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Corroded ITF PST Mod

Moonshane

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Moonshane

Moonshane

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Until recently, I have been using my 100mm PST Tal based stage 2 mod happily in mono and getting excellent detail. Over the past few months I have been working towards using binoviewers and initially was very impressed with the views as they increased the level of contrast and viewing was so much more comfortable. I was though getting slightly frustrated as the views were getting much darker than I was used to in mono even at lower powers.

At the DIY Peak Star Party at the weekend, I had a slight epiphany when I looked through Stu’s similar 102mm Vixen based PST mod. The views were really quite phenomenal compared with mine. Even at really high magnification (perhaps around 150x-200x) the views were bright and detailed. Mine were dim and with less detail in the proms but possibly dimmer but more detail in the surface detail, almost like when thin cloud passes across the sun while observing.

We tried all combinations, and the views did not change. That was until we tried Stu’s whole stage one assembly in my focuser. This then provided comparable views at last! This to our eyes meant that there were fewer ‘culprits’ causing the differences: the blocking filter, the etalon or the internal workings of the PST box. We considered the latter to be unlikely so the etalon or the blocking filter were thought the more likely two.

Along with another SGLer, I consider fellow member Peter Drew a genuine expert of all matters solar and he built Stu’s version so I sent Peter a message and described the symptoms. Peter highlighted a more likely cause and one that matched my symptoms well. His prognosis was that my ITF (the little reflective filter at the base of the PST eyepiece holder) was the problem. I could not really see any issues in situ but on removal (it was pretty firmly held in its cell with loctite) and holding against a light box I could immediately see the apparent signs of rusting/heat damage. I suspect this has always been there (since I bought my PST mod parts anyway) but as I have only used it in mono (and those views have not changed) the brightness levels have been high enough for lower power views anyway. I attach pics below which show slight milkiness in all but a clear ring about a third out from centre.

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As I use a Baader DERF, the ITF is not actually necessary (but the blocking filter definitely is necessary) as all the heat and UV are removed from the path before even going through the etalon. Therefore I have removed the ITF and now hope that the BV view will be more like those I had through Stu’s at least at higher powers. Lower power viewing might result in an image too bright that will wash out the contrast on the full disk so I may need to add a further Baader Ha filter (I have one) into the train, possibly between the etalon and blocking filter or if it works on the nose of the PST mod scope side.

I’ll report back in due course regarding results but thought I’d start this post to encourage others to check their ITF if their views are not as expected or if they have deteriorated.

Big thanks to Stu and Peter for helping me with this – I hope!

 

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Peter Drew

Peter Drew

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Its a commonly held belief that the degradation of the ITF is caused by the ingress of moisture rather  than heat damage. For some reason, manufacturers seem unable to reliably seal the edges of these filters.  :hmh:

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Merlin66

Merlin66

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Yeah,

It comes as quite a shock to some users who feel they paid "top dollar" for an Ha filter/ scope to find the ITF fails so quickly......

I suppose it's like buying the top of the line APO with "special" super ED glass and finding out after a year or so the objective clouds over and needs replacement!

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Moonshane

Moonshane

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I presume that the BF is not at risk with a Baader DERF in place? I also use a Baader 35nm Ha filter in a 60mm PST mod. Would both of these adequately protect the BF?

I am please to say that the bf and etalon were both clear as a bell when I checked them.

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Paul73

Paul73

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Great to know that they can be replaced. I'm a bit worried about the view though my PST.

After looking through Stu/Peter's Stage 1 mod, I can sense a project coming on..... Phenomenal views for the outlay (particularly if you already own the PST part).

Paul

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Moonshane

Moonshane

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Well I have just had a few odd seconds between the clouds using my BVs, the 1.7x GPC and the 25mm plossls. This gave the equivalent of a 15mm plossl so 67X. The view of the full disk was evenly detailed with no hot spot discernible and showing incredible amounts of detail even in poor seeing and transparency. The view was a little bright as the clearer spells rolled through so I added the 35nm Baader 2" Ha on the nosepiece (scope side of the etalon). This reduced brightness enough to be more relaxed and enjoyable view. To say I am happy is a very big understatement. The views are even better than the mono views I was getting when I first bought the bits.

Seriously guys, this has transformed my Ha solar observing. My only problem now is that I used to struggle to sketch the detail I could see previously. I doubt that I'll even get close to sketching what I can see now. :happy7:

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spaceboy

spaceboy

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They should call them Coranado POS not PST. They do seem to have their share of problems but you rarely hear of any with the Lunts other than with the new pressure tuners failing. Hopefully Meade offer their customers PST's that have longer shelf lives. Although saying that I am sure I read somewhere that all etalons have expiry dates of around 8 years or so.

Glad you got it sorted Shane and sounds like an easy if not expensive fix.

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spaceboy

spaceboy

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To be fair back in the day they could be had new for only £450 and at f/10 they were perfect for mods but lets be honest, customer seemed to have an array of faults to look forward to after parting with their hard earned cash. Rusty objectives/ ERF, completely rubbish focusers, eye watering sweet spots and failing ITF's. Sorry but as a stand alone PST they wouldn't get my vote. PST mods are their only saving grace IMHO and even then as you have found they can still be troublesome.

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Paul73

Paul73

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Ooooo. We have a PST hater!?? Only joking.

When you can't afford a Lunt, a PST makes a great intro to Ha. Compared to Quarks, the PST's are bulletproof!!

It has to be said, that Lunt products always  feel like quality items.

I love my PST. With the Double Stack, it out-performs a naked Lunt 50mm, albeit with a dimmer image.

I love my PST so much that I'm going to chop it up for a Mod........ 

Paul

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spaceboy

spaceboy

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1 hour ago, Paul73 said:

Ooooo. We have a PST hater!?? Only joking.

When you can't afford a Lunt, a PST makes a great intro to Ha. Compared to Quarks, the PST's are bulletproof!!

It has to be said, that Lunt products always  feel like quality items.

I love my PST. With the Double Stack, it out-performs a naked Lunt 50mm, albeit with a dimmer image.

I love my PST so much that I'm going to chop it up for a Mod........ 

Paul

Sorry to be dragging this off topic here Shane but still loosely on topic to your ITF troubles all the same.

I had this discussion in another thread. Back in the day yes a PST offered the average Joe a step in to Ha solar for an affordable price at £450 even back in 2011. But these days at £880+ for a PST compared to £1K for the LS50Tha pressure tuned (not convinced PT offer anything other than problems with seals) I couldn't justify all the short comings of the PST for the measly saving of £120. The LS50 has arguably "a better focuser" but upgradeable none the less. 10mm extra aperture, far better control over the sweet spot (more a sweet banding than a spot) no doubt due to the slightly larger etalon. PT which the jury is still out on??, superior build quality and a better reputation. No better example of this than when they delayed release or the LS50 for several months because they had problems during manufacture and didn't want to allow sub standard scopes being released to customers. Daystar should have maybe took a leaf out of Lunt's book on that one!!

BTW I'm not a PST hater :evil4:. As part of a solar mod they offer breath taking detail that if had it been a dedicated Ha scope would be far beyond the disposable budget of all but the super rich. In all honesty a stand alone PST, once you get a hang of that sweet spot I couldn't knock the visual performance of them as the views aren't bad at all given the small 40mm aperture. It's the poor shelf life and to some degree build quality that lets them down. Price a PST competitively or build them to last is my view. Don't just sell sub standard poop to people because they (at the time) had no other "affordable" options available to them.

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Paul73

Paul73

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Good point. There is no excuse for sub standard poop at £880 plus the £700 for the Double Stack needed to challenge the Lunt 50.

Hang on! £1600 is virtually a new 60mm Lunt. I know which I'd go for ......

Paul

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