PST double stack question

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PST double stack question

Post by Montana »

Just thought I would ask the experts. I enclose an image taken with my double stack PST, I always have the feeling that it seems a bit like a double image but I can't quite put my finger on why it feels wrong. Is this normal? Smerral's (Brian) images always seem so razor sharp and single looking. I'm probably not describing it very well. Is there anything simple I could try to improve it or is it just that perhaps it isn't well matched, or as I have adapter the distance is wrong for the filters?
Please can any answer be really simple and not with techno speak as otherwise I will be clueless :blush:

Thanks for any help




Kind regards
Alexandra


colinsk

Re: PST double stack question

Post by colinsk »

Focus is hard to achieve. That would be my first guess. Once you know you are in focus then processing is the next guess. The astroimagers here are really good at sorting out this stuff.


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Re: PST double stack question

Post by Merlin66 »

Geee, that doesn't look too bad to my eyes!!
Always comes down to the tuning and focus.
(The pentaprism alignment in the PST can also give "smearing")


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Re: PST double stack question

Post by swisswalter »

Hi Alexandra

that is a very contrasty shot, but i agree, it is not razorsharp.

How does it look like on the monitor while capturing? Is it sharp? If it is, it may be the capturing. How many frames in which time do you capture.


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Re: PST double stack question

Post by marktownley »

Hi Alexandra,

It's not razorsharp like walter says, maybe it could be the penta prism - that would be something I would explore further if you repeatedly get this blurring. If you get chance email me a 16bit unprocessed version of it - lets really push the sharpening and see what it shows.

Mark :)


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Re: PST double stack question

Post by peter drew »

In view of your proven expertise in focusing and processing :bow: I would suspect the pentaprism as mentioned by Merlin66.


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Re: PST double stack question

Post by robert »

I would check the processing too. I find that using the iterative unsharp mask method in Photoshop can produce a double image effect if it is pushed too far.
Robert

PS very nice contrast and 3D effect even so


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Re: PST double stack question

Post by Montana »

May be I am just being fussy? I'll present the single stack and double stack close up of the same area, I'm sure the double stack spicules seem to come in identical pairs? I also find focusing with the double stack next to impossible whereas single stack it always seems sharp.





Mark, I'll try and e-mail you tonight but I know you are going to be busy with holidays etc. If it were the pentaprism wouldn't the single be bad as well or does it only show up when adding a double?

Many thanks
Alexandra


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Re: PST double stack question

Post by robert »

May be I am just being fussy? I'll present the single stack and double stack close up of the same area, I'm sure the double stack spicules seem to come in identical pairs?

really does look like processing there
Robert


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Re: PST double stack question

Post by rsfoto »

[quote]May be I am just being fussy? I'll present the single stack and double stack close up of the same area, I'm sure the double stack spicules seem to come in identical pairs?

really does look like processing there
Robert

Hi,

I agree with Robert that it is more a processing issue then acquire issue.

The contrast and details in the image are great.

Can you upload somewhere your original AVI in order to do some process trials.

Thanks :thanx:


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Re: PST double stack question

Post by Montana »

Thanks Robert and Rainer :)

Rainer I have absolutely no idea how to do that as the original avi's are 1GB?

Alexandra


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Re: PST double stack question

Post by rsfoto »

Thanks Robert and Rainer :)

Rainer I have absolutely no idea how to do that as the original avi's are 1GB?

Alexandra

Hi Alexandra,

Try it with this page https://www.wetransfer.com/ and my e-mail adress is

rsfoto at rsfotografia.com

As soon as it is uploaded I will get an email and then I can download it

Thanks :thanx:


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Re: PST double stack question

Post by marktownley »

Here's my take on the pic Alexandra emailed me...


Sun_R_24_06_2012_161828_g3_b3_ap1304 processed by Mark Townley, on Flickr

There is a shift in focus from left to right across the frame.

I don't think the processing is at fault at all, look at any of Alexandras images - they are all spot on.

The left to right shift says a penta prism that is starting to mis-align to me. The same thing happened with my CaK PST. The glue on the prism 'flows' with time and causes the original alignment of the prism to shift.

If you're feeling brave Alexandra, get an Allen key and take off the side of the black box on the PST and see if the top face of the prism is perfectly parallel with the top face of the inside of the black box. This will give you a definitive answer as to what is going on.


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Re: PST double stack question

Post by Montana »

Thanks Mark,

Oh my! the hubby pointed at the Allen keys last night and laughed, I can see me having to use them at the weekend :blink:

I guess this is what happens if you cart it around all over the place, I hope it's not terminal.

Alexandra


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Re: PST double stack question

Post by rsfoto »

Thanks Mark,

Oh my! the hubby pointed at the Allen keys last night and laughed, I can see me having to use them at the weekend :blink:

I guess this is what happens if you cart it around all over the place, I hope it's not terminal.

Alexandra

Hi Alexandra,

If you decide to open the BlackBix be very carefull with the heads of the screws eg. the Allen key should have a nice tight fit. It looks like Coronado uses LocTite in order to secure the screws and you really have to use force to open them.

At least that happened to me when I tried to open a PST BlackBox :blink: The last screw just did not want to get loose and voila the Allen key seating stripped, the hole in the screw, and is now round. The only way to get it out would be with a special tool which means to drill a hole into the screw, screw in the special tool and get the screw out.

I do not want to scare you. Just telling what happened to me :blush:

http://en.wikipedia.org/wiki/Screw_extractor

http://www.screwfix.com/p/screw-extractor-set/18643


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Re: PST double stack question

Post by Montana »

Oh my god...

Alexandra


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Re: PST double stack question

Post by rsfoto »

Oh my god...

Alexandra

Hi Alexandra,

I knew it :) Do not be afraid. If the first screw goes out look at it. If you see something like a white residue then you have LocTite on them. In that case try to heat up locally the screw as so the LocTite gets softer and manageable.

If you could send me the AVI file it would be nice. The image you posted has very similar symptoms as a few solar imagers do have here in Mexico. If there is exactly a twin of all the details for me it is still a sign of processing.

If I could get the AVI file I can say more, before you start opening your PST BlackBox.


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Re: PST double stack question

Post by marktownley »

Correct sized allen key is obviouslly important, don't worry Alexandra, just unscrew the Allens and take the cover off it will be fine.


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Re: PST double stack question

Post by peter drew »

If your Allen keys have a ball end, use the other end, the ball end is not such a secure fit. :)


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Re: PST double stack question

Post by Montana »

Thanks Rainer, but I am on limited broadband usage and I can't justify uploading 1GB data as this will use a significant proportion of our monthly allowance :( but thank you for the help. I will try and have another go at re-processing but I fear it is something wrong. My CaK PST is so easy to get crystal clear focusing and the Ha PST I am just really struggling with and I never had a problem before. In September I gave up as I couldn't find focus at all and I thought seeing was good. When it is cloudy I will brave a peek and update you all.

Kind regards
Alexandra


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Re: PST double stack question

Post by rsfoto »

Hi Alexandra,

No problem. Understand.


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Re: PST double stack question

Post by swisswalter »

Hi Alexandra

just take it easy, we are here for a while, not for a day ;)

when I opened my PST case, I had to drill open one screw only. I guess your hubby will assist, if not, get another (case or screw) ;)


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Re: PST double stack question

Post by colinsk »

Thanks Rainer, but I am on limited broadband usage and I can't justify uploading 1GB data as this will use a significant proportion of our monthly allowance :( but thank you for the help. I will try and have another go at re-processing but I fear it is something wrong. My CaK PST is so easy to get crystal clear focusing and the Ha PST I am just really struggling with and I never had a problem before. In September I gave up as I couldn't find focus at all and I thought seeing was good. When it is cloudy I will brave a peek and update you all.

Kind regards
Alexandra

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Re: PST double stack question

Post by Montana »

OK, plucked up the courage and took the screws out. I had a little help with 2 as the allen key got stuck on the screw and couldn't get it off the end.

So to the PST doctors here are a couple of photos of the patient:

I can't it says Kunena has a fatal error?


?

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Alexandra


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Re: PST double stack question

Post by robert »

Have another go later. I think it will be part of the problem and various people have fixed the astigmatism successfully.
Robert


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Re: PST double stack question

Post by Montana »

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 »

smaller file or just a link from flikr perhaps?


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Re: PST double stack question

Post by Montana »

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

Alexandra


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Re: PST double stack question

Post by Montana »


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)

Alexandra


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Re: PST double stack question

Post by swisswalter »

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 »

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 »

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 »

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 »

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 »

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 »

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 »

Hi Keith

that is a good point. Thanks for the pic


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Re: PST double stack question

Post by Merlin66 »

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 »

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 »

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 »

: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 »

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 »

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 »

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 »

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!
Robert


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Re: PST double stack question

Post by rsfoto »

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 »

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 »

Hi Mark,
Here is a picture when tightened.


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

Alexandra


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