27.10.2025 102mm APO and Lunt 60MT etalon spacing.

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 Monday 27th. Sunny for most of the morning. Then clouded over in time for lunch. The forecast was a sunny afternoon.

 I dragged the tripod, mounting and 102mm APO refractor outside. There followed some hours of experimenting with etalon distance from focus. I was able to focus the red sun but couldn't see any H-alpha detail in the image. There was one major spot and two smaller ones.

 After lots of trial and error I rechecked where the focus lay relative to the refractor's focuser mounting ring. Only to discover I had been using the wrong measurement. The 60MT etalon was about 41mm too far in towards the objective. I needed the etalon nose to be 264mm from focus. I made some more adjustments and finally saw some filaments. 

 13.30 Lunch over and the sun seems to have come out again. I shall go back out and do some more testing.

 There was only a little sunshine after lunch. I double checked the focus relative to the focuser adapter ring. 290mm. The clouds actually helped to identify sharpest focus. 290-264=26mm. So the original position of the etalon wasn't too far out. Though that doesn't explain the lack of H-alpha detail. 

 Coarse changes in etalon spacing are possible. By moving the pipe clamps in the dovetail slots. While fine or coarse changes are easily achieved. Simply by loosening the main tube in its rings. 

 I spotted several proms on the limb after lunch. A double hump and a triangular sail. The latter looked as if it was about to detach. 

 Screwing in the pressure tuning knob made the proms disappear as expected. So the tuning is working and the instrument must be showing some H-alpha. Otherwise these proms would not have been visible at all. Though strangely, I was no longer able to see the filaments. Nor any H-alpha, surface texture. 

 Increasing cloud and wind made it pointless to continue. Tomorrow looks like rain.

 

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26.10.2025 PT. 2 102mm H-alpha APO Lunt 60MT etalon.

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 Sunday 26th. Wet and windy with fleeting sunshine after 15.00. Sadly it is far too cloudy to be worth trying the telescope on the sun. With only a few second visibility in the last hour.

 I shall share some images of my Lunt 60MT H-alpha conversion of the 102mm Tecnosky APO refractor. It is all mounted on a PrimaLuce 50cm long [Losmandy style] dovetail plate. The APO uses its original tube rings. Though only the single, M6 central screw holes can be used. The spacing of the holes does not match between these rings and the dovetail plate. 

 The obscenely high price of PrimaLuce tube rings. Prevents me from buying a pair. Just to ensure the correct spacing. To allow pairs of screws to be used for attachment. A spacer block adapter with suitably spaced holes. Would allow two screws, per ring, to be used for extra security. I'll see if I have the patience to make a pair of adapter blocks from aluminium.

 The bosses of the tube clamps can be seen bolted directly to the dovetail plate. As mentioned in previous posts: I needed to buy 6 to 10mm thread inserts. To adapt the 6mm screws for the dovetail plate. To match the 10mm pipe clamp threads. Note the thick rubber gaskets. Which nicely protect the high quality finish on the Lunt telescope parts. 

 The plywood ring was turned on my lathe. To centre the Lunt 60MT etalon on the OTA focuser ring. Except, of course, I have removed the complete focuser from the Tecnosky OTA. The focuser ring shown is from a 2.5" Feathertouch focuser. 

 I sent the Tecnosky focuser back to Telescope-Service for "adjustment." It was far too rough to be used as supplied. [Brand new!] They promised me a few days turnaround. It has been three weeks. 

 Fortunately I am in no hurry. I am not using a focuser in this H-alpha modified refractor. It was sheer luck that the FT ring also fitted the APO tube thread of M91. The main focuser had to be removed anyway. To allow the etalon to function correctly at 461mm inside focus of the APO. Which has a stated focal length of 617mm.

 I have retained the Lunt 60MT R&P focuser behind the etalon housing. The knurled thumbscrews are an addition. To replace the pointed grub screws supplied by Lunt. More pointed thumbscrews would have been better. 

 The focuser houses the Lunt BF1200S blocking filter. Which has a nominal 2" body. Most solar telescopes employ a blocking filter based on a star diagonal. I chose a straight through model when purchasing it alone. To obtain a larger clear aperture. The tail end of the adapters has a T2 thread and 1.25" clamping ring to hold en eyepiece or my ZWO ASI174MM mono astro camera.

 

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26.10.2025 Down with the bosses!

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 Sunday 26th Another wet and windy day in series. 

 I have not made much progress on the pipe clamp mods. For holding the Lunt etalon onto the extended dovetail plate. Which in turn is holding the 102mm APO without its focuser.

 Clamp bosses with threaded 10-6mm inserts/reducing adapters.

 The bosses are slightly too tall at 20mm. Relative to the OTA in its tube clamps. Made far worse by the threaded inserts. Which convert the bosses to a useful M6. Anything larger in diameter. Would mean modifying the dovetail plate by drilling. 

 Having tightened the threaded inserts fully. I must now saw them off flush with the bosses. This would still demand that the 102mm OTA's tube rings must be packed up sightly. To ensure the etalon is concentric with the OTA. I would rather not shorten the bosses. To avoid shortening the threaded inserts any further. Though I may be worrying unnecessarily.

Bosses reduced to 18mm tall. The reducer inserts are brass. With plenty of thread left. I used an angle grinder to make quick work of it. Followed up with an orbital sander to smooth the bosses.

 I had intended to bolt 18mm plywood to the dovetail plate. To sit under the etalon. With two, 15mm Ø holes at 22mm centers. Drilled in the plywood for the clamp bosses.

 When I should really be using 20mm ply. To bring the bosses to the height of the dovetail plate. While using 18mm ply and filing the bosses down to 18mm. Which should bring the etalon to OTA level. Too much text and not enough sawing! 

 I assembled the H-alpha telescope without the plywood as seen here. The dewshield is fully retracted. I still need to confirm etalon position and the resulting camera focus. Which will require some sunshine. We are going through a long run of changeable weather. Mostly gales and rain. 

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8.10.2025 Etalon adapter ring.

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  I needed to be able to use M6 machine screws. To hold the pipe clamps to the dovetail bar. Without having to make oversized holes and enlarged slots. The clamps were threaded both 8 & 10mm in the bosses. So I bought some threaded adapter bushes. 6-10mm. 

 They are rather long to fit deep enough into the clamp bosses. I have yet to see how high the bosses will stand off from the dovetail plate. Once the etalon nosepiece is centered in the FT rotation ring. If the spacing is enough then I need not modify the threaded bushes. If too short a spacing. Then I must saw off the excess protrusion of the bushes. 

 If I am lucky I can use a plate of some sort. To maintain the clamps distance from the dovetail plate. With the bosses passing though suitably sized holes. The bosses can then be pulled down tight onto the dovetail plate. Rather than onto the protruding threaded bushes. The measurement to the centre of the ring and to the centre of the nosepiece. Is really quite close to each other at 60mm. 

 15.30 I have turned a marine plywood ring. This fits snugly in the FT rotation ring. The plywood ring is being secured by the FT rotation ring's pointed screws. While holding the etalon nosepiece centralised. The pipe clamps' bosses are actually close to the correct height. Unfortunately, the threaded bushes project much too far. 

 So, should I saw off the projecting threaded adapters? It makes the most sense. I then make a plate, probably in plywood first, to ensure accuracy. To support the pipe clamps and to fix the lateral location of the projecting bosses underneath. This plate would be bolted to the long dovetail bar. With the provision to allow the Lunt etalon to slide along the axis during fine tuning. 

 I can make an aluminium plate afterwards. Once the exact location of the etalon, relative to the APO telescope's focus, is confirmed. There is considerable weight involved in the Lunt etalon housing and the following focuser and camera supports. This must all be held firmly and safely on axis. Without sag or twisting under its own weight.

 

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3-8.10.2025 APO focal plane and Lunt 60MT H-a etalon positioning.

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  After lunch I set up the bare Tecnosky OTA on the long [Losmandy] Prima Luce dovetail bar. Then mounted the telescope on the ZWO AM5N on the AVX tripod. With the telescope tracking the sun I was able to make repeated measurements to the sun's sharpest image as accurately as possible.

 I wanted to know the distance from the focal plane to some fixed reference point on the telescope. I chose the tip of the retracted dewshield as my reference point. 

 Mock-up of the position of the Lunt etalon. Relative to the 102mm APO telescope. Primary focuser removed.

 After repeated measurements I decided the sun's image was sharpest at 735mm. With the focal length stated as 714mm this placed the objective 21mm behind the forward edge of the retracted dewshield. 

 The rear of the Feather Touch focuser's rotation ring is 460mm from the front of the retracted dewshield. This second reference point would be 439mm from the objective. 460 - 21mm. It was this ring which would support the tubular adapter I intended to turn on the lathe. 

 The FT focuser body would be removed entirely. Leaving the distance to the Lunt etalon to be determined by the 264mm from focus rule. To match Lunt's own measurements in the 60MT. The Lunt etalon will also be supported on three, radial, pointed screws in a circumferential, dovetail groove. Just like the FT focuser.

 714 - 439 = 275mm. So the etalon must be supported at 275 - 264mm = 11mm behind the FT rotation ring. The tubular adapter will be approximately 85mm long. To reach to the front face of the Lunt etalon housing.     

 If I can find, or make, a supporting ring for the 76mm Ø, Lunt etalon housing. Then this would allow fine tuning of the etalon position relative to the focal plane. The exact, linear position of the etalon, relative to the focal plane, is important for the best possible H-alpha image. It is this position which sends parallel light through the etalon. Which also needs to be concentric of course.  

A simple, rubber lined, pipe clamp [or two] can be used to hold the Lunt etalon for tuning trials. The etalon will be attached to the Lunt focuser. Making for a rather weighty assembly. So stiffness in support is vital. A pair of clamps in tandem can be bolted to the dovetail bar. Which has useful slots for linear adjustment.

 If the pipe clamps are successful. Then a simple, bored disk of marine plywood can be clamped in the FT rotation adapter. Using the pointed dovetail screws for clamping. This will provide perfect concentricity and support. For the 52mm nosepiece of the etalon. While simultaneously allowing for sliding, linear adjustment. 

 Here is an image of the two 76mm pipe clamps in place. Attached to the Lunt etalon housing. They are quite smart and don't look too out of place in this context. 

 These clamps are usually fixed to the supporting wall with M10/M8 bolts. Which is much larger than the M6 standard of the dovetail bar. I may saw off the attachment bosses. To allow an M6 countersunk head screw. To allow an M6 fixing through a spacer. 

 Or, the rubber sleeve can be lifted. To allow M6 CSK screws to pass right through the bosses from the inside of the clamping ring. This will leave the bosses to provide very firm support. Without any loose parts. 

 A smaller clamp on the 44mm Ø camera extension. Could provide further support at the tail end. There is no room on the Lunt, R&P, compact, focuser body for clamps. The camera's exact position is still an unknown. Which is where the Lunt focuser comes into play. With assorted 2" and T2 extensions available.

  I think I am finally getting there. With a safe and working trial assembly for H-alpha etalon, linear tuning. I like the fact that no precise machining or expensive accessories are needed.   

  

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1st October 2025 Sun photos with phone

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  After hours of waiting for the clouds to clear I captured some phone "snaps" of the sun. T-S binoviewer, 26mm Meade Plossl, GPC, Lunt 1.25" Solar Wedge, Bresser phone clamp. I was using slight digital zoom on the phone's camera to better fill the field of view of the screen.

 It would have been better to have used a single eyepiece. The binoviewer has optical limitations and adds a lot of glass to the light path. Which must be addressed with a GPC. Glass Path Corrector. 

However, the binoviewer adds a great deal of  enjoyment to the visual view. It gives a real sense of floating above the solar surface. The detail seems to be greatly improved in comparison with a single eyepiece. In fact I struggled to see as much detail with only my dominant right eye. It seemed as if it was going repeatedly out of focus. Perhaps a symptom of my age. I struggled to see the numerous pores. Which were easily visible when using both eyes.

 I have yet to enjoy binoviewing in H-alpha with the new telescope. This is a tricky subject and requires strict adherence. To the optical spacing of the elements involved. My choice of an f/7 refractor was deliberate. To match the original Lunt 60MT. 

 The problem is getting the Lunt etalon far enough inside focus. Which would require the complete removal of the focuser. Not easily achieved but not impossible. I shall have to resort to my vintage metal lathe. To make an aluminium adapter ring. The rest of the optical chain will be hanging from this ring. So it needs to be accurate. 

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