Going H-alpha: ITF/BF stack/eyepiece holder adapter.

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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!

 

 

I have had a look at the PST's ITF and Blocking filter components. These two could easily be fitted into a bored out 2" to 1.25" eyepiece adapter. A cheapo, plain one will do. No need for a brass compression ring. Though one could easily be incorporated into a larger diameter slot cut in the adapter's newly increased bore.

It won't take much time to bore out the adapter to increase the usual 31.8mm EP size to the 36mm Ø of the PST's ITF/BF5 tubular section. The adapter's usual pinch screw will provide a lock on quite a useful sliding fit.

The Blocking filter [with EP holder] from the PST will screw onto its original thread on the ITF to make the standard PST filter stack.

Which cannot escape from its adapter thanks to the shoulders at each end. While providing nearly 1/2" of [focusing] travel if required. More, if the adapter is counter-bored to clear the shoulders.

After cleaning the threads on the ITF/BF I was able to screw them back together after slipping them both into the bored out [now] 50mm : 36mm adapter. Turps substitute worked as well on the gummed up threads as it did as a cutting fluid on the lathe. It is quite magical how well turps works on aluminium being turned in the lathe.

The problem with this 2" adapter idea is that it demands a 2" star diagonal and I'm not sure it should be necessary given the modest diameter of the PST's filter elements. The complete 'plug-in' set-up is shown in the image right. The Etalon to EP distance of 20cm is also achieved. Though 2" extenders are readily available if they were needed. Or the long and sturdy AOK female adapter could be slightly shortened if need be.

Since it is so reflective, it is quite hard to judge where the mirror surface lies inside the 2" star diagonal. So measuring the exact, optical path is prone to personal error. I don't have a calibrated feather to avoid damaging the delicate coating.

Silly me! I just need to lightly pencil crossing lines centered on both 2" spigots. The reflection point [at the mirror surface] is then indicated by the corner of the square formed by these pencil lines and the diagonal's own body casting. So I can now confirm that the optical path length from the far side of the etalon to the lower rim of the eyepiece is close to 20cm when the [now sliding] ITF/BF filter stack is pushed right down.

The focal plane [at the field stop] of the Cemax 12mm  EP lies deep inside its chromed barrel.  25mm from the bottom rim, in fact, from examining a pencil using the EP as a magnifying glass. This almost coincides with the lower edge of the gold band plus about 5-6mm, or so.

I wonder whether I need to compensate for this small difference in the 20 cm measurement? The answer is probably to see where the system focuses when connected to the refractor and go on from there. I can counter-bore the top of the already bored out adapter to reduce the optical path length, if needed, by sinking the top [BF] shoulder. Making the path length longer is very much easier. I just lift the filter stack and re-tighten the pinch screw.

Click on any image for an enlargement.

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