PST tweaks and improvements

[Reeny]

Below is a summary of some easy gains on Coronado PST 40mm solar scopes.

My apologies if some of the terminology is incorrect

 

1) Elaton - had backlash, slack and wobble. The M2 screw is located in a countersunk threaded hole, with very little of the Aluminium ring to grip onto. I managed to strip the threads on mine.

Solution = Tap out to M3 so the hole is "less countersunk", and can grip the lock screw better. The new M3 screw should still fit nicely into the tuning ring below for a tighter fit. If not, you can thread that hole as well.

 

2) Small finder prism - comes loose easily if knocked while fixing the other faults. From the factory, mine was way off centre and needed fixing.

Solution = Clean off all of the old loose clear silicone sealer with a knife / use a removable glue such as impact adhesive to re-attach the prism (fix the prism flush with the rear and the front inner surface of the case)

Note: I avoided using 2-pack Araldite to allow re-adjusting at a later date.

 

3) Focus adjuster - had slack and sideways wobble. It was getting worse day-by-day as the hole in the case appeared to wear out on the steel bar.

Solution = Pull the circlip off the shaft (just above the spring loaded brass spacer on the threaded bar) / remove the assembly / Take up the slack with a sleeve or spacer - e.g. a strip of white plastic, cut from a milk carton. Wrap the shim around the shaft. / reassemble.

 

4) Large focus prism adjustment - the usual mis-alignment, and the reason the side panel was removed.

Solution = Twist the assembly back into shape, using the top surface as a guide. The soft tacky adhesive (holding the prism to the back plate) has loads of stretch - so do not be too gentle.

Note: if the gold tube can be unscrewed from the front - you can check the alignment quickly & easily, without the need to remove the side panel.

Edited September 3, 2023 by Reeny
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[Reeny]

The thread could have placed this in equipment - but I thought solar observing would be a better place to find the advice for PST users.

[Peter Drew]

Good tips but I would question the "problem" of the tilted main prism.  I have worked on around 20 PST's and each one has exactly the same forward tilt.  It is my belief that this is intentional to avoid unwanted reflections.  The prism is a pentaprism which produces constant deviation of the light path so a small tilt does not affect the optical performance.    🙂

[Reeny]

The optical view was not good.

The other thread I opened suggested that excessive tilt can cause banding - the picture of the prism above is with the tilt reduced.

Either I am getting used to the "hit & miss" view, or the low powered 18mm supplied eyepiece and 12mm Plossl are helping to mask the problem.

I don't notice any reflections at all, and the visual image is half decent.

Note, As supplied new, the visual image needed to be moved left to right within the view finder to avoid looking at the burry bits. The prominences on the edge would completely disappear in certain dead spots within the eyepiece.

Edited September 4, 2023 by Reeny

[Elp]

Did you change the etalon tuning ring position (ie take the screw out, find a nice median on band point, then put screw back into nearest hole)?

When I first got mine I was wondering why I could get sharp prominences but not the best surface detail, the above fixed that.

Also, adjusting the pentaprism, I think it's supposed to be like that (on tilt), otherwise they wouldn't fit rails under the prism to make sure it doesn't go out of alignment.

[Reeny]

Peter - my mistake, you are correct about the prism making no difference to the view.

The black-outs have gone, but the view is just as bad - even with the prism adjusted.

The left side of the view = 30-40% faded and blurred / The right side = 10% at the edge with missing detail.

I found that lower magnification eyepieces can work around the problem by keeping a decent portion of the sun inside the sweet spot.

 

ELP - most of the advice suggested to leave the settings alone, so I only moved the ring one hole - then I needed to move the ring back a few weeks ago.

I have a hunch that squaring up the main prism has adjusted the light path. The elaton needed a tweak each time / both times.

The main elaton on the gold tube appears to be OK now - but I will attempt a few adjustments beyond the limits of the factory settings in search of a better view.

The double stack elaton has terrible slack & wobble on the adjuster threads as the ring screws out - the threads tighten up again as the elaton is screwed back towards the inner stop.

Once the PST scope starts to work as it should, I can try to make improvements on the double stack unit by finding a decent setting further down the adjuster threads to stop the slack & wobble.

 

For anyone considering the PST without the double stack unit - I can recommend binoviewers on a single stack PST to reduce light transmission, while increasing detail and contrast.

I have a cheap pair of bino's which work well with the 1.85 barlow supplied, and 23mm Aspheric 62deg eyepieces (= 32x mag). 

The Bino's are used more than the £700 double stack unit.

Edited September 5, 2023 by Reeny