My 70mm PST Stage 1 Mod

[michael8554]

Here's my PST Stage 1 mod.

Donor scope is a Meade 70mm F10 Achromat, ERF is a secondhand Baader 70mm C-ERF, both found on UK astro Buy & Sell.

My local hardware shop had a Boa Baby wrench for £6, but not the larger ones.

With that I managed to remove the PST front element, and the etalon and gold tube from the black box, but I couldn't seperate the etalon from the gold tube with the Baby Boa.

This didn't matter because I reckoned that the f10 lightpath from the 70mm scope should pass nicely down the F10 gold tube, if correctly located.

I had a couple of ashwood doughnut ring adapters turned for me, 50mm long, 70mm OD, and 50mm IDD.

These could also be made in aluminium, from 70mm bar bored out 50mm.

One of these rings was located flush with the end of the gold tube, held in place by silicon sealant on the gold tube, and its face blacked to minimise reflections back to the objective.

The other ring was located close to the etalon, leaving room to turn it, again held on the gold tube by silicon sealant.

The 70mm OTA was shortened and the PST inserted into the rear and held in place by woodscrews 120 degrees apart into the rear wood adapter.

At the front I found the 70mm doublet was held in place by a male threaded plastic ring with quite a lot of thread on it.

I blackened the edges of the two elements and then placed the ERF in front, held in position by a carefull choice of nitrile rubber O-rings.

The doublet was actually 73mm in diameter so I inserted one 2.5mm thick O-ring, giving me a 68mm ring to sit the 70mm ERF on.

To keep the ERF centralised I put a couple of 1.5mm thick O-rings between the ERF edges and the OTA.

Finally another 4mm O-ring and then the threaded ring to hold everything in place - simples!

Finally to mount the OTA onto my driven EQ mount I needed two 70mm split OTA rings.

Orion USA make these and they cost £29 plus postage.

Instead for a tenner I bought two Canon lens tripod mounting rings from Hong Kong.

These come in many sizes and I simply chose ones with 70mm ID.

Costs:

Meade 70mm scope £20

C-ERF £120

Wood adapters £20

O-rings £6

Canon tube rings £10.30

Total £176.30

What I like most about this mod is that the only metalwork involved was shortening the 70mm OTA.

I'm considering shortening the OTA some more and then elongating the 3 rear securing screw hole into slots to give two working lengths, one for optical and one for webcam setups.

The OTA has some redundant screw holes that need properly covering - has anyone got a good method for filling holes in OTAs?

Michael

[JamesF]

The OTA has some redundant screw holes that need properly covering - has anyone got a good method for filling holes in OTAs?

Saw the heads off some domed bolts and stick them (the heads) over the top with the silicon sealant?

It looks very neat though. I like the idea of using the lens mounts for tube rings.

James

[DrRobin]

Very nice Michael.

To cover the holes, just put the screws back in?

Robin

[michael8554]

I forgot to give credit to Merlin66 for his excellent PDF on modding PST's, without which I would never have started or completed my mod.

Thankyou Ken

Michael

[michael8554]

Although I'm a hardcore bodger I don't like the idea of nuts sticking out inside the OTA.

JamesF has got me thinking of glueing those plastic domes you put over screwheads on packflat furniture.

[lukebl]

Nice cheap mod (unlike the 500 notes i spent on my D-ERF filter, when a smaller one have been just as good!) I agree, Merlins instructions are excellent.

Well done.

[JamesF]

JamesF has got me thinking of glueing those plastic domes you put over screwheads on packflat furniture.

Or you could use the chromed mirror screw covers. They'd probably look smarter.

James

[Zakalwe]

Lovely mod!

To blank the holes you could use a pop rivet (blacken it inside). Look for ones with collapsible stem like these:

http://www.shop4fasteners.co.uk/acatalog/Aluminium_Steel_Multigrip_Rivets.asp

Or do a search for blanking grommets, like these:

http://www.mossexpress.co.uk/grommets-strain-relief-and-blanking-plugs?gclid=CKjiu5PP8LgCFafLtAodFBAAjQ

Halfords or a motor factors will probably have what you need.

[Merlin]

I filled the holes in on my stage 1, 70mm mod, with plastic wood from Wilkies. The wood squeezes out of a tube like toothpaste and can be smoothed down and painted when dry. There's also plastic metal ( from engineers' suppliers ) but this is more expensive.

[Merlin66]

Looks pretty good!

Just one comment..you don't mention tilting the ERF.....

Any results yet?

[michael8554]

Ah, someone's paying attention!

Yes, I tilted the ERF, provisionally with two guesstimated pieces of cardboard until I get it right, you can see one at the top of the ERF in my 3rd image.

Weather has been too much of a mix of sun and cloud to do any serious tests.

Under clear skies I would have time to do back to back tests with and without tilting so I can see what effect I'm chasing.

Initial results are promising, I can focus and tune for proms and sunspots, but I may have a bit of internal reflection to sort.

Is there any gain in taping over the port on the black box where the finder light comes in?

And blacking the edges of the prism?

Michael

[Merlin66]

I don't believe blacking the edges does much...

The internal reflections shouldn't be any different from what was seen in the original PST set-up.

Check the internal baffles ( if there are any (!)) in the OTA and any bright edges.....