Removing a stubborn PST etalon assembly - help needed

[DJD]

Dear All,
I have just got an adapter from AOK ( and fallen foul of Customs and Banking systems in the process, but that is another story ). I am having trouble getting the etalon asembly free from the black box. I have removed the knurled rubber ring assembly, but despite breaking one of the rubber boa type wrenches and using a pipe wrench I cannot get the damn thing
to start unscrewing.
Any suggestions gratefully accepted as I am in a really bad mood now .
Best wishes,
David

[Merlin66]

David,
Are you using the heavy duty BOA?
It's usually easier to separate the etalon from the black box as the box can be held firmly in a padded vise. I've experienced some difficult customers (!!) but haven't failed as yet to get them separated.
You can make a longer tourniquet using a piece of rubber belting and a length of timber.....

[hunterknox]

Hi David,

It's not something I ever tried (the gold tube came off quite easily) but I could fetch another couple of Boas over and we'll see how many we can break! My only thought is whether a warmer or colder environment might make a difference?

Tom

[marktownley]

Mine was the same.

Take off the knurled rubber band from the tuning ring, take out locating screw from tuning ring (make note of the hole the screw sat in), remove tuning ring, use the boas to apply the force directly on the etalon assembly housing. Can you fix the black box in a vice while you apply the torque? This should allow you to remove it.

[swisswalter]

Hi David

I understand you well on both counts. My PST was very stubbern too. I used two strategies:

first: place the body with the etalon in the oven at 80 °C for an hour then try again, if that is not helping, apply some drops of kreep oil in between the body and the etalon housing, let it soak for at least 48 hours, then try again.

I had to do both, which one really worked at the end I do not know but finally I was able to seperate the etalon from the body.

good luck

[DJD]

Thank you all for the suggestions -they will all be tried one by one until the
b*gger throws in the towel :-) I am clamping the black box, but keep finding that the rubber ( boas ) or padding
to stop wrench marking the barrel ( too late for that now :-( ) keeps slipping when I apply force.
I can't apply too much force at the moment as I am a bit worried about hurting myself - waiting on possible neck surgery.
But I am encouraged that I am not on my own with the difficulties and that that have been overcome by yourselves.
Tom, if I haven't sorted the problem ( will report back here when I succeed ) when you come over, please bring the boas.
Thanks everyone,
David

[marktownley]

Something else to try - wrap the tuning ring-less etalon housing in a tea towel / cloth and with a pin hammer gently but sharply hit a small block of wood onto the etalon housing, gradually rotating a full 360 degrees around the housing. Your looking at doing this at 30 degree intervals. The aim is to loosen / break the bond the loctite has on the threads.

I had to do this on the second PST I dismantled.

[solarchat]

***be advised that NOTHING in this forum has been safety tested and you are reading and using these posts at your own peril. blah, blah, blah... dont mess around with your eyesight when it comes to solar astronomy. Use appropriate filtration at all times...

[DJD]

Indeed Stephen, timely reminder .
Just to say success! some rubber with a better grip wrapped around the etalon housing - minus tuning ring and
two rubber O rings and a hefty bit of elbow grease ( and a pipe wrench ) finally made the Loctite lose its grip.
The new adapter fits , the mini ERF and BF unit screws nicely into my Revelation dielectric diagonal, now just need
to measure and re-space everything for the DSLR. Sorry folks but I guess it will be raining for the fore-seeable
future once I am ready to look at the sun ;-)
Thanks for all the suggestions, hope they save someone else some heartache.
Best wishes for the New Year,
David

[swisswalter]

Hi David

congratulations on the removal. Now we are waiting for the first light pic ;-)

[DJD]

Still no sun, so trying to work out spacings to get the DSLR in an optimal position.
Does anyone know what the light path distance is through the PST pentaprism ?
I am being a bit lazy hoping to save some time measuring and calculating :-)
Thanks,
David

[Merlin66]

If the etalon is positioned correctly in the donor scope, then the distance from the rear of the knurled ring to focus is 200mm.

[DJD]

Thanks Merlin, but I must be missing something.
I have opened up my PST black box and measured the set up as at last viewing.
Front face/back of etalon to front face of pentaprism = 75mm approx.
three reflections in pentaprism ( middle point of faces ) = 35mm + 30mm + 35mm (approx ) = 100mm.
Top surface of prism to black box port approx 5mm
That would put the focal point sapprox 20mm inside miniERF/BF stack.
Does that sound about right ?
Best wishes,
David

[robert]

For what it is worth, the focal plane on my PST was at the top end of the black eyepiece holder tube, almost exactly in line with the top edge.
Robert

[Merlin66]

David,
Your measurement sound close....
Focusing is achieved by moving the pentaprism to "reduce/ increase" it's effective length.
Robert's comment on the final focal surface is pretty close, hence the promblems with imaging!