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Re: Two simultaneous pst etalon

Posted: Thu Mar 12, 2020 6:33 am
by marktownley
Nice unit. RG630 is an absorptive filter, it will absorb heat used behind the objective and could well crack. You might want to consider using a dielectric filter or using a uv/ir cut filter ahead of the rg630.

Re: Two simultaneous pst etalon

Posted: Thu Mar 12, 2020 10:05 am
by AndiesHandyHandies
Yes a dielectric 35nm Baader filter is the best to have first.
For visual I use a Beloptik KG3 with IR and UV coatings on the eyepiece for safety and catch the long IR the Baader passes. Could use it in front of a CCD as well. I think removing the long IR helps visually for me, 2" BeloptiK on front of PST visual, and Mark T says for imaging.

On Maksutov telescopes I used a modded PST on I came up with the idea of putting the 35nm Baader after the moved forward PST Collimating lens needed to reach focus. Worked for two years on a Vixen VMC110 and seems to work on my Santel 180mm Maksutov, its got a Pyrex secondary I discovered. The Baader sees collimated light and then the reflected light is spread back out again by the Collimting leans as it came in so no hot spot on the secondary or outside of the telescope so safe for casual users.
I use a Omega 40nm ERF in place of the removed Collimating lens as a dust blocker and backup ERF. As its a USA filter its 25mm like the lens. Have to get round to replacing my moved re-focussing lens, for double stacking or other larger blocker, with a KG1 from previous experiments as a dust cover.

AstroRod changed configuration on his Omega filter set up, changed barlow, and cracked a red filter BUT can still use it as its well before focus so the hairline cracks do not matter. Looks like a red filter on or between a clear glass. So put a Baader 35nm before the existing red filter and you should be safe enough. You need to position the Baader to be fully illuminated as possible ideally but take care where a reflected focus is in terms of being visible outside of the telescope or near to a lens or mirror.


Re: Two simultaneous pst etalon

Posted: Fri Mar 13, 2020 9:54 am
by AndiesHandyHandies

Thanks for the advice.

AstroRod had the filter crack and he did not notice for a bit. Then stopped using it, my bad there.
He was using it in the same place you have your red filter.


Re: Two simultaneous pst etalon

Posted: Fri Mar 13, 2020 12:28 pm
by MalVeauX

Interesting build; any examples of its output? Transmission? Double-limb suppression?

Very best,

Re: Two simultaneous pst etalon

Posted: Thu Mar 19, 2020 7:31 pm
by marktownley
These are linear polarisers, circular polarisers work.

Re: Two simultaneous pst etalon

Posted: Tue Mar 24, 2020 4:14 am
by Astrophil
Quite the experiment you have in progress. It's been fun to follow but I'm not quite sure why multiple circular polarizers are being used.

A circular polarizer consists of a 1/4 wave plate plus a linear polarizer fused together. The linear polarizer side will take unpolarized light (random) and filter or convert it to "polarized light" in a particular plane. This will reduce transmitted intensity by 50%. The wave plate will then take the linear polarized light and cause it to rotate in a circular fashion, hence the name. Keep in mind for later that a 1/4 wave plate converts circular polarized light back into linear. This circularly polarized light when reflected off the etalon will change its direction of rotation as it bounces back. Now the useful part, since the reflected circularly polarized light has changed direction of rotation and passes through the 1/4 wave plate again it converts the light back to linear but with a 90° shift and is blocked by the linear polarizer side. No transmission of the reflection it is absorbed.

To see this in action, shine a light through the CP at the etalon while looking through the CP at the same time and direction. Flip the polarizer and see the difference.

Polarizers made for photography will not be 100% effective at this wavelength (Ha) but useful. There will also be a reflection off the polarizer itself depending on the AR coating. Tilting may also help.

I have used circular polarizers to reduce reflections from a Quark Combo blocking filter and between etalons on a Lunt.

Good luck with your setup,


Re: Two simultaneous pst etalon

Posted: Tue Mar 24, 2020 9:21 am
by Rusted
A fascinating discussion and beautifully illustrated, but may I ask a rather mundane question?

Where do you find wedge or tilting adapters or filter rings?
I've tried searching with different terms but came up completely blank.

I was thinking it might be worth trying to tilt the "heat protective" UV/IR filters in front of a PST mod etalon to kill reflections.
It is not possible to tilt them "by hand" [just as a proof of concept] with a typically closed OTA.

Re: Two simultaneous pst etalon

Posted: Tue Mar 24, 2020 5:52 pm
by Rusted
VitaminSun wrote:
Tue Mar 24, 2020 4:28 pm

you can make a tilt adapter with good old fashioned elbow grease.

Take a 48mm filter cell, and push down the female end of it on a belt sander. (take out the retainer ring(

if you do not own a sander, rough concrete works great; Make sure you wear gloves... You can use sandpaper, but you will end up tearing a few sheets before you find the right rhythm.

if your filter cells are tall enough, and the filter glass is thin enough you can use the same method on the retainer ring themselves to make shims, but you have to sand away the threads first.
Thanks. I thought that was what you had done judging from the appearance.

However, I must be dense:

I can't see how wedge sanding a filter housing allows you to screw anything to its remaining thread.
The filter glass must remain axial to its housing. The machined glass seating will not allow any other angle.
Even [half] sanded away at an angle the thread remains perfectly axial to the housing.
It can't be any other way unless you re-cut the thread at the new tilt angle. [Very unlikely!]

Even an empty, but wedged filter housing, would have to be glued to something else to achieve a change of angle.
AND, to provide a useful thread at that new tilt angle to attach your tilt adapter to the etalon nosepiece.
What am I missing here? :?

Re: Two simultaneous pst etalon

Posted: Tue Mar 24, 2020 10:31 pm
by Merlin66
Bart and others have previously set up double PST etalons...the circular polarizer worked...
viewtopic.php?f=9&t=25524&p=229597&hili ... ck#p229597

Re: Two simultaneous pst etalon

Posted: Wed Mar 25, 2020 9:08 pm
by Merlin66
All's well that ends well..........

Re: Two simultaneous pst etalon

Posted: Thu Mar 26, 2020 7:20 am
by marktownley
Good progress!

Re: Two simultaneous pst etalon

Posted: Sat Mar 28, 2020 4:01 am
by Solar-Cologne
Please post pictures of your complete H-Alpha conversion with the two PST etalons.

And a list of your used components.

Thanks a lot.