Televid 77 can't focus properly (1 Viewer)

[Hakan Granbohm]

I am a swedish birder, that bought a Televid 77 sometimes in the 1990s. Problem is that I could never get a sharp view in it. It is a subtle, but yet noticeable differences to my friend's Televid 77. (one friend even compared this Televid 77 to the new Televid 82, and found the differences so smail, he kept his 77)
Very annoying. I have looked through this forum and other forums on the Internet, to see if someone else has had similar problems, but not found anything. The closest I have come, is people having problems focusing, but on my scope the focusing knobs are easy to operate.
I have tried sending it to Leica, but they claim it would be a rather high cost to get it repaired. And what do I know, maybe there is an individual variation between scopes and no real fault on mine, but rather I received on that was very close to not pass quality control.
Does anyone have any ideas what to do?

 

[Mike Crawley]

I have a good friend (he's a Swarovski guy like me but I think this helps)
He allowed his 'scope to over balance in a hotel room. The 'scope landed quite gently on the eyepiece.
However, it was a while before he realised what had happened.
He had to re-focus regularly to get a sharp image and it wasn't until he looked through my 'scope he realised there was a problem.

He had strained the thread where the eyepiece fits into the 'scope body.
This was diagnosed by a very helpful guy in our local optics shop.
The answer was - buy a new eyepiece or live with it.
Given the cost of Swarovski stuff, he has got used to fiddling with his focus.

This might not be your problem but hope you find out what is going wrong.

 

[jring]

Hi,

first of all, welcome to birdforum!

If I read you correctly, the focus drive on your non-apo Televid 77 works (knob spins and focus changes) but the image is never quite sharp? The question is at what magnification does that happen - on a non-apo unit I would expect this behaviour above 35 or 40x due to longitudinal chromatic aberrration. Does your friend have an Apo Televid 77 maybe?

There could be other aberrations present of course, regardless of CA which also result into this. For this doing a star test (either at night with a real star or an artificial one made from a tiny pinhole in aluminium foil and a led flashlight at 30m or so) could help to shed some light on the quality (or lack thereof) of the scope. Focus as good as you can on the star at maximum magnification and defocus a tiny bit in each direction. Ideally you should see identical images of concentric circles. Ellipses mean astigmatism, non-concentric circles mean coma and well defined circles on one side and a fuzzy disc on the other means spherical aberration.

Here is some example images (along some theory) - look for the unobstructed example images:

https://www.telescope-optics.net/star_testing_telescope.htm

Some difference in color between both sides is expected in a non-apo unit.

Joachim

 

[garymh]

Hi,

I have never heard of a strained eyepiece fitting thread - the eyepice is a bayonet fitting.

If the focus wheel turns but the focus does not change it could be because the screw that attaches the focus wheel to the inner focus shaft has become loose.


Gary