PST Mod C11

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PST Mod C11

Post by Montana »

Hello,

I had an investigation of my PST mod and the C11 on Friday. It is absolutely rock solid and impossible to cause any tilt anywhere with every piece being either screwed on or click locked on. I did however turn the etalon around in circle and came up with this. I centred the spot each time and rotated the etalon around by 45 degrees each time. No matter where I put it I cannot get the on-band region centred. What could be the problem?
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If only I could get it centred then at 2x I would fill the field of view better.
Sun_102549_31_07_2020_a.jpg
Sun_102549_31_07_2020_a.jpg (596.8 KiB) Viewed 3048 times
Alexandra


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Re: PST Mod C11

Post by marktownley »

You can set the position of the ROI manually in Firecapture by click and dragging the red box. It shouldn't matter the orientation of the offset sweetspot, you can just drag the box to fit.


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Re: PST Mod C11

Post by Montana »

Really?

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Re: PST Mod C11

Post by Montana »

Thinking about this while nodding off last night and this fix doesn't help, all these images were taken at full size (not ROI). When I add a 2x Barlow the sweetspot isn't in the middle so I can't move the camera to the side. How do I straighten up the sweetspot or is this impossible?

Alexandra


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Re: PST Mod C11

Post by marktownley »

Ahhh, understand. There is no obvious way to do this, maybe a tilt tuner on the camera nosepeice and try and orientate so it is looking off axis into the direction of the sweetspot offset, but, I don't know if this would work and it would be a faff.

I bet if you look on the exit side of the etalon, up into the body of the etalon itself, you will see the little foam spacers packing it up. This is what coronado do to get the etalon on band - it is essentially tilting the etalon, that's why you see the gradient across the sweetspot (think how you get a similar thing with an external etalon when tilt is applied).

Dismantling and reseating the etalon in the chamber will certainly change the position of the sweetspot, however whether this is for better or worse won't be known till after the event.

Think my PST etalon was number six when I finally found the one...

Mark


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Re: PST Mod C11

Post by Montana »

Thanks Mark, yes Apollo said that too and showed me your video. I think that is a little out of my league and risk :(

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Re: PST Mod C11

Post by AndiesHandyHandies »

Hi

I used a tilter, T2 or M48, on my 127mm ES and PST mod with auto-collimation and got the sweet spot centred.

Punched a hole with a binder punch in old Xmas Card and did a 1/4" V with pen centred on the hole.
Stand 20' infront of the scope and check with PST on with no pre-filters. Move from side to side and check edge vignetting is balanced of the PST reflection when looking through the hole and the V to the front.

Cheers. Andrew.


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Re: PST Mod C11

Post by banjo »

hello Alexandra,
if you use the black body of the Pst, you must check that the pentaprism is not slightly inclined.(sorry for my English translators google :mrgreen: )

Paul

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sorry for my English translators google :mrgreen:
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Re: PST Mod C11

Post by Carbon60 »

Hi Alexandra,

Sorry, I missed your original post back on Aug 3rd. There is definitely some eccentricity somewhere in the setup. The assumption is that screwing and click-locking everything together automatically brings everything into alignment, but this is not necessarily the case (machining tolerances, droop....whatever). I think a way needs to be found with your setup to allow the whole rear assembly to be manoeuvred (tilted) to move the sweet spot around. This is what I do with my RCT and 150mm scopes where the Lunt 35s are fully tilt adjustable. When I tried making a machined ‘solid’ system I couldn’t move the sweet spot to an optimal position as I can now to an accuracy of a fraction of a degree of tilt. Something we can take a look at once COVID restrictions are relaxed.

Stu.


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Re: PST Mod C11

Post by Merlin66 »

Mark, et al,
As recently reported the tuning of the PST etalon is by pressure, distorting the plates.
Any additional knobs on the orange ring are there to “equalize” this pressure nothing to do with tilt.


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Re: PST Mod C11

Post by AndiesHandyHandies »

Hi

Its an urban myth about the PST etalon being out of alignment.

All the pictures I have seen on the web show it like that.

It probably done to tilt reflections out of field.

There is an abberation penalty but Coronado/Meade must consider it a reaonable trade off.

Have you tired adding a tilter before the PST and auto-collimating as I advised above?

Cheers. Andrew.


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