No adapters, no tube cutting: the easiest 80mm "stage 1" Lunt LS50 mod possible!

[Roel80]

I recently bought a Visionking 80mm f/7,5 refractor on Ebay to use it as a donor scope to mod a Lunt LS50 to an 80mm h-alpha scope. It's a Chinese, but very well made telescope with a retractable dewshield, 1:10 Crayford focuser, tube rings and dovetail plate included. It even comes with some kind of sports bag (low quality, but still).

When the refractor arrived a few days ago, I started to measure things, to determine whether I had to cut the tube and/or make an adapter to mount the Lunt's etalon unit at the "magic" distance from the focal plane.
I discovered that when you remove the focuser, the remaining mounting ring has an internal diameter of 70mm. That's quite convenient, because the etalon unit has M68 male thread, and will fit snugly when a 1mm thick strip of plastic (or tape, or whatever material available) is placed around the piece of thread. Just tighten it with the three allen screws in the ring, and it looks like the two are made for eachother. A one minute job: nice!

But that was not the only surprise. It turned out the ring places the etalon unit at the exact same distance (-168mm) from the focal plane as the original 50mm Lunt OTA. And I mean EXACT! I double checked by measuring again, and by switching between the original OTA and the 80mm OTA, and I didn't have to adjust the focuser AT ALL for a sharp image. I mean, this is incredible, right?

All I basically had to do, is mount an ERF (inside the tube, for it is a 75mm), and that's it!
With this refractor you can basically built a 'stage'1' 80mm Lunt mod it the time it takes to mount an ERF. It even looks great:

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[Roel80]

But there is more! If you would like to replace the original helical focuser, it's possible to use the Visionking's 1:10 Crayford, although the drawtube needs to be shortened and an adapter will be needed to mount in onto the back side of the etalon unit. The focusing part is even shorter than the helical focuser and a travel of 12mm is possible when you cut the drawtube. It is way cheaper to use the Visionking's focuser with a custom made adapter than buy a $$$$$$ Feathertouch.

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This is something I'm planning to do in the weeks to come. To be continued!

The Visionking refractor is easy to find for around US$200 when you search for "Visionking 80mm" on Ebay.

[marktownley]

Very nice!

[MalVeauX]

Looks nice, any images or opinion on performance? Potential to double-stack? Transmission?

Very best,

[Roel80]

Thanks!

Well, I intend to use the telescope for visual observations only, so unfortunately I can't show any images.. All I can say (with the Sun barely 15 degrees altitude at noon these days) is that it works like a charm! I got some great details in small proms some weeks ago. Surface details are nice, detailed, and contrasty. The B600 in combination with the faster f/ gives me a significant wider true field of view compared to my old 70mm PST mod with its BF5.
The sweet spot is fairly large, round and centered, so everything appears to be in line. And that's without one bit of machining!

The mount I use is from my 3" Polarex/Unitron refractor

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[marktownley]

Nice setup!

[dzaucha]

Is there anything similar to this that would allow an easy LS50 mod? This scope is unfortunately no longer made.

[Serg]

I suppose the same result could be obtained using e.g., a typical ED80 refractor: same aperture, same focal length and ratio. Of course, with a flange (3d printed?) to attach the Lunt 50 etalon to the back of the tube?

[marktownley]

Is there anything similar to this that would allow an easy LS50 mod? This scope is unfortunately no longer made.

You just need a scope that has a male m68 thread to go onto.

[dzaucha]

Would I need to get an ERF with an 80mm if using the LS50 talon? Thanks in advance!

[Vinch]

Is there anything similar to this that would allow an easy LS50 mod? This scope is unfortunately no longer made.

You just need a scope that has a male m68 thread to go onto.

So, if the scope has f/7, for example a 100/700 or a 110/770 and if it has the right M68 thread, I could simply screw the etalon into the scope and it would work without having to cut the scope? Thanks!

[marktownley]

Yes, in theory.