Finder scope assembly...

[jamieren]

Well, I decided to take a chance and picked up a second hand 6" reflector (after selling my ridiculously heavy 12") and already I'm revelling in discovery I never would have even attempted with an expensive scope. To be more precise, I went to align the scope with the finder and realized I couldn't see through the finder scope. I tried focusing it and even taking it off the mount and walking towards an object, but I can't get the slightest view through it. I've now taken it apart and cleaned the very dirty lenses, and it's got me thinking perhaps they have been assembled incorrectly, either from the factory or from a well intentioned user (there was a fingerprint in there...).

This is what I have:

Starting from the 'eye side' is the small lens, convex on one side and flat on the other - convex side to the eye. Next is a larger lens also convex / flat and oriented the same way (convex toward the eye). At the far 'light gathering' end is another large lens convex on both sides, which I suspect can go in either way. Can anyone make sense of this? A few 'exploded view' pictures would be great. I suppose I could just fiddle with it, but there's another piece that fits down inside that will be a pain to reassemble so I'd rather just do this once...

[jamieren]

Okay, I've tried fiddling now and have come to no positive conclusion. I've attempted to orient the lenses in different configurations, but the same event occurs. I get an upside - down image that comes into focus with about 3" distance between the assembly for the eye side containing two lenses and the other convex lens. No matter which way I orient the eye side lenses, this happens. I've looked at various things and cannot see any evidence of magnification either. The length of the finder scope is about 7" and I have found no way to make it focus at that distance. The best guess I can offer is that it is Keplerian in design, but has an extra lens?? I'm feeling a bit lost here, and I'm dying to hear the solutions you pros have to offer

Here's a link to the Keplerian pic: http://en.wikipedia.org/wiki/Refracting_telescope

[Moonshane]

I'm no expert but I am not sure what you mean re the the lack of focus. Are you saying that the tube appears too long and when you dismantle the finder and hold the objective 3" away from the eyepiece that's the only way it will focus?

I may be wrong but I'd have thought the flat surfaces in the eyepiece would be on the 'outside' i.e. the convex surfaces go toward each other. Try this way and see what happens. There are only a total of six combinations for the lenses concerned (ignoring the one inside the tube which presumably has not been messed with previously? Hold then between finger and thumb in both hands if you can and see which focuses at approx. the right focal length for the tube.

if all else fails, there's lots of cheap finders here AstroBoot

good luck!

[jamieren]

Are you saying that the tube appears too long and when you dismantle the finder and hold the objective 3" away from the eyepiece that's the only way it will focus?

That is exactly what I'm saying. I'll try the way you suggested and see if I may have missed that configuration though. Thanks!

[Moonshane]

no worries - hope it works out.

it could of course be something else entirely but worth ruling this out first.

when I bought my (new) refractor the finder had the opposite prblem. I had to hold the objective off the end of the tube by about 20mm to focus. the supplier provided me with a replacement and suggested it happens very occasionally in the factory that someone puts on th wrong objective.