19.08.2022 Lunt 60MT mod. Etalon extension.

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 The Lunt 60MT provides easily separated modules. To allow a larger aperture telescope "Mod" or conversion to H-alpha.

 The entire Lunt 60MT telescope weighs close to 4kg. Of which the objective, tube and clamshell add 1.5kg. 

 I decided to attach the etalon and focuser to the 2.5" FT focuser of my 150mm [6"] f/10 iStar via the etalon nose-piece. The thread of the nose-piece in the etalon housing is of adequate length to carry the load. I just needed to make a conversion of the [51.8] 52mm nosepiece to a 2" push fit. [50.8mm] I shall bore out a standard 35mm long x 2" extension to achieve this. I used an Omegon extension which had to be ordered online.

 The 35mm x 2" extension has arrived in the post. I lost no time in chucking it in my lathe. To bore it out by ~1mm to 52mm. Unfortunately the depth of the groove for the bronze clamping ring was much reduced. 

 There followed much fiddling with a small boring bar. By twisting the tool horizontally I was able to deepen the groove while maintaining square edges. An internal grooving bar would have helped but I don't have one. 

 Eventually the bronze ring of the extension sank low enough that it fitted over the Lunt's filter nose-piece.  This image [left] shows the interior of the extension turned to the larger internal diameter. There is no need to blacken the bright metal because the light will not reach it.

 I could then refit the thumbscrews to achieve a firm and even clamping pressure on the etalon nosepiece. I did not want a sloppy fit. Which might have caused flexure.

 The second image shows the extension in place fitted onto the Lunt 60mm MT etalon nosepiece. 

 This allows the Lunt etalon to fit securely into any 2" focuser adapter. Further extensions can be screwed into this etalon extension. 

 This is also the same thread as 2" filters. So these too can be added to provide thermal protection for the Lunt etalon, blocking filter and camera. 

 The Lunt etalon housing is a sturdy lump of heavy metal. Weight 1.3kg by itself. The Lunt etalon, focuser, blocking filter and ZWO camera will be cantilevered off the etalon extension. The all-up weight being 2.5kg. So it needs to be adequately strong and stiff. 

 The entire H-alpha assembly will be inserted into the iStar's beam at 264mm inside focus. Measured from the front collimating lens. The face of this lens is level with the large, black face of the etalon housing. A handy reference. 

Achieving this position may require the main tube of the modified telescope to be shortened. Not something to be taken lightly where a high quality telescope is involved. This would probably cause a serious loss of its resale value.

The 60mm Lunt objective is not needed for this "Mod." So it and the main tube will be stored away safely.

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