New Lunt 35 etalon housing for RCT

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Carbon60
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New Lunt 35 etalon housing for RCT

Post by Carbon60 »

I've redesigned a new 'back end' for my 250mm RCT solar telescope which houses a Lunt 35 etalon and refocusing lens behind a plano-concave lens that feeds the etalon with substantially parallel light from the secondary mirror.

The main feature of this is design is that I can now tilt the etalon through a much wider angular displacement than before (up to 5.5 degrees). Not that this is needed in full, but I can now go across a range of wavelengths either side of Ha more so than I could previously with the stock tilting mechanism. To achieve this I dismantled the original tilt tuner wheel and 'spring plate' (on the opposite side) and installed a couple of 3mm diameter ball bearings 180 degrees apart on the perimeter of the etalon mount so as to form a rocking pivot line. The etalon is now located inside a machined aluminium tube just a fraction larger than the diameter of the etalon housing. A couple of spring loaded small diameter bolts holds the main body of the etalon onto the rocker bearings. Tilting is achieved using a couple of small screw jacks mounted 180 degrees apart across the diameter of the housing. Using these in a sort of push pull fashion gives full control of the angle of tilt and allows the etalon to be secured by 'nipping up' the jacking screws.

Here's a montage of the new arrangement. The result is a sleeker design with much improved angle of tilt (positive and negative) and better concentricity between the RCT and etalon housing/extension tube.
Montage.jpg
Montage.jpg (789.33 KiB) Viewed 2599 times
Poor conditions have limited my imaging time, so I don't have any good quality hi-res images to share and the inset image was the best I could do under dreadful seeing. What I can say is that the Ha view is beautifully even and so are the off band views that I've managed to see so far (no images yet, though)

Hopefully we'll have better weather and periods of good seeing in the coming days/weeks to give it a proper test run.

Watch this space.....


Thanks for looking.

Stu.


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Re: New Lunt 35 etalon housing for RCT

Post by marktownley »

Tweaking is always fun! I'm guessing it's all backwards compatible with the 150?


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Re: New Lunt 35 etalon housing for RCT

Post by Carbon60 »

marktownley wrote: Fri Jul 02, 2021 7:57 pm Tweaking is always fun! I'm guessing it's all backwards compatible with the 150?
Hi Mark,

Fun?…yes, but also a bit nerve racking at first light given that I irreversibly cannibalised the existing etalon housing for this scope, so there was no going back :o . Thankfully the results so far look extremely promising. I just need a good session to really test it out.

This is completely independent from the 150mm, which I’m very reluctant to further modify, so this has not been touched at all, although in principle the new arrangement could be fitted to the 150 using an adapter.

Cheers.

Stu.


H-alpha, WL and Ca II K imaging kit for various image scales.
Fluxgate Magnetometers (1s and 150s Cadence).
Radio meteor detector.
More images at http://www.flickr.com/photos/solarcarbon60/
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Re: New Lunt 35 etalon housing for RCT

Post by EGRAY_OBSERVATORY »

Interesting project Stu.

Terry


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Re: New Lunt 35 etalon housing for RCT

Post by MalVeauX »

Looks fantastic Stu!

Care to share how your 150mm is setup for this? I'm looking to do something similar with a 150mm frac and an internal Lunt 60mm's pressure tuned etalon from the new modular series.

Very best,


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Re: New Lunt 35 etalon housing for RCT

Post by Valery »

Really impressive Lunt35 mod for a big RC scope!

The only question remains: is this a standard location of this instrument during solar imaging session?

I see that the scope placed at a surface of concrete plates, which can be too hot during sunshine and this can easily preven any hopes for good seeing. I have found that my scope C11 should be places ON a grassy lawn and there must be grassy space at the bottom and in a front of the scope.

Valery


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Re: New Lunt 35 etalon housing for RCT

Post by Carbon60 »

MalVeauX wrote: Mon Jul 12, 2021 11:28 am Looks fantastic Stu!

Care to share how your 150mm is setup for this? I'm looking to do something similar with a 150mm frac and an internal Lunt 60mm's pressure tuned etalon from the new modular series.

Very best,
Hi Marty,
For my 150mm/Lunt 35 mod I use a Plano concave lens located part way up the rear of the OTA to create, essentially, a parallel beam that feeds into the Lunt 35 scope mounted on a rail using mounting rings which is bolted to the rear of the scope. The lens is 50 mm stopped down to 30mm and is -300mm from the natural focus point for the f10 objective. It’s crude, but effective when using a 1.5x, or higher, Barlow on the nose of the camera. I’ve actually got two Lunt etalons stacked in my current setup. The standard compound objective lens in the Lunt body serves to refocus the light rays emerging from the etalons in the normal way. Naturally, I have an ERF in the optical train.

Hope this helps.

Stu.


H-alpha, WL and Ca II K imaging kit for various image scales.
Fluxgate Magnetometers (1s and 150s Cadence).
Radio meteor detector.
More images at http://www.flickr.com/photos/solarcarbon60/
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Re: New Lunt 35 etalon housing for RCT

Post by Carbon60 »

Valery wrote: Mon Jul 12, 2021 11:48 pm Really impressive Lunt35 mod for a big RC scope!

The only question remains: is this a standard location of this instrument during solar imaging session?

I see that the scope placed at a surface of concrete plates, which can be too hot during sunshine and this can easily preven any hopes for good seeing. I have found that my scope C11 should be places ON a grassy lawn and there must be grassy space at the bottom and in a front of the scope.

Valery
Hi Valery,

Unfortunately this is my only option. I can’t use the grass because there’s a tree in the way. I have to rely on being very selective, when the conditions are just right, early in the morning when the ground around me is still cool.

Thanks for your comment.

Stu.


H-alpha, WL and Ca II K imaging kit for various image scales.
Fluxgate Magnetometers (1s and 150s Cadence).
Radio meteor detector.
More images at http://www.flickr.com/photos/solarcarbon60/
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