range & size of the target being observed, you can use geometry to get a fair estimate of the degree(s) of divergence &/or dipvergence present. Typical standards for field binocs are about 15 arcmin. dipvergence, ~30 arcmin. divergence. My own personal standard is *zero* dipvergence (or darn near it) & <15 arcmin. divergence.

Indeed, it's never a bad idea to check with David or Al or anyone else at Tele Vue.

Best wishes and luck.
Posted 09/25/2003 06:38PM #2
Doug I read about your reluctance to apply the scotch tape as a permanent fix.It is understandable,however if you can shim the binoviewer and eliminate the problem then this tells you how much smaller the 1.25in. adapter's inside diameter needs to be
in order to eliminate the problem.Buy a micrometer if you don't already own one, and measure the old and new exterior diameter of the barrel. This will tell you in thousandths of an inch how much smaller the new adapter you will have machined will have to be.Avoid anodizing,this is partially how you got into this in the first place.The etching process necessary before anodizing can take place shrinks the surface minuetly in an unpredicatable manner and ruins carefully calculated tolerances.It is an anathema to machinists.All of the best after market focusers avoid it in locations where tolerances are critical.Ask Bill Burgess or Van Slyke Engineering about it.
Posted 09/25/2003 08:54PM #3
Re: collimating with or without diagonal/binoviewer in place

If the diag/bv ever needs to be rotated (GEM, Dob) then the draw-tube has to be true. If the scope is an alt-az SCT with moving-mirror focus and optically stationery diag/bv/ep assembly, then either way should be fine.

-- William