Best donor scope for pst mod?

Frankenscope? Let's see it!***be advised that NOTHING in this forum has been safety tested and you are reading and using these posts at your own peril. blah, blah, blah... dont mess around with your eyesight when it comes to solar astronomy. Use appropriate filtration at all times...
Post Reply
Fenrir
The Sun?
The Sun?
Posts: 10
Joined: Mon Sep 14, 2015 7:32 pm

Best donor scope for pst mod?

Post by Fenrir » Tue Sep 15, 2015 8:53 pm

Hello everyone,

This is my first post on this forum. I've had plans for a long time to try a pst mod,
but I am now seriously starting to look at my options. I have recently acquired a second hand pst and
an 100mm ERF filter.

Now I am looking what the best donor telescope would be. I was considering a 150 mm achromat.
I know there are mods in existence using fast f/5 achromats where the lightcone
is made parallel again with a different front lens in the etalon assembly.

I started by doing some drafting in 3D to check the lightcones and the amount of vignetting this would cause.
I dissasembled my pst and measured everything and made a 3d model of the etalon assembly with the front/rear lenses.
All this was done with keeping the backfocus exactly 200mm behind the rear lens of the etalon assembly.
This was my reasoning ...

This is the pst lightcone:
A total length of 400mm with the etalon assembly right in the middle.
Image

A view of the 100% f/10 lightcone coming in the front lens of the etalon:
No vignetting at all at the 20mm entry lens.
Image

Then I sketched out an 150mm f/5 refractor lightcone:
The lightcone would be corrected by using a -100 mm correcting lens (replacing the pst -200 mm lens)
to get parallel lightrays going through the etalon.
Image

But as you can see a f/5 lightcone is causing very serious vignetting at the etalon itself. The
light hitting the front face of the etalon is a 46.38 mm circle, while the entry lens is only 20mm wide.
So only about 18% of the front objective goes into etalon. Which seems like a waste of having such a large objective!
Of the 150mm objective only 64mm is used.
Image

The next popular 150mm achromat is an f/8 (fl 1200mm). The problem here is, there is not a correcting frontlens available to do the same
trick as for an f/5 scope. I could stop down to f/8.5 then I can use a -170 lens.

But I was thinking to add a 120mm front aperture baffle to bring the scope to an f/10 system.
Then no changes would be needed on the pst etalon + lenses. It would be an upscaled pst objective train.
Image.
The thin blue line shows the stopped down objective.
Image

It seems a contradiction that a 120mm f/10 could be better than a 150mm f/5 (with modified etalon lens), does this make sense?


Thanks for any advice!

User avatar
Merlin66
Librarian
Librarian
Posts: 3169
Joined: Tue Dec 13, 2011 1:23 pm
Location: St Leonards, Australia
Contact:

Re: Best donor scope for pst mod?

Post by Merlin66 » Tue Sep 15, 2015 10:51 pm

Fenrir,
Your thought process is correct!
The etalon assembly acts as an intermediate aperture stop in systems <f10. Theres no need for the objective baffle.
Using the PST etalon on other systems requires either telecentric lenses (to provide f10 or greater) or a change to the front Barlow element.
HTH
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.yahoo.com/neo/groups/ast ... scopy/info
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer

Sungazer
Ohhhhhh My!
Ohhhhhh My!
Posts: 50
Joined: Wed Apr 02, 2014 1:59 pm
Location: N.Gloucestershire

Re: Best donor scope for pst mod?

Post by Sungazer » Wed Sep 16, 2015 1:44 pm

Hi Fenrir, I have had this problem but was able to find a -98mm lens, which can be made to fit into the front of a PST etalon. These were, and most likely , still are available from Surplus Shed, in the U.S.A. at a very modest cost. Also -129 mm ,which seem to have less distortion than the -98 mm . This lens I found avoids shortening the scope tube, as most modern refractors have a fairly long focus tubes ,in the2" units.
Have a look and see if they are still for sale, I am sure an Email will bring a reply. Good luck with the project. Kind regards, John.

User avatar
marktownley
Librarian
Librarian
Posts: 24536
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Contact:

Re: Best donor scope for pst mod?

Post by marktownley » Wed Sep 16, 2015 3:31 pm

I bought a variety of negative lenses from Edmunds Optics if this helps.
Image
http://brierleyhillsolar.blogspot.co.uk/
Solar images, a collection of all the most up to date live solar data on the web, imaging & processing tutorials - please take a look!

Fenrir
The Sun?
The Sun?
Posts: 10
Joined: Mon Sep 14, 2015 7:32 pm

Re: Best donor scope for pst mod?

Post by Fenrir » Wed Sep 16, 2015 6:46 pm

Thanks for the suggestions, much appreciated!

A question to "Merlin66"

So if I understand you correctly, the vignetting at the etalon front lens in combination with the 150mm f/8 (fl 1200mm) scope will
turn it into an f/10 automatically? That's why no front baffle of 120mm would be needed and the full aperture can be used ?

Going back to the 150mm f/5 scope:
Could you also replace the rear lens to reduce the backfocus of 200 mm? You would be able to grab a lot more light from the steep lightcone if you could bring it down to 100mm. Or would this degrade the view too much?
That could help improve the 150mm f/5 alternative a great deal, the used aperture would rise from 63 to 112mm.


Image
Image



Neo: Very impressive work you have there! Indrukwekkend!


Thanks!

User avatar
Merlin66
Librarian
Librarian
Posts: 3169
Joined: Tue Dec 13, 2011 1:23 pm
Location: St Leonards, Australia
Contact:

Re: Best donor scope for pst mod?

Post by Merlin66 » Wed Sep 16, 2015 10:11 pm

Fenrir,
Yes that is correct.
The critical component is the front negative element which must be positioned at it's focal length in front of the donor telescope prime focus. (That's where the majic -200mm spacing comes from with the original PST etalon assembly.)This then provides the parallel collimated beam to the etalon.
The rear refocusing lens is independent. It has no impact on the positioning of the etalon assembly in the donor scope. Almost any suitable lens could be used - the 200mm chosen by Coronado was to provide a final effective focal length of 400mm to allow full disk viewing with a 5mm blocking filter.
Hope this helps.
Ken
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.yahoo.com/neo/groups/ast ... scopy/info
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer

Solar-Cologne
Ohhhhhh My!
Ohhhhhh My!
Posts: 57
Joined: Tue Nov 10, 2015 8:57 am
Location: Cologne-Public-Observatory / Germany

Re: Best donor scope for pst mod?

Post by Solar-Cologne » Sun Nov 15, 2015 2:08 pm

Hello Fenrir,

Is so easy.
I f you want to use the original PST-Etalon and the BF5 you need a 100mm f10 FH-Rafractor not an ED or APO !
An FH is cheaper and most not corrected of the green line, they are better in the red line and this is much better for visible h-alpha.
Most APOs and EDs are visible bad in h-alpha, because they are corrected in the green line !
If you want to tuneup your FH-Lens, you need other spacers between the two lenses and then you correct it to the red line.....wonderful for h-alpha.

You have an 100mm ERF, i prefer Front-ERF Mods.
If you want to build the ERF inside the Scope you can use a Refractor with larger Aperture.
This Mod is for Photographs ok but not so good for visible watching.

sunny Greetings :D
Achim

Post Reply

Who is online

Users browsing this forum: No registered users and 1 guest