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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