Going H-alpha: Correcting etalon: focus error.

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 WARNING: SOLAR OBSERVATION REQUIRES GREAT CARE AND SAFE FILTRATION.
INSTANT AND PERMANENT BLINDNESS CAN EASILY RESULT FROM SIMPLE MISTAKES. NEVER LOOK AT THE SUN THROUGH ANY LENS, MIRROR OR INSTRUMENT UNLESS IT HAS BEEN FULLY TESTED AND APPROVED FOR SUCH USE. YOU FOLLOW MY EXAMPLE ENTIRELY AT YOUR OWN PERIL!

My first trial on the sun yesterday required I swap the correct 80mm extender for a 40mm to reach focus. This introduced a 40mm error in the PST etalon position relative to the focus of the 6" objective. The rule is to match the PST's 20cm inside focus position for the leading edge of the etalon tuning ring.

My error was in using a 2" star diagonal which has far too long an optical path length.

There seemed to be no obvious way to connect the ITF/BF filter stack to a 1.25" star diagonal. Then I remembered I had a second, no-name 1.25" star diagonal.

Unscrewing the eyepiece holder provided a female thread which exactly matched the ITF's tubular holder. The 1.25" chromed spigot fitted a 2" to 1.25" adapter.

Using the 40mm extender after the AOK etalon adapters gave me the final piece of the puzzle. The 80mm extender can go back into the plywood ring in the tailpiece of the refractor to bring the etalon to the correct position.

I now have an awful cold so testing the new layout may be slightly delayed despite the usually rare sunshine.



Despite my cold the bright sunshine meant I dragged myself outside to set up the H-alpha modified 6" refractor. It was soon obvious that neither of my 2" extenders was of the right length. I had a choice of 35mm or 80mm This gave me 97 or 102cm instead of the desired 100cm to the etalon from the objective. So now I have ordered a WO 2" x 50mm extender for the tailpiece ring. The best option was the 102cm [8cm extender] which gave me simultaneous prominences and some surface texture. In both cases the post etalon, optical path length was also badly wrong. Which is hardly surprising since it was working on the wrong angle and length of light cone.

I also climbed the stepladder to check how much of the D-ERF was fully illuminated. The bright circle of the light cone was falling over the surrounding baffle slightly. So I shall have to move the D-ERF even nearer the tailpiece. With perfect hindsight I should have bought the 110mm D-ERF despite the extra cost. That would have placed the filter more centrally instead of right down near the tail end. The brilliant spot of reflected, re-focused light from the D-ERF was sitting on the objective lens. I just hope the heat doesn't do any damage.  

UPDATE: I had ordered a 2" William Optics extender and expected it to be 50mm or 50.8mm . Which is the metric equivalent of 2".  It turns out to be 60mm long. I am awaiting a response from the dealer because there is no mention of length on their website. Nicely made and finished, though. With a compression ring and two decent thumbscrews for a firm grip on the AOK etalon adapter.

Click on any image for an enlargement. 

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