Basic question about PST etalon to focus spacing.

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Basic question about PST etalon to focus spacing.

Post by Rusted » Fri Oct 09, 2020 8:30 pm

Hi,

I am trying to get my head around my use of several different GPCs used as [single] Barlows.

When there is no GPC/Barlow involved the focal plane and PST etalon are spaced at 200mm.

BUT: What spacing is required when a GPC or Barlow are added in front of the camera?

The focal plane is thrown further out. So our carefully measured 200mm spacing back to the etalon is now much too long.

Should the spacing between the new focal plane and the etalon be reduced to bring it back to 200mm?

You obviously can't move the camera towards the etalon or it will lose focus. So the etalon must be moved towards the camera.

Or should it? Is it necessary to cling to the 200mm standard with each change of Barlow [or GPC]?


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Re: Basic question about PST etalon to focus spacing.

Post by marktownley » Sat Oct 10, 2020 5:37 am

Hi Chris.

I'm assuming all this in the context of your 6" f10 scope?

The collimating lens ahead of the etalon should always sit 200mm ahead of focus. You should always use the glass path on the nose of the camera. There should be the ability to focus (independent of the scopes main focuser) behind the etalon and ahead of the camera, this distance will vary depending on the glass path you use.

Mark


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Re: Basic question about PST etalon to focus spacing.

Post by Rusted » Sat Oct 10, 2020 6:56 am

Thank you Mark! :bow

This is precisely the information I needed to confirm my own fuzzy thoughts on the matter. :bow2

While moving various components around in the filter stack I noticed the discrepancy in increased spacing with different GPCs.
It has been staring me in the face for ages and I never thought to check the spacing.

My 2" helical focuser couldn't cope with the cantilevered etalon and filter stack load. I have never had a conventional focuser.
So I've ordered a "proper" focuser to go on the backplate and will now move the helical to where I can fine adjust the etalon to camera spacing.

I have been struggling with ring shaped sweet spots and have been tilting the exposed etalon to change the view to the barely acceptable.[IMO]
Getting the spacing correct really matters. I only had the correct spacing with the bare camera.[i.e. No Barlow/GPC] Which I rarely use.

I had even chopped off the T2-1.25" camera nose adapter [where I fit the TS GPCs] to try and reduce the error.
This was based on a completely false assumption. Since the camera still had to remain in focus.
So I was having to "trombone" the shortened T2-1.25" camera adapter as a crude push-pull focuser. :oops:


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Re: Basic question about PST etalon to focus spacing.

Post by Rusted » Sat Oct 10, 2020 12:46 pm

Sorry Mark. I was probably too keen to agree with what I thought you had written.

To avoid any further confusion:

The etalon spacing is fixed at 200mm from the focal plane.

BUT NOT a fixed distance from the telescope objective.

The position of the focal plane changes with each change of GPC.

So, with each change of GPC, I must readjust the etalon to focal plane spacing back to 200mm.

Focus must then be achieved by using the telescope's normal [back plate] focuser.

Do we agree? :)


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Re: Basic question about PST etalon to focus spacing.

Post by Montana » Sat Oct 10, 2020 4:44 pm

I always wondered about this question too and never understand the answers, it always seems to be in another language to me :( reading this post is still as clear as mud to me.

Alexandra



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Re: Basic question about PST etalon to focus spacing.

Post by Rusted » Sat Oct 10, 2020 8:07 pm

Hi Alexandra,

Our modified PST telescopes are based on the same geometry as the little PST but usually made much larger in scale.
The little PST objective lens is an f/10. So we aim for f/10 in our enlarged objectives.

The entire PST is designed around the geometry of its f/10 objective.
If our bigger objective is too "fast" then it must be made "slower" to match the PST's f/10 requirements.
Either by stopping down the objective with a ring baffle. Resulting in loss of aperture and loss of potential resolution.
Or by fitting a suitably [weak] Barlow lens before the etalon. A 1.125X GPC is used for converting popular f/8s to f/10.

The light cone of any modified PST telescope must strike the PST etalon at the same, converging angle as the original PST.
The PST etalon is sandwiched between two lens. The first [a negative lens] makes the objectives converging beam, parallel.
The etalon needs parallel light to function properly.
The second lens [a positive lens placed after the etalon] makes the parallel light emerging from the etalon back into an f/10 cone.

After the etalon a series of filters narrow the passband, reduce the intensity and reject unwanted wavelengths.
The original ITF and IBF filters are very small in the original PST. While our scaled up PST has a much larger image.
So we pay the price of our "cheating" in a much narrower field of view.
The larger image of our scaled up telescope has the edges blocked off by the tiny PST filter housings.
Blocking filters are sold which are much larger and much more expensive but do provide a wider field of view than the tiny PST originals.

The eyepiece or camera following the filters see an f/10 converging cone of light coming to a focus at the focal plane.
We know that if we place the camera or eyepiece 200mm from the etalon that we have satisfied the f/10 requirement.
The focused beam exiting the etalon/lens group will come to a focus 200mm after the etalon.

But what happens if you add a Barlow or GPC to the eyepiece or camera nosepiece?
The light cone will be stretched beyond the original focal pane. Beyond the 200mm rule.

Does the camera or eyepiece really care whether the final light cone is f/10? Or some other ratio?
If they do then we must adhere to the 200mm rule. We must move the camera or eyepiece nearer the etalon to maintain the 200mm spacing.
Which will throw the camera or eyepiece well out of focus. Which can only be corrected by a focuser before the etalon.
I hope you were following all of this because I shall be setting homework. :D


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Re: Basic question about PST etalon to focus spacing.

Post by Montana » Sun Oct 11, 2020 2:25 pm

Thanks Rusted, that is all very clear. However, when I add a Barlow I just refocus with the telescopes focuser and it all seems OK? Also, I tried measuring 200mm but it doesn't seem to matter where I put it as I can just focus with the telescope focuser anywhere. So I have no idea where 200mm is?

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Re: Basic question about PST etalon to focus spacing.

Post by Rusted » Sun Oct 11, 2020 6:07 pm

Montana wrote:
Sun Oct 11, 2020 2:25 pm
Thanks Rusted, that is all very clear. However, when I add a Barlow I just refocus with the telescopes focuser and it all seems OK? Also, I tried measuring 200mm but it doesn't seem to matter where I put it as I can just focus with the telescope focuser anywhere. So I have no idea where 200mm is?

Alexandra
You measure the 200mm from the telescope side of the PST etalon adjustment band to the focal plane. Focal plane = Point of sharpest focus. Where a film or piece of paper would show the sharpest image. Apologies for my pedantry but I'm not sure who else will be reading this.

With my ZWO '174 camera I measure from the far side of the etalon band to the front shoulder of the camera casing. Where there is thin black ring covering the red main casing. The sensor is actually sunk below this point by 6.5mm [according to ZWO] so we ought to take at least that much off the shoulder measurement. Say 193mm to the [thin black] shoulder ring? It depends whether you screw a T2 - 1.25" adapter into the camera as I do. Then you can ignore the 11mm deep black ring into which the adapter screws. The adapter stops against the thin black ring I think. I shall have to check tomorrow. My adapters are all safely tucked in for the night. ;)


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Re: Basic question about PST etalon to focus spacing.

Post by marktownley » Mon Oct 12, 2020 7:14 pm

Just to clarify.

The front face of the collimating lens of the etalon assembly should be positioned 200mm inside the focal point of the objective. This is the native focal point of the objective, not after any barlows etc are used ahead of the camera. Most people achieve this by adjusting it using the primary focuser on the telescope. When this position is found the scopes primary focuser should be clocked and then not be touched at all.

If you are not at 200mm inside the focal point one of 2 things will result. If the collimating lens is positioned more than 200mm ahead of the plane, it will act as a stop and the resolution of the system will be reduced. If the collimating lens is placed less than 200mm inside focus then the full diameter of the etalon is not being used; as sweet spot size is a function of aperture of scope to effective etalon aperture, this will mean that the sweet spot (jacquionot spot) is smaller than it needs to be and the resultant field of view will show more banding, gradients in brightness and detail etc.

I can't stress how important this 200mm figure is in a PST mod, regardless of the scope it is used in. I'm talking in the context of a native f10 system here.

There should be a secondary focuser after the etalon and before the camera. Once this -200mm has been set then this secondary focuser is the only one that should be adjusted to reach focus with the camera. This is the case regardless of what glass path barlow etc is used.

When a PST mod is not performing ideally bad metrics are often the case, normally the -200mm figure.


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Re: Basic question about PST etalon to focus spacing.

Post by marktownley » Mon Oct 12, 2020 7:15 pm

Montana wrote:
Sun Oct 11, 2020 2:25 pm
However, when I add a Barlow I just refocus with the telescopes focuser and it all seems OK?
Absolutely not the thing to do. Focus using the secondary focuser after the etalon.


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Re: Basic question about PST etalon to focus spacing.

Post by marktownley » Mon Oct 12, 2020 7:25 pm

Montana wrote:
Sun Oct 11, 2020 2:25 pm
So I have no idea where 200mm is?
You need to find this by projecting an image of the sun / moon onto some white paper. You need this to be 200mm out from the back of the eyepiece holder of your SCT, then, when you put in your etalon assembly it will naturally be sat 200mm ahead of the focal point. Do this by altering the focus using the focuser on the C11.

When the etalon assembly is now in place, any focusing to get a sharp image at the camera needs to be done using the secondary focuser only.


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Re: Basic question about PST etalon to focus spacing.

Post by RodAstro » Tue Oct 13, 2020 4:15 pm

Hi Mark

Just one question to confuse everyone, how would you work out the position of the SCT primary mirror, as its position changes the focal ratio of a SCT so wouldn't the 200mm in focus change?
Also after a certain point of in focus most SCTs secondary mirrors start to stop the primary down and again add to the confusion of the actual focal ratio.
I am sure this has been talked about before but some of us need further clarification.

Also it would be best once you have found the f10 mirror position to mark it somehow so you can always come back to it if you use the scope for other viewing ie white light or night time astronomy.

On a SCT you could then make up a set of extension tubes to the correct length so the PST mod just slots into or screws onto the correct 200mm position.
And for a refractor do the same thing but with the focuser racked completely inwards at its most stable point.

Cheers Rod



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Re: Basic question about PST etalon to focus spacing.

Post by marktownley » Tue Oct 13, 2020 6:37 pm

RodAstro wrote:
Tue Oct 13, 2020 4:15 pm
Just one question to confuse everyone, how would you work out the position of the SCT primary mirror, as its position changes the focal ratio of a SCT so wouldn't the 200mm in focus change?
Don't think you can work out the position of the primary, important thing is the focal point is thrown sufficiently back that the -200mm point is accommodated with the etalon / focuser / blocker module of the PST mod.

Lots is touted that SCTs are the magic pill for large aperture solar, however like you say there are numerous complications that an imager has to overcome to get performance to an ideal.

No such thing as a free lunch in solar! ;)


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Re: Basic question about PST etalon to focus spacing.

Post by RodAstro » Tue Oct 13, 2020 8:06 pm

Hi

Yep Mark after some reading it seems quite hard to find an easy answer to this but from what I can find, is it may be reasonable to assume that the centre of focus travel may be the actual point where the optical system is at a true f10, as either side of the f10 design the optics incur spherical aberration and the further from the optical design the worse it gets.
Also the internal baffle is probably designed to be optimum at the centre of focus travel as well so best contrast without vignetting.
These are just my assumptions that the optics were designed correctly and not just thrown together.

It would be nice if someone in the know can shed a little more light on SCT telescopes for solar and how best to set them up.

Rod



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Re: Basic question about PST etalon to focus spacing.

Post by Merlin66 » Wed Oct 14, 2020 12:17 am

KISS
The PST etalon assembly is designed for an f10 input beam and to get optimum functionality/ performance the front lens of the etalon assembly should be placed at -200mm inside the prime focus of the telescope. That's it.
(if you're using something like a SCT you can quickly determine if the etalon is at the -200mm position - How? Well, when the etalon is in the correct position the final focus will be +200mm behind the rear lens of the assembly. Just set up a spacer to give this distance, when in focus - using the SCT focuser, the etalon is correctly spaced. Lock and load!
Remember that in the original PST the gold tube is a fixed length - there's no focuser before the etalon, and in a Mod, like wise, there's no need to have one.
What happens after the etalon ( between the rear lens of the etalon and the final focus) is of no real consequence. Sure, you can add reducers/ barlows etc etc.


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Re: Basic question about PST etalon to focus spacing.

Post by Rusted » Wed Oct 14, 2020 2:49 pm

Thanks to all who responded. :bow

The PST BF and ITF are obviously sized for their position in the original PST f/10 beam.
So moving them away from focus may cause problems.

I have this vague memory of the PST BF and ITF working by multiple reflection.[Like the etalon]
So they may well be optimised for an f/10 converging beam to attain their designed bandpass.
Though this may be a false memory.


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Re: Basic question about PST etalon to focus spacing.

Post by marktownley » Wed Oct 14, 2020 4:12 pm

Rusted wrote:
Wed Oct 14, 2020 2:49 pm
Thanks to all who responded. :bow

The PST BF and ITF are obviously sized for their position in the original PST f/10 beam.
So moving them away from focus may cause problems.

I have this vague memory of the PST BF and ITF working by multiple reflection.[Like the etalon]
So they may well be optimised for an f/10 converging beam to attain their designed bandpass.
Though this may be a false memory.
Any image amplification should be going after the blocking filter and before the camera so won't affect the blocking filter performance.

How is your scope setup Rusted?


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Re: Basic question about PST etalon to focus spacing.

Post by marktownley » Wed Oct 14, 2020 4:17 pm

RodAstro wrote:
Tue Oct 13, 2020 8:06 pm
Hi

Yep Mark after some reading it seems quite hard to find an easy answer to this but from what I can find, is it may be reasonable to assume that the centre of focus travel may be the actual point where the optical system is at a true f10, as either side of the f10 design the optics incur spherical aberration and the further from the optical design the worse it gets.
Also the internal baffle is probably designed to be optimum at the centre of focus travel as well so best contrast without vignetting.
These are just my assumptions that the optics were designed correctly and not just thrown together.

It would be nice if someone in the know can shed a little more light on SCT telescopes for solar and how best to set them up.

Rod

I would say they're reasonable assumptions Rod. ;)


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Re: Basic question about PST etalon to focus spacing.

Post by Montana » Wed Oct 14, 2020 5:03 pm

I'm more confused than ever after this post :?

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Re: Basic question about PST etalon to focus spacing.

Post by RodAstro » Wed Oct 14, 2020 5:06 pm

Hi

Merlin yes that is so obvious now I think of it that way around (backwards) nice explanation many thanks, so I could use a PST mod to get my SCT to exactly f10 for star measurements COOL.

I know this was not the original point of the thread as Rusted's scope is f10 but I was thinking of Alexandra's problem with the C11.

The best way for both systems would be to use a fresnel screen set a 200mm behind the PST unit (last surface of the last lens) and focus the image with the front telescope focuser and lock this focuser in that position. Then you can use a second focuser after the PST unit for focusing whatever equipment you want to use.

It would be interesting to test a few SCTs to see where the exact f10 best mirror position actually is,and if it is at the mid point of focus travel, another project for me one day.

So much of this telescope making information has disappeared from magazines and the internet (and my brain) over the past few years, as people just move into buying what they are given and shy away from modifying equipment, very sad.
Nice to still have it in the solar section of our hobby thanks to this site.

Cheers All



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Re: Basic question about PST etalon to focus spacing.

Post by RodAstro » Wed Oct 14, 2020 5:11 pm

Hi
Alexandra have a read of the third paragraph in my previous post, from what Merlin said and it makes sense this should be the easiest way to get everything in the correct position on your C11,

I hope I am correct anyone please correct me if I got it wrong.

Cheers Rod



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Re: Basic question about PST etalon to focus spacing.

Post by RodAstro » Wed Oct 14, 2020 5:51 pm

Hi

I Just realised I have said what Mark said previously much further up in the tread, but hadn't quite got it in my head yet sorry Mark.

Cheers



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Re: Basic question about PST etalon to focus spacing.

Post by Rusted » Wed Oct 14, 2020 6:14 pm

marktownley wrote:
Wed Oct 14, 2020 4:12 pm

Any image amplification should be going after the blocking filter and before the camera so won't affect the blocking filter performance.

How is your scope setup Rusted?
Thanks Mark.

Ironically, I have just invested in a "normal" focuser for the first time on my H-alpha 150/10.
My 2" helical focuser used up far too much space in my 200mm limit.
It has too much friction when supporting the whole H-a stack. So it's being "retired."
Now Ken has undermined my cunning plan but I use the scope for Lunar imaging occasionally. So all is not lost. :D


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Re: Basic question about PST etalon to focus spacing.

Post by marktownley » Wed Oct 14, 2020 7:32 pm

Just going to throw a 'thinker' in here for you all.

The original (unmodified) PST has the etalon that sits -200mm ahead of the focus of its f10 optical system, as would a mod with a 150/f10 or a C11, however, neither of these (modded) systems would be used without a barlow etc so as to dig into the sweetspot when imaging (The PST we will happily image a full disk at f10).

So, it is not as simple as saying the focal distance after the etalon in a mod is 200mm, it is not, as a barlow / powermate etc is being used ahead of the camera. This will make that '200mm' distance longer than that (as we are using an image amplifier). The 'secondary' focuser is so important in achieving this exact distance, rather than using the primary focuser on the scope.

As our PST mods get larger (6"/C11 etc) then these metrics become all the more important, as, the size of sweetspot (and we want as large a one as possible) is a function of effective etalon aperture vs objective size.


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Re: Basic question about PST etalon to focus spacing.

Post by marktownley » Wed Oct 14, 2020 7:37 pm

Rusted wrote:
Wed Oct 14, 2020 6:14 pm
Ironically, I have just invested in a "normal" focuser for the first time on my H-alpha 150/10.
My 2" helical focuser used up far too much space in my 200mm limit.
It has too much friction when supporting the whole H-a stack. So it's being "retired."
Now Ken has undermined my cunning plan but I use the scope for Lunar imaging occasionally. So all is not lost. :D
Still not sure how you have it all setup? How do you focus the camera independently whilst keeping the etalon fixed at the -200mm focal position in the objectives f10 beam?


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