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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.
Or not? The focal plane is where it is relative to the dewshield. Regardless of any adjustments to this measurement for the exact position of the objective lens. So 735 - 264 = 471mm from the front edge of the retracted dewshield. This is where the nose of the etalon should lie. About 20mm more than the present position. Unfortunately I shall have to wait until tomorrow afternoon's promised sunshine to prove it.
The image [top right] shows my original assembly. With the etalon nose extension inserted and centered. By a marine plywood ring in the empty, FT focuser, rotation ring. The extension is a standard, short, 35mm, bored out 2" fitting from Omegon. The nose of the Lunt etalon is rather oversized for a 2" standard push fit extension. So I bored out this extension in my old lathe. To increase the internal diameter to 52mm. Only to a depth of 25mm in the female half of the extension. I also had to increase the depth of the groove which houses the bronze clamping ring. Which was much more fiddly.
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. The old fashioned way of using radial fixing screws has been replaced by precision, CNC machining.
My final solution will be to turn a thick, aluminium ring. To replace the temporary plywood ring with something more solid. Probably with a turned V-groove. To match the Feather Touch adapter and its nylon pointed rotation and clamping screws. Most Feather Touch focusers have this V-groove turned in the bases. To allow secure and silky smooth rotation of the entire focuser. With no sloppiness or loss of concentricity.
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 camera's field of view.
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