PST swap mod?

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twhite
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PST swap mod?

Post by twhite » Wed Apr 15, 2015 4:04 am

Modders,

I recently picked up an early PST with a rusted objective and ITF but a good etalon, etc. Have an idea I'd like to run past you modding gurus.

I'm wondering if it would be possible and practical to build a setup where I could 'switch' between using the PST essentially 'stock' and what would amount to a Stage 1 mod without a great deal of work.

I understand that the PST optical train with the standard collimating lens is optimized for an f/10 system and requires a specific spacing from etalon to eyepiece. I also understand that the Stage 1 mod doesn't alter this in any way, so that doesn't concern me too much. What I'm thinking, though, is to 'dedicate' another f/10 scope (probably a 4" f/10 refractor of some sort) with a 2" focuser to the purpose. (Note: I think I've seen someone do something like this already, shortening a refractor with an adapter for use as a solar scope then setting it back to standard length to use for non-solar astronomy -- which is an idea I like, but let me just continue to make sure my thinking is right). I would have an adapter machined to thread into the front of the etalon holder (where the gold tube threads in now) to fit into a 2" focuser or visual back (probably better a threaded visual back to replace the gold tube for a dedicated scope so that the PST focuser can be used rather than a focuser on a scope which will change the distance between the objective and the etalon -- again, just for the sake of argument here). And, of course, adding an ERF to the front of the system (permanently mounted for safety -- think custom objective cell with a slot for the filter in front or behind the objective -- not sure it matters if I'm going to set it up as a dedicated system; if it does please correct me.

If I wanted to use the PST version for an ultra-portable setup, I'd put the original gold tube back on; if I wanted more aperture and had the ability to set up and use a larger EQ mount, then I could 'swap' the built tube with the gold tube.

I may be crazy to think this is practical and easy(ish) to implement, but it seems like it would be.

Thoughts? If I'm nuts, don't hesitate to tell me I'm nuts. I'm not in a rush to do this soon, just in the thought stage. The PST was too good a deal to pass up, but I had already been thinking in this direction anyway, and acquiring said PST has now accelerated the thought process.

Thanks all.

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Re: PST swap mod?

Post by swisswalter » Wed Apr 15, 2015 4:36 am

Hi Tony

that is duable. No crazy thing. good luck
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Re: PST swap mod?

Post by marktownley » Wed Apr 15, 2015 5:32 am

All do able :)
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Solar images, a collection of all the most up to date live solar data on the web, imaging & processing tutorials - please take a look!

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Re: PST swap mod?

Post by twhite » Wed Apr 15, 2015 7:36 pm

Very good. I'll start gathering the various pieces and parts and get to work. Will keep things posted here.

Another question -- what negative effects will I see if the spacing from the objective to the etalon is incorrect? I understand that the etalon needs to be 200mm inside prime focus of the optical system -- correct? That's true of all optical trains, correct? I assume that prime focus is, by default, the actual focal length of the objective?

Thanks again,

Tony

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Re: PST swap mod?

Post by Merlin66 » Wed Apr 15, 2015 11:44 pm

Yes, the focal ratio and the distance for the etalon is pretty critical.
If you're working at f10 or greater - OK
The -200mm is mandatory to achieve a collimated beam at the etalon. This gives the best performance. I've found +/-3mm to be about the limit.
I have a TS102 f11, where I've fitted an adaptor ring to the end of the OTA - this ring allows me to quickly change from white light (using a Baader Herchel wedge) to the modded PST set-up.
Works very well.
ken
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Re: PST swap mod?

Post by twhite » Thu Apr 16, 2015 12:43 am

Ken,

Thanks for the comment. If you're using a TS102, did you have to shorten the OTA to make it work? I assume you did.

Still wondering if the -200mm means 200mm shorter than the focal length or 200mm shorter than prime focus? And my understanding of prime focus being that point at which an in-focus image can be projected from an objective without the aid of an eyepiece or any other optics.

Thanks again,

Tony

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Re: PST swap mod?

Post by bandazar » Thu Apr 16, 2015 1:52 am

Suspect it means adding about 200mm to where the telescope focuses. I guess that is why my pst mod is not working correctly at some of my other apertures quite correctly, although works well with my 90mm.

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Re: PST swap mod?

Post by twhite » Thu Apr 16, 2015 2:30 am

Not adding -- subtracting is my understanding. The site I looked at showed the modder removing a 75mm section from his OTA to make the PST mod come in to focus.

This is also why I'm making sure I have clarity with respect to exactly what the requirement is -- if it's shorter than the focal length or prime focus. Those are two different things. I don't know that I'll want to do what Ken has done and make the scope usable for anything other than the PST mod, but I certainly could. My current solar setup is an SM60DS for Ha and a Carton/Parks 60mm f/7 with a Herschel wedge -- nearly identical focal lengths, so similar eyepieces yield similar images. I have also been known to use my bigger refractors (especially my TMB 152/1200) with a solar pentaprism setup, especially at large star parties. My thoughts here are to have a scope in the 4" range that I can side-by-side with that TMB and do Ha on that scale affordably. The PST mod seems to be a reasonable approach to that.

Almost all the refractors I have now are either too high end to think about modifying (AP, TMB, Vixen fluorite and ED), classics (Unitron, RAO, etc.), or too small aperture (<=80mm). I have only one scope now that would be suitable in my mind, and that's an Antares 4" f/15 Elite -- which is too *long*. So I'm seeking an f/10 achromat of some flavor, or the parts to build one.

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Re: PST swap mod?

Post by twhite » Thu Apr 16, 2015 2:39 am

One other thing that came to mind while I was typing the above.

I realize that most of the stage 2 modders use a BF15 or larger diagonal, which I assume is due to the focal length of the optical train involved.

The PST I got is a fairly early pre-Meade scope, using what amounts to a BF5 at the eyepiece holder. I've ordered a replacement ITF from Maier which is significantly larger diameter than the little ITF the PST was equipped with originally. I've already done a refit of my SM60DS's ITF with great results and think this will be the same, but I know that at larger diameters the exit diameter of the PST's stock BF will be inadequate.

There's no reason why I can't just drill out the holes in the eyepiece holder before installing the new (larger diameter) ITF and continue using it on either the PST or the larger optical tube, right? I can't think of one, based on what I've seen tearing into my BF10, but I haven't torn into all the various parts of the PST so I don't know whether or not it will be an issue there. I do want to be able to get a full-disk image at lower powers if possible.

Also, won't the etalon holder vignette the light cone significantly if I go 'too' large? I haven't measured or drawn out the light cone, but I have to think that there is definitely a point of diminishing return with this mod. Comments?

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Re: PST swap mod?

Post by Merlin66 » Thu Apr 16, 2015 4:51 am

The etalon nosepiece if 2" fitment will not vignette in a f10 beam....
The limiting factor in the original PST blocking filter is not the "mini-erf"/ ITF filter element.
(BTW The Maier alternative still fits the PST) but the final blocking filter. This has an aperture of 5mm.
IMHO it's much better to dump the Black Box and pentaprism etc. and go for a Stage 2 mod using a larger BF....
EDIT:
Re the earlier questions:
The -200mm is measured from the prime focus position of the objective. If you have say a 100mm f10, the distance to the focus will be 1000mm. The etalon should then be placed at 800mm from the objective (1000-200)
Focal ratio's above f10 work well - no issues
On the TS102 I was lucky... with the Baader steeltrack focuser removed from the OTA it only needed +10mm spacer to drop the etalon exactly 200mm inside the original prime focus position.
Last edited by Merlin66 on Thu Apr 16, 2015 4:57 am, edited 2 times in total.
Reason: added info
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
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"Astronomical Spectroscopy for Amateurs" and
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Re: PST swap mod?

Post by twhite » Thu Apr 16, 2015 5:09 am

Ken,

So I was under the impression that the final blocking filter in the PST *was* the ITF, just like it is in the BFx diagonals. Am I wrong about that? I thought I had seen something documented about folks who had taken the ITF/holder part of a PST and swapped it with one from a larger BF and had that work. Am I delusional? (Quite possibly.)

I can use my BF10 for a stage 2 mod, at least temporarily, should I decide to go that route, though.

Thanks for the other information. I think I have everything I need now. Just need to find a scope an a full-aperture ERF (either the Baader D-ERF or the Lunt 100mm ERF, probably). I've identified a few possible scopes or objectives that I can build around. Now the fun begins. :-)

Again, thanks.

Tony

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Re: PST swap mod?

Post by swisswalter » Thu Apr 16, 2015 5:19 am

Hi Tony


good luck and have fun
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Re: PST swap mod?

Post by Merlin66 » Thu Apr 16, 2015 5:20 am

Tony,
The newer PST's (with the Blue objective) have a blocking filter assembly identical to the Coronado BF.
There's an upfront ITF followed by the blocking filter/ trimming element.
On the ol' original PST the ERF coating (Gold coloured) on the front of the objective acted as an "ITF" and hence the eyepiece holder did NOT have the added ITF, this then just leaves the blocking element (5mm) close to the eyepiece.
Hopes this helps.
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Re: PST swap mod?

Post by twhite » Thu Apr 16, 2015 5:56 am

(Heavily edited post)

Ken,

I've gone back and done more research and we're on the same page. You're right in that the blocking filter (the trimmer filter) can't be modded except by a larger filter, and that the original PST like mine had coatings that mimic the function of the ITF. I was confused.

But, that still raises the question: If there is no ITF behind the etalon in the early PST, how can it be modded for use with another scope with only an ERF in front? The obvious answer is that it can't -- there has to be another filter in the train -- something like the ITF or another narrowband blocker, like the 35nm Baader narrowband, or even the Maier unit installed in the eyepiece holder as they are with the newer blue-lensed PSTs, right? I would just need to come up with a way to mount it in there, but a simple holder should be fairly easy to have machined and installed.

And I can (temporarily, at least) use the trimmer filter and holder from my BF10 initially, or at least until I decide to acquire a BF15 or do a full-blown Stage 2 mod.

So I think now that getting the answer with respect to the 'second' filter -- whether it be a narrowband blocker on the nosepiece or the Maier unit installed in section in front of the eyepiece holder, as Meade appears to be doing -- should give me all the information I need to proceed.

Again, thanks for your patience with me. I don't know how I jumped so far overboard on this, but obviously I did. Just needed a good sorting out.

Tony

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Re: PST swap mod?

Post by Merlin66 » Thu Apr 16, 2015 9:00 am

Tony,
If you mod the PST using a Baader D-ERF or another ERF which give good IR blocking then the ITF in the eyepiece holder is not required.....
I have modded dozens of PST like this with no issues.
"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

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Re: PST swap mod?

Post by Moriniboy » Tue May 19, 2015 8:12 pm

You can always convert your BF5 into a BF6 with the careful use of a pillar drill. Need to make sure it is 100% blocked around the edges though as the BF is 6mm square so the tolerance is non existent.
Attachments
BF5 uncovered.jpg
BF 6.0mm.JPG
Hoping for clear skies.....lol

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Re: PST swap mod?

Post by swisswalter » Wed May 20, 2015 4:16 am

Hi Nigel

that is a precision job, well done
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 :)

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