Coronado PST testing

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Coronado PST testing

Post by Montana » Sat Nov 30, 2019 5:49 pm

Miracles happen, it was actually sunny on a Saturday! I had only the usual hour between the gaps in the upstairs bedroom window (still no sign of the new one yet). I bought a new PST a couple of weeks ago at the International Astronomy show and I wanted to test its performance against my original one.

The first thing I noticed was that it was a dream to move the etalon, silky smooth and in no time I had it on band. I then changed to my old one and it was terrible, it was a way off band (oops haven't changed it for a while). It then took 20 minutes to try and get it on band as the etalon is so stiff and won't move and drags the mount with it. After a fight I think I got it right. So I took an image with both in single and double stack. The new PST is twice as bright as the old one, for instance it was 1/300 (or thereabouts with the old) and 1/900 for the new PST in single stack mode with the DMK41.

Single stack old
Image2019-11-30 13-14-08_SS Old f colour by Alexandra Hart, on Flickr

Single stack new
Image2019-11-30 12-51-10_SS New f colour by Alexandra Hart, on Flickr

Double stack old
Image2019-11-30 13-42-15_DS Old f colour by Alexandra Hart, on Flickr

Double stack New
Image2019-11-30 13-01-03_DS New f colour by Alexandra Hart, on Flickr

One thing I do need to get right is finding the sweet spot for the new PST and rotating the DS unit to try and get a more even field. I also need to get the Chameleon out too. That will hopefully be tomorrow if I get more sunny weather.

Overall I am very pleased indeed with the new PST and I think it might be the old one that goes to the breakers yard for the PST mod unless anyone else has any ideas why not.

Alexandra



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Re: Coronado PST testing

Post by eroel » Sat Nov 30, 2019 7:00 pm

Alexandra:
In your "comparo" test I like better the old one, looks like it has a shorter band, aside of giving a more even image.
You can try to get the best adjustment on your new one and then compare it with the old one.
More brightness can be because the BF (filter) of the new PST is more transmitting, (I have BF10, BF15, BF30 and all give different brightness at the focal point).
You can also try the PST etalon with a bigger scope.
Hope that some forists with more experience on PST etalons can give you a better idea.
Best wishes,
Eric.



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Re: Coronado PST testing

Post by MapleRidge » Sat Nov 30, 2019 11:09 pm

Hi Alexandra...

Glad you had a clear day to try the new PST, and these images look great :bow

They look close to my eye, but I'd give a slight edge to the old one as well.

Brian


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Re: Coronado PST testing

Post by Bob Yoesle » Sun Dec 01, 2019 12:28 am

Hi Alexandra,

I too think I prefer the old PST image, even though it is dimmer - it appears a bit more uniform.

Just a thought - have you checked the ITF for rust? Replacing it in the older PST might give you a considerable bump in brightness. Better yet, using a Baader 35 nm Ha filter as an objective ERF and using a BelOptik UV/IR KG3 as a replacement for the ITF could be a significant improvement... Might be worth a try.

Don't have much input on how to fix the stiffness issue, perhaps some others have some experience there.


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Re: Coronado PST testing

Post by Astrophil » Sun Dec 01, 2019 2:39 am

Alexandra,

The old PST has always produced what I consider the Gold Standard of PST images. Maybe once you get the new one dialed in, it will perform better. The filters in the PST seem to have a limited lifespan and Bob's suggestion may give new life to the old one. If both scopes are the newer design, with both filters in the eyepiece holder, swapping them would tell you if the old ITF has deteriorated. A strap wrench will be needed 'cause they're tight.

All in all, both sets of images are very good.

Phil



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Re: Coronado PST testing

Post by Merlin66 » Sun Dec 01, 2019 7:11 am

Alexandra,
I agree with the above Re the ITF.
The stiffness could be the grease/ O Rings hardening.....clean and re-grease May be needed.
Ken


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Re: Coronado PST testing

Post by Rusted » Sun Dec 01, 2019 10:02 am

With the greatest respect, I found no difference from applying white grease or silicone rubber oil [for push fit plumbing gaskets] to the original PST o-rings.

Going back to the original 42mm x 2mm o-rings [well greased] still makes my etalon band very difficult to turn.

I have tried a range of 1.5mm o-rings [45, 46 and 47mm Ø] but they were all too thin. Causing major slop in the etalon band.

Changing the o-rings to monofilament nylon [1.3mm and 1.6mm] didn't help very much.
One is too small [too sloppy] and the other no better than the original 2mm o-rings. [Too Tight!]

I'm now looking for 1.7mm o-rings. Or lengths of continuous 1.7mm, o-ring material. None of these exist at the usual stockists.

BTW: The rings do not need to be continuous. Just enough length for slightly short of a circumference. Overlaps would bulk up.
The material is pressed into the groove with a thumbnail as the outer, etalon shell is slipped back over.

I have also confirmed that the drive screw does not spoil the image, nor alter the sweet spot, if the etalon band is far too sloppy.
The etalon itself seems immune to any rocking of the outer shell.


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Re: Coronado PST testing

Post by rsfoto » Sun Dec 01, 2019 4:10 pm

Hi Alexandra,

Interesting and yes that is not an easy task to get the Hot Spot in place ...

I have mine rotated by 90°


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Re: Coronado PST testing

Post by marktownley » Sun Dec 01, 2019 8:08 pm

I think the old etalon is the clear winner. I bet the ITF on that PST is a rusty as heck, given what you're saying about exposure times; I also bet the same thing on you CaK PST too.


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Re: Coronado PST testing

Post by MAURITS » Sun Dec 01, 2019 8:49 pm

Alexandra, for me is the oldest etalon is more on band.


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Re: Coronado PST testing

Post by Carbon60 » Mon Dec 02, 2019 6:52 am

Hi Alexandra,

Difficult to say for sure based on this one test. I would suggest that there’s no rush, given that you won’t be able to use the larger scope for a few months, so try the comparison again when the conditions improve in spring and you get more of a chance to ‘fiddle’ with the settings and properly optimise each. In the meantime, as Mark and others have suggested, maybe check the ITF for rust and try to loosen up the old etalon.

Stu.


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Re: Coronado PST testing

Post by marktownley » Mon Dec 02, 2019 7:26 am

The 'stiff tuning' of the old etalon assembly will be caused by the foam gasket that the etalon sitting on in it's chamber compressing over time, I seem to remember you had to adjust the position of the screw to be able to get on band a few years ago? The new PST will have a nice soft and springy foam gasket still, hence why it is easier to tune; give it several years and the chances are you'll need to adjust screw position on that as the gasket naturally compresses. The tighter tuning isn't a bad thing and will prevent flop of etalon, which can be a frustrating thing in itself.

Whichever etalon you use with the C11 you'll find there is a clocked position that produces best result with flattest field and least gradient across the fov. Like Stu says, won't be till next year when you can do some meaningful testing at large apertures, till then use the winter period to assemble all the bits needed.


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Re: Coronado PST testing

Post by Montana » Mon Dec 02, 2019 7:49 am

Thanks guys for all the info.

My main object I think was to leave myself with a good PST for visual and quick full discs and I think the new PST will do this once I optimise it. I think the bandpass seems very similar to the old one and if anything it seems to have more contrast. It also works well with my DS unit which is also pleasing. I also have doubts about the ITF and blocking filter of the old one so if I break this one apart for the PST mod I will only be using the etalon so I will remove the faulty bits yes? so that solves another problem. I think the stiff etalon is going to be an nightmare to fine tune on the C11 though but I guess once it is done hopefully it won't move much. These are my ideas anyway.

I bought a rubber boa last week, never seen one in my life before so I didn't know what I was looking for but I found one in the local shop. So I am ready I think :) I can see myself putting this job off for as long as possible, I might stare at it for hours too, the whole thought sends me a quiver.

Alexandra



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Re: Coronado PST testing

Post by marktownley » Mon Dec 02, 2019 8:46 am

You need to take off the tuning ring and locating screw when you use the boa, grip directly to the housing...


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Re: Coronado PST testing

Post by Rusted » Mon Dec 02, 2019 9:54 am

marktownley wrote:
Mon Dec 02, 2019 8:46 am
You need to take off the tuning ring and locating screw when you use the boa, grip directly to the housing...
At the risk of further pedantry, but for absolute clarity, you must remove the knurled rubber band first.
Followed by the cross-head screw and then the outer, metal ring [or sleeve] lying over the o-rings.
Only then can you use the boa on the inner etalon housing.

My etalon housing was so tight I had to hold down the body on a bench with two brand new, carpenter's F-clamps.
Then the boa strap was fed through a plastic plumbing pipe to provide a little more leverage.
The tube also provided more hand comfort and to keep the boa's strap firmly in its track after an hour of intense struggling.

Despite decades of hands-on, mechanical and engineering experience, I very nearly gave up on my PST ever coming apart.
It was so tightly bonded with rock hard, red, thread locker I feared the etalon housing would disintegrate under the pressure!
The thread locker was so hard that it does not respond to sharp hand tools in its later attempts at removal to clean the threads.
Even when I heard the first loud crack from the locked joint being "broken free" it took another hour to finally remove the etalon.
Some illustration of the dismantled parts:

https://fullerscopes.blogspot.com/2018/ ... parts.html

https://fullerscopes.blogspot.com/2018/ ... strap.html

Here's an image of my rusty ITF as purchased [secondhand.]

https://fullerscopes.blogspot.com/2018/ ... prism.html

I purchased a Maier ITF and it was worth every penny.


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Re: Coronado PST testing

Post by Montana » Mon Dec 02, 2019 12:16 pm

Oh good grief!!! :shock:



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Re: Coronado PST testing

Post by Montana » Wed Jan 01, 2020 3:35 pm

Thanks guys for the information. We have managed to get the gold tube off the PST, the rubber ring off the etalon and removed the screw and the housing. There was a lot of grease under the ring which I cleaned off with a towel. However the rubber boa still keeps slipping, there is no grip. What is the best way to clean the grease off to get a good grip? any tricks or tips? otherwise it doesn't look like we will shift it.

Alexandra



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Re: Coronado PST testing

Post by marktownley » Wed Jan 01, 2020 7:48 pm

If you have a black and decker work bench thingy you can try clamping the greasy end in this and using the boa to twist the black box.


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Re: Coronado PST testing

Post by Montana » Sat Jan 04, 2020 6:44 pm

Well,
I don't know how so many people do PST mods? it should not be recommended on this forum. I think I have just wasted more than a £1000, this is too much money!
We bought a new rubber boa today, a large one as obviously the one we bought before would not go round the main body as Mark suggested. After an hour on the work bench we managed to get the etalon around 1 and 3/4 turns. It never got any easier and is still as firmly stuck as it was before. We now snapped the extra big boa. This etalon will not turn and will not come off.
We give up.
Alexandra



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Re: Coronado PST testing

Post by MapleRidge » Sat Jan 04, 2020 7:52 pm

Hi Alexandra...

Disappointing to hear that the disassembly is such a challenge.

I may be mistaken, but I thought that I read in the forum a while back that the newer PST's had a 'lock tight' type of material applied to the threads. If this is correct, that could explain why you are having trouble.

Hopefully others can offer some suggestions,

Brian


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Re: Coronado PST testing

Post by MalVeauX » Sat Jan 04, 2020 8:13 pm

Alexandra,

Sorry to hear you're having such trouble. Some PST's are very hard nuts to crack open, you may just have one of those. If there's anyone local to your country that has ripped a few open, you could reach out and have them do it maybe? There is a bond in the threads, heating up the pipe with a hair dryer and then using a strap-wrench should do the trick. Get it real hot. I've torn up a few PST's now and am comfortable opening them up. Some really are glued and require a lot of work to open them. I destroyed one to get the etalon (a good etalon, it was fine of course, but the brass tube and black box were junk so I didn't care about ripping them up).

Very best,



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Re: Coronado PST testing

Post by pedro » Sat Jan 04, 2020 11:10 pm

Hi Alexandra

I managed to dismantle a few (3) PSTs. Old models are easier to crack open. There is a strong Loctite glue in all of the threads. Try to apply some heat and using a strap-wrench should work. Best of luck
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Re: Coronado PST testing

Post by marktownley » Sun Jan 05, 2020 11:08 am

If you've got the etalon turning 1 and 3/4 turns it will come undone; but will probably cost another boa or 2. I think i've bought 4, breaking 2 of them. Key to success will be to keep slackening off and then tightening the etalon housing back down, repeat ad infinitum, this will allow the etalon housing to unscrew slowly but surely. . This will move the flakey loctite around the threads - the same way as using a tap to put a thread in a piece of metal, as the threads come out of the housing if you can see any loctite in the threads scratch it out with a pin. What you're experiencing is perfectly normal for a PST mod with loctite threads.


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Re: Coronado PST testing

Post by Montana » Mon Jan 06, 2020 10:12 am

It has taken me 1 1/2 days to calm down.

How do you apply heat Pedro? my niece says I need at least 70 degrees C to soften loctite. I don't think my husband's shoulder can use a boa again. I am not willing to spend £10,000 on another shoulder rebuild.

Alexandra



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Re: Coronado PST testing

Post by pedro » Mon Jan 06, 2020 1:43 pm

Use lots of pressure and a hair drier, it should work

best of luck



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