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Author Topic: OG - etalon spacing  (Read 2332 times)

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Offline JohnM

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OG - etalon spacing
« on: 16:17:48, 10 March, 2011 »
While I wait for the Sun to clear a corner in my obs I have a question that I've been curious about.
Ideally the etalon should be spaced far enough from the objective that its front face is in a 20mm diameter point in the light cone?
Should it be pushed further out to a point, say, 19mm to avoid edge aberations that might be there?
What effect does having it closer, say, at a point where the Solar disk would be plus 20mm on the etalon?

Offline Merlin66

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Re: OG - etalon spacing
« Reply #1 on: 16:45:59, 10 March, 2011 »
John,
The etalon has a -200mm barlow lens fixed to the front, so it's designed to be positioned EXACTLY 200mm inside the prime focus.
The size of the etalon is also designed specifically for an f10 beam, so at this distance the illumination will be 20mm.
At the rear of the etalon Coronado fitted a +200mm lens... so the focal point is ALWAYS 200mm behind the etalon.
Hope this helps.
ken
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: OG - etalon spacing
« Reply #2 on: 17:01:42, 10 March, 2011 »
Ah, OK then, that -200mm lens IS THE critical measurement.
Thank you, for all the reading I've been doing that 200mm measurement keeps confusing me when I measure the solar disk at the etalon vs various spacing.

Offline JohnM

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Re: OG - etalon spacing
« Reply #3 on: 16:11:24, 14 March, 2011 »
You all have seen my attempts at obtaining a good sharp image with this Denk supplied 60mm OTA.
With full scale ray traces and the assembled scope sitting on top of the paper, I come to the conclusion that the OTA is ~20mm too long.
Could having the etalon 20mm too far out, that is, the front of the etalon only  180mm from the point of prime focus instead of 200mm, cause the soft focus I keep getting?
I'm almost to the point of thinking I invested in an expensive paper weight.

Offline Merlin66

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Re: OG - etalon spacing
« Reply #4 on: 16:23:41, 14 March, 2011 »
John,
In all the testing I did on the PST etalon re mods etc I found that the "optimum" results ie best performance minimum sweet spot issues etc was when the etalon was within 5mm of the design 200mm position.
I'm sure the guys at Denk know all this...it's not really rocket science and if the original 40/400 objective is being replaced with a 60/600 it should be easy to cut the tube to the correct dimensions??????
A very easy way of confirming your measurements would be to remove the new tube and focus the sun on a piece of card. Measure the distance from the end of the tube to the focus. This should be close to 200mm

 
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: OG - etalon spacing
« Reply #5 on: 16:30:54, 14 March, 2011 »
I've measured that many times.
It always comes out right close to 180mm.
Ever since I took delivery, I've wondered why with the black box installed in the normal manner I could never get a focus with the usual set ups I always used with the standard 40mm PST.

Offline Merlin66

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Re: OG - etalon spacing
« Reply #6 on: 17:05:48, 14 March, 2011 »
Hmmm
Can you get a replacement tube from them??
BTW do you still have the original PST tube? If so you can check the back clearance on that and then put it back into the blackbox to verify the problem...
The last resort would be to get the tube cut back and re threaded with the M50 x 1 PST thread.....don't know what else to suggest...
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: OG - etalon spacing
« Reply #7 on: 17:41:44, 14 March, 2011 »
Can't check what the old 40mm PST is because I loaned that objective to a friend, it was magnitudes better than the one on his PST.
I'll be back later, getting ready for another experiment.

Offline JohnM

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Re: OG - etalon spacing
« Reply #8 on: 19:34:49, 14 March, 2011 »
More snow storms rolling in Ken, so I'm shut down for a while.

Offline Nomis Elfactem

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Re: OG - etalon spacing
« Reply #9 on: 21:05:26, 14 March, 2011 »
John - I'd drop Russ a line and ask him.  He got bac to me within half an hour of me sending a question and in teh following email conversation he clearly knows his stuff.  The 200mm focal distance is well known so I'd be surprised if they got it wrong (and would be sending it back !!)

Good luck.

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: OG - etalon spacing
« Reply #10 on: 16:47:25, 16 March, 2011 »
Yesterday I went together as a visual rig.
The amount of detail observed was excellent.
Fine surface detail, including watching a flare taking place in Ar11169.
And fine views of a lot of great prominences including one detaching.
I used the 10mm BF, it required a lot of moving of the disk around to find the best spot to get the best views.
Wanted to have a go with the 5mm BF again, but clouds have ruined things again.

 

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