PST Query

[robindurant]

On removing the side plate of my PST I see that there is a Red vignetting on the clear glass of the EP holder. Is this as it should be ?

[NGC 1502]

Hi, are the views thru the scope ok ? If so, I'd be happy myself. I have looked down the eyepiece holder on a PST, from the other side of your pic,

and the filter that is visible is quite tiny, far smaller than the red area.

Regards, Ed.

[Merlin66]

Hmmm,

You're looking at the mini-ERF on the bottom side of the eyepiece holder.

It looks like it is seriously deteriorated and suffering from "rust" - this unfortunately is a common occurance on these filter elements.

If you have the original purchase paperwork and the PST is less than five years old, this should be replaced under the Coronado warranty.

Otherwise, a replacement will be needed to return the instrument to full working condition.....

[Merlin]

I thought CORONADO had sorted out the problem of rusting. Is this one of the older PSTs? What actually caused the problem in the first place, humidity?

[Merlin66]

The original problem of PST objective "rusting" is long gone.....

Unfortunately the replacement "mini-ERF" mounted close to the final blocking filter still takes a fair amount of energy and breaks down after a while.

The internal ERF in the Lunt also fail.......

Seems to be energy x exposure = failure

A good up front D-ERF (ie Baader) seems to last forever - better interference coatings? and this could/ should provide more protection to both the etalon and the final blocking filter.

[robindurant]

Thanks Guys. I have contacted the Importers, Telescope House who inform me that as I opened the plate to inspect and find the ERF fault, it makes my warranty nul and void !!

How do they know I opened it. How would I know I had a problem without opening it up. Anyway I needed to adjust the prism which was out of line.

I bought this PST from Bern, Modern Astronomy July 2008.

As the centre of the ERF is clear then maybe as Ed say, it's not a problem.

[Merlin66]

Yeah,

A typical catch 22...

Jack Martin went through a similar problem....finally got it repaired...and the "new replacement" mini-ERF was as bad as the original!!

He ended up returning it for credit!

It will work, but won't be 100%

Sorry.

[Merlin]

If any forum members are confused about which "Merlin" is which, Ken is the solar expert and I'm plain old Wizard Merlin. My raven has just whispered in my right ear as to whether a BAADER ERF mentioned by Ken could be placed in front of a PST to compensate for a failed internal ERF?

[Merlin]

More bird-brained queries. Can the failed ERF be left inside the PST? If so, would the addition of the BAADER ERF not darken the final image too much?

[Merlin66]

IMHO, it would be much more cost effective to do at least a Stage 1 mod on the PST; remove the gold tube, remove the mini-ERF and replace with a 60mm+ f10 donor scope (the bigger f10 the better!) fitted with a Baader D-ERF. It's like giving it laser eye surgery....the full potential of the etalon comes to life!

I have a detailed wtite-up available on PST mods- PM me an email address for a copy.

[Merlin]

I have a SKYLUX 70mm/f.10 'scope that I could use for a mod Ken, but I'm satisfied at this point in time with the views I get with my unmodded PST.

I'm more concerned to extend the life of the PST by tasking your advice of placing a 75mm BAADER D-ERF in front of the objective.

What concerns me here is that having three EDFs in the optical train ( one over the objective and two inside the PST ) the final image might be too faint, especially with respect to proms.

What would you advise?

[Merlin66]

An external ERF combined with the "mini-ERF" at the blocking filter is the usual set up after a mod. These two ERF's seem to work well and visual/ imaging is not compromised.

It really comes down more to the sensitivity of the imaging camera (DMK etc)

The final blocking filter (BF5/ BF10/ BF15 etc) must always be there to act as a sorting filter for the etalon "spikes"

[Merlin]

Thanks Ken. I was under the impression that there are two internal ERFs in the "improved" PST. One behind the objective and also the mini-ERF at the eyepiece end. You seem to be saying that there's only the mini-ERF, unless I've misunderstood you.

I'll purchase a 75mm ERF ( BAADER of course ) and, for the time being, make a cell and fit it over the front of the 40mm objective, with the option to mod later. As a matter of fact, the amount of detail I can see with the unmodded PST is pretty astounding in itself.

I'm hoping that by adding the BAADER D-ERF, I'll be able to maintain the integrity of both the etalon, mini-ERF and blocker for considerably longer than normal.

[Merlin66]

Yes,

The orginal PST had the ERF coating on the objective, and no interior "mini-ERF"

When Coronado dumped the front coating and went to the "blue" objective, they added the "mini-ERF" to the eyepiece holder.

If you do use the Baader up front, you certainly don't need the rusty mini-ERF - it can be safely removed......

[Merlin]

I don't have a "rusty" mini-ERF, my PST is brand new. It's going to be a bit awkward making a cell for the BAADER 75mm ERF because I no longer have access to the local F.E. college's engineering workshop ( it ran model engineering classes ), they've demolished it! It's the end of an era.