1st July 2026 Summary of the modified 102mm H-alpha telescope.

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 Wednesday 1st. Mostly overcast. I had to wait at home for a delivery. So used the intervening time to rebuild the 102mm TecnoSky APO refractor as a modified H-alpha telescope. Much of the detail here is a duplication of a number of earlier posts. Posted as I developed my ideas for modification. I have summarized the details here so they are hopefully much easier to follow in one place.

 After my last viewing of the sun in white light. I had left the Lunt 60MT etalon module and focuser unattached. The Feather Touch 2.5" focuser had proved to be ideal for white light and had the same thread size as the TecnoSky focuser. 

 I still wanted a more stable connection between the two major components. The Lunt 60MT etalon module and the TecnoSky 102mm APO telescope. Which involves removing the focuser from the APO telescope. To allow the Lunt etalon module to be attached more closely at the rear. The Feather Touch adapter ring is left in place. The TecnoSky focuser's base ring/rotation adapter is not as useful as the Feather Touch. Though it could be adapted if desired. The base ring is also deeper than the FT adapter. This might not matter. Provided there is still room for the Lunt 60MT etalon at the correct spacing.

 The etalon's nosepiece must be placed 264mm inside the APO's focal plane. The focal plane is 735mm from the front edge of the fully retracted dewshield. The focal length is quoted at 714mm. The APO's main tube is 460mm from the retracted dewshield tip. To the edge of the Feather Touch focuser rotation/adapter ring. So the Lunt etalon's nosepiece should be 11mm beyond the adapter ring.

 By a happy coincidence the bored out extension has a shoulder at 35-36mm. Which should be perfect spacing with the extension shoulder butted up against the plywood ring.

 The image [top right] shows my original assembly. With the etalon nose extension inserted and centered. By a marine plywood ring in the empty, focuser, rotation ring. The extension is a standard, short, 35mm, bored out 2" fitting from Omegon. The nose of the Lunt etalon is oversized for the 2" standard push fit extension. So I bored out this extension in my old lathe.  To increase the internal diameter to 53mm. Only for a 25mm depth in the female half of the extension.

 I really wanted to fit a permanent spacer. To ensure the two components would always be close to the desired spacing. Unfortunately there isn't room for a focuser in the gap between them. Which would have been preferable. To allow fine adjustment and more accurate centering. With a positive clamp on the 2" etalon nose piece once the best position was found. It would take a brave and skilled man to shorten the APO's main tube and re-thread it to match the original. 

 My final solution will be to turn a thick, aluminium ring. To replace the temporary plywood ring with something more solid. Probably with the V-groove turned. To match the Feather Touch, nylon pointed screws. 

 Before that happens I must absolutely confirm the required spacing. The quality of the H-alpha image depends entirely on this dimension being correct. Not just to tune closely to the H-alpha wavelength but to obtain even lighting and detail across the entire sun's disk. Or field of view. 

  

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