PST PentaPrism Problems and Adjustments

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colinsk

PST PentaPrism Problems and Adjustments

Post by colinsk »

I wanted to break this off the other thread.

My pentaprism was sagged. When opening the box it was quite clear that the input facet was not aligned to the optical beam. This made the focal point across the field of view change. Since I am a visual observer I found my eye would compensate but it was tiring. I adjusted the pentaprism by removing it from the double stick tape and setting it up on a optical bench with a Laser on axis. I rotated the prism till the laser exited from the center of the last filter and then stuck it in place. The focus improvement was stunning.

What have other peoples experiences been?


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Re: PST PentaPrism Problems and Adjustments

Post by marktownley »

My experiences mirror yours exactly Colin. I did the same as you apart from didn't use a laser, just wiggled it till I got the best view via the CCD camera. In the end I ditched the penta prism assembly completely as I believe it is one of the major limiting factors of the PST design...


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Re: PST PentaPrism Problems and Adjustments

Post by sullij1 »

Hi Colin,

You stated (Here we go again :pinch: :lol: ) "This made the focal point across the field of view change. Since I am a visual observer I found my eye would compensate but it was tiring."

Merlin specifide blurring. Can you elaborate more on what you see?

I just purchased a PST last week and did some visual and imaging experiments with it last Sunday. I am encouraged by the early imagery but did notice that as I shift my eye/head about or use averted vision, I am able to better identify some surface detail. I also noted that by moving the scope back and forth in RA, detail would change depending on position of the sun disk on the filter. I have also seen others in the forum specify that they get a better image(cam wise)with the sun disk in the lower right of center.

Is this what you are referring too?


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Re: PST PentaPrism Problems and Adjustments

Post by colinsk »

No, What I found if I set the focus so one side was just inside focus the other would still be blurry. If I set them both to inside focus I had to use my visual accommodation to correct the focus. This gives me a headache. I have a stereo microscope that I have had to adjust and I am really sensitive to focus issues. (And rotation for that matter in stereo vision.)

In other words: To relax your eyes the most you make the instrument as long as possible and just bring it shorter till it achieves focus. This leaves your eyes the most relaxed. When I did this on the my PST one side would come into focus before the other. When I passed it, then both sides would focus as long as I used the muscles in my eyes to correct as I scanned the field of view.

I should note that my eyesight correction is -.5 diopters and has no astigmatism so I view with no glasses.


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Re: PST PentaPrism Problems and Adjustments

Post by Merlin66 »

In simple terms....
The front and top face of the pentaprism should be square and parallel in the inside surfaces of the black box.....


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Re: PST PentaPrism Problems and Adjustments

Post by peter drew »

Everyone of the PST's that I have opened has revealed that the pentaprism front face was slightly tipped downwards leading me to suspect this was an attempt to shift an internal reflection from the field of view. On one of my mods I replaced the pentaprism internally with an adjustable RA prism with which I was able to optimise the setting, later mods with larger BF's just used the Coronado diagonal.


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Re: PST PentaPrism Problems and Adjustments

Post by DavidG »

I was up at Stellafane last weekend and a good friend had a PST that showed an image that was not fully in focus at every location. I removed the cover and as I have seen with many other PSTs including both of mine the prism was tipped out of alignment. It took a few minutes of slowly rotating it back into place so the two 90 faces were parallel to inside of housing and then letting it sit to be sure that the prism was holding position. We all saw a huge improvement in the image quality. Having the prism out of alignment introduces a large amount of astgimastism into the image.

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Re: PST PentaPrism Problems and Adjustments

Post by Merlin66 »

Dave,
As I mentioned previously, this is not an uncommon problem. The solution is a Stage 2 mod.


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Re: PST PentaPrism Problems and Adjustments

Post by colinsk »

I was up at Stellafane last weekend and a good friend had a PST that showed an image that was not fully in focus at every location. I removed the cover and as I have seen with many other PSTs including both of mine the prism was tipped out of alignment. It took a few minutes of slowly rotating it back into place so the two 90 faces were parallel to inside of housing and then letting it sit to be sure that the prism was holding position. We all saw a huge improvement in the image quality. Having the prism out of alignment introduces a large amount of astgimastism into the image.

- Dave

Dave, thank you for teaching me this when I first got a PST.


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Re: PST PentaPrism Problems and Adjustments

Post by Northern Cross »

Mark.
Would it be poss for you to make a video of taking out the pentaprism and raligning it, sure this would go down a treat, like the video of the taking apart the PST?


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Re: PST PentaPrism Problems and Adjustments

Post by marktownley »

Mark.
Would it be poss for you to make a video of taking out the pentaprism and raligning it, sure this would go down a treat, like the video of the taking apart the PST?

Yeah, can do that, won't be for a week or so, away in snowdonia atm...


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Re: PST PentaPrism Problems and Adjustments

Post by DJD »

Mark,
did you get round to doing the alignment video ?
best wishes,
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Re: PST PentaPrism Problems and Adjustments

Post by marktownley »

No, never got around to it :whistle: With winter coming and cr@ppy weather there's more chance of me doing it tho...


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Re: PST PentaPrism Problems and Adjustments

Post by Northern Cross »

Looking forward to your video. thanks in advance.


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Re: PST PentaPrism Problems and Adjustments

Post by occamfan »

I'm also looking forward to Mark's next video; until then here is some food for thought. I have an empty PST box with pentaprism to play with, so I had a closer look. My prism also droops in the body, but I don't think this matters so much. The droop looks deliberate and probably prevents reflection back towards the etalon: this should not cause astigmatism. What will definitely cause astigmatism is any sideways mis-alignment,and looking at the focusing mechanism reveals where this can take place. The pentaprism slides diagonally with a pin along a slot in the body -see pic.
(sorry about the poor picture but you can just make out the pin and track -I'm just as bad at imaging!)
The pin and slot are both metal and are well greased; this is tensioned by a small piece of velcro on the opposite face, and as is shown on the pic. this does not even cover the whole bracket and the assembly could rock sideways here.

. There is plenty of scope here to do improvements just by replacing the velcro pad with something more accurate. This still leaves the accuracy of the glue joint between the pentaprism and bracket, but that would involve complete dis-assembly is a matter for more experienced people than me.


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Re: PST PentaPrism Problems and Adjustments

Post by colinsk »

On mine the glue was double sided foam tape. Not the best solution.


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