Refurbishing of Prinz 660 classic refractor continues...

[digital_davem]

The first set back

I purchased an 0.956 to 1.25" adaptor plus a 1.25" mirror diagonal. And I've hit focusing trouble.  I can focus on terrestrial targets but the stars are great big blurry circles. The problem seems to be that I can't rack the focuser in enough for infinty focus.

I suspect the combination of the adaptor and the new diagonal is the problem.  With my old 0.956 diagonal + 1.25 adaptor there is no problem.  My guess is that my mirror diagonal as a different path length to the old diagonal which uses a single prism.  Do prisms lengthen the focal length so the eyepiece needs to be further away?

If this is the case I seem to have a few options beyond using the old parts:

- get a 1.25 prism diagonal

- get a hybrid diagional and dump the adaptor

- dump the adaptor and use my mirror diagonal by replacing the original 0.956 eyepiece holder (is this called the visual back) with a new one that accepts 1.25 eyepieces 

I'm trying to do this with zero expenditure or as near as zero as possible  and I favour the third option as it seems the tidiest.  But are such eyepiece holders available?  The focusing tube has a female thread on the inside but I don't know what type it is. I've measure the (quite worn and loose) male thread on the original eyepiece holder and if I'm measuring it right it seems to be around 35.4 to 35.6mm.  I see that there is a fitting called the "small vixen" that used to be used on some Japanese scopes and you can get an adaptor that allows the use of 1.25 eyepieces. But the specs for this say it has a 36.4mm thread.  Is it likely that mine is this but just worn down? It is a very loose and uncertain fit. 

I've attached pic of the original.

Thanks and cheers

Dave

[Roy Challen]

I went through all this a few months ago! It's worth persevering with though as you will end up with a scope that rivals most modern scopes for clarity of view and colour correction. And looks better too!

The thread inside the focuser drawtube is indeed a 36.4mm, which is fairly standard on old Japanese scopes. It may well be referred to as the "Vixen" thread. You need an adapter with this thread on one end and a 1.25" fitting on the other. And it needs to be as short as possible. Tring Astro do one actually made by vixen for around £20-25. I have one spare due to "upgrades" so PM me if you're interested. I can't show you any pictures at the moment as my scope is away having the upgrades done to it as I type.

You also need a diagonal with as short a light path as possible. This has two suitable options IMO. The Tal mirror diagonal as sold with Tal 100RS (not the 100R) is usable (it has quite a short light path compared to most mirror diagonals). The better, but more expensive, option is the Takahashi Prism diagonal. Prisms generally have a shorter light path than mirrors and the Tak is around 63mm if I recall correctly. The Tal mirror is around 68-69mm. Doesn't sound like much, but trust me, it all helps. Don't bother with a hybrid diagonal.

Even after all this, some EPs still don't come to focus. You'll either have to cut the tube down or get some different EPs. But most EPs will be fine eg Tele Vue Plossls, Explore Scientific, MaxVisions, BSTs, Meades etc, all the modern ones basically.

[Ricochet]

There must be a lot of these old Prinz scopes still hanging around. I've got a 500R (Newt) but my EP holder thread is ~30mm.