Going H-alpha: Celestron 150mm OTA mods.

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WARNING: SOLAR OBSERVATION REQUIRES GREAT CARE AND SAFE FILTRATION.
INSTANT 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!

This morning I put my old Celestron up on the Fullerscope's MkIV mounting to check focal distance behind the OTA. The mounting saved me messing about with props while trying not to drop the OTA. I just had to align the sun's image on a piece of dark sandpaper then measure back to the tailpiece adapter with a steel rule.

The previous owner of the Celestron was an imager. He had already shortened the main tube of the 6" refractor and fitted a Vixen 2" focuser. I found I could not get focus until I added long extension tubes and/or a star diagonal. [See old image right.]

It seems I have 26cm from the back of the focuser adapter casting to the focal plane. Which would allow adequate clearance for the tuning ring if the PST is added straight onto the tail end without its objective. So no main tube cutting is required to achieve the 20cm inside focus requirement for the Etalon.

Now I need to provide a 10cm, 4" long adapter to the OTA which will screw into the female, etalon thread at the other end. The extra length [over 6cm] will be required to attach the tubular adapter to the refractor. I shall turn an adapter ring to replace the Vixen, 2" focuser. 

As already discussed the plan is to use an internal 90mm Baader D-ERF to reflect the sun's heat away. This will be fitted some distance from the objective, inside the OTA. It also doubles duty as a UV/IR rejection filter.

I imagine a 10mm thick aluminium ring, with spacers at 120° around the rim, will allow much easier handling within the confines of the main tube. Compared with a solid, closely fitting ring, it will also be easier to align radial, fixing screws.

Now I have finally decided to discard the PST's main housing I shall have to arrange the filters at a suitable distance from the etalon. Then add a star diagonal to bring the eyepiece to a comfortable viewing angle. Star diagonals have the advantage of using up more of the focal length than straight tubes. The ITF and BF5 Blocking filter, tubular fittings can go on the eyepiece end of the 2" star diagonal just as they do in the PST.

All this will make for a rather long and unwieldy tubular structure projecting from the tail end of the 150mm refractor. Though there is really no need to use 2" astro fittings at considerable expense and possible flexure. I can use larger diameter aluminium tubing with turned inner rings to support vital components at the correct distance.


I have now ordered the PST Etalon to 2" adapter set from Beat Kohler of AOKSwiss. Real images to follow on arrival.

http://www.aokswiss.ch/d/zub/adapter/50.8-pst.html

 http://www.aokswiss.ch/d/zub/adapter/pst-2''.html

AOKswiss online - deutsch

Use Google Translate. I found an English version of Beat's website by accident but can't find my way back again. I could have been using Google Translate in Firefox and produced a false memory.

Click on any image for an enlargement.

 

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