Worthwhile experiment or wild eyed nonsense?

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JohnM
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Worthwhile experiment or wild eyed nonsense?

Post by JohnM »

Below is a photo of the assembly that fastens to the rear of my 4" OTA.
I'd been wondering if it might be possible to internally mount the front DS etalon from a PST.
I figured if done the ERF from that etalon would need to be removed to eliminate heat build up and allow more light transmission.
That took a while to figure out and if anyone's interested I'll post some more pics and explain what I found out to do it.
So now! Here is the set up.
The full PST tube passes through the draw tube of my Crayford focuser, I had to use the original objective housing, with OG removed for attachment, even with it and the tilt mechanism attached, the etalon still can pass the light cone without vignetting and will all slide into the 4" tube without reaching the rear baffle.
The ink mark on the yardstick indicates the proper position for the regular PST etalon.
The big question now is, the external etalon is affixed to its housing with silicone and the etalon itself has a plastic button in the center.
Would it be safe to even put this assembly together or would the concentrated light beam hitting that front etalon generate enough heat to destroy it?
Any thoughts much appreciated!


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Re: Worthwhile experiment or wild eyed nonsense?

Post by swisswalter »

Hi John

what a mod. It would help me to understand it better if you could mark the different pieces explaining the function


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Re: Worthwhile experiment or wild eyed nonsense?

Post by JohnM »

Walter,
I use 2 focusers for my stage 2 mod.
The front one is a standard 2 speed Crayford attached the the 4" tube, I use it to set the distance between objective and the standard PST internal etalon, attached the that etalon is an SCT focuser used for general focus.
To attach that external etalon I just used a complete PST gold tube which will pass all the way through the Crayford draw tube to the interior of the 4" tube.

I haven't reassembled and tried it yet till I get some ideas about heat effect.
I screwed on the tilting plate just to see if it would still keep the light cone inside all that stuff.
If this is a viable plan it would be possible to make an access hatch in the 4" tube to reach the tuning knob on that front etalon adjustor.


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Re: Worthwhile experiment or wild eyed nonsense?

Post by marktownley »

Do you have a collimated beam hitting the external etalon on the left hand end of the picture when all is assembled? It needs it to work.


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Re: Worthwhile experiment or wild eyed nonsense?

Post by JohnM »

Hmmm? No, as assembled now it would just be in the light beam coming from the 4" objective.
Why would the beam need to be collimated?, in a normal application that etalon is on the front of the PST.


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Re: Worthwhile experiment or wild eyed nonsense?

Post by Zakalwe »

Hmmm? No, as assembled now it would just be in the light beam coming from the 4" objective.
Why would the beam need to be collimated?, in a normal application that etalon is on the front of the PST.

As far as I understand it, you consider the light falling on the objective to be collimated due to the great distance to the Sun. Once the light goes through the objective it is refracted into a cone. You would need to put a collimating lens in front of the Etalon and another lens behind the stack.


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Re: Worthwhile experiment or wild eyed nonsense?

Post by JohnM »

Yeah, I follow that.
I don't know what I would have that would recollimate the beam to that forward etalon.


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Re: Worthwhile experiment or wild eyed nonsense?

Post by swisswalter »

Hi John

I tryed to sort some of the stuff out







please explain more of the details. Where is the etalon ?


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Re: Worthwhile experiment or wild eyed nonsense?

Post by marktownley »

Hmmm? No, as assembled now it would just be in the light beam coming from the 4" objective.
Why would the beam need to be collimated?, in a normal application that etalon is on the front of the PST.

Light coming from the sun is at f109 (essentially collimated - as good as!), the light that would hit that etalon in the configuration you propose is f10 (I guess?) - what you would find is it would perform at considerably wider bandpass and with sweet spotting due to the increased field angles of the light hitting it.


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Re: Worthwhile experiment or wild eyed nonsense?

Post by JohnM »

OK walter, where you have the question marks is a PST DS etalon with the front red ERF removed and the gold colored outer shell removed.
What you have marked as 40mm lens is just the shell with the OG removed, the only way I have to match up threads.


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Re: Worthwhile experiment or wild eyed nonsense?

Post by swisswalter »

John

is that the place of the etalon ?







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Re: Worthwhile experiment or wild eyed nonsense?

Post by swisswalter »

Hi John

so I updated the pic with your information. Any more details ?







Only stardust in the wind, some fine and some less fine scopes, filters and adapters as well. Switzerland 47 N, 9 E, in the heart of EUROPE :)

from 7 am - 7 pm http://www.nanosys.ch

from 7.01 pm - 6.59 am http://www.wastronomiko.com some times vice versa ;)
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Re: Worthwhile experiment or wild eyed nonsense?

Post by JohnM »

Thanks Mark.
Got any ideas what would work to collimate the beam?
And I'm still worried about heat effect on that etalon since it's assembled much differently than the internal one.
That is, could the thing just self destruct if I tried it to see what I got from it?


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Re: Worthwhile experiment or wild eyed nonsense?

Post by swisswalter »

Hi John


here another piece updated




Only stardust in the wind, some fine and some less fine scopes, filters and adapters as well. Switzerland 47 N, 9 E, in the heart of EUROPE :)

from 7 am - 7 pm http://www.nanosys.ch

from 7.01 pm - 6.59 am http://www.wastronomiko.com some times vice versa ;)
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Re: Worthwhile experiment or wild eyed nonsense?

Post by JohnM »

This is what that etalon looks like stripped of parts.
The thing was all bonded together with thread locker, plus a tiny set screw runs down into the threads of the housing to lock them.
The ERF unscrews from the inside, it was held from moving by great gobs of black silicone.
The etalon itself is held into its mounting ring by clear silicone sealant.


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Re: Worthwhile experiment or wild eyed nonsense?

Post by sullij1 »

Good thread, I have rigged a doner scope in the same fashion in preparation to do the same thing you are attempting. Or I plan too, one day when Ihave the funds to complete the project.

It seems it may work.I think you need to keep the ERF in place as an internally mounted ERF and get the magic 200mm spacing right. I used anoter posters model that is here in the threads somewhere. He has already sucessfully done this. Slit in tube and all.

It is in the very early mod threads. I would help get you there but my main computer is down and I am working off a netbook. So I cannot easley navigate the site. My big fat fingers have trouble pushin these lil keys.

If I remember correctly it is DSobserver who has an internal ERF mod instruction on his website (somebody here may have a link to what I am writing about). That also heped guide me. He is NOT the only person who did this type of mod. There are a couple of examples in the early threads.


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Re: Worthwhile experiment or wild eyed nonsense?

Post by JohnM »

Thanks Joe. The reason I thought I'd better figure out how to get that red ERF off the assembly was that I ran across a bit about doing something like this and one of out knowledgeable guys thought if it was left in place it would act as a heat sink.
:?
Anyway, now that I've figured it out and cleaned off all the adhesives it's an easy matter to put it back together.

It's clear out at the moment, I suppose I could put it all together and do a quick test.


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Re: Worthwhile experiment or wild eyed nonsense?

Post by JohnM »

Nope, that won't work at all.
Curiosity taken care of. :P


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Re: Worthwhile experiment or wild eyed nonsense?

Post by Merlin66 »

To get a parallel beam through the front etalon you really need to consider a Marcon type solution, where a second objective is placed after the first with it's focal point at the focal point of the first objective, to provide the collimated input beam.
This solution tends to lead to VERY long optical systems!!!
I tried it once (EC90) with a Big Boy 2" barlow....didn't work for me!)


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Re: Worthwhile experiment or wild eyed nonsense?

Post by Merlin66 »

John,
I've got to say that's one of the grubbiest etalons I've seen!
I've never come across one with clear silicon holding the etalon (always seem to the black RTV) and with so much "excess"...


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Re: Worthwhile experiment or wild eyed nonsense?

Post by JohnM »

Yeah, it's just filled with silicone!
The thing is an outstanding performer on my 40mm PST though.


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