How close to F10 must a PST mod truly be?

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How close to F10 must a PST mod truly be?

Post by MalVeauX » Fri Jan 10, 2020 8:23 pm

Hey all,

I'm curious, how exactly close to F10 must a PST mod conversion truly be?

For example, a F7.5 scope with a 1.25x glass path corrector would be F9.38. Is that close enough, or does it truly have to be F10 on the nose?

Also, how about when things are F11? F12? F13? Does it matter with the F10 collimating lenses? Or does it effectively just become a field stop? As in, if we're longer than needed, is there any negative effect at the collimating lens and etalon, or is it just amping up image scale and nothing more?

Very best,



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Re: How close to F10 must a PST mod truly be?

Post by Merlin66 » Fri Jan 10, 2020 9:52 pm

Marty,
The from collimating lens on the PST etalon assembly is designed to work in an f10 beam. If the incoming beam is faster than f10, say f8 (80/640) then the etalon will stop the beam down to an f10 beam - you end up with an effective 64/640 system.
With systems >f10 the diameter of the collimated beam to the PST etalon is slightly reduced (ie less than 20mm), but this seems to have minimal impact on performance.
What is more important, is positioning the PST etalon assembly at the majic -200mm inside the prime focus. This is the optimum (and design ) position. If you move far from this position the beam to the etalon is no longer fully collimated, and performance can suffer.


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Re: How close to F10 must a PST mod truly be?

Post by AndiesHandyHandies » Sat Jan 11, 2020 10:07 am

Hi

Because of the -200mm backfocus requirement for the collimating lens the barlow can be giving more magnification because it is further in than the designed back foucus to give the stated magnification.

So you will get some vignetting.

I have been using the element out of a ED Barlow to try to get focus without cutting tubes.

AstroRod did some experiements and moving the barlow further into the focusser gives much higher magnifications.

Makes a 2x Barlow into a 4X.

Andrew



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Re: How close to F10 must a PST mod truly be?

Post by Merlin66 » Sat Jan 11, 2020 11:10 am

Andrew,
You can easily measure the focal length of a Barlow or any negative lens...
Measure the clear aperture, draw a circle twice this size on a piece of card, point the lens towards the sun with the card behind, when the image fills the circle, the spacing is the focal length of the lens.


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Re: How close to F10 must a PST mod truly be?

Post by MalVeauX » Sat Jan 11, 2020 4:19 pm

Thanks Ken,

I currently have my collimating lens forward with adapters so I can insert it into virtually any refractor to get it to the -200mm point and it works great on all my refractors and most of them can be masked to hit F10 or F11 no problem. It has been doing great.

Here's what came out of my single PST etalon with a 52mm F11 masked ED80 refractor (its a mosaic) and an ASI290MM (2.9um pixel):

Image

I'm thinking of using the full 80mm aperture of my ED80 to do the same thing, but at a larger scale, but it's F7.5 so I would need to get it closer to F10, and was thinking of the 1.25x Baader GPC to get it close at F9.4 rounded up a bit. But wasn't sure if the etalon would perform differently with that slight difference or not. I would just try it of course but I have to buy the 1.25x GPC and don't have that yet, so was questioning before shelling out for one.

I have other scopes I could do this with, but I don't want to use a 102mm or 120mm at F10 for this because the scale is too fine and would require too many panels to do a mosaic that size with variable seeing. I think 80mm F10 would be enough to get a bit more resolution over my 52mm setup, and still have a reasonable amount of panels to perform a mosaic with and it not be impossible. So far its been reliable to get a mosaic done with my 52mm F11 and this PST etalon. Just looking to see if I can do it with my 80mm F7.5 with a 1.25x GPC too.

Either that or maybe I'll just hunt down a ~800mm achromat with a 2" focuser and mask it to 80mm. I just would like to not buy another scope of course, just for this, it's more of an experiment really.

Very best,



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Re: How close to F10 must a PST mod truly be?

Post by rsfoto » Sat Jan 11, 2020 4:35 pm

Hi Marty,

Great image. Especially that Mushroom prominence at 5 o'clock


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Re: How close to F10 must a PST mod truly be?

Post by AndiesHandyHandies » Sun Jan 12, 2020 9:15 am

Hi Merlin

Should the circle be the Barlow aperture x the magnification factor, 2x for 2x barlow, 1.25x for 1.25 barlow?

The ED barlow looks like 100mm as it gives 4x 200mm plus a bit before focus.

Andrew



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Re: How close to F10 must a PST mod truly be?

Post by bart1805 » Sun Jan 12, 2020 9:30 am

When a telescope is f/8 you place the glasspath (mounted right before the etalon) at minus 160mm. More about this in this thread:
viewtopic.php?f=9&t=26808



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Re: How close to F10 must a PST mod truly be?

Post by christian viladrich » Sun Jan 12, 2020 10:04 am

Ken said it all :-)
It does not matter whether you have to add some Barlow lens (or equivalent) to reach F/10. The important thing is to have an incoming F/10 beam at the entrance of the PST collimating lens.


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Re: How close to F10 must a PST mod truly be?

Post by Merlin66 » Sun Jan 12, 2020 10:07 am

Andrew,
Nothing to do with the quoted magnification, just the clear aperture of the negative lens.
It will give you the focal length, you can use this to calculate the magnification from the thin lens formula.


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Re: How close to F10 must a PST mod truly be?

Post by AndiesHandyHandies » Mon Jan 13, 2020 8:04 pm

Hi

Oh I see. You place the etalon at the position for the F8 lens so 160mm and by placing the 1.25GP right in front of the etalon it changes the F8 beam to F10 without vignetting the objective. The 1.25x and the Collimating lens are then -160mm in effect.

If you want less glass replace the -200 collimating lens with a -160mm lens for an F8. For and F5 -100 and so on.

Edmund and Sigma make suitable lenses and I have a thread for them.

Andrew.



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Re: How close to F10 must a PST mod truly be?

Post by bart1805 » Mon Jan 13, 2020 8:55 pm

Yes Andy, that is how it works. Here is an example of a result with a 150mm f/8 Skywatcher, mounted in the nosepiece of the etalon was the glass path.
viewtopic.php?f=4&t=25937&p=233583#p233583
Idea comes from banjo, he produces a steady stream of beautiful images with (I believe) the same set up.



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Re: How close to F10 must a PST mod truly be?

Post by MalVeauX » Thu Jan 16, 2020 2:26 pm

So to ensure I understand, if I use my ED80 with a 1.25x corrector, and it's delivering a F9.4 beam to the lens & etalon, the etalon will stop down the beam, which would be 75.2 at F10, so essentially it would work fine, but it would operate as a 75mm aperture scope at F10. If this is accurate, that's great, I'm totally ok with a 5mm difference in aperture.

Thanks for any confirmation or correction!

Very best,



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Re: How close to F10 must a PST mod truly be?

Post by MalVeauX » Thu Jan 16, 2020 8:09 pm

New question:

I have the Baader 1.25x GPC. How the heck does everyone mount this thing?

Very best,



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Re: How close to F10 must a PST mod truly be?

Post by Merlin66 » Thu Jan 16, 2020 9:37 pm

Marty,
To answer your first question, yes. You end up with a 75mm f10 system.
Can't answer the second question - never tried it.


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