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Easy PST Module with ERF (non-destructive, no cutting of OTA, & no-black-box rebuild)

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Easy PST Module with ERF (non-destructive, no cutting of OTA, & no-black-box rebuild)

Post by MalVeauX » Fri Sep 06, 2019 5:07 pm

Hey all,

++++++++++++++++++
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Update, jump straight to the final module & results:
viewtopic.php?f=9&t=26560&start=25#p239784

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I've been looking at my PST again and thinking of doing two things. One, just keep the etalon and adapters and use it as a stage 2 PST mod and double-stack option with other etalons as I currently have it. Two, I'm considering keeping it intact but I want to replace the black box. I really dislike the black box and the prism and all that. It's too much to fiddle with and keep aligned right and proper. I'd rather get rid of the black box completely and instead put a different kind of focuser and do a straight-through design using the existing blocking filter. I have an adapter on the way to step the threading of the eyepiece holder of the PST to a standard threading that can work with other eyepiece stuff and 1.25" gear. My main goal is to get rid of the sloppy focuser and prism. I've seen people do this with an SCT focuser and an adapter of some sort. I'm also considering doing it with a good helical focuser if it can be simpler to adapt (again the goal is straight through, no diagonals, even the blocking filter will be straight through).

Granted, I can already do this. I have my adapters and can insert my PST etalon into another scope's existing focuser with 8" of extension with the collimating lens in an F10 scope (such as ST80 stopped down to 40mm) so that I don't have to cut a tube and simply insert that 8" extension with colimating lens into the scope's focuser and be set. Then I have a good focuser and I can adapt the blocking filter no problem with the step down threading in a straight through design. But, this makes it impossible to double-stack the system with a SM40II module at any point since without a costly adapter that wouldn't work on "all" scopes either, which I want to avoid (being stuck with one possible way to use it, I like things modular). That is, of course, unless two PST etalons can be double stacked successfully. But that seems harder to do without tilt mechanisms. So I probably will not pursue double-stacking it again unless I keep the gold tube and stock lens for that purpose. I've double stacked it in the past with a SM40II and liked it visually but imaging wise I had issues with it at the time.

So again I'm looking at two potential projects and directions.

A) 8" of 2" extensions with collimating lenses with PST adapters so that it can be inserted into any scope without cutting the scope, and simply adapt my blocking filter in a straight through with the step down threading I have coming, making it a hybrid PST stage 1 mod. No more black box at all. But uses the stock blocking filter. This is the cheapest thing I can do with it right now. ERF will be a Baader Red CCD imaging filter (2").

B) Or, find a way to replace the black box with a different kind of focuser, keep the golden rod and stock lens and straight through adapt the blocking filter. The question becoming, what to use for the focuser.... SCT crayford adapted or maybe a helical (a good one though), whatever is easier to adapt. I would love to avoid more custom adapters since I already have the PST adapters for front/rear that convert the etalon to work with any 2" typical gear if that helps.

Thoughts?

Very best,
Last edited by MalVeauX on Wed Sep 18, 2019 2:35 pm, edited 3 times in total.

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Re: PST, how to replace that black box?

Post by MalVeauX » Fri Sep 06, 2019 5:39 pm

While figuring ideas... I have a M36 to 2" adapter coming to convert a helical focuser to be able to simply insert into standard 2" gear. I will try to do a straight through design and the adapter blocking filter will simply insert into the helical focuser and I'll just measure out the needed spacing from the rear collimating lens to where focus would be achieved without a diagonal/prism and go from there. This would allow typical use and still allow double stacking with the SM40II.

And of course, I will do the obligatory Quark test with it. I have double stacked my Quark with the PST etalon (without its collimating lenses) with ok results, but not great results. Once I have the focuser, I will see what it's like if I put the Quark inline with 2" gear, operating at F42, without the PST blocking filter, with the Quark in 2" extensions behind the PST etalon and the helical focuser on the camera side after the Quark. This may not work. I'm not sure. I will use a 0.5x focal reducer after the Quark (F21). If it does work, I will post results.

Very best,

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Re: PST, how to replace that black box?

Post by Merlin66 » Fri Sep 06, 2019 11:10 pm

The removal of the PST black box - is a Stage 0.5 mod.
The back focus from the rear lens of the PST etalon assembly is 200mm.
You 're looking at a Stage 2 rear end. I have a M50 to T thread adaptor some T thread spacers then a Borg Helical focuser. I use a BF10, but you can use the original PST blocker in a suitable adaptor.
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Re: PST, how to replace that black box?

Post by MalVeauX » Fri Sep 06, 2019 11:31 pm

Interesting thanks! I'll measure things and see what all options happen at 200mm.

Very best,

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Re: PST, how to replace that black box?

Post by MalVeauX » Sat Sep 07, 2019 2:09 am

Super cute! That hardware looks familiar! Great way to start!

My understanding is that the piece that secures with the front collimating lens can thread into a 2" M48 extension, so 8" of those should put it into position without having to cut a tube. Right?

Very best,

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Re: PST, how to replace that black box?

Post by Merlin66 » Sat Sep 07, 2019 3:53 am

BP,
The closer you achieve the majic -200mm the better the performance. Don’t compromise.
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Re: PST, how to replace that black box?

Post by MalVeauX » Sat Sep 07, 2019 1:54 pm

I just tested it and the inside of the PST etalon housing that faces the incoming light beam threads out with the collimating lens still threaded into its center. That threading is the same as 2" standard filter threads (M48 I think?). So 2" eyepiece extensions have this threading and 8" (200mm) of that can be threaded into place and then the plate with the collimating lens can thread down into the barrel. If I extend the extensions a little bit more, I can thread a 2" ERF (Baader Red CCD filter) in front of that so it's ready to go in any refractor upwards of 150mm or so and deep into the light cone. The F10 beam will be the same from the collimating lens whether its up close or 8 inches away I think? This simply inserts into the focuser and stops at the etalon itself. Post etalon will be the PST adapter with 2" entry. I need 200mm (8") out focus from it, at this point I can slide in a Quark with 2" skirt and go from there. Or, leave out that, and just have 2" extensions that end in 1.25" with a helical focuser and the stock PST blocking filter adapted to straight through on the helical focuser and it can be used as a standalone straight through instrument by putting the golden tube back on with its lens, or it can be inserted into another OTA without the golden rod. Two potential ways to use it. I'll try it all out. Some of it might work. Might not. I have two PST etalons to fiddle with, one good working one, and one bad one (decontacted I think?).

Any of that make sense or anything off?

Very best,

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Re: PST, how to replace that black box?

Post by bart1805 » Sat Sep 07, 2019 2:06 pm

Don't understand completely what you are saying. In a f/10 telescope the front lens should be placed at minus 200mm from the focal point. It is true that you can mount the etalon (without the front lens) further back. The distance from the backlens of the etalon to the focal point is about 200mm.

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Re: PST, how to replace that black box?

Post by MalVeauX » Sat Sep 07, 2019 2:44 pm

bart1805 wrote:
Sat Sep 07, 2019 2:06 pm
Don't understand completely what you are saying. In a f/10 telescope the front lens should be placed at minus 200mm from the focal point. It is true that you can mount the etalon (without the front lens) further back. The distance from the backlens of the etalon to the focal point is about 200mm.
Trying to make it so that I don't have to cut any refractor tubes. So my understanding is that I can extend the collimating lens in front of the etalon by extending it out 200mm since all the threading is M48 and 2" this is common stuff and doesn't need custom work. Then at the back end its business as usual as a stage 2 but with the stock blocking filter instead of buying a new expensive one.

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Re: PST, how to replace that black box?

Post by bart1805 » Sat Sep 07, 2019 2:59 pm

Ehm. The threading of the etalon is not m48. The front part of the front AOK adapter is M48. You could indeed try to install the front lens of the etalon in the nosepiece of the AOK adapter. If the telescope you are talking about has not enough back focus.

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Re: PST, how to replace that black box?

Post by MalVeauX » Sat Sep 07, 2019 3:02 pm

bart1805 wrote:
Sat Sep 07, 2019 2:59 pm
Ehm. The threading of the etalon is not m48. The front part of the front AOK adapter is M48. You could indeed try to install the front lens of the etalon in the nosepiece of the AOK adapter. If the telescope you are talking about has not enough back focus.
Maybe I have it wrong. The internal disc that threads into the PST housing with the first collimating lens threaded into its center, it comes out, and literally threads into the end of any typical 2" accessory I have just like a standard 2" filter does. That's not M48? I might just have that wrong. Either way, it threads into typical 2" stuff so I can extend it out using standard 2" extensions. I just hope that putting it 200mm forward with extensions allows it to still work appropriately to be able to use it without having to cut tubes.

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Re: PST, how to replace that black box?

Post by bart1805 » Sat Sep 07, 2019 3:44 pm

Hi Marty, can you post a picture what you mean with "internal disc". Thanks!

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Re: PST, how to replace that black box?

Post by MalVeauX » Sat Sep 07, 2019 3:56 pm

bart1805 wrote:
Sat Sep 07, 2019 3:44 pm
Hi Marty, can you post a picture what you mean with "internal disc". Thanks!
Yep, thanks!

Here's the internal disc that threads out from the lower internal threads, this threading happens to also thread into any 2" threading like a standard 2" filter does. This is not the same threading as the upper threading which is unique to the PST housing where it connects to the gold tube. Moving this disc forward can be done with 2" extensions.
pst_01.jpg
pst_01.jpg (57.56 KiB) Viewed 796 times

Here's the disc threading into a 2" extension tube, not planning to use this, just to show it threads onto typical 2" stuff.
pst_02.jpg
pst_02.jpg (43.1 KiB) Viewed 796 times

I have these 2" spacer rings, that are 2" threaded. I can combine a few to make the total 200mm needed with the collimating lens threaded at the end.

I just connected 2" threads on both sides and the disc and collimating lens are sitting inside a 2" housing threaded together. So I can put another short extension at the end and place a 2" ERF (Baader Red CCD ir block filter).

https://agenaastro.com/blue-fireball-2- ... nsion.html

Very best,

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Re: PST, how to replace that black box?

Post by MalVeauX » Sat Sep 07, 2019 4:22 pm

Hrm,

Here's a couple of those eyepiece extenstions (2") thread into the PST front adapter and the collimating lenses's plate and the colliamting lens threaded into the end. This is only 6 inches long, so I need another 2 inches to make it 8 inches total.

I'm not entirely sure where to measure from to start the 8" (200mm) point. Is it at the original point where the collimating lens was and then go 200mm out?

Also, how to figure in the focuser in this, if applicable in terms of the distance too?

The PST adapter (AOK) nose is about 1.5" long from where the lens would normally sit behind it. So 4x 1.5" spacers would add 6" to this for 7.5" and then I have a 0.5" extension to finish it off. Thread on the plate/collimating lens. Then a gender reversale. And one more 0.5" extension and then thread on a 2" ERF filter to complete the module for small refractor use. Behind the PST housing, the AOK 2" adapter with 2" extensions as long as it takes (8") to terminate to a typical 2" to 1.25" adapter for the helical focuser and then the blocking filter and all that as a straight through. I will also see how much distance a 1.25" diagonal adds, as the eyepiece holder threads directly into a standard old celestron 1.25" star diagonal which could be used for visual purposes. Just waiting on the helical focuser and M36 -> 2" adapter to arrive.

Forgive my obvious ignorance on this, I just haven't found much chatter/documentation on doing it this way versus cutting tubes or using SCT scopes for this purpose. :oops:

The idea is to simply insert this into a focuser and it's 8" forward, rather than cutting a tube.

Think this would work?
pst_03.jpg
pst_03.jpg (41.81 KiB) Viewed 793 times
Very best,
Last edited by MalVeauX on Sat Sep 07, 2019 5:15 pm, edited 1 time in total.

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Re: PST, how to replace that black box?

Post by bart1805 » Sat Sep 07, 2019 5:11 pm

Aha, you have taken the whole piece out, not just the front lens. It goes like this:
Define the original focal point of the f/10 telescope. 200 mm before that point there has to be the front lens of the etalon. Sometimes it means cutting the telescope 4 to 6 cm, because the backfocus is somewhere between 14 to 16 cm. Let's assume it is 16 cm. Instead of cutting the telescope, you can also move just the front lens 4 cm into the telescope. Etalon is placed at minus 16, but that does not matter. There is no need to put the front lens 200mm before the etalon because that would mean your telescope has no backfocus at all. If that is the case: send it back. (-;

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Re: PST, how to replace that black box?

Post by MalVeauX » Sat Sep 07, 2019 5:22 pm

bart1805 wrote:
Sat Sep 07, 2019 5:11 pm
Aha, you have taken the whole piece out, not just the front lens. It goes like this:
Define the original focal point of the f/10 telescope. 200 mm before that point there has to be the front lens of the etalon. Sometimes it means cutting the telescope 4 to 6 cm, because the backfocus is somewhere between 14 to 16 cm. Let's assume it is 16 cm. Instead of cutting the telescope, you can also move just the front lens 4 cm into the telescope. Etalon is placed at minus 16, but that does not matter. There is no need to put the front lens 200mm before the etalon because that would mean your telescope has no backfocus at all. If that is the case: send it back. (-;
Thanks, starting to clear up!

So the front collimating lens needs to be 200mm forward from where ever the focus point is behind the focuser. So for that, I can just point it at the sun and move an object that won't melt back and forth until it's focused. Then from that point, measure 200mm forward into the system with the focuser and all that in place and then look at where the etalon would be sitting in this train. That distance difference is how long the extensions need to be then.

Sound right?

Thanks!

++++++++++++++++++++++

So here's a not so elegant approach. I just quickly pointed a scope that I'd use towards the sun and took everything off and wracked focus tube in as much as it will go. I measured from the closest point of the focuser where something could be, to the focus point behind it. It seemed to be about 5" (120~127mm) from the focus tube to that point. This is a ST120 with a GSO 2" Linear bearing focuser, pretty short. So, knowing my focus point is 120~127mm behind that focuser how does that play into this exactly? Does that mean I only have to add extensions that reach another 75~80mm into the OTA to hit the 200mm mark?

I tested two scopes, both were around 5" and 5.5" behind the focuser achieving focus.
focus_point.jpg
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Re: PST, how to replace that black box?

Post by bart1805 » Sat Sep 07, 2019 5:45 pm

Yep, that is how it works.

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Re: PST, how to replace that black box?

Post by MalVeauX » Sat Sep 07, 2019 5:54 pm

bart1805 wrote:
Sat Sep 07, 2019 5:45 pm
Yep, that is how it works.
Interesting, thanks!

So, if my focus point behind my focuser is 120mm, I need the front collimating lens to be 80mm inside the focuser tube. Now, (A) does my etalon have to be sitting at the focus point and extensions go from there? Or (B) do I place the etalon assembly into the focuser as far as in as it all goes, then put enough extensions to push the collimating lens 80mm deeper into the focuser tube?

Thanks!

Very best,

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Re: PST, how to replace that black box?

Post by bart1805 » Sat Sep 07, 2019 6:34 pm

The front lens has to be minus 200mm, not the etalon. So in this case you position the etalon with the foucusser fully racked in. 80mm extensions before the etalon. With the front lens mounted right there.
About 200mm after the etalon will be the new focus point.

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Re: PST, how to replace that black box?

Post by MalVeauX » Sat Sep 07, 2019 6:37 pm

bart1805 wrote:
Sat Sep 07, 2019 6:34 pm
The front lens has to be minus 200mm, not the etalon. So in this case you position the etalon with the foucusser fully racked in. 80mm extensions before the etalon. With the front lens mounted right there.
About 200mm after the etalon will be the new focus point.
Thanks!

I think I got it then. I'll try to do a more precise measurement of the focus point on the scopes I'll test to get it closer and more accurate. Extensions to bring the front collimating lens -200mm into the OTA from that focus point, unique to each imaging train and I'll just keep some documentation on the scopes so I know what extension mount is needed. Post etalon, I will do +200mm extension, minus a bit to allow for 2" to 1.25" conversion and bits and pieces for visual/imaging.

I'm curious if it's possible to slide my Quark into the post-etalon 2" extensions....

Very best,

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Re: PST, how to replace that black box?

Post by Merlin66 » Sat Sep 07, 2019 11:47 pm

Marty,
Just check the baffles and their position inside the OTA....
I had an issue mounting a 150mm long 2" CaK module I constructed into the refractor due the the rear baffle.....
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Re: PST, how to replace that black box?

Post by MalVeauX » Sun Sep 08, 2019 1:22 am

Merlin66 wrote:
Sat Sep 07, 2019 11:47 pm
Marty,
Just check the baffles and their position inside the OTA....
I had an issue mounting a 150mm long 2" CaK module I constructed into the refractor due the the rear baffle.....
Good point, will check it out to make sure there's not something stopping it!

Very best,

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Re: PST, how to replace that black box?

Post by marktownley » Sun Sep 08, 2019 6:31 am

No reason why it won't work Marty, i've done this.
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Re: PST, how to replace that black box?

Post by MalVeauX » Mon Sep 09, 2019 12:37 am

marktownley wrote:
Sun Sep 08, 2019 6:31 am
No reason why it won't work Marty, i've done this.
I believe I used your disassembly/reassembly of a PST etalon housing that I used as reference to NOT cut an OTA and just explore extensions!

Very best,

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Re: PST, how to replace that black box?

Post by MalVeauX » Mon Sep 09, 2019 4:47 pm

While on the subject of the PST in general, I have two elements. An old one that is reddish colored, where I think it was coated to be the ITF itself. And I have a newer element, that is a blue color, where an actual ITF filter was later installed before the eyepiece holder and blocking filter.

Very best,
Last edited by MalVeauX on Tue Sep 10, 2019 12:46 am, edited 2 times in total.

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