PST double stack question

Use this section to discuss "standard" Baader/Coronado/ Lunt SolarView/ Daystar, etc… filters, cameras and scopes. No mods, just questions/ answers and reviews.
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Montana
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Re: PST double stack question

Post by Montana » Mon Oct 22, 2012 5:06 pm

Still no photos, it says the server is full and there is no space left on the error message. It can't be my pics as one is 900kb and the other about 980kb.

Alexandra

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Re: PST double stack question

Post by robert » Mon Oct 22, 2012 5:10 pm

smaller file or just a link from flikr perhaps?
images and animations http://tinyurl.com/h5bgoso
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Re: PST double stack question

Post by Montana » Mon Oct 22, 2012 5:14 pm

I may perhaps have to do that but I hate to put on Flickr pictures of a sad PST in my Photostream :(

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Re: PST double stack question

Post by Montana » Mon Oct 22, 2012 5:19 pm


PST problems by Alexandra's Astronomy, on Flickr


PST problems by Alexandra's Astronomy, on Flickr

A link instead.

Diagnosis and treatment (I hope not terminal)

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Re: PST double stack question

Post by swisswalter » Mon Oct 22, 2012 7:44 pm

Hi Alexandra

thank you for the pics. The pentaprisma seems not to be aligned. It should be parell to the housing
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Re: PST double stack question

Post by Merlin66 » Mon Oct 22, 2012 8:32 pm

The front edge of the pentaprism, ideally, needs to come up, so that the top surface is parallel to the housing. However, I've seen MUCH worse than that.
IMHO the impact of that degree of tilt will be marginal, it should still perform reasonably well.....
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Re: PST double stack question

Post by peter drew » Mon Oct 22, 2012 10:15 pm

This degree of tilt of the pentaprism is similar to every PST I've opened up, surely the assemblers can't be getting it "wrong" every time. I'm just in the process of making an adjustable pentaprism to verify the effect of tilt. The pentaprism was quite difficult to remove from its mounting block which suggests that the observed tilt might not be due to post assembly movement.

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Re: PST double stack question

Post by thenews24 » Mon Oct 22, 2012 11:59 pm

I've had the same frustrations with the double stack myself Alexandra, I'm sure you've seen my post in stargazers. I appreciate the increase in contrast it brings, but it is not worth the retail price whatsoever imo. I am actually looking to sell mine, or maybe trade for a 70-90mm erf for the pst mod.

As for the pentaprism, I seriously doubt there is a pst out ther with a perfectly aligned prism, parallel to the balck box. Please somebody post a pic if it exist. My pst stock is excellent, maybe a 8 out of 10, but I have just as much angle on my pentaprism as the photo you posted. And unless I moidfy it, or add some kind of spacer, no amount of adjustments by hand will change it.

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Re: PST double stack question

Post by Montana » Tue Oct 23, 2012 9:19 am

Thanks everyone, it's a relief to hear it's OK :)

I just want my pictures to be as good as Smerral's :( I think I need to work harder!

Alexandra

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Re: PST double stack question

Post by keithatrochdale » Tue Oct 23, 2012 1:33 pm

This is my best PST and it works staggeringly well grafted to a TAL100.

Peter Drew and I have both compared it to various PSTs and PST conversions.

It seems strange that each PST I have seen opened are all offset by roughly the same amount; if it was poor assembly you would expect them to be all over the place, not almost identical.

This leads me to believe that they are set this way on purpose, whatever that may be.

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Re: PST double stack question

Post by Spectral Joe » Tue Oct 23, 2012 5:39 pm

Just stumbled onto this thread, and have never seen the inside of a PST back end before. I think the use of a pentaprism in that way is quite ingenious, they are using a particular characteristic of the pentaprism to their advantage, that being the 90 degree constant deviation of the incident beam regardless of the prism's orientation. A ray entering a pentaprism will exit at 90 degrees to it's entrance even if it is not entering at 90 degrees to the prism's face, within limits posed by the critical angle within the glass. The angles of the reflecting surfaces must be correct to get the 90 degree deviation to be exact. Tilting the prism will shift the optical axis laterally, but not tilt the focal plane. Moving the prism at a 45 dgree angle shifts the focal plane axially using just a simple screw, and a little wiggle in the prism angle won't matter. In fact, the tilt noted is almost certainly deliberate, as it avoids having the prism entrance and exit faces at right angles to the optical axis, avoiding glare from the surfaces and unwanted etalon effects (Newton rings). They built it that way for a reason. The only significant drawback to using prisms like this is that the extra glass thickness adds negative spherical abberation, but this is hardly noticable at high F numbers and can be corrected for (if it's a problem) by designing in some positive spherical elsewhere in the system.
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Re: PST double stack question

Post by swisswalter » Tue Oct 23, 2012 5:40 pm

Hi Keith

that is a good point. Thanks for the pic
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Re: PST double stack question

Post by Merlin66 » Wed Oct 24, 2012 2:29 am

Interesting stuff......
I have to say, in all the PST's I've pulled apart, the prism is always slightly tilted (sometimes a lot!) forward. I've never seen one tilted backwards....
I don't think you need a 10 degree tilt to obviate glare etc. But it could be, that it's used as a means of shifting the sweet spot slightly off axis and giving a better visual outcome.
I do know, that a severely tilted prism causes some astigmatism.
(If you remove the BF and set up a laser collimator to fire back through the optical train the "spot" never seems to emerge in the centre of the objective...doesn'r move with tuning - this was communicated in a recent discussion on the UK Stargazers Lounge forum)
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Re: PST double stack question

Post by robert » Wed Oct 24, 2012 3:10 pm

What does an astigmatic prism look like? Is it dropping away from the side of the body? or...?
People have reported fixing CaK PST's with this effect and this may be the same problem?
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Re: PST double stack question

Post by Merlin66 » Wed Oct 24, 2012 8:17 pm

I agree the prism can tilt off-axis in both x-y...
The one's I've seen and improved show the front top edge strongly tilted downwards.
(I've never seen any astigmatism in a Stage 2 mod)
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Re: PST double stack question

Post by sullij1 » Fri Oct 26, 2012 7:10 pm

:lol: We digress from Alexandra's question. :) Although I do not own a double stack unit such as her, I have to ask the question; Can or “is” there an adjustment similar to the 60mm pin adjustment that allows the etalon to be tilted to compensate to altitude? Such as Markmanner had to do with his 60mm (er whatever) before he got it to behave.
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Re: PST double stack question

Post by sullij1 » Fri Oct 26, 2012 7:19 pm

Cannot remember who but in another post (perhaps in solar mod) didn't kornfeld1 (or somebody) have a similar problem in which the solution was a wedge or similar device placed between the primary stack/objective and the double stack module such that the physical tilt of the top module eliminated the reflection? I will look for it later, help would be appreciated.
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Re: PST double stack question

Post by Merlin66 » Fri Oct 26, 2012 8:01 pm

I use the SM60 double stacked and just make use of the T-Max tilt adjuster to tune the etalons. Only takes a few mm of rotation of the adjusting knob to make a difference...
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Re: PST double stack question

Post by Montana » Tue Oct 30, 2012 12:30 pm

Just to keep you updated, it is definitely a double stack problem

Single stack


2012-10-21 13-42-37 Single stack f colour by Alexandra's Astronomy, on Flickr

Double stack


2012-10-21 13-53-01 DS f Colour by Alexandra's Astronomy, on Flickr

The single stack is crisp all over, the DS version is only crisp on the right hand side.

Any ideas / theories?

This weekend I will rotate the DS unit 180 degrees (advice from Merlin) if the Sun comes out.

Kind regards
Alexandra

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Re: PST double stack question

Post by robert » Tue Oct 30, 2012 12:46 pm

Very interesting. It is quite a difficult thing weighing up contrast which is very pleasing against sharpness which is also very pleasing...
There is a double image effect in the DS which is from the processing being applied more strongly... (IMO)

I am intrigued by the problem but don't know what it is. Rotating the etalons in relation to each other is very significant, my two only work in one orientation.

Good luck. I think both images are good by the way!
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Re: PST double stack question

Post by rsfoto » Tue Oct 30, 2012 8:29 pm

Hi Alexandra,

I think that you are being mislead to this subjective seeing that the double stack is not sharp on the left side.

If you take a close look at your single image you will see that the contrast is mainly in the center and all the Sun is well iluminated.

Now taking a look at the double stack the higher contrast is slightly shifted right and so the left side is darker and this gives you the impression that it is not as sharp as the right side.

I know it is very hard to get the HotSpot of a double stacked filter into the center but it is possible.

Your idea of turning one of the filter 180° to the other one can help you. Try many positions not only 180°. A friend here in Mexico has his filters in a 90° configuration.

I have my DS60mm in a 180° configuration.
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Re: PST double stack question

Post by marktownley » Sat Nov 03, 2012 5:49 pm

An interesting thread to jump back into Alexandra...

The problem we have in the UK is such brief opportunities to image, let alone to experiment. I'm in the same boat as you with various solar 'projects' I want to play with that I need to further experiment with to optimise...

I think, when the clouds allow...

Try experimenting by rotating the external etalon with respect to the PST tube... Do you have a pic that shows where the thumbwheel on the external etalon is with repsect to the PST tube? I think you will get best results with it at a 12 o'clock or 6 o'clock position with respect to the PST tube...

mark :)
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Re: PST double stack question

Post by Montana » Sun Nov 04, 2012 3:59 pm

Hi Mark,
Here is a picture when tightened.


CaK PST + DS Ha PST by Alexandra's Astronomy, on Flickr

Alexandra

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