Help needed with Cassegrain assembly

Started by macdonjh, 05/01/2011 03:04PM
Posted 05/01/2011 03:04PM Opening Post
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?
Posted 05/01/2011 04:02PM | Edited 05/01/2011 04:21PM #1
The hyperbolic secondary has two foci. One should coincide with the focal point of the primary, and the other should be the focal plane of the image behind the primary. If you know or can measure the radius of curvature of the secondary mirror, then the first foci is located at approximately 1/2 the ROC behind the secondary.

If the mirrors were an ATM project, these given focal lengths and multipliers probably have a large tolerance, for example the f/4 primary might actually be f/4.2. Don't assume any of the given numbers are correct - verify them. An error of 3 mm in the spacing will shift the focal plane by more than 70 mm. You probably don't want to hear this, but your best bet is to build a horizontal test bench setup from plywood and noodle it out by trial and error.

Jim McSheehy
Posted 06/05/2011 09:22PM | Edited 06/05/2011 09:23PM #2
John,

Any equations are assuming you have exact data on the mirrors, which you don't. You don't have exact Radius of Curvature data for either mirror, just rough design points.

I just ran through this with a set of old 10.25" DK optics and built an adjustable truss for the front ring and secondary holder using telescoping tubing from www.dxengineering.com with hose clamps on the slotted ends, and assembly with star nuts (tube connecting nuts) from www.mcmaster.com. I was then able to get everything dialed in, and actually the open tube is great for cool down!

This is probably the best way to go, and the woodwork does not need to be pretty to get spacing down.

Dave


John MacDonough said:

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?


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dmcgough's attachment for post 140970
Posted 07/05/2011 07:01PM #3
Hey!
Many years ago I had the same problem, so what I did is set up a simple optical bench using home made mirror cells out of plywood, bolts, wing nuts and compression springs, all found at ACE hardware. I also fashioned a mini one for the secondary and placed the whole thing on a board, which I cut in a slot along the middle of. Then I placed that whole arrangement on a GEM mount from 2 inch pipe parts so that I could "track" the moon. I tried to do all the positioning of the optics in daylight but failed miserably, so I had to wait until dark. One thing that would have helped was a very far off bright street light to zero in on.
Anyway, just by a bunch of trial and error I finally came up with the spacings. All the math in the world did not help one iota. I just monkeyed around with it until it all came into focus. Once I had the spacings set, I then bolted everything in place, measured everything, and built a tube assembly to fit it, at first using Sono tube which I later replaced with a fiberglass one.
The bottom line is that you just have to "play" with it all until you get what it is you want. If the optics get to a fairly good focus, but the images are crap, then the problem is with the optics alone. Altho mirror tilt and alignment may play a key role.
I am terrible with math, so I build my 'scopes by just playing around with things until it comes together. Over the years, I have a "feeling" of how it goes together and, just about always, it does. Yes, I screw up a lot, but I also learn. It is my hobby and my passion and my therapy.
I also try to build my 'scopes as something to look at as well as through.
So, you might try building a stage to just mess around with and who knows, maybe it will work! It did for me.
If you have a hard time visualizing what it is I am talking about, let me know and I could send you a drawing through snail mail or e mail.
Starman 1000