Collimation problem: Prism-tilt adjustment inaccessible

[michael.h.f.wilkinson]

Two days ago I tried to get the sickle of Venus in my old Bresser 10x50 (these were 399 Dutch guilders (near 180 euro) at the time, and quite a bit better than the current cheap ones). They have served me well for 17 years (gift from the missus at my PhD defense). However, someone (everybody in the house looked innocent ) must have given them a really nasty knock, because they are out of collimation for the first time, one sickle of Venus appearing to the upper left of the other . Looking at terrestrial objects doesn't show this effect up that clearly, but the tell-tale eye strain and ensuing headache after just a minute or two are enough to realize there is a serious collimation problem.

I checked whether the EPs were to blame by rotating them, but the effect stayed the same. The objectives seem seated firmly in their mount, and the exit pupils look circular. I looked up the usual sources of information on collimation, and was hoping to use the instructions on changing prism tilt yesterday evening. Though the instructions say that the adjustment screws should be accessible by gently lifting up the rubber armour of the bins, this does not work here, because the rubber is in a single, seamless piece. I fear the damage cannot be undone.

Any suggestions?

[michael.h.f.wilkinson]

Here are some images of the bins. Should anyone recognize them, and know how to collimate these, I would be most grateful. As you can see, the rubber armour shows no seams on the prism housing. The feature is that they are labelled Bresser Speed 10x50, and have a massive 122 m at 1000 m field (7.0 degrees). In the lower image they seem to have ruby coating, but this is just reflection from the table. In white light the coatings are a subdued indigo-purple.

[rory]

sorry to hear that michael. when i had to do my pentax bins the small alan key grubs were under the rubber too. where you see the bresser spped and opposite side 10x50, the rubber armour curves with the diameter of the eyepieces.thats where i found mine. i had to undo a couple of screws to let the rubber be gently prised. are there any screws visible at all ?

[michael.h.f.wilkinson]

I found no screws, but based on your suggestion I did undo the EPs and found a seam where the rubber was held by double-sided tape. I have now removed the rubber from the one side and found the collimation screws. Many thanks.

[michael.h.f.wilkinson]

Just to let you all know that I have been able to rescue them. By first removing a metal plate that was snapped onto the base of the prism housing, I could remove the rubber (which as mentioned was held in place with double-sided tape). Two 0.050" size Allen screw heads could be discerned in the usual places. I first had to get myself such a small Allen wrench as 1.5 mm was my smallest. I first sort of colimated it on a distant terrestrial object, and then had to wait a few days before I could do fine adjustments on a star (Arcturus). I find a residual double image much more easily seen on a single bright star than on any other object. They now seem to work better than they have done in quite a while, as I also took the top part of the focuser apart, and when reasembling tightened various bits which had become wobbly over a 17 year time span. The focusing is much crisper now, as the play in the focuser has all but been removed.

Thanks to everyone for putting me on the right track!

[AndyH]

Great result !!