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Commercial PST mod kit..

pauldaviesuk

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pauldaviesuk

pauldaviesuk

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Hi

I've been planning/scheming/daydreaming(!) about a Ha scope since the SGL Imaging day (TBH..its been at the back of my mind for years.. the imaging day just upped it profile!) and spent a reasonable time looking into the various options on the market, trying to gauge the all important functionality/cost' ratio.

I've looked at Coronado's PST and Lunt's LS35, 50 & 60 models.

That said Nick's presentation at the Imaging Day, specifically his almost evangelical portrayal of his PST80, really resonated with me.

I've read various accounts of how these mods have been achieved, which seem to vary from the high end of the quality spectrum (as seen on SGL) to the other...some even employing gaffer tape and coffee cans!

So...(jeez.. I waffle at times)...I've just come across the PST modding 'kit' that Pulsar Optical are selling. I've just called and spoken to them and they were very helpful. It includes an ERF, PST etalon->2" focuser adapter and a pre-cut tube for a Z80(type) OTA....It would appear to be a fairly comprehensive set up...just add 1 80mmf10 OTA and 1 PST and go...

Any one with any experience of this particular kit?

I understand that the ideal setup would be to have just the etalon from the PST fixed to the tube and used with a separate BF and focuser... but there is nothing to preclude that in the future...

On that subject, can any PST modders out there confirm the PST etalon -> 'black box' thread? Is it the same as the etalon -> 'gold tube' thread? M50 male?

cheers

Paul

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Merlin66

Merlin66

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The rear thread is the same as the front thread = M50 x1

The available parts: Baader D-ERF in a cell

a 2" to PST adaptor - available ex- Telescope Austria

only need a f10 OTA ( IMHO anything different is only a compromise and will NOT perform at optimum) obtaining the 200mm critical dimension inside focus is the only currently outstanding issue.

A suitable aluminium adaptor to fit the end of the OTA ( remove the focuser) would NOT require cutting the donor tube and could easily be machined ( I have sketches available to anyone who needs them)

That's it for the #1 Mod.

The #2 mod ( dump the black box) only needs a SCT>2" adaptor which almost ( near as d&*^%) fits the etalon rear thread, a couple of spacers and a BF10. The BF5 can be adapted to a 1.25" nosepiece/ diagonal and used until the larger BF filters become available.

Hope this helps,

Ken

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pauldaviesuk

pauldaviesuk

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Thanks for that, Ken. That a male thread? on both side of the etalon?

TY also for the note about the etalon thread matching the 'standard' SCT visual back thread. I have (in the mail) an SCT helical focuser. If that will screw onto the etalon, I would imagine it'd be a fairly simple task to get the 200mm needed by the etalon witht a BF diagonal and maybe a spacer.

TBH the pulsar kit includes all you mentioned in 1 package - the ERF in a (tilted) cell and a M50x1 2" tube

cheers

Paul

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Merlin66

Merlin66

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The front thread is a M50 x1 female and the rear a M50 x1 male.

The SCT thread is 2" x 24tpi - so very close, a bit of pipe tape on the threads and a couple of nylon lock screws usually does the job.

Just watch, if you attempt to screw the SCT focuser directly onto the rear of the etalon that the draw tube etc doesn't impact with the rear lens.

All we really need is to find a machinist to make up the rear adaptor plate for the donor OTA........

Ken

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Helen

Helen

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Hi Kev,

Here's a link to the one I've just bought

Baader D-ERF Energieschutzfilter - D = 90mm - Teleskop-Express: Astro-Shop + Fotografie + Naturbeobachtung

Great service - ordered Wed, with me Saturday, fully tracked too!

I'm going to have a word with a friend to see if he'll make up some adapters (SGL preparations etc have stopped me doing it so far..). I'll let everyone know!

Helen

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NickH

NickH

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I'm saying nothing about the incorrect focal length and non optimal ERF solution using the astigmatic prism body of a PST solution being sold as a kit..by a dealer with a notorious track record for delivery of goods...all I will say is ask anyone at SGL 5 what the real deal looks like :-)

The blanket I had on site was known as the "******* hell" blanket by the end of the weekend, as EVERYONE who had a look at the laptop screen buried beneath it blurted out some expletive or other when they saw a LIVE view of the Sun through a properly modded PST80

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NickH

NickH

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Vixen A80M...by hook or by crook...unless you find a TMB achromat...at F11 that's the one to get..

SW909's are also okay, but variable quality. The Vixen's are always pretty bang on..

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pauldaviesuk

pauldaviesuk

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how critical is it that the donor OTA is f10?

Ken has said 'IMHO anything different is only a compromise and will NOT perform at optimum', but Nick is advocating the Vixen A80M which is f11.4 as the optimum solution.

How much flexibility is there here? I've found a possible candidate in an 80mm f8.75 achromat. would this be worth investigating?

I guess an alternative would be an Evostar90, though that'd ofc require a larger ERF...more $$$

Paul

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Merlin66

Merlin66

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Critical!

The original PST etalon is designed and configured for an f10 beam.

I'd give you f9 to f11 ( I know from first hand experience that the Vixen A80Mf works 100%) I'd tend to go towards the f11 rather than the f9 or lower.

Just my 2.34c (after tax) based on modding some 12 PST's so far - and the count is rising.....

Ken

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NickH

NickH

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The Vixen is not F11.4 when you chop it :-)...(edit...doh oh yes it is...it's just the back focus that changed), even though the focal length of the tube has been reduced, the focal beam is just brought forward to accommodate the lens on the PST...

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NickH

NickH

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aye...that's true, but the focal length back focus will bring it inside the requirement needed... was on crossed purposes (edit DOH!)...thinking that by reducing the focal length, it would adjust ...but you're right...the lens assembly is an F11.4 setup......me being a numpty not thinking about the actual lens setup on the PST and why it's brought forward in the first place to accomodate

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pauldaviesuk

pauldaviesuk

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So.. I think I have a plan...I have an old 60mm f/11.7 Achromat, which I'll use as a test bed. Its a little longer focal ratio than the A80M, but not by much, so if the Vixen works, I guess this'll be close enough for me to use to sort out the engineering details, before scaling up.

Paul

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Peter Drew

Peter Drew

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I think a lot of this is to do with what level of performance you are looking for. I currently have 4 scopes all modded to what is now becoming known as "stage 1". They are a 6" F8 Skywatcher stopped down to F10, a 4" Vixen flourite operating at F9, a Skywatcher 100ED operating at F10 and a Sky 90 operating at F5.6. Despite the obvious divergence from the optimum in some cases, each has given performances vastly superior to the original PST's. The Sky 90 combination, the worst case senario of the four was at Kelling last Autumn and matched the performance of two other Sky 90's fitted with 90mm Coronado front etalons.I,m convinced that the "stage 2" modification will be even better in due course but for now I'm enjoying the easy route.

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