PST conversion?

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garymck
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PST conversion?

Post by garymck »

Hi first post here!

Have a chance to buy a PST double stack (used) for an extremely favourable price. Have seen the conversions to 80mm and 100mm done with the single stack, but can the double stack be used in the conversions also?
TIA
Gary


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Re: PST conversion?

Post by marktownley »

Hi Gary, welcome to the forum. You can't realistically use a external double stack etalon in a PST mod.
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Re: PST conversion?

Post by daslolo »

Is it because the external second stack is 40mm?


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Re: PST conversion?

Post by marktownley »

daslolo wrote: Sun Feb 16, 2020 7:43 pm Is it because the external second stack is 40mm?
No, they have a central obstruction that will be visible in a collimated beam.


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Re: PST conversion?

Post by daslolo »

marktownley wrote: Thu Feb 20, 2020 8:57 am
daslolo wrote: Sun Feb 16, 2020 7:43 pm Is it because the external second stack is 40mm?
No, they have a central obstruction that will be visible in a collimated beam.
I've seen that happen on my SCT when out of focus but it goes away when in focus (still don't know how).
Can you explain?

I have :
beams entering the tube are parallel, go through the second etalon parallel which punches a hole in the middle with that obstruction, then telescope lens converges them to the correcting lens before the etalon, turn them parallel again through the etalon then back to converging after the second correcting lense, that hole is still there, same ratio to the image. So yeah that makes sense.
Why don't SCT not show the obstruction?


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Re: PST conversion?

Post by AndiesHandyHandies »

Hi

The black hole does not normally show.

For stars it inside the diffraction pattern.

I lent a F5 Newtonian to a friend and he noticed the dark hole in the middle in daylight and thought there was a fault.

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Re: PST conversion?

Post by daslolo »

AndiesHandyHandies wrote: Fri Feb 21, 2020 1:15 pm For stars it inside the diffraction pattern.
Stars wavelength is shorter than the obstruction?


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Re: PST conversion?

Post by AndiesHandyHandies »

Hi

The obstruction blocks light in the middle of each diffraction pattern, which a lot of make up the image.

On night time objects like star fields it not noticible.

In the day you can see the field has a dark part in the centre due to the obstruction of the light in the centre.

Andrew.


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Re: PST conversion?

Post by marktownley »

marktownley wrote: Thu Feb 20, 2020 8:57 am
daslolo wrote: Sun Feb 16, 2020 7:43 pm Is it because the external second stack is 40mm?
No, they have a central obstruction that will be visible in a collimated beam.
I was thinking about this since, i'm wrong in saying the central obstruction will be visible in a collimated beam, it is visible in a telecentric beam. Think about the Coronado SMII scopes, their etalon has a central spacer in the etalon which sits in a collimated beam.

Practically, can you double stack a PST etalon with a SM etalon? Realisitically no, the central spacer in a SMII etalon is ~12mm(? not measured it?), PST etalon is 20mm diameter so would eat a chunk off it. You could mount it offset so the central spacer is not in the light path, but then you're in the realms of custom made parts.


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