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Author Topic: PST tuning ring performance  (Read 4571 times)

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Offline JohnM

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PST tuning ring performance
« on: 14:28:02, 27 February, 2011 »
Operation of the tuning ring on my PST has always given very smooth movement across the band.
From finding a position of best surface detail to best prominence detail.
Suddenly it's become very erratic.
The slightest touch seeming to disturb the etalon wildly.Has anyone seen this happen before?
We've had a lot of extremely cold temperatures this winter, -30°F and colder, but then the scope has been through those temperatures in the past.
I'm mystified.

Offline Roger Banks

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Re: PST tuning ring performance
« Reply #1 on: 10:52:09, 28 February, 2011 »
The screw under the rubber ring isn't loose is it John?

Offline Nomis Elfactem

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Re: PST tuning ring performance
« Reply #2 on: 11:12:05, 28 February, 2011 »
Good call Roger, that's what I was thinking too  :yes:

-30deg is pretty cold John... might be worth making it a little dufflecoat  ;)  I'm guessing it's not the absolute temp that will cause problems but the speed and severity of temp change.  The etalon is quite a precise piece of kit so worth keeping temp fluctuations to a minimum I'd say.

S.
Simon

Scopes: Astro-Tech AT-111EDT Triplet, TS65ED Quad, Orion ST80, Modded PST-90 Solar Scope, PST Cak (on loan)
Cameras: SXVF H694, Atik 16ic, Canon EOS 600d, DMK41, DMK21, QHY 5L-II (mono & colour)
Accessories: SX USB Filter Wheel, SX OAG, Baader LRGB Ha SI OII Filters, SharpSky Focuser
Mount: EQ6 (EQMOD), SW Star Adventurer, plus a lot (and I mean a lot) of other bits and pieces

Offline Merlin66

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Re: PST tuning ring performance
« Reply #3 on: 12:12:57, 28 February, 2011 »
I'd certainly check the small screw first....It could be that the foam pressure pads have weakened and softened....
C11, C9.25 SCT, 4" Genesis, NEQ6pro, modded 1000D,  ATik314L+, DMK41AF04, SM60DS/BF15, and Spectroscopes (many!).
http://tech.groups.yahoo.com/group/astronomical_spectroscopy/
"Astronomical Spectroscopy for Amateurs", "Grating Spectroscopes-How to Use them" -Springer

Offline JohnM

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Re: PST tuning ring performance
« Reply #4 on: 13:26:10, 28 February, 2011 »
All very mysterious. The ring screw is tightly in place.
A ring rotation of, say, a 1/16 or less, was having more effect than a full rotation. max to max.
I've had things all apart inspecting the parts and pieces.
I can detect nothing different with the Etalon itself.I'm suspecting the foam ring now.
Perhaps it's not smoothly contracting and expanding any more. From the extreme cold that we had more of than usual?
If I have any sky today, I'm going to wrap the Tuning Ring area wit a Kendrick EP heater belt, I have those all over the place to keep frosting and stiffness of parts down during the winters.
Also, have you all noticed the tiny bits of additional foam glued to one side of the ring?
The first time I saw those I thought it was a defective assembly, then decided it must provide the tilt.
I have quite a variety of different foams, I might experiment with some different rings.

Offline JohnM

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Re: PST tuning ring performance
« Reply #5 on: 19:32:00, 01 March, 2011 »
OK, I still can't get normal response from the tuning ring.
If I go back together with the house all seems normal.
When I first got this Denk 60mm upgrade, it didn't seem that they had not kept the focus point the same.
I couldn't reach a good focus with my normal set up using the DMK21 as I did with the stock 40mm rig.
The prism had to be run all the way up till it ran into the house and focus still didn't come in.
I removed the bottom section of the EP holder and then got prism travel to reach focus.
From the final position of the prism it looks like they were off by maybe a half inch.
So, could it be that with the Etalon assembly attached to the tube in the normal manner, the spacing is incorrect enough to cause the tuning adjustment to be totally out of range when I take out the house and add the 200mm spacing?
I've been rotating the plate full travel with a pin and it's a no go.
Any ideas?

How can I determine if the etalon position is correct relative to the objective on the 60mm f10 system?

Offline Merlin66

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Re: PST tuning ring performance
« Reply #6 on: 19:41:44, 01 March, 2011 »
The "barlow" lens on the front of the etalon needs to be exactly 200mm inside the prime focus to give a nice collimated beam through the etalon. if you can find the prime focus and measure back 200mm; this is where the etalon should be positioned.
Don't mix up tuning with positioning... when the etalon is correctly positioned, the sweet spot will be maximised.. the tuning really depends on the tilt of the etalon plates.....
C11, C9.25 SCT, 4" Genesis, NEQ6pro, modded 1000D,  ATik314L+, DMK41AF04, SM60DS/BF15, and Spectroscopes (many!).
http://tech.groups.yahoo.com/group/astronomical_spectroscopy/
"Astronomical Spectroscopy for Amateurs", "Grating Spectroscopes-How to Use them" -Springer

Offline JohnM

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Re: PST tuning ring performance
« Reply #7 on: 20:05:29, 01 March, 2011 »
Hmm, OK, I just went out and removed every thing from the tube.
It looks like prime focus is 190mm from the very end of the tube.
That would put the furthest face of the etalon at around 185mm I think.
I need to go back through your paper to look just where on the etalon the 200mm was supposed to touch.

Offline Merlin66

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Re: PST tuning ring performance
« Reply #8 on: 06:05:38, 02 March, 2011 »
The optimum position is the front of the rubber ring at 200mm - I found a leaway of +/-5mm before it became noticable in the sweet spot.
C11, C9.25 SCT, 4" Genesis, NEQ6pro, modded 1000D,  ATik314L+, DMK41AF04, SM60DS/BF15, and Spectroscopes (many!).
http://tech.groups.yahoo.com/group/astronomical_spectroscopy/
"Astronomical Spectroscopy for Amateurs", "Grating Spectroscopes-How to Use them" -Springer

Offline Nomis Elfactem

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Re: PST tuning ring performance
« Reply #9 on: 10:05:57, 02 March, 2011 »
John - there's a calculator here (attached) which may help you !

S.
Simon

Scopes: Astro-Tech AT-111EDT Triplet, TS65ED Quad, Orion ST80, Modded PST-90 Solar Scope, PST Cak (on loan)
Cameras: SXVF H694, Atik 16ic, Canon EOS 600d, DMK41, DMK21, QHY 5L-II (mono & colour)
Accessories: SX USB Filter Wheel, SX OAG, Baader LRGB Ha SI OII Filters, SharpSky Focuser
Mount: EQ6 (EQMOD), SW Star Adventurer, plus a lot (and I mean a lot) of other bits and pieces

Offline JohnM

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Re: PST tuning ring performance
« Reply #10 on: 13:49:59, 02 March, 2011 »
Thanks ken and Simon.
This all gets because I don't have any other PST parts to use for comparison.
I'll see if I can understand how to use your chart Simon, I'm a sheets of paper and pencils guy.
When it gets light enough to see I'm going to check again just where the front of my Etalon is relative to the OG,
but I'm pretty sure it's going to be somewhere around 185mm.
At times like this I really miss my lathe.
I think I will go back together with the house and prism again just to make sure the etalon is performing correctly.
I read that someone ran into a decontacted one, so I need to check again that mine hasn't come apart with all my fiddleing.
I should soon have more parts to work with and should be able to install the focuser to the tube and etalon,spacers,BF; after it.
Trying to use my existing 60mm tube and move the forward etalon position a cm or so is going to be tough.
As I go through things again today a light might come on.

Offline JohnM

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Re: PST tuning ring performance
« Reply #11 on: 16:50:27, 02 March, 2011 »
Simon,
Using your calculator, I come up with the etalon at the wrong distance from the OG with my 60X600 OTA.
Now it appears it needs to be about 1.25" further away than I have at the PST thread on the end of the tube.
I thought I had used up all the 2" plastic tube I had been using to make spacers, but just found my stash.
So the next project is to devise an extension for the tube to move out to the distance indicated and find out if that is where all the trouble lies.
Interesting experiments here.

Offline JohnM

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Re: PST tuning ring performance
« Reply #12 on: 21:11:15, 03 March, 2011 »
Success at last, success at last!
All the formulas I have for positioning components in this sort of major PST mod all indicated the etalon was in the wrong position.
I kept coming up with a figure of around 35mm it needed to be moved away from the objective.
Simple deal, yeah
I could make the connection to the male 50mm PST thread easy enough, but no way to connect female SCT t0 the male PST thread without cutting and threading a special adapter.
And I needed a way to make adjustments to that setting.
Past the etalon is easy and when I get the focuser I'll be all ready for that part.

I had been wracking my brain to come up with a solution that didn't mean sawing that nice tube on the 60mm in half
Looking at an old 2" draw tube I had flocked gave me the idea and nerve enough to saw off the tube.
The tube is 50mm, a fraction too small for a good fit in a 2" draw tube, but the flocking made it a snug fit.
Sawing off the compression ring collar of the old draw tube gave me a heavy walled tube a little over 4" long.
Drilled and tapped for 3 10-32 lock screws at each end finished it off.
I could still mount the scope in the Losmandy rings I was using.
Put back together I pulled out the back end of the 60mm PST tube 35mm and locked it down.
A calculator Simon sent me gave 452 to 462mm back from the objective.
Right now it's at 454mm.
Voila' !!!

I could mount the DMK from prime to any of the various barlow and powermate combination I want.
And the tuning ring which was out of wack what ever I had tried immediately returned to normal operation.

I had just a few minutes of fairly light overcast to look through, but on the monitor I even saw proms the like of which I've never detected before.
With all the extension sleeves and stuff hanging on, focus is just too difficult to do much, come on focuser!

Offline Merlin66

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Re: PST tuning ring performance
« Reply #13 on: 04:57:38, 04 March, 2011 »
I'm glad it all worked out for you and that the etalon is now working.
In all the PST mods I've done, the positioning of the etalon in an f10 beam is  the critical factor for success. I've just trialed an SM60 front etalon with a rear PST etalon and the results are stunning.....
C11, C9.25 SCT, 4" Genesis, NEQ6pro, modded 1000D,  ATik314L+, DMK41AF04, SM60DS/BF15, and Spectroscopes (many!).
http://tech.groups.yahoo.com/group/astronomical_spectroscopy/
"Astronomical Spectroscopy for Amateurs", "Grating Spectroscopes-How to Use them" -Springer

Offline Nomis Elfactem

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Re: PST tuning ring performance
« Reply #14 on: 09:08:32, 04 March, 2011 »
Yeap great news John - well done !!

Which focuser did you go for ?  When the sun came out a couple of days ago I did some testing with the Borg and I had a few problems with flexing so not sure it is going to be a good enough solution.  May need to find a "plan B" !

S.
Simon

Scopes: Astro-Tech AT-111EDT Triplet, TS65ED Quad, Orion ST80, Modded PST-90 Solar Scope, PST Cak (on loan)
Cameras: SXVF H694, Atik 16ic, Canon EOS 600d, DMK41, DMK21, QHY 5L-II (mono & colour)
Accessories: SX USB Filter Wheel, SX OAG, Baader LRGB Ha SI OII Filters, SharpSky Focuser
Mount: EQ6 (EQMOD), SW Star Adventurer, plus a lot (and I mean a lot) of other bits and pieces

 

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