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How long do the filters in a PST last?

keithatrochdale

By keithatrochdale
in Observing - Solar

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S.A.M

S.A.M

Posted
Posted

I used to have a PST, brought it about five years ago and had no trouble with the filters. I sold it to a fellow astronomy friend about two years ago and he says there is nothing wrong with the filters. I think the serial number was around 101000? The PST was brought in the US, not that should make any differents. Could also be the way the PST is stored when not in use, that might decrease the life of the filters?

Peter

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Merlin66

Merlin66

Posted
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I've stripped and modded many PST's...

So far I haven't seen any deterioration problems with the etalons.. but I have seen some pre-filters (in the eyepiece holder below the blocking filter) which have failed.

I've also found a couple where the blocking filter as started to deteriorate.

I have no idea whether it's due entirely to usage or just the "normal" life.

HTH

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Merlin66

Merlin66

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In the original design the front objective was coated to act as an energy rejection filter (ERF) this Gold coating reduced the amount of energy going in to do two things...reduce the overall brightness of the image and add some protection to the Blocking filter....this concept failed! The ERF coating deteriorated quickly (rust) and was replaced with a normal objective. The ERF was then redesigned and mounted just in front of the blocking filter in the eyepiece holder.

Note - that there's now no ERF before the etalon....

HTH

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keithatrochdale

keithatrochdale

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So if there is no ERF in front of the etalon, when converting a PST there is no need to have one on the front of the donor scope? This could save some money.

Comparing two older PSTs and one newer PST, there is quite a difference in what they can see, the newer 'seeing' more; would this not indicate that something goes 'off' with time?

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Merlin66

Merlin66

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NO, NO, NO

I strongly (like DO IT!) recommend that any front aperture above 50-60mm be fitted with an ERF. The Lunt has a 60mm without ERF, but I honestly believe this to be a limit.

There's a lot of energy going into the scope and the etalon passes about 10% through to the blocking filter - that's still a lot of heat and power!

The pre-filter blocks the energy and the blocking filter only passes a bandwidth of 1A which dramatically, obviously, reduces the throughput to very safe and acceptable limits.

Seriously, any PST mod MUST include an up front ERF.

Over the years I have found no significant difference in performance between the original "gold" objective and the later "blue" objective.

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keithatrochdale

keithatrochdale

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With respect, is there any data or research to back-up your opinions?

Have you done any testing to see what is being passed and measurements to see if there is any heating of components?

Have you tried a modified set-up without a front ERF?

Please understand I am trying to learn and obviously want to be safe.

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Merlin66

Merlin66

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Yes, to all of the above.

I've modded quiet a few PST's and have done spectroscopic measurements of bandwidths etc.

If you can find anyone with more experience...I'll listen to them.

My 6.5Mb write-up on PST mods explains the various filters used in the system...the ERF, the etalon and the blocking filter.

HTH

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Merlin66

Merlin66

Posted
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Yes, V5e is the latest available..an update and conversion to PDF is in progress. There's also a review and assessment of ERF filters available.

Just drop me a PM with an email address (needs to be able to accept a 6.5Mb attachment) and I'll send a copy.

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Smeg

Smeg

Posted
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In the original design the front objective was coated to act as an energy rejection filter (ERF) this Gold coating reduced the amount of energy going in to do two things...reduce the overall brightness of the image and add some protection to the Blocking filter....this concept failed! The ERF coating deteriorated quickly (rust) and was replaced with a normal objective. The ERF was then redesigned and mounted just in front of the blocking filter in the eyepiece holder.

Note - that there's now no ERF before the etalon....

HTH

So these could be returned under the "not fit for the purpose it is sold for" and "should last a reasonable amount of time (even outside of warranties)" clauses in the trading laws.

Things like this just put more uncertainty and doubt into potential new purchases such as me lol

If they have a lifetime of 5 years, thats 100GBP per year rental :glasses2:

This would probably rust even quicker where I am, even our door handles rust fast here.

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Merlin66

Merlin66

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A "reasonable amount of time" in this case is five years.

Meade/ Coronado do replace the failed "gold" objectives within five years of purchase. No argument...just takes a lot of time to happen.

As I said, I've seen and used many older PST's (with the blue object changeover) and they are still going strong.

I recommend them as a very good Ha solar telescope.

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Smeg

Smeg

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A "reasonable amount of time" in this case is five years.

Meade/ Coronado do replace the failed "gold" objectives within five years of purchase. No argument...just takes a lot of time to happen.

As I said, I've seen and used many older PST's (with the blue object changeover) and they are still going strong.

I recommend them as a very good Ha solar telescope.

So a telescope shouldnt last more than 5 years?

There has been many cases in the UK where a car gearbox failed outside the manufacturers warranty and it had to be replaced by them, FREE. Because a gearbox should last more than a year or two reasonably.

So if the PST's only "reasonably" last 5 years, I wont be buying one. Thats another lost sale. WHere I am currently rusting is an even bigger problem, they would degrade faster.

I think I will spend my 500 GBP on a telescope that lasts more than 5 years.

Seems PST's are not mature enough, I will let the current set of adopters iron out the bugs.

They can afford to be a tester for them, I cant.

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spaceboy

spaceboy

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I gather you are talking about purchasing new Smeg??? The rust issue was not an issue with the newer models only the older ones with the Gold objective (and apparently a replacement purple one??? from what I have read). While I agree it's simply not worth the risk buying a PST S/H I would not have the same level of concern buying new but that's just my opinion.

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Smeg

Smeg

Posted
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I gather you are talking about purchasing new Smeg??? The rust issue was not an issue with the newer models only the older ones with the Gold objective (and apparently a replacement purple one??? from what I have read). While I agree it's simply not worth the risk buying a PST S/H I would not have the same level of concern buying new but that's just my opinion.

So how does the new coating last and work?

It is as good as gold? Probably not.

Still there is no proof these will last longer, I might wait a little longer. Let the rich people test them :glasses2: The old saying, a fool and their money is easy parted :rolleyes:

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keithatrochdale

keithatrochdale

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This is interesting as I just bought a s/h PST and saved a good deal of money - so who is the fool? :-)

Today I checked the serial number of the other two PSTs I have access to and was surprised to find my (104490) PST was only 174 from one of the others (104324), the other was (0995574) which had the rusting lens problem.

Now the seeing through my PST is noticeably better than both the others so I was expecting a bigger s/n gap.

We were expecting both the other PSTs to be much older and the filters 'going off', but it would seem that this may not be so. So it does raise the question of 'Are all PSTs equal?'

We have tried swopping individual parts of the PSTs to see if there is any improvement, it seems it is the etalon that is not performing as well.

Now can anybody help by letting us know what serial number their PST has and when it was bought? It would be interesting to try and build a rough timescale of serial numbers so we can judge the rough age of PSTs, thanks.

Hopefully next week we may have more data when my researcher returns from trying other PSTs at Kelling Heath. :-)

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ScubaMike

ScubaMike

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I've stripped and modded many PST's...

So far I haven't seen any deterioration problems with the etalons.. but I have seen some pre-filters (in the eyepiece holder below the blocking filter) which have failed.

I've also found a couple where the blocking filter as started to deteriorate.

I have no idea whether it's due entirely to usage or just the "normal" life.

HTH

Merlin,

as you appear to be an authority on this......

How would you tell that the blocking filter was failing? (without stripping the scope).

How dangerous is this? (would it be an eyesight risk?)

Can you do anything once it fails?

Mike

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