Buying a 2nd hand PST

[Uplooker]

I may have the opportunity to purchase a used PST. It is one with the red ERF at the front of the gold tube. Is it only the ERF that suffers from “rust”? What about the etalon itself, does that suffer from “rust”? I will not have the opportunity to dismantle the PST to examine it.

Is there anything else I should check for, other than “rust” on the ERF on the front of the gold tube?

I plan to use this for a PST mod project, so if there is “rust” on the ERF would it still be a reasonable buy as to do a stage 1 or stage 2 mod the front ERF is not utilised.

Ian / Uplooker

[John]

I'm not an expert in what to look for but the Cloudynights forum run a register of PST's that are known to have issues:

http://www.cloudynig.../23#Post4275375

It seems to get updates fairly regularly. When I had a PST (a non-rusty one) I posted the serial number there.

[hunterknox]

If you're modding it then rust at the front end, which would be immediately apparent, shouldn't put you off. I'm sure you could negotiate a better price for a rusty one than a non-rusty one. If it's got a blue lens then that's from the post-rust era.

Never heard of an etalon rusting. I think blocking filters can degrade but I think that's more apparent from the underside, which you can't see without taking it apart. The odds of you getting a decent one are still strong, but ultimately if you're buying second hand you need to be comfortable with some level of risk - that's why you pay less than new.

[Peter Drew]

For a PST mod, stage 1 or 2 the condition of the front objective is imaterial as this component is not used. As Hunternox says, there is little evidence of etalons deteriorating but the condition of the 5mm blocking filter is important and should be examined for corrosion like appearances.

[Merlin66]

As per the above....

I certainly try to look through it before buying, if you can. Even with a "rust" objective you should still get a good view of the solar surface and visible proms (albeit with less constrast)

The heart of the PST is the etalon. Up until last week I would have said they are generally "bomb proof" - if you have a good image in the PST you'll have a good image in the Stage 1/ 2 mod.

( The reason for the comment is that someone posted a image of a PST etalon which appeared to show deterioration of the front lens coating - fungus? In all the PST's I've modded and stripped down this is something I've never seen......)

[Merlin]

I recently bought a secondhand PST from a shop similar to "Cash Generator" for £300. Of course, as Ken said, we should always try out the 'scope before purchasing, which is what I did with success.

It was easy to do a stage 1 mod and, on Ken's advice, I removed the mini-ERF from the PST for better clarity. I have a 75mm BAADER ERF over the front of the 70mm objective of the donor 'scope.

Now, I'd like to replace the PST's pentaprism with a right-angled prism. Good luck with your mod and looking forward to seeing your observation reports.

[Uplooker]

Thank you very much to everyone for your replies and information.

Well, i am now the proud owner of a PST. It does suffer from "rust" on the main objective. The limited views that have had through it are good. I certainly feel that I have got a lot of scope for not a lot of money. From what others, here and elsewhere have said, there may be a slight loss of contrast - it is certainly not evident to me.

All in all I am very happy.

Ian

[Merlin66]

Well done!

Enjoy.

[jimmyjamjoejoe]

I have a rusty one too, looking through just the objective, it looks like water marks left a ring around the outside about 20% of the way in.

Not sure if it effects views really, but there does seem to be a "sweet spot" for viewing proms right in the center of the fov. They tend to fade if i centralise the disk rather than the proms.

[Peter Drew]

Pretty normal for a PST, "rusty" or not.

[Merlin66]

Jimmy,

Does your PST have a "blue" objective or does it look more "gold colour"?

The "blue" ones don't deteriorate ie rust

[jimmyjamjoejoe]

Hmmmm... Mine's blue. The "rust" just looks like rust coloured powder, on what i thought was the back of the objective. After removing the objective however, it seems that it's between the lens group...

[Merlin66]

Any chance of a photo?

I can add it to the "list of things that can go wrong......"

[jimmyjamjoejoe]

Here's a couple of images - http://imgur.com/D37k5&Xre1U#0

serial #99404.

Views still seem ok, but i've not used a new pst so i wouldn't know how good they are supposed to be. Most importantly though, is it still safe?

[Merlin66]

Hmmm,

With a serial # like that I'd expect a "gold" objective...unless it's already been repaired under warranty.

I assume we're looking up the tube from the rear towards the objective? Why the yellow colour?

My blue objectives are only AR coated - and look clear just like any other refractor lens....

In either case it's stil 100% safe, you'll loose some contrast.

[jimmyjamjoejoe]

yeah it was from the rear. it's red in there, i guess the camera messed with the colour but it was just to show the "watermarks".

The lens is a cyan kind of colour just like in the photo, unless tilted in such a way, then you see the rust.

It's not even mine, so not too worried, just borrowed it from my dad who seems to have forgotten about it.

Maybe when i find myself with too much cash i'l go for a stage 1... So long as it's safe until then... Thanks Merlin