Resuming PST mod

[GlassWalker]

I got a 2nd PST with the intent of taking it apart for playing with and modding... that didn't get very far as I couldn't get the front off. It got forgotten for a while, and today I had another go with help of someone at work. Nope, still not able to get the front tube off short of destroying it in the process. Is there any trick we might have overlooked? We had the scope in a vice, and nothing would grip it enough to turn it. Everything slipped.

If that doesn't go anywhere, I thought there's another easy option. Cut the tube off leaving a short length attached to the etalon. The tube appears to be 50mm diameter, which is near enough to a 2" eyepiece fitting. At worst I can wrap something thin around it to pad it out to a full 2". Also saves me looking for an adapter to go on the front had I got the tube off! Is there anything I'm missing here? Note for now, I'm only thinking of a stage 1 mod - replace objective only.

And finally, what scope? I've only got a few candidates: a ST80, ST120 and 8" SCT.

Firstly the ST120, at a standard f/6 that's far too fast for the etalon I know... but it also has a smaller 53mm aperture on the front cap to give a f/11 system. On the flip side, 53mm doesn't sound like much more than the standard PST at 40mm. But ERFs are affordable in this size. The ST80 would have a similar problem which may be easier, and I'm thinking a 2x barlow would resolve that? Actually, can you get barlows in 2" fit? Not sure I've ever seen one.

The other is the 8" SCT... it's already f/10 so that's perfect. At 5x the focal length of the SCT that should be able to give some nice close up views. But where can I get an affordable 8" ERF!? I don't want to go partial aperture again, as that would negate the point of doing the mod again... I need to look up notes again, does the coating on the PST original front objective have any significant filtering properties? If not, then in theory I shouldn't need one on the SCT either, since the same focal ratio would give the same light intensity through the etalon and black box. Failing that, I have some ideas involving lighting gels...

I suppose there is a 4th option: new scope! Any low budget contenders? I'd like to aim for 80mm to give a tangible increase over the original PST.

[keithatrochdale]

The TAL100RS makes a good conversion, with an 75mm D-ERF part way down the tube.

This site may be of use; http://solarchat.natca.net/index.php/en/?func=listcat

[GlassWalker]

That scope seems to hit the right buttons in spec and price, and that forum... I just spent a while browsing on there... could suck me in!

[keithatrochdale]

They really do make a good conversion, performance is out of this world.

Shame you are so far away or you could try it.

[Merlin]

Thanks mainly to Ken ( Merlin66 ) and Peter Drew, I did a stage 1 mod using a 70mm/f.10 SKYWATCHER refractor. I fitted a BAADER 75mm D-ERF over the front of the objective.

The assembly works well, allowing me to get up closer to the proms. On Ken's advice, I removed the PST's mimi-ERF for a brighter image.

[GlassWalker]

I think I'll eventually push for the bigger size of the TAL. I'll have to remove the mini-ERF on mine anyway since it has degraded already.

The first step is now done... the PST is on the desk next to me in two parts. Looking at the etalon I suspect I have fungal growth But it looks light enough that I think I'll just try to kill it and leave it.

[Merlin]

I doubt whether I'd want to do a sub-aperture mod because of the heat being more concentrated on the internal D-ERF. I'm no expert, but it seems to me that the life of the D-ERF would be shortened in this mode.

Incidently, besides having a D-ERF over the front of the modded PST, I've also fitted one over the front of the unmodded PST; this should extend the life of the etalon, mini-ERF and blocker.

[keithatrochdale]

I doubt whether I'd want to do a sub-aperture mod because of the heat being more concentrated on the internal D-ERF. I'm no expert, but it seems to me that the life of the D-ERF would be shortened in this mode.

Incidently, besides having a D-ERF over the front of the modded PST, I've also fitted one over the front of the unmodded PST; this should extend the life of the etalon, mini-ERF and blocker.

Where would this heat build-up be? After hours of my TAL tracking the sun, I can find no temperature change.

It seems with a Baader D-ERF D=75mm at 15" down the tube, any heat is reflected back out of the tube. And using a 75mm instead of a 110mm saves nearly £200 - got to be worth considering!

[Uplooker]

Hi Keith,

How do you fit / attach the D-ERF D=75mm at 15" down the tube? Apologies for my ignorance.

Ian

[Merlin66]

Glad to see there's still interest in PST mods.....

Removing the gold tube can be a PITA, but I find a good heavy duty BOA ALWAYS removes it. Shock treatment works!

Glasswalker: The PST etalon cannot deteriorate...it's just two pieces of coated glass VERY close together...do you mean the mini-ERF??

The SCT solution seems to work, there's enough backfocus to get the -200mm but finding a good full aperture ERF is another matter!

Quote:

"" the coating on the PST original front objective have any significant filtering properties? If not, then in theory I shouldn't need one on the SCT either, since the same focal ratio would give the same light intensity ""

The newer PST has no ERF coating on the objective. The energy build up is directly related to the aperture, the 8" will give x25 more energy concentrated onto the etalon!!!! Definately needs at up-front ( or sub-diameter) ERF fitted....

[GlassWalker]

I think sawing the gold tube was a great idea. If I had got the tube off the etalon, I'd be hunting around for a new adapter to make it fit a scope about now. The 50mm tube is a nice fit to a 2" focuser, although I still intend to pad it slightly to make it tighter.

What I see on the etalon of my mod-PST looks like initial stages of fungal growth. It is not the degradation of the ERF since it is physically at the etalon, and the ERF isn't even fitted on the scope at the moment. There are what appears to be fine thread like structures visible. Fungus can grow on optics if kept in the wrong conditions for long enough, primarily dark and humid ones, where they feed off the lens coatings and/or glue. I got this scope used so I don't know the storage history of it. It's almost ironic that one common way to kill fungus is to expose it to sunlight (specifically UV)! The previous owner did complain about not getting much of a chance to do that...

I speak with a photographic hat on when I talk about the light intensity. I don't think the aperture is the key factor here, as the focal ratio is that which determines the image brightness, and therefore also the amount of light passing through. That might sound counter-intuitive, as a bigger opening must mean more light collection right? What I think is happening is that it is offset by the much longer focal length. You're looking at a much smaller patch of space, thus the light is spread out more.

I was thinking of playing a middle ground anyway, have a partial ERF on the front. That is still under consideration.

[keithatrochdale]

Hi Keith,

How do you fit / attach the D-ERF D=75mm at 15" down the tube? Apologies for my ignorance.

Ian

Slot cut in tube under the holder cover

Holder to hold D-ERF in tube at correct position; insert into slot in tube and held with two screws

[hunterknox]

I've been wondering how Keith's sub-aperture mod works. Neat-o!

[Merlin66]

Glasswalker....both the original PST and any mod i.e. C8 would work at f10. The etalon being at -200mm means that they would both fully illuminate the 20mm disk - there would be far more energy concentrated by the C8.

In all the PST's I've modded over the years I've never come across one which showed deterioration of the etalon...

Is it possible to get a photo?

[GlassWalker]

Hard to get a shot of it, but if that isn't fungus, I don't know what it is.

[Merlin66]

Not clear from the image...does it appear to be on the external surface or internal to the etalon??

Very strange..............

[GlassWalker]

I can't be sure. To me it looks like the symptoms of fungal growth. I've seen many similar cases on photographic lenses.

[allcart]

This is an interesting concept. I had been looking at the PST's but didn't know you could mod them for use with a bigger scope. I need to look into this more.

First question...can you attach a dslr to the PST for imaging?

[GlassWalker]

The PST doesn't have enough focus range to use a DSLR at prime focus without modifying the scope. The two ways around that are to either use eyepiece projection (some eyepieces have a T-thread so you can connect it directly to the camera via T-ring) or use a barlow which can extend the focus distance.

[allcart]

Thanks. Using a barlow doesn't seem much of a problem. Did you finish your mod yet?

[GlassWalker]

Not really, I still need to buy another scope and ERF. Not in any hurry as right now the sun is too low for me to get anyway, so this is something to pick up again in a few months.

[allcart]

Same here, but come the summer the sun will be high enough for me to spend some time on it when I am at home during the day. ( I work a split shift so have 3 hours doing nothing from 11am till 2pm )