Going H-alpha: PST dismantling: A serious work-out!!

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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 was lucky enough to find an original Constrictor strap wrench at a rural timber yard. This saved me a journey into the city.

The PST needs to be very firmly held if any progress is to be made at dismantling it. With the prism already removed I could clamp the open body down on a B&D workbench with a Bessey F-clamp. Cardboard packaging between the bench and the PST's housing is also a very good idea to avoid cosmetic damage. Why didn't I think of that first?

Note that the tuning ring was always placed well clear of the bench surface to avoid crushing forces doing serious damage!I started with the eyepiece holder overhanging the edge of the bench.

The sum of the PST parts [so far] warming up indoors.

To say I was shocked at the force required to start unscrewing the upper section is an understatement. It took all my weight pressing repeatedly before it would even start unscrewing.

There followed a series of loud and violent "cracks" as the locking medium gave way only very reluctantly. This went on repeatedly for several minutes until the top section of the eyepiece holder finally relented. It was shocking how much force was required until the very last! 

The lower eyepiece section contains the ITF and this was even more difficult to start moving! I almost gave it up as completely impossible until I was finally rewarded with the first loud CRACK! There followed another series of pressing my full weight [now over 12 stone] on the Constrictor's lever.

This section alone took about ten minutes of repeated loud CRACKS with every tiny movement of the part a serious battle of wills! One could say the locking compound was really earning its keep!

The two halves of the eyepiece holder posing as newly separated Siamese twins. The ITF is orange from rust. I have ordered a new ITF from Maeir in the USA. The smaller glass element in the top section is the BF5.

 
If I have not fully conveyed the incredible effort required, against an immovable object, then let me just say I was sweating, while lightly clothed, in my sub-zero workshop.

If you cannot fix your PST down as firmly as I had done and then used a serious amount of muscle then your PST will never come apart! Though it should be said that some PSTs easily unscrew. Presumably from a complete lack of locking compound during assembly. I was not so lucky.

Image left: The newly visible Etalon optical group after the gold tube is removed. This surface has to be placed 20cm [200mm]inside the focus of an f/10 objective. You can buy special focuser sized extensions/adapters with the special male thread of the gold tube. [50mm x1mm pitch.] I'm not sure why the etalon looks like it has orange rust.

From my own experience I really do not think any person standing on the housing would be enough resistance to the strap wrench. A lesser F-clamp than the sturdy Bessey would not have been enough. A 6" [or larger] woodworkers professional, iron G-cramp [C-clamp] would also have sufficed. Two would be much better.

Unless you are very serious about dismantling I really wouldn't bother. I felt totally committed to my project so had little to lose provided I didn't actually break anything. That would have been a very expensive disaster! I have had a long lifetime of breaking things thanks to countless hobbies, DIY and building projects. It's amazing I have survived this long!

Note that the 'poorly' bits, like replacing a rusty ITF, can be accessed without dismantling the eyepiece holder. The prism will have to come out first though, to allow you room to work without causing very serious damage.

I have seen a very large hex nut with pins projecting from one face suggested as a driving tool for the ITF locking ring. Or you can modify the spring from a clipboard style, paper clip. The blue/black metal kind with two wire handles sticking out. You'll need pliers to bend the tips at right angles to fit the holes in the ITF retaining ring. The you'll need lots of patience to unscrew the ring by turns. Repeatedly reversing the spring clip. Be careful about the glass ITF filter dropping out!

You can even saw the gold tube in half and use it as a sloppy drawtube in 2" focuser without removing the etalon from the black casing. Do the sawing neatly and you can always resurrect the two halves of your Frankenstein PST with a close fitting sleeve. A wrap of paper will give you a nice straight line to carefully saw around the tube with a junior hacksaw. Don't forget to remove all the metal swarf BEFORE you tip it up to have a look inside and coat the costly and inaccessible optics in fine metal dust!

Click on any image for an enlargement.

 

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