PST Mods - getting sufficient in-focus

[astroshot]

I'm kinda thinking again of trying out the PST mod.
I have a TEC140. However, I know that the PST won't come to focus with it.
Needless to say, I wont be cutting the OTA! ;-)

Can I use a 2x Powermate in front of the PST etalon/lens set?

I already have a 4x Powermate.
Is it possible to pop off the lens directly in front of the PST etalon and operate it at f/28?

Thanks,

Michael.

[RobBower]

Have you watched the video from Mark Townley? He does mention moving the front lense of the pst etalon assembly inside the tub by extending it from the housing. Virtually making it like a barlow. This way you dont need to cut off that beautiful 140!

[marktownley]

I would have thought it will come to focus, it will just vignette it to an effective aperture of 98mm.

You could replace the negative lens at the front of the etalon assembly for one with -140mm fl to operate at the full 140mm aperture of the TEC

[Allcart]

Or you could use the method that Colmic uses and add a Glasspath lens in front of the etalon. I don't know which Glasspath lens you would need, but I bet Colmic could advise you.

[colmic]

Yes, you have a TEC 140, with 980mm FL no? So F/D=7.
You can buy a 1,25x or a 1,7x Baader Glasspath (http://www.baader-planetarium.de/sektio ... 08.htm#+04)

With 1,25x you goes to F/8,75
With 1,7x you goes to F/12

And the glasspath are telecentric, and have a fixe negative FL. (-20mm for the 1,25x and -35mm for the 1,7x)

No need to remove the inside lens of Etalon, i only add the Glasspath in front of.

For the 140, you have 2 choices of ERF's (the best ERF is the Baader one with lambda/10 optic):
- Baader D-ERF 135mm
- Baader D-ERF 160mm (more expensive)

If you use the 135 ERF + adapter you'll get 130mm diameter. So the F/D will goes to 7,53. With the 1,25x Glasspath, you'll almost reach the F/10 needed by the etalon.

If you need to keep all the 140mm of the refractor, you'll better to buy the 160mm ERF and then use the 1,7x Glasspath.

[marktownley]

And the glasspath are telecentric, and have a fixe negative FL. (-20mm for the 1,25x and -35mm for the 1,7x)

Now this I dis not know. I need to do some research here

[astroshot]

Thanks for the replies.

I already have a Baader 135mm DERF in a cell for the front of the scope,

Michel - do you use the badder lens?
Looks quite good value for money.

The native focal length of the scope is 980mm.
I would like to be imaging at 2x to 4x this focal length.
Is there any benefit in using a 2x or 4x powermate in front of the PST etalon and removing the front lens?
It would keep the glass down to the minimum.
I.e, powermate -> etalon -> lens -> camera
Otherwise, I will need this:
Barlow -> lens -> etalon -> lens -> barlow -> camera

Thanks.

[colmic]

The real question is: what quality do you need?

You have a TEC140 which, IMHO, is one of the best refractor of the world (as my Tak !).

So what is the quality of these cheaper lenses?

I use since 7 years some Glasspaths on all the refractors I used:
- Tak Sky90 + ERF 70 + GP1.7x
- Tak FS60 + ERF70 + GP1.7x
- Tak TOA130 + ERF135 + GP1.25x
With best results

I never removed the etalon's lenses. Only added a Glasspath in front.

NOTE: I always put a glasspath with the goal to use the etalon at F/10. If you need a bigger sampling, add a Barlow AFTER the BF, not BEFORE.

Baader ERF's are lambda/10, Baader Glasspath's are lambda/10.

Here is 2 images from my Tak FS60 mod: I use the 1.7x Glasspath in front of the etalon. I use this FS60 only in visual with bino, and I have a full disc entirely in halpha, what a beautiful image with bino! ERF is inside the Dewcap (old C-ERF70 Baader)




And now the mod on my Tak TOA130, I just replace the 1.7x Glasspath by the 1.25x and I use the Baader D-ERF135:



Results with this setup are wonderful: Baader D-ERF135 + TOA130 + Baader Glasspath 1.25x + PST etalon + BF15 + Barlow 1.8x + PGR 2.8Mpixels camera

[astroshot]

Thanks Michel.
Much appreciated.

[astroshot]

Michel,
Just to confirm, does the Baader glass path screw directly onto 2" filter threads.
Thanks.

[colmic]

Glasspaths have a M34 and a nylon ring which can be inserted onto T2 accessories, but you can buy the 2"/T2 adapter.

Take a look at all the Baader accessories here: http://www.baader-planetarium.de/sektio ... tm#tabelle

[astroshot]

Thanks Michel.
The Baader adapter thread system has always confused me.

[swisswalter]

Hi Michel


there is another solution to your TEC/PST mod. Use a 1,6 TAK extender then the PST mod, works like charm

[colmic]

Yes it works. This is the first mod i used with my old Sky90: extender Q-1.6x Takahashi. It worked well.

[astroshot]

Thank you Walter and Michel.

In terms of the stage II mod, are there any safety issues as to whether an old-type or new-type PST is used?
I would presume not - I will be using a Coronado BF.

[marktownley]

Thank you Walter and Michel.

In terms of the stage II mod, are there any safety issues as to whether an old-type or new-type PST is used?
I would presume not - I will be using a Coronado BF.

Nope

[astroshot]

Glasspaths have a M34 and a nylon ring which can be inserted onto T2 accessories, but you can buy the 2"/T2 adapter.

Take a look at all the Baader accessories here: http://www.baader-planetarium.de/sektio ... tm#tabelle

Perhaps I am over-analysing it and apologies if I am...getting confused now.

I bought the front and back PST adapters from AOK in Switzerland. Arrived today.
With regard to the one on the objective -ide of the PST etalon, it has 48mm female threads.
If the glasspath is M34 and needs to attach to a t-thread (42mm), then how does that fit into 48mm threads?

[astroshot]

OK, I think i have found the adapter I need

https://www.teleskop-express.de/shop/pr ... apter.html

That look correct?

[colmic]

Sorry for the late answer.

I have this adapter 2" to T2, and the M48 thread is in front, not in back. In the back you can insert the Glasspath, but it will not work with your PST adapter.

Can you show me some photos of your adapters?

[astroshot]

I emailed Baader to see if they could offer advice.
They are saying it is not possible, even with an adapter.
I will have to look at an alternative solution.

[swisswalter]

Hi Michael

what exactly do you wish from baader ?

[astroshot]

I have a 2" adapter for a PST with 48mm female threads.
I was looking into buying a glasspath.
I thought I could use a t-2 to 48mm adapter.
However it appears this won't work.
This is the response from Baader:

sure we do have T-2/M48 Adapter but this may not help you as you need to clamp the Glaspath in the T-2 Thread.
And this converts it only. We have not "adapter" special to adapt the GPC into a 2" Extension

[swisswalter]

Hi Michael

thank you very much for the fast answer. I'll look into it with my GPC 1,7x from the bino I have. So the Setup is as following if I understand it correkt

lens, tube, 2", GPC, T2, Ha filter etc. .... o.k. ?

[astroshot]

The 2" adapter which I bought from AOK Swiss screws into the PST etalon.
This adapter allows attaching the PST into a 2" focused.
The adapter has threads on the objective end on the inside (female).
These threads are 48mm in diameter.
Apparently, the glasspath is intended for t-2.
I will post a pic.

[astroshot]

The end of the adapter on the table screws into the PST.
The side nearest the camera fits into the telescope focuser.
This is where the 48mm female threads are located.

[swisswalter]

Hi Michael

as soon as we have daylight again here I can check a possible solution

[peter drew]

Michael. Are you sure that the 48mm female threads are "active" or just an antireflection detail?

[swisswalter]

Hi Michael

here are the pics

assembled





disassembled





Note the front lens of the PST is now up front the 2" Extension


but it works even easier the other way round

the overview






the GPC fits very nicely in ;-)





As soon as we get some sunshine here again (may take some months or so ) I'll shot AR's with that configuration and different scopes

[astroshot]

Thanks for taking the time to post the pics Walter.
So, the GPC fits just in front of the etalon.
I see it comes with a white (nylon?) retainer. My understanding is that this retainer is designed for t-2.
The PST adapter which fits over the GPC, is this the AOK Swiss version?
What stops the glasspath falling through?
As I understand the GPC doesn't gave any threads of itself.
Many thanks,
Michael

[swisswalter]

Thanks for taking the time to post the pics Walter.
So, the GPC fits just in front of the etalon.
I see it comes with a white (nylon?) retainer. My understanding is that this retainer is designed for t-2.
The PST adapter which fits over the GPC, is this the AOK Swiss version?
What stops the glasspath falling through?
As I understand the GPC doesn't gave any threads of itself.
Many thanks,
Michael

Hi Michael

yes that is the aok swiss pst Adapter

The GPC is bigger in diameter and therefore holds just by squeesing inbetween the PST and the Adapter

Yes the GPC has no threads at all

[astroshot]

Thanks Walter.
Ill order it so.
Once I have that, I just need to find good PST.

[swisswalter]

Hi Michael

A PST is for sale at astromart

[astroshot]

Two problems:
1. Shipping costs from the US to Europe
2. Taxes
Might be better getting a s/h one locally which I can test??

[peter drew]

A better plan Michael. The PST's vary from OK to excellent, you need a good one if it is to be modified for the best results.

[swisswalter]

Hi Michael


you are so right. Locally is much better, but availability

[astroshot]

Availability and the weather to test it are the problems I have to deal with alright.