Question about PST mod: keeping collimator lens at -200mm during focussing

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Question about PST mod: keeping collimator lens at -200mm during focussing

Post by gabrieli »

I've had enough experience so far with both my LS80 + LS60F front mount double stack and my 102 apo + LS60F + PST etalon
double stack to see double stack effects (although with uneven brightness with the 102).

With the 102 apo, the focus point is 14.5 cm outside the stationary part of the focuser. This means that the collimating lens
has to be located 5.5 cm inside the tube (not a problem). I attached the collimator to the 2" PST adaptor. When I crank the focuser outwards, the collimator is carried outwards away from its -200mm position.

I'm looking for suggestions on how I can focus while keeping the collimator in its required position. Should I make up the
14.5 cm out to the focus point by using extension tubes and crank the focuser as little as possible?


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Re: Question about PST mod: keeping collimator lens at -200mm during focussing

Post by marktownley »

Do you have a picture of this so we can see what you mean. Do you have a second focuser on the other side of the PST etalon?


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Re: Question about PST mod: keeping collimator lens at -200mm during focussing

Post by swisswalter »

Hi Lou

if you do not have a second focuser behind the PST mod then go with the extensions so close as possible. I do not see really a big difference when I have my collimator lens sitting at -195 or -205 mm


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Re: Question about PST mod: keeping collimator lens at -200mm during focussing

Post by gabrieli »

It didn't cross my mind that, if I used the assembly in the image as a module in the draw tube and focussed, the collimator would move out of its -200mm position.

One solution I thought of would be to mount the collimator on the 2" barrel extension apart from the rest of the module and just fix that at the -200mm position inside the focussing tube(easy enough). Then I could attach the rest of the pst/blocking filter module into the draw tube and use the existing focuser without a second focuser without changing the collimator position. It shouldn't matter how far the collimated rays travel before they are re focussed.

I do have a small focusser but that rotates the camera and has a T mount attachment.

I'm just looking for what other people have done if there is a simpler solution apart from a second focuser.


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Re: Question about PST mod: keeping collimator lens at -200mm during focussing

Post by swisswalter »

Hi Lou

a fine pic of your mod. I love mods ;-)


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Re: Question about PST mod: keeping collimator lens at -200mm during focussing

Post by RobBower »

I have a small helical focuser where the eyepiece holder use to be. I get about 2mm inside focus with that when my scope focus is all the way in. Then i use my regular scope focuser to move out the 2mm to achieve focus. Maybe its not perfect... But it works! This keeps your collimator lens within 2mm of the magic number!


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Re: Question about PST mod: keeping collimator lens at -200mm during focussing

Post by Allcart »

I think your first step should be to measure the length of the drawtube. Based on your measurements, the front lens needs to be 5.5cm into the drawtube to to be at -200mm when at focus. If your drawtube is very much longer then the lens may be too far from the etalon to operate efficiently and may even cause severe vignetting.


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Re: Question about PST mod: keeping collimator lens at -200mm during focussing

Post by Merlin66 »

A common problem/ issue....
The focuser on the OTA side of the PST etalon is NOT required. It will NEVER be used to focus etc with the PST mod (think about it...there's no focuser on the gold tube of the PST!!?)
You can certainly leave it in place if you can still achieve the correct positioning of the etalon at the -200mm, but you will need some form of focusing between the etalon and the eyepiece/ CCD.
I recommend and use the Borg Helical focusers (#7315) on the PST mods.
(Walter: I disagree...probably 2-3mm max is the leaway...outside that I start to see issues with hot-spots etc.)
EDIT: I see you have a 2" blocking filter....Hmmm...Ralph has come up with a great solution for that on his current PST mod...I hope he will post details.


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Re: Question about PST mod: keeping collimator lens at -200mm during focussing

Post by gabrieli »

The problem I was thinking through never concerned having enough in focus. There is more than enough infocus to reach -200mm. I was wondering whether people with working mods were using the pst adaptors + etalon as a single module
and whether there was any problem focusing where the module moves as a unit with the collimator moving in and out of the -200mm position.

With the two barrel extensions I've used, I can place the collimator at the -200mm point but, as you can see in the image,
the focus point is inside the blocking filter. One barrel extension doesn't take the collimating lens deep enough. I guess this means that I can't do prime focus imaging here but would have to use a barlow to pull the image outside of the blocking filter.




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Re: Question about PST mod: keeping collimator lens at -200mm during focussing

Post by gabrieli »

Hi Ken

What you say, of course, makes sense. After all the emphasis on the "magic" -200mm position, I shouldn't have expected
that it would be ok to move the collimator away from this point a significant amount. I checked on the Borg focuser and it looks
like a doable solution although I would prefer a bit more travel. Thanks.


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Re: Question about PST mod: keeping collimator lens at -200mm during focussing

Post by Merlin66 »

Gabrieli,
I have a 10mm T thread spacer in the arrangement....this can be removed to give more back focus for camera or left in place for visual....

EDIT: Just looking at your image of the set-up....the final focus position is incorrect!
Unless you have also relocated the rear lens on the PST etalon assembly, the final focus will be 200mm BEHIND the etalon...You can safely measure 225mm from the rear of the knurled ring.....


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Re: Question about PST mod: keeping collimator lens at -200mm during focussing

Post by gabrieli »

Hi Ken

I'm a little confused about your last observation.

I found the prime focus point by using baader film on the front of the scope and measuring how far right of the stationary part of the focuser the camera sensor was when the white light image was in focus.
I've labeled the point "A" (14.5 cm to focus point) in the image.

The collimator lens from the etalon was relocated to the left most barrel extension.
Am I measuring the focus point the right way?

Can you use directions right and left of the rubber ring in your answer? By "rear" and "behind", do you mean on the eyepiece side of the etalon?

In the image, the pst module and draw tube are lined up as if they were in use and assembled.


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Re: Question about PST mod: keeping collimator lens at -200mm during focussing

Post by Merlin66 »

Gabrieli,
The distance between the etalon and the final focus is 200mm.
Reading the scale in your image it seems to show a spacing of approx 120mm between the right hand side of the etalon and the focus....
I think you are showing the focus to be 200mm from the collimating lens...this is not correct.There's a re-imaging 200 mm focal length lens on the camera/eyepiece side of the etalon.


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Re: Question about PST mod: keeping collimator lens at -200mm during focussing

Post by Allcart »

Ken is right Lou. Your method of finding the initial focus point and then finding the position of the collimating lens is ok. However, when the etalon is inserted into position, the focus point then moves and becomes 200mm from the rear lens of the etalon.
The rear etalon sits between the black arrows and your new focus point should be 200mm behind it. Its then a question of adding extention tubes or a focuser so that you can reach that 200mm.

See attached file.




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Re: Question about PST mod: keeping collimator lens at -200mm during focussing

Post by gabrieli »

Many thanks, Ken and Allan

Clarity hit me after a good night's sleep


I realised my mistake just as I woke up this morning.

When I determined the prime focus point of my 102 mm apo, that point would obviously not be the same as the focus point of the optical system with the collimator in place and the refocusing lens focusing light an extra 200 mm beyond the rubber ring.


My own rough diagram misled me into confusing the prime focus point with the "new" focus point with all the extra optics in place. Light travels an extra distance

(a) after it is collimated and then
(b) a further 200mm after being refocused.

On the plus side, this realization does give more flexible working distances.

Thanks for pointing out this oversight.


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Re: Question about PST mod: keeping collimator lens at -200mm during focussing

Post by Merlin66 »

That's OK mate - we "hold hands in the traffic" and help each other!


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Re: Question about PST mod: keeping collimator lens at -200mm during focussing

Post by swisswalter »

(Walter: I disagree...probably 2-3mm max is the leaway...outside that I start to see issues with hot-spots etc.)
EDIT: I see you have a 2" blocking filter....Hmmm...Ralph has come up with a great solution for that on his current PST mod...I hope he will post details.


Hi Ken

thank you very much for your comment. I still disagree, but I have to wait some months to proof it on real sun shots :-(

I hope Ralph will chim in here to show his great solution


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Re: Question about PST mod: keeping collimator lens at -200mm during focussing

Post by Merlin66 »

Ralph has agreed to me uploading the image of his adaptor set-up for his stage 2 mod.
He will prepare a detailed write-up...







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Re: Question about PST mod: keeping collimator lens at -200mm during focussing

Post by swisswalter »

Hi Ken

thank you very much. Looks interesting but I do need a write up to understand it. I do not see many focusers ;-)


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Re: Question about PST mod: keeping collimator lens at -200mm during focussing

Post by Merlin66 »

Walter et al,
From the left hand side...
The white OTA, focuser removed and an adaptor plate fitted, this then holds the PST etalon (knurled ring visible) at the majic -200mm.
To the right of the etalon is a M50 spacer which then holds the electric focuser (black rectangular box), the next spacer holds the 2" Blocking filter (black cylinder visible) and finally the camera adaptor and camera.
A very neat arrangement!


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Re: Question about PST mod: keeping collimator lens at -200mm during focussing

Post by swisswalter »

Hi Ken

thank you very much for your fast response and explanation, now I understand


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Re: Question about PST mod: keeping collimator lens at -200mm during focussing

Post by Allcart »

Wow thats a neat little focuser. I wonder how that works. Maybe Ralph could enlighten us about that as well.


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Re: Question about PST mod: keeping collimator lens at -200mm during focussing

Post by gabrieli »

Given the very generous back focus indicated in the last image, it looks like I'll be able to use my B1200 if there are any reflection issues with the B3400 in the image. Initially, I thought that the overall focus point would be further in. That's why I opted for the shorter B3400.

I have a helical focuser on order from Hultech which has 10 mm of travel and use it for focussing beyond the blocking filter.
The main focuser will stay fixed in position to maintain the collimating lens at -200mm.

I really appreciate all the illuminating teaching moments from many people that have run through this thread.


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Re: Question about PST mod: keeping collimator lens at -200mm during focussing

Post by Solarhound »

I have recently re configured my mod using a Moomlight focuser , this is the result

[map][/map]




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Re: Question about PST mod: keeping collimator lens at -200mm during focussing

Post by marktownley »

That's exactly the same configuration I use, my moonlite is orange...


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Re: Question about PST mod: keeping collimator lens at -200mm during focussing

Post by swisswalter »

Hi Mike

that looks very good


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Re: Question about PST mod: keeping collimator lens at -200mm during focussing

Post by Solarhound »

It was late when I posted the image last night and I didn't include an explanation . My first configuration used 2" extensions in place of the focuser . The mod was functional this way but not optimum and the sweet spot was difficult to deal with when the etalon was moving with the focuser . In my opinion for best results the etalon needs to be fixed at the 200mm location . I use this in a Celestron 102 Omni XLT with 3" removed from the tube , I lock the original focuser extended 1" and this places the etalon at the magic 200mm . This is the same mod using a B1800 straight through BF .




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