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Old 18-04-2022, 01:51 PM
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mura_gadi (Steve)
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cass/mak mirror set etc for a Mersenne telescope.

Hello,

Could you use a Mak/Cassegrain mirror set or any convex mirror set for a Mersenne design? Keeping to the pre-set F/l etc...

Recently reading about Clyde Bone and his Mersenne twins, and for someone who hates heights it seems like a killer design. If you could include the obs chair on the lazy suzy base, be close to the bee knees of viewing imo.

Does anyone even make these?


https://televue.com/televueopticstal...ne-telescopes/

Last edited by mura_gadi; 19-04-2022 at 04:00 PM.
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Old 24-04-2022, 08:53 AM
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dave brock
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Both primary and secondary have to be a parabola and the same
focal ratio so no, you can't use "any" convex mirror set.
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Old 24-04-2022, 12:21 PM
Rod
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Hi Steve

To add to Dave’s comments, the Mersenne does not produce a focus on its own. It produces a parallel light beam. Clive used a very expensive apochromatic refractor to get focus. It’s visible in the pictures. Apparently when you view through a scope like this you see the mersenne field of view inside the refractor field. It would be interesting but unusual.

If you want a compact cassegrain design, I suggest you look at this one:

http://www.loptics.com/ATM/telescope...12in_cass.html

You can have the focus come out the centre of the altitude bearing if you want the eyepiece height to remain more constant.

There is also an old design called the ‘beaver tail’ cassegrain. It is essentially a Springfield mounted cassegrain. The eyepiece position never moves.

Cassegrain optics tend to be custom made and expensive. You need a bit of experience, preferably with faster mirrors, to make your own. You also need to read up on baffling which is essential in a cassegrain.

Rod.
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Old 24-04-2022, 12:43 PM
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Thanks for the replies,

Poorly worded...
I meant that the mirror set would match the refractor component in F/L as well. I thought that the refractor was the D of light cone and the F/L of the larger mirror set.

Read about Mr Seronik's shortly after that post, another very useable design and great use of the secondary shadow I thought.

I have Texereau vol2 for the cass making aspect but years away from my skill levels atm.

I was going to make a few reflector mirrors and then try my hand at a refractor lens. Seemed like it might be an idea to make something with an end plan along the way, on a far smaller scale.

Ps. While reading some of Gary's pages I noticed one of his first scopes was very close in design to mine. A long F/l newt, while I don't plan to keep refiguring till I hit 1/29th wave, I do plan to plagiarise a few ideas from that scope design. SWMBO goes away for a week soon, I am hoping to establish most of the lounge room to polishing and testing while she's gone...

Last edited by mura_gadi; 24-04-2022 at 12:53 PM.
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Old 24-04-2022, 10:56 PM
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OzEclipse (Joe Cali)
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Steve,

Thanks for the link. Interesting reading.

A lounge room is a naturally very dusty environment. A laundry or bathroom makes a much better polishing room in a domestic environment especially if you close it up when not using it. A board with down-facing door stoppers can fit snug onto your laundry tub and hold the polishing lap while polishing. You also have water on hand.

If you need the length in the lounge room for testing, dust is less of a problem for testing providing you clean off the mirror afterwards before resuming the polish.

I have a very precisely hand figured 6" f7. Mine is set up for visual with a 3.5% secondary obstruction. Gives great visual quality.

Joe Cali
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Old 25-04-2022, 05:57 AM
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Your secondary is tiny, even by old school standards. Was that selection by trial and error or a set design method?


Think I'd be shot if I went near a sink with grits/rogues... The lounge room is the only 1 climate room I have big enough to setup the Ronchi test.

the other option is the garage... then I have to keep a jug going for warm water and temp's vary a lot over the day etc... The smoothing onwards was in the kitchen as I try and not have carpet under foot while working.

Can I ask what you used as your spider? Atm I'm tempted to use either .7mm thick 90cent steel ruler from office works and go a single curved arm. Or get some sub .1mm black wire and do a suspension job.
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Old 25-04-2022, 06:57 AM
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OzEclipse (Joe Cali)
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The scope was built 44 years ago. I was a member of a telescope makers club. Cliff Duncan mentored me and suggested that diagonal size. I was 15, who was I to argue. The spider body is a brass bar with tapped holes to hold the 3 adjustment screws and a ball washer on a central thread that the whole thing rotates upon. There are two tabs solders on the brass tube to stop the mirror falling out and it's held in place by cotton wool and crumpled tissue paper.

The vanes are made of spring steel silver soldered (brazed) into thin slots cut into the brass body and brazed into the heads of brass screws to make the tensioning threads. I can't remember how thick they are and can't put a vernier on them unless I remove the whole spider. From memory, the flexibility feels like 22 gauge or thinner at a guess. Thin vanes on a small diagonal are fine but you have to untwist & straighten them after tensioning the tube nuts to minimise their cross section.

Definitely don't flush grits and rouges down sinks but it's handy to have water nearby. Dump the waste in the garden on the lawn. Mineralogically, abrasive wastes mix well with and are similar to some components of soils and are not toxic.

Joe
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