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They don't 'arf glue them PSTs...

Andrew*

By Andrew*
in DIY Astronomer

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NickH

NickH

Posted
Posted

Peter, that system does rely on a perfect prism though and is limited by the 5mm BF on the PST to some extent (did try one similar)

Andrew...great that your ED120 is awesome...it really does make huge difference having both a good etalon and good donor scope...you'll surely get some great results

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Merlin66

Merlin66

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For anyone following this thread; the latest revison (R3) of the draft write-up on how to mod your PST is available.

Just drop me your email address and I'll forward a copy (about 2Mb)

Ken

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NickH

NickH

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Ken...I have had a few PM's about optimal spacing from Andrew and Gordon. I have referred both of them to your excellent xls sheet which you posted on UKAI. I don't want to repost your work, so do you think it would be good to put it on here (and mods make it a sticky...trust me...it's gold-dust!)

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Merlin66

Merlin66

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Nick,

The current illustrated write-up I'm preparing is much better!

It not only explains the critical spacing of the etalon but gives some practical hints and tips about physically transforming the PST to a PST80 or PST120 Ha solar scope.

This is available on request, just need your email address and I'll forward it to you.

Ken

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Andrew*

Andrew*

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Andrew,

email dropped!!

Ken,

Could you send a new one - the dropped one got damaged! :D

No, seriously, it doesn't quite answer my question. I was basically wondering what to look out for when I fine tune the etalon position using the ED120 focuser. How much, and what kind of improvement shall I expect getting closer to the magic 200mm?

This question may well answer itself on first light, but I would rather spend less time faffing to get it all right if I can avoid it!

Cheers

Andrew

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Merlin66

Merlin66

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Andrew,

If you're using the focuser on the 120 to set the final position of the etalon, there will be no ambiguity if you are using the black box with the eyepiece/ camera on the existing PST eyepiece holder.

When you have the "black box" PST focused at it's "normal" position the etalon must be correctly placed!! As simple as that.:D:)

If the focus position on the PST black box is too far out, or you find you can't get into focus, then the etalon via the 120 focuser need to be moved either in or out.

Depending on the final f ratio, and focal length of the "donor" OTA, the "sweet spot" will vary. If the ERF is not tilted enough you'll have a background glow.

All these things you'll sort out in the first five minutes!

Remember the PST tuning ring is VERY sensitive, it doesn't take much to move between a detailed surface view and prominences; or going from a good image to a poor image......

Getting accurate focussing for good imaging, may take a little longer.:D

I'll send you a copy of the latest Version 4!!

Ken

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Andrew*

Andrew*

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Ken,

I see that, but I am not using the black box. I'm using an SCT focuser and BF-10. As I see it there are three ways to get focus:

- the ED120 (etalon) focuser too far in, and the SCT focuser (eyepiece) too far out. Etalon too close to objective (poorly positioned)

- ED120 focuser too far out, SCT too far in. Etalon too far from objective (poorly positioned)

- Both focusers just right. Etalon ideally positioned.

I tried this the other day on a very distant terrestrial object, and a regular 1.25" diagonal in place of the BF-10. What I found was that changing the position of the etalon (i.e. everything) altered the focus at the eyepiece, but I could get clear focus no matter where the etalon was by countering the spacing with the other focuser.

At the least it would be good to know precisely what part of the etalon assembly should be at 200mm.

By the way - the tutorial is coming along very nicely.

Cheers

Andrew

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Merlin66

Merlin66

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Andrew,

Sorry mate! I thought you were doing the Stage 1 mod first!

The front edge of the knurled rubber ring is as close as needed to the position of the front barlow element. If this is 200mm inside focus then you're very close to the "optimum" position.

I found the size and intensity of the "sweet spot" changed with etalon position (and tuning!!); start where you think the 200mm mark is then work from there.

Ken

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Merlin66

Merlin66

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Based on the mod I'm doing:

If the etalon is <200mm inside the prime focus, the final focus distance is <200mm

If the distance from the etalon to the prime focus is >200mm then the final focus moves outwards and is >200mm

I'll do a sketch and the optical layout with calcs and add them to the write-up.

Ken

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NickH

NickH

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Excellent..

Interesting discussion at the momemt about the use of alternate blocking filters. I know from speaking to Ken at Solarscope that cross matching his and Coronado is a definitive no-no... but it seems that people are pushing the idea of using the B1200 with the PST etalon mod. It could work, but I don't know anyone who has tested it for eye safety, and if the trims "match" so to speak

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