Have Etalon Will Travel

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Have Etalon Will Travel

Post by Okay-1 » Mon Aug 07, 2017 12:01 am

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Ever wish you had a portable etalon? One that you could move from one scope to another?
If so, please read on… as I show what I did to achieve this.

My Story
I have a LUNT LS50 solar telescope.
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I specifically bought the Lunt LS50 after measuring the diameter of the tube and verifying that this scope, with objective lens removed, would slide into the 3 inch focuser of my Canadian Telescope 152. Here’s my LS50… breach-loaded into the massive 3 inch focuser of my CT152 (f/5.9, 900mm). The LS50 tube is held inside the focuser of the CT152 by three O-rings… shown in the previous illustration.
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Here’s a close up of the Lunt etalon attached to the focuser of the CT152. Attached by friction of the three O-rings around the LS50 tube inserted inside the focuser of the CT152.
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This worked well, giving me the ability to use the LS50 alone (350mm for full solar discs) or in combination with the CT152 (900mm for solar close ups). The only draw backs seemed to be:
1) The combo works with only one telescope – my CT152
2) The combo configuration requires any Barlow to be located after the LS50 etalon.

I thought, wouldn’t it be nice if the LS50 etalon (without scope tube and objective lens) could be moved from one telescope to another. I could use the focuser of the main telescope and not be limited by the shortcomings of the LS50’s helical focuser. Also, I could insert a Barlow into the main telescope and then insert the etalon into the Barlow… a lot more freedom for choice of Barlows and no heavy weight loading on that helical focuser. e.g. There is no way to put a 2 inch Televue Power Mate onto the output (helical focuser side) of this etalon.

Here’s my LS50 etalon. See those naked threads… they are about 68mm x 1mm pitch – totally non standard!
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What I wanted to end up with was an adapter to a 2-inch focuser, something like this…
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I figured I could make the adapter using inexpensive photographic step-up rings and step-down rings. The problem was that step rings come in standard sizes like 67mm and 72mm. I could even find 69mm rings. But 68mm? On eBay, I found one 68mm ring but at high price of US$35 and a thread pitch of 0.75mm, not the 1mm pitch that I needed. Quite frustrating.

How was I going to obtain a ring with ~68mm diameter x 1mm pitch thread to match the Lunt etalon?
Alas, I was going to have to make it – myself. How? I didn’t have a lathe to cut threads.
Here’s how I managed to do it.

I bought a metric M6 (1mm thread pirch) socket head bolt and used a tap to extend its threads all the way to the socket head. I then used two disks of 1/8 inch thick aluminum on either side of a 25.5mm to 37mm adapter ring. Tightening the nut sandwiched the assembly tight together.
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The aluminum disks were cut using a hole-saw with a drill press. Slightly smaller than a perfect fit, I used 0.005 inch brass shim stock to fill the space between the disk and ring.
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This two disk/ring combo threaded onto a stack of step up/down rings.
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Here is the final assembly, terminating on the right with a 72mm to 67mm step-down ring.
Note the three additional nuts threaded onto the M6 socket-head screw – they form my lead screw.
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Time to go to my make-believe lathe!
On the left, one regular vice holds my lead-screw assembly. On the right, a cross-slide vice holds an M6 x 1mm pitch tap. I carefully adjust the fit of the tap to the existing 1mm pitch threads on the 72 to 67mm diameter step-down ring – ensuring a tap cutting edge engages the right side vertical of the ring.
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To cut the threads, I applied cutting oil to the ring threads and by hand I turned the ring assembly clockwise. This drove the assembly out with a pitch of 1mm toward the tap which cut the threads. After a few turns, I turned the assembly counter-clockwise to the starting position. I’d repeat this a few times until there no more pressure between on the ring and tap – threads were cut. Then, I would adjust the cross-slide vice to move the tap against the ring. I would then repeat the process.
Slick eh? Yes but this takes time, in my case… a few hours – and I would have to stop and remove the assembly for a trial fit on the etalon. Slowly but surely, I got closer and closer to a perfect fit.
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TA-DA… finally… it fitted!!
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I removed the rethreaded 72mm to 67mm (now 72mm to 68mm) step down ring and combined that with a selection of rings, etc. to form an multiple-ring adapter that takes my etalon to a 2-inch focuser.
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Here is the final product of my labour…
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The portable etalon is likely to be used on telescopes with larger aperture than the 50mm objective lens of the Lunt LS50. This means more light striking the etalon and some form of additional energy rejection filters are required. Up to 152mm in aperture, I like to use standard 2 inch (48mm thread) filters, like a UV/IR cut plus Wratten 25A threaded onto the 2 inch adapter. I also have a 50mm Lunt ERF mounted in a 52mm filter holder; to use that I can insert it between the rings – between the 49 to 52mm step-up ring and the 52 to 55mm step-up ring.

I hope you enjoyed my story… comments & suggestions most welcome!

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Re: Have Etalon Will Travel

Post by marktownley » Mon Aug 07, 2017 9:50 am

Very cunning! I like the rethreading trick.
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Re: Have Etalon Will Travel

Post by Okay-1 » Mon Aug 07, 2017 12:06 pm

Jeez Mark, it's like you are everywhere! :lol:
Once the rain and clouds disappear I will be able to try this out... should prove interesting.

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Re: Have Etalon Will Travel

Post by _Zakalwe » Tue Aug 08, 2017 8:20 am

Thats probably one of the most ingenious things that I've ever seen.

M68 is reasonably common though...one of these might have saved you a lot of work?
http://www.teleskop-express.de/shop/adv ... suchdas/OK

Maybe this one?
http://www.teleskop-express.de/shop/pro ... x0-75.html

or this one?
http://www.teleskop-express.de/shop/pro ... 58225.html

Moonlite Focusers also carry a lot of M68 (they call it WR25) adapters for their new Nitecrawler focuser.

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Re: Have Etalon Will Travel

Post by Okay-1 » Tue Aug 08, 2017 8:39 pm

Yikes... how did I miss all those in my search?
Maybe I was blind to adapters in the 38 Euro (CDN$57) range? :lol:

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Re: Have Etalon Will Travel

Post by Okay-1 » Tue Aug 08, 2017 8:43 pm

Oh, today I got a chance to test the portable etalon on my CT152, with an without a 2 inch 2X Televue Powermate.
All worked out OK... needed some extensions but I expected that. I'm happy!

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