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Is it safe? (PST)


markse68
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So I just picked up a used PST that I'm very excited about and it seems to sort of work, but how do I tell if it's really safe?

It's a very old one- one of the first type which had the gold coloured objective that acted as an ERF so in the ep holder there's just one filter-- the blocking filter.

But the objective although sort of gold, isn't very gold. I've seen images online that show the gold objectives to be pretty opaque when looking from the front but I can see through it into the tube. But these are usually images of the failed ones that have oxidised.

4C339EDE-6295-4EBC-B0CA-A056353FA75C.thumb.jpeg.a81886e5967940f6269a2f062cb1ff41.jpeg

35EBB7AE-246B-4516-BE10-F18CA19B9784.thumb.jpeg.e7e1433bc01d3c1f14d9fbc1a75a0455.jpeg

I pointed a spectrum analyser down the tube and got this- there seems to be a lot of infra red getting through? 

64AE7F8E-46BA-4772-A40F-E58E754958DE.thumb.jpeg.ab8b1c52478c99dc0dc033d7356b38ab.jpeg

This is pointing the analyser directly at the sun (no scope)

2E10E8C1-0DFE-4174-A90B-26D111AE50BF.thumb.jpeg.9d3a34463acaaa7452572f7ec2ec8767.jpeg

When I look through it I see a bright yellow sun with spots and by tuning the etalon I see clear dark red prominences around the edge, but not a lot of detail on the bright disk.  I tried to take a photo with my phone but it’s rubbish.

E1B124D9-FFB4-4CA3-8B4D-8725F0714C60.thumb.png.730a95102cd36685dc6c34f7aebb91f4.png

But I don't want to look anymore unless I know its safe!

Mark

Edited by markse68
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8 minutes ago, globular said:

[ Now we know why you pictured a colleague (expendable) using it on another thread. ]

😬 I looked through it a lot more than he did! Doesn't seem to be any UV getting through which is good and the scale on the spectrometer suggests the IR is diminished a lot but would be good to hear from someone who knows these things

Mark

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The main purpose of the coating was the same as that of an ERF, to reduce the thermal loading on the etalon and blocking filter.  Failure or more likely reduction in efficiency should not present a danger situation, just sub par performance.  Unfortunately you can't replace anything easily on the early PST's, later ones have an internal filter that can.  If you are concerned about IR leakage you can screw an IR cut filter into the eyepiece.     🙂 

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Thanks Peter, for peace of mind I was thinking of investing in one of the 2" Beloptic KG3 UV/IR blocking filters for the front of the scope- to protect the scope as much as anything

https://beloptik.de/en/uv-ircut-on-kg3-filter/

One other thing- I read about the PST "sweet spot" but what I'm seeing is a circular aperture with a distinct cuttoff. It only just clears the suns disk and some of the proms are cut off- it would be nice if it were wider. Is that the limitation of the small blocking filter or the aperture of the etalon? Is it normal? And should I be able to see granularity on the disk?

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@markse68.  I've just read through your other thread,  you have a 4" Tal refractor and a poorly PST?, the time is ripe for converting them into a Ha mod.  Apart from a bit of mechanical work and the purchase of a Baader 2" 35nm filter you have the makings of a 100mm Ha solar telescope with the performance of a Lunt 100 and look at the difference in price!.  I've made a couple and they work really well.    

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2 minutes ago, Peter Drew said:

@markse68.  I've just read through your other thread,  you have a 4" Tal refractor and a poorly PST?, the time is ripe for converting them into a Ha mod.  Apart from a bit of mechanical work and the purchase of a Baader 2" 35nm filter you have the makings of a 100mm Ha solar telescope with the performance of a Lunt 100 and look at the difference in price!.  I've made a couple and they work really well.    

Hi Peter, the thought had entered my mind before I got it. But I'm not sure it is poorly? I guess I need to look through another to see

Mark

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50 minutes ago, markse68 said:

Thanks Peter, for peace of mind I was thinking of investing in one of the 2" Beloptic KG3 UV/IR blocking filters for the front of the scope- to protect the scope as much as anything

https://beloptik.de/en/uv-ircut-on-kg3-filter/

One other thing- I read about the PST "sweet spot" but what I'm seeing is a circular aperture with a distinct cuttoff. It only just clears the suns disk and some of the proms are cut off- it would be nice if it were wider. Is that the limitation of the small blocking filter or the aperture of the etalon? Is it normal? And should I be able to see granularity on the disk?

I'm not sure about the attenuation of a KG3 filter, you won't want to make the image too dim, on the other hand your description of the current performance suggests it might be too bright and this is what is swamping your surface detail.  The 5mm blocking filter usually provides enough field to cover the full disk plus enough for average size prominences, the "sweet spot", the area on full Ha band, is smaller than the low power field and an object needs to be centred for best effect.  PST's have a manufacturers acceptance tolerance of between 1A and 0.7A which can affect the resolution and contrast of surface details, the lower number is the best.  Tuning of the etalon by rotating the tuning ring is another important consideration when judging performance.     🙂      

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You can try adjusting the tuning ring locking position which is on the main tube just in front of the prism box. The knurled what looks like plastic ring is rubber and slides off the tuning bit easily, underneath you'll see a single screw, you can screw it out and the top housing can be moved off to reveal the etalon tuning housing which is a lot of holes set at equal distances apart, by moving the top housing to one of the other holes and reinserting the screw so you can effectively "overtune" or "undertune" from the default you can try to see if more detail can be seen (you'll feel it getting tighter or looser the more you turn it as it's applying bending force to the etalon optics). I found by leaving it fully disassembled and using a pin (or anything a similar diameter to the locking screw) you can trial this in the field, then lock it off when you're happy with a screw position. I've struggled to see surface detail myself but by moving the screw position I have been able to tune past the original fully left/anticlockwise position (looking down the gold tube toward the objective at the far end) which was where the best prominence detail was seen, now some more surface detail can be seen at the same time. I used to believe you can either see prominences or surface detail, but I think now once one is tuned in, the whole sun is on band. Also I find looking for around 2 minutes unbroken helps your eyes/brain to resolve detail, for reassurance use a UV/ir block filter on the end of an eyepiece too. Bear in mind not all PSTs (or solar equipment in general) are the same as each other.

 

Also, make sure the prism tuning knob under the prism box is turned so the details are as sharp as possible, if you open the side of the prism box you may find the pentaprism inside is not parallel (the two faces which are 90 degrees to each other) to the walls of the prism box housing, some people try to adjust this but I find the prism wants to follow the guide groove underneath the prism and will require a complete disassembly and reglue to fix properly, when I tuned it's position via the knob it gave sharp enough views for not for me to take mine apart and reset, yet.

 

Edited by Elp
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1 hour ago, Elp said:

You can try adjusting the tuning ring locking position which is on the main tube just in front of the prism box. The knurled what looks like plastic ring is rubber and slides off the tuning bit easily, underneath you'll see a single screw, you can screw it out and the top housing can be moved off to reveal the etalon tuning housing which is a lot of holes set at equal distances apart, by moving the top housing to one of the other holes and reinserting the screw so you can effectively "overtune" or "undertune" from the default you can try to see if more detail can be seen (you'll feel it getting tighter or looser the more you turn it as it's applying bending force to the etalon optics). I found by leaving it fully disassembled and using a pin (or anything a similar diameter to the locking screw) you can trial this in the field, then lock it off when you're happy with a screw position. I've struggled to see surface detail myself but by moving the screw position I have been able to tune past the original fully left/anticlockwise position (looking down the gold tube toward the objective at the far end) which was where the best prominence detail was seen, now some more surface detail can be seen at the same time. I used to believe you can either see prominences or surface detail, but I think now once one is tuned in, the whole sun is on band. Also I find looking for around 2 minutes unbroken helps your eyes/brain to resolve detail, for reassurance use a UV/ir block filter on the end of an eyepiece too. Bear in mind not all PSTs (or solar equipment in general) are the same as each other.

 

Also, make sure the prism tuning knob under the prism box is turned so the details are as sharp as possible, if you open the side of the prism box you may find the pentaprism inside is not parallel (the two faces which are 90 degrees to each other) to the walls of the prism box housing, some people try to adjust this but I find the prism wants to follow the guide groove underneath the prism and will require a complete disassembly and reglue to fix properly, when I tuned it's position via the knob it gave sharp enough views for not for me to take mine apart and reset, yet.

 

I've had dozens of PST's apart and every one has had a tilted prism, either someone is assembling them incorrectly or, as I'm convinced, they are purposely tilted to avoid unwanted reflections.  The prisms are pentaprisms, a property of which is constant deviation so a small amount of tilt does not affect the optical performance.  This also results in the image orientation being retained as astronomical with NSEW reversed despite the 90 degree attitude of the eyepiece.     🙂 

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28 minutes ago, Peter Drew said:

I've had dozens of PST's apart and every one has had a tilted prism, either someone is assembling them incorrectly or, as I'm convinced, they are purposely tilted to avoid unwanted reflections.  The prisms are pentaprisms, a property of which is constant deviation so a small amount of tilt does not affect the optical performance.  This also results in the image orientation being retained as astronomical with NSEW reversed despite the 90 degree attitude of the eyepiece.     🙂 

Mine was tilted so i twisted it back. It was stuck on with some gooey double sided tape by the look. Could they just sag under their own weight? I should have left it?

Mark

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3 hours ago, Peter Drew said:

@markse68.  I've just read through your other thread,  you have a 4" Tal refractor and a poorly PST?, the time is ripe for converting them into a Ha mod.  Apart from a bit of mechanical work and the purchase of a Baader 2" 35nm filter you have the makings of a 100mm Ha solar telescope with the performance of a Lunt 100 and look at the difference in price!.  I've made a couple and they work really well.    

Hi Peter, I was under the impression that the PST mod for the Tal would require a large (expensive) D-ERF filter to safely attenuate the light before it reached the PST etalon etc. Are you saying it just needs a 2” Ha filter and that’s all? And that’s safe? Would i also need the erf filter that modern PSTs have in the ep holder (that mine doesn’t) or does the 35nm Ha replace that too?

Mark

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Yes, in principle once you have a PST and a suitable donor telescope, all you need extra is an ERF.  A full aperture front fitting ERF is best but very expensive, a smaller internally fitted ERF provides a very similar performance and is much cheaper and better shielded from damage and contaminants.  A 2" fit Baader 35nm filter is an ideal ERF for a 4"Tal/PST mod.  The only other extra you need is adequate metalwork facilities and the experience to use them.  If you decide to go down this route I will always be pleased to advise if necessary.    🙂

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I think it is safe :) I put my Astronomik 2c L filter (UV/IR cut) on the ep and it got rid of that IR.

Here without filter

5119E90D-201C-4484-95B2-D077EB1CB49B.thumb.jpeg.f1174b907ab67a25e81994dcda08b421.jpeg

Here with filter- only the Ha getting through :)

420B0B2F-C630-4AFC-8A0C-BBEE40741DB2.thumb.jpeg.e39e1f913e863e5dbfcd0d184e7f06e6.jpeg

I was just being paranoid but that’s probably a good thing when looking at the Sun! 

Mark

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17 hours ago, Peter Drew said:

The 5mm blocking filter usually provides enough field to cover the full disk plus enough for average size prominences

Here’s the thing that i can’t get my head around. What i see looking through the ep is the suns disk framed by an oval sort of mask that cuts off anything beyond it. It is quite tight on the sun on the sides but has more room top and bottom. BUT it is fixed relative to the image of the sun! How can that be? When i pan or tilt the scope, the image of the sun AND thus frame move together. So i can’t centre a prom in that frame- it’s always at the edge of the frame. Is this normal and how does it work? It’s most peculiar. Is it some sort of angular cuttoff feature of the etalon? That's all I can think of

Mark

PS there’s a very cool feature visible today- like a huge fountain extending from a very narrow stalk and fanning out like tree roots. 

Edited by markse68
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Sounds odd, as far as I'm aware some PSTs have off band falloff in that you'll get a visual dead region around the sweet spot which will be on band. What you describe is very peculiar, is it the same with different eyepieces, eyepieces do make a massive difference with PSTs, some very good ones for night are no good for solar I've found.

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Hi ELP, I did try several eps- mainly pseudo Masuyama/astroplan types and yes it’s always there. Difficult to take a photo with the brightness of the disk but you can make it out in this image. Changing to a higher power view just magnifies this and like I say this “mask” effect is locked to the disk so it can’t be vignetting caused by something in the scope i think?

Mark

 

62163487-4999-4117-833E-6EB963450742.jpeg

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36 minutes ago, Elp said:

I'm no expert but it looks to be some sort of optical train issue like a decontacted etalon, it certainly should not be there.

If it was decontacted, would I be able to see the proms that are very clear to the eye when tuned though?

Mark

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I think you still can as the etalon is still being bent to tune to the HA wavelength, the symptom of seeing double vision in effect is usually to do with an issue with the optics. If you do a quick search for it this issue comes up with the PSTs, bear in mind your issue could be something else entirely. We've eliminated the eyepieces so what's left is within the scope.

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From what i’ve read, if the etalon was decontacted there’d be multiple images of the disk and only photosphere would be visible but 🤷‍♂️ Maybe it’s a reflection off something somewhere in the optical train? Will have a look and clean surfaces of prism etc.

Mark

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35 minutes ago, markse68 said:

From what i’ve read, if the etalon was decontacted there’d be multiple images of the disk and only photosphere would be visible but 🤷‍♂️ Maybe it’s a reflection off something somewhere in the optical train? Will have a look and clean surfaces of prism etc.

Mark

Correct.     🙂

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My first reaction was to mention both Exposure and Gain. Whatever software you are using to see through the scope (assuming you are looking at astrophotography) you will become a friend to gain and exposure. For prominences, gain and exposure are down quite far. For interior of sun, they are up higher.

If you are using an etalon filter, the one I use on my Lunt had to be tightened pretty far down. If you have one, tighten it all the way down, then back off of it in small amounts. 

Good luck.

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