Help needed with Cassegrain assembly

Good morning all,

I've about "trial and errored" myself to death, so I'm asking for some help. I purchased a classical Cassegrain mirror set hoping to assemble a telescope. What I'm told by the person who sold the mirrors to me is this:
Primary: 200 mm diameter, f/4
Secondary: M = 5
EFL: 4000 mm, f/20

My problem, assuming the above is correct, is mirror spacing. I was told to target between 628- 630 mm for a back focus of either 275 mm or 210 mm. That hasn't worked out. For one thing, with the mirror cell, focuser and diagonal I have, I need approximately 339 mm of back focus. I've also consulted the VanVenrooij book and calculated a mirror separation of 614 mm for the back focus distance I want. I've currently got the scope assembled with 611 mm separation and have experimented with separations all the way up to 635 mm without being able to achieve focus.

Can someone help me out?

Try it without the diagonal. And even without achieving focus, the varying size (as you manipiulate the focuser back & forth thru its' range) of an extrafocal star image should inform as to whether you need more or less back focus distance. It may be that mechanically you cannot use a diagonal with the present mountings. Or perhaps the mirror itself needs to be shifted for focus adjustment vs a separate focusser behind the mirror (& in that case there might be adequate back focus for a diagonal).

I finally got the scope to work. Thanks to those that sent suggestions. I built a sliding jig so I could change the mirror separation easily and quickly. I then tried forming an image of the sun on a white card. The sun was too bright, of course, so I had to wait for a full moon. Who would have expected that full moon nights would have been clouded out six months in a row?! The full moon turned out to be perfect and now the mirrors have been installed in the "permanent" OTA so I could verify that everything is working. Now, of course, everything is disassembled so I can do the painting and wiring (secondary dew heater and fans).

I took pictures, but they're too big to post. I'll have to post some of the finished scope.

John,

Here are the numbers from ATMOS using your information. I don't think your mirrors are close enough to each other. Hope this helps.

Keith

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Yes, I know, it's been almost a whole year, but a lot of non-astronomy stuff has happened. I finally did finish the Cassegrain this past weekend. Images of the moon at 200x are spectacular. They were good even at 380x despite the average or below average seeing. Assembly was much more difficult than I thought it would be, but collimation was much simpler than I made it. The scope isn't as light-weight as I'd hoped, but because of that I got to get a new mount. It is fairly compact (as compared to my refractor) and easy to mount. The views are great. I've learned enough that I may try this again (help me, I must be insane).

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