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23.10.20 Today's only image of AR2776 through a teaser.


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A brief glimpse of the sun fooled me into setting up.
One quick "snap" and the overcast returned.
Not so much a work of art as a humble record of the event.
The spot had left quite a trail of destruction in its wake!

23.10.20 15.56 b1200 2x GPC blog 2.jpg

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Man that etalon you have is begging to be fixed!   open it up and let  foam gasket expand.   It has taken memory shape, and the etalon has sunk to the left or right.  This is why your sweetspot is so offset!

 

Easy fix,  and you will be quite pleased and amazed once you get it shifted to the right.      Just be careful removing the etalon /    I use a film canister to pop it in and out.   Just flip it over gently, so it falls onto the canister lid.  (anyone remember film?)

 

Your etalon , where it IS  onband ;  clearly -  extremely well performing , 

 easily in the half angstrom range,   this is a phenomenal pst etalon=-  it just needs a tender moment with your hands.

 

 

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8 hours ago, Kitsunegari said:

Man that etalon you have is begging to be fixed!   open it up and let  foam gasket expand.   It has taken memory shape, and the etalon has sunk to the left or right.  This is why your sweetspot is so offset!

Easy fix,  and you will be quite pleased and amazed once you get it shifted to the right.      Just be careful removing the etalon /    I use a film canister to pop it in and out.   Just flip it over gently, so it falls onto the canister lid.  (anyone remember film?)

Your etalon , where it IS  onband ;  clearly -  extremely well performing , 

 easily in the half angstrom range,   this is a phenomenal pst etalon=-  it just needs a tender moment with your hands.

Thanks. :)

My etalon is already, half exposed. i.e. Without its usual metal or rubber band.
I adjust the etalon tuning and tilt frequently with a rod in one of the usual screw holes.
But I have always suffered the same off axis "illumination."
There is one setting where the entire field darkens and becomes more even.
That is my preferred setting but it is not very stable.

I have previously removed the etalon element, examined it and its sponge support ring.
Sadly I was none the wiser over what could actually be done to improve matters.
There is no separate packing piece. Which is mentioned occasionally in solar circles.
Just a plain, sponge rubber ring.

Any specific advice would be very welcome. :thumbsup:

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7 hours ago, Rusted said:

Thanks. :)

My etalon is already, half exposed. i.e. Without its usual metal or rubber band.
I adjust the etalon tuning and tilt frequently with a rod in one of the usual screw holes.
But I have always suffered the same off axis "illumination."
There is one setting where the entire field darkens and becomes more even.
That is my preferred setting but it is not very stable.

I have previously removed the etalon element, examined it and its sponge support ring.
Sadly I was none the wiser over what could actually be done to improve matters.
There is no separate packing piece. Which is mentioned occasionally in solar circles.
Just a plain, sponge rubber ring.

Any specific advice would be very welcome. :thumbsup:

okay, i think i know whats going on.

 

You have lateral shift (up and down) of the glass in its cell due to the rod.   If you gently push forward on the pin;   i bet you  can move that sweet spot.      If true you may be able able to just add some paper sheets to the side of the glass plates so it cant "up or down angle"   

 

Dont give up!

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3 hours ago, Kitsunegari said:

okay, i think i know what's going on.

You have lateral shift (up and down) of the glass in its cell due to the rod.   If you gently push forward on the pin;   i bet you  can move that sweet spot.     
If true you may be able able to just add some paper sheets to the side of the glass plates so it cant "up or down angle"  

Don't give up!

Thanks. Give up? Never. I am fascinated by your suggestions for improvement.

What are you suggesting precisely? That I shim the outside of the etalon glass block to stop it rocking sideways in its cell?

BTW: I use the rod to deliberately tilt the etalon to find the most even illumination as I simultaneously rotate the etalon inner band.
The rod is not remotely a tight fit in the screw holes and I often use anything to hand which will fit easily in one of the numerous, tuning, screw holes.
I presume, from what you say, that I am manually centring the sweet spot by tilting? That makes sense.

There is also the matter of the freedom of movement of the etalon sandwich itself.
The glass must be quite loose in its cell if it allows such tilting.  Presumably due to years of heavy, foam ring compression causing it to flatten.
It seems unlikely that it would revert to its original state and dimensions, in a relatively short time, simply through unloading the pressure.

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An update: Mark Townley's piece on the PST etalon:

I'd forgotten about his blog as an excellent solar resource.

Brierley Hill Solar: Equipment 

Mark suffered from a floppy etalon. Just as I do.
I use the freedom to manually tune and re-collimate my etalon but it is not at all stable.

Mark used an acrylic ring to restrain the base of his etalon.
Plus some PTFE plumber's tape on the threads.

I have a lathe and measuring equipment so I'll try turning a base, restraining ring first.
Clear polycarbonate, or even black HDPE, would probably suit this exercise.
I may even have a suitable O-ring.

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This thread has reminded me of this issue which was raised on Solarchat some years ago now.  With regard to sponge ring compression memory, which is a separate issue to loose etalon "up-down" fit, it was advised that if a PST etalon is to be left unused for a considerable time, then the tuning adjustment should be backed off via the tuning ring to release the pressure on the sponge ring.      🙂

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Thanks Peter. :thumbsup:

The truth is out there.  It is a matter finding it by means of simple search terms.
Which even Google can understand.. without trying to sell me solar panels.. :wink2:

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I've just  been over to the observatory [in the rain and the wind] to dismantle my PST etalon for science. :rolleyes2:

It seems I have been fooling myself about tilting the inner, tuning ring.
The undersized implements, I have used to tune, have fooled me into believing I was tilting.

The etalon glass sandwich is held firmly in its cell, albeit with some freedom.
Not even enough for electrical tape tape to close the gap.
I would have needed PTFE plumbing tape.

The etalon element is under pressure from a PTFE ring lying very close to the etalon's rim.
So it is difficult to see how one can externally apply any useful tilting.
The pressure is perpendicular, applied at the rim and therefore [probably] self-aligning.

Regarding the sponge ring. This sits squarely in the bottom of the etalon cell.
If it is certainly being compressed, over a long period of time, but so what?

Assuming the sponge took on a memory of being compressed then why would it matter?
One merely adds a slight extra turn on the tuning band to take up any "memory" displacement.
There is no change in the pressure required because the extra space is simply taken up manually.
The centre tuning point might be displaced, but again, is unimportant.

The acid test requires a good dollop of sunshine.
I shall find a rod which fits snugly in an inner ring screw hole.
Then attempt to tilt the etalon's inner, tuning ring while the image is centred on the well-focused sun.

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2 hours ago, Rusted said:

I've just  been over to the observatory [in the rain and the wind] to dismantle my PST etalon for science. :rolleyes2:

It seems I have been fooling myself about tilting the inner, tuning ring.
The undersized implements, I have used to tune, have fooled me into believing I was tilting.

The etalon glass sandwich is held firmly in its cell, albeit with some freedom.
Not even enough for electrical tape tape to close the gap.
I would have needed PTFE plumbing tape.

The etalon element is under pressure from a PTFE ring lying very close to the etalon's rim.
So it is difficult to see how one can externally apply any useful tilting.
The pressure is perpendicular, applied at the rim and therefore [probably] self-aligning.

Regarding the sponge ring. This sits squarely in the bottom of the etalon cell.
If it is certainly being compressed, over a long period of time, but so what?

Assuming the sponge took on a memory of being compressed then why would it matter?
One merely adds a slight extra turn on the tuning band to take up any "memory" displacement.
There is no change in the pressure required because the extra space is simply taken up manually.
The centre tuning point might be displaced, but again, is unimportant.

The acid test requires a good dollop of sunshine.
I shall find a rod which fits snugly in an inner ring screw hole.
Then attempt to tilt the etalon's inner, tuning ring while the image is centred on the well-focused sun.

I had attempted gasket replacements , and have had some success actually.   Even replaced the white teflon ring above the etalon with a thicker one with smaller diameter to centralize compression more.   All of the information was removed from solarchat but a bit was left on flickr.    

 

https://www.flickr.com/photos/163396781@N07/with/48932719731/

pst sponge face 33mmpst sponge side 4.5mm

the tilting actualy comes from the coarse threads.  The top portion of the filter cell is what has the lateral play.

 

Yor etalon is designe to operate at 0 degrees to the objective, so flush.       

 

The thing with the foam memory,   its exactly the same property of carpet under your bed frame or night stand.   Move it, and you will see a divot and impression ring in your carpet.      

 

That impression ring on the memory foam is permanent until the gravity can release the load.     After all, it is called "memory foam" because it will revert to its original dimension.

 

 

The rod method you speak of, i do the same thing.  Infact i never detune my h-alpha etalon anymore.      I use a small eyeglasses screw driver, to rotated.   

 

this is where things get notable for investigation.  The bottom portion of the filter cell, is much larger than the etalon glass plates;  So take the etalon out and examine the rotation of the coarse threads.  PTFE tape on the threds might do the trick.

 

The clear aperture of the etalon is 21mm     The etalon glass itself is 33mm in diameter.

 

This oil gasket replaces the orange ring perfectly, but it has too much resistance so tuning range is eliminated. It will still come on band, but its too tight of a fit because of the rubber density.   The foam ring has almost zero resisitance .  

https://www.ebay.com/itm/Nitrile-U-Cup-25mm-ID-x-33mm-OD-x-5mm-Seal-HT-Price-for-1-pc/163464651129

 

Would be awesome to attack this hack again to get the pst "mended" :)

 

 

 

This is what i used to replace the white teflon ring.  I happened to have a slip bearing that fit nicely, and it was thicker than the OEM teflon ring.

 

The new pressure ring

 

 

First etalon 1 with white ring inplace,  You can see it in there.

The new pressure ring

 

 

second etalon with the white ring replaced with the slip bearing.   ( white ring is no longer present.)

The new pressure ring

 

Edited by Kitsunegari
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Thanks.

I should at least try to pack out the etalon sides to better match its cell.
Teflon/PTFE plumbing tape makes most sense. It is very thin and easily adjusted for thickness.
Electrical tape was far too thick for this job. One small tab and the etalon no longer rattled but was off centre.

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