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Author Topic: Question for you guys who have done PST mods  (Read 2849 times)

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Offline JohnM

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Question for you guys who have done PST mods
« on: 00:01:53, 19 August, 2010 »
Friend of mine just got a CaK PST back from Meade after 8 months for astigmatism problems.
He said it is still almost as bad as it was when he first bought it.
He's in the eastern US and I'm in the west so we're doing things by phone.
I asked him to shake the objective end to see if he could detect any rattle of the glass in the cell.
He said there was none, so I had him loosen the retaining ring and then reset it. he said it was locked down mightily.
Now that he has some room for thermal expansion of the objective, I think that might help him.
We won't know till tomorrow.

All this has got me to thinking.
I'm wondering if the objective and the penta prism is identical between the 2 models, and possibly also the blocking filter.
The etalon is what does the control of spectrum that passes through the OTA.
Any of you guys gotten into them enough to know if that could be the case?

Offline Merlin66

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Re: Question for you guys who have done PST mods
« Reply #1 on: 06:44:19, 19 August, 2010 »
The CaK and the Ha PST use completely different arrangements in the optics. The Cak gold tube is much shorter than the Ha tube ( there's no etalon) and I believe that the Cak only uses a narrow band filter in the diagonal, so there's no Ha type blocking filter involved.
All in all I don't think there are any worth while components which could be salvaged from a CaK unit to help with a Ha PST mod.

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: Question for you guys who have done PST mods
« Reply #2 on: 14:28:47, 19 August, 2010 »
Hi Ken,
They're both 40mm f10s and use the same penta- prism focusing mechanism.
I still think the object glass with its ERF component could be the same.
I've completely disassembled and put back together the Ha version.
That was a really interesting project.
I'll soon have a CaK version to take apart for study, I hope to learn a lot from that too.
John

Offline Merlin66

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Re: Question for you guys who have done PST mods
« Reply #3 on: 21:51:48, 19 August, 2010 »
The same objective ( but no ERF!) and the tube is shorter because the etalon and associated lens are missing.
I agree the penta prism and focus arrangement is the same but there's no Ha bloccking filter....
For a stage one Ha mod ( dump the gold tube), you would still need an etalon and a blocker....
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: Question for you guys who have done PST mods
« Reply #4 on: 22:17:42, 19 August, 2010 »
Ah, so no ERF coatings on the objective. That answers on question I was wondering about.
So, without an etalon there is no need then for the forward telecentric lens.
And there's also no barlow element at the rear for correcting focal point for the penta prism,
taken care of by the different tube length?
I'm not interested in this as part of an Ha mod, I'll be receiving this defective CaK model before long and I'm just trying to get ideas about what all is in there to give me some ideas about maybe getting the thing to work right.
After waiting since last January for Meade to repair bad astigmatism and to get it back only marginally better, he's not about to send it back to them. So, he wants me to have a look at the thing.
I've heard many stories about astigmatism in the PST CaK.
He did say after he loosened the OG in the cell it did improve some.
More pinched optics?

Offline JohnM

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Re: Question for you guys who have done PST mods
« Reply #5 on: 13:58:03, 20 August, 2010 »
One more question.
Has anyone ever taken a look at just how the narrow band filters are mounted?
Shouldn't both filter elements be parallel to each other?
OK, 2 questions :)
Thanks,
John

Offline Merlin66

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Re: Question for you guys who have done PST mods
« Reply #6 on: 10:48:33, 21 August, 2010 »
John,
I haven't seen inside a CaK PST so I don't know for sure. I'd always assumed the filter bandwidth requirement for CaK line imaging was no where near as severe as that for Ha. I'd say a bandwidth of 1 or 2 nm would be enough (CaK at 393.4nm).
This could be achieved with a wider blocking filter ( reduce the UV loading) followed by a narrowband tuning filter ( I'm almost sure there's no F-P etalon involved).
It will be interesting to see what you find.......
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: Question for you guys who have done PST mods
« Reply #7 on: 15:10:48, 21 August, 2010 »
When I get this scope I'll take photos of the assembly.
I'm hoping to be able to identify what each optical element is in the scope.
I'm thinking now the first elements after the OG must be ITF elements.
With no ERF at the front there has to be something early in the train to control excess IR/UV before it gets to the CaK band pass filters.
From the descriptions on the phone, it sounds like 2 elements inside the blue band have metallic reflective surfaces.

 

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