PST Mod Reflections back off the Blocking Filter

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AndiesHandyHandies
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PST Mod Reflections back off the Blocking Filter

Post by AndiesHandyHandies » Wed Mar 06, 2019 6:38 pm

Hi,

I am moving forward slowly with the Vixen VMC110L PST Mod.

I have a bodged up front baffle.

The correction lenses in the secondary mirror holder did not have blackened edges so RodAstro made a little baffle for them.

And a little baffle for the top of the primary baffle to only let the bright Sun image through, masks of a little of the top inside.

The primary baffle has Black2 paint or Black2 painted 180 Emery in it.

The adapter holding the moved forward PST Collimating lens, and a 1.25" Baader 35nm and a Beloptic KG3, has knife edge baffles in and Black2 paint.

With all that when the sun is moved just out of the field the field is black. Without the front baffle a little glow opposite where the sun is. As the internal extra baffles still let some off axis skylight in.

I had a carefull clean and seating of the lens and filters in the adapter to seat them square as there were some slight off axis reflections.

Taking the side off the prism box we found grease on the prism which probably contibuted to background glow, cleaned up.

Also RodAstro made a baffle at the entrance to the prism box after the etalon. 15mm and now the beam fits neatly in the prism face.

Do you get reflections off the back of the blocking filter showing? Clearly there is a bright reflection from the eyepiece part as it shows in several places on the sides, opposite the eyepiece and back up to the side of the eyepiece entrance.

On the last try there is a second quite bright ghost about 40% off set and a dimmer ghost about 10% outside of that.
Is a reflection off the back of the blocking filter, which is quiet tilted, being reflected back off the prism?

Andrew.
Last edited by AndiesHandyHandies on Thu Mar 07, 2019 3:32 pm, edited 1 time in total.

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Re: PST Mod Reflections back off the Blocking Filter

Post by Merlin66 » Wed Mar 06, 2019 10:40 pm

Andrew, how square and parallel to the top of the black box is the prism sitting?
Is the tilt of the ITF at right angle to the prism face?
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Re: PST Mod Reflections back off the Blocking Filter

Post by AndiesHandyHandies » Thu Mar 07, 2019 10:02 am

Hi

The prism of course flops about. So its not parallel to the top of the box.

We have not unscrewed the eyepiece part yet.

Andrew.

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Re: PST Mod Reflections back off the Blocking Filter

Post by marktownley » Thu Mar 07, 2019 12:07 pm

The prism will not be helping things then!
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Re: PST Mod Reflections back off the Blocking Filter

Post by AndiesHandyHandies » Thu Mar 07, 2019 2:51 pm

Hi

I was not aware that a mod needed the prism to be in a certain position.

I thought the penta-prism design meant its orientation did not matter.

I forgot I have an original gold/reddish objective so there is no ITF. !!!

Andrew.

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Re: PST Mod Reflections back off the Blocking Filter

Post by AndiesHandyHandies » Thu Mar 07, 2019 4:22 pm

Hi

Doing some research.

My pentaprism seems to have flopped down.

DavidG on CN just twisted his back and let it settle down till it was parallel to the top of the housing.

Whilst I can twist mine it still settles down as before.

What is actually twisting?

There seems to be a base square of metal with two studs out the bottom to engage in the slot in the housing.

The penataprism with the metal on top with the velcro stuck on it.

Is the pentaprism glued onto the sliding block with elastic adhesive?

Andrew.

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Re: PST Mod Reflections back off the Blocking Filter

Post by marktownley » Thu Mar 07, 2019 6:20 pm

Personally I would just do away with the black box. If the goo has gone on the prism you will struggle to get it back square, you'll just get astigmatism with it as is now.
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Re: PST Mod Reflections back off the Blocking Filter

Post by Merlin66 » Fri Mar 08, 2019 6:49 am

The prism is mounted to the block with a black flexible mastic....I’ve repaired a couple where the prism was removed from the block, (note the relative position) and remounted using super silicon to get a better positioning and orientation.
If you don’t have an ITF in the eyepiece holder then I’m not sure why you would have excessive ghosting.
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Re: PST Mod Reflections back off the Blocking Filter

Post by Merlin66 » Fri Mar 08, 2019 6:51 am

Andrew,
I never saw any slot in the housing for the pips on the block.....
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Re: PST Mod Reflections back off the Blocking Filter

Post by AndiesHandyHandies » Fri Mar 08, 2019 5:05 pm

Hi Ken,

There is a photo of a totally removed prism system in here with the two pips, near the end. There has to be some mechanical guidance.

fullerscopes.blogspot.com/2018/02/

I have not done any investigations yet, I just assumed it could be off the blocking filter as well baffled into the block.

The grease on the pentaprism may have scattered enough light to mask the ghosting before.

Andrew.

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Re: PST Mod Reflections back off the Blocking Filter

Post by Merlin66 » Fri Mar 08, 2019 11:15 pm

Andrew,
I can seem to find the photo.....
Can you help?
Ken
OK, final found your PST images in your Blog....one shows the pips on the prism block but no image of the "groove" in the housing.
On the couple I stripped there was plenty of grease on the housing with track marks of the pins but no physical groove.
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Re: PST Mod Reflections back off the Blocking Filter

Post by AndiesHandyHandies » Sat Mar 09, 2019 9:33 am

Hi Ken

Its not my blog.

The prism assembly is upside down. Clearly the pins must slide in a slot and the block sit on the flat base.

In this Post there is an image of the base with the prism assembly removed, the screw is still there, and you can see the slot.

www.cloudynights.com/topic/530890-newbi ... onado-pst/

People say that all the original PST pentaprism they see are at a slight angle to the beam. So I assume the Coronado optician does that for a reason. Research shows that having it at an angle will introduce some astigmatism so there must be a trade off, perhaps reflections off the seemingly uncoated prism faces being off axis?

Andrew.

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Re: PST Mod Reflections back off the Blocking Filter

Post by Merlin66 » Sat Mar 09, 2019 9:55 am

Andrew,
I can assure you on the ones I stripped down there were no guide grooves in the housing......
I think the angled prism comes from "Basic" assembly skills used in Mexico.......
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Re: PST Mod Reflections back off the Blocking Filter

Post by AndiesHandyHandies » Sun Mar 10, 2019 11:28 am

Hi Ken,

If there are no pins and groove in some how is the prism assembly constrained to be in the right position as it slides?

Were the first ones with the Gold/reddish objectives made in Mexico?

Does the serial no indicate where they were made? And the mechanism details inside?

Andrew.

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Re: PST Mod Reflections back off the Blocking Filter

Post by Merlin66 » Sun Mar 10, 2019 9:12 pm

Andrew,
The pins are still there but just run on the interior surface of the housing.
The prism is held by the focussing screw and the size of the block used.
All the PST’s as far as I know were assembled in Mexico.
Due to warranty returns etc etc there’s no meaningful relation between the serial and the build.
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Re: PST Mod Reflections back off the Blocking Filter

Post by AndiesHandyHandies » Thu Mar 21, 2019 4:59 pm

Hi

Had some Sun yesterday, Normal contrail based clouds over Derbyshire and some cloud.

I had tried moving the PST etalon screw as adjustment near the one end but decided to move it back as reflections moved about a lot.

I removed the Beloptic KG3 from before the etalon and put it on the eyepiece which removed one reflection. Just there for eye safety.

One reflection remained which was removed by replacing the Baader 35nm behind the collimator lens with the original 110mm Lunt ERF on the front of the telescope. The 35mm behind the collimator lens is safe as its in the parallel beam and the lens spreads the light back out again as it goes back out.

Originally I had a lot of red flooding light with the Lunt, dispersion in the glass I presume, but extensive baffling seems to have mostly removed it.

A bodged up a front hood from floor-underlay foam and cardboard baffles.

I found that the correction lenses in the Vixen Maksutov secondary mirror holder did not have blackened edges, pretty poor, so AstroRod made a baffle for that.
He had a small bit from a barlow which just fitted over the front of the mirror baffle tube. I found out how big the front of it needed to machined out to just take the fully illuminated beam, and it was turned to that.
With those baffles you can just see sky round the edge at the back of the mirror baffle tube, but the front hood baffle removes that.
With the Baader 35nm moving the suns image just out of the field gave a black field.

I have painted the baffles with Black2 paint and the inside front of the mirror baffle. I made a 180grit emery paper tube painted with Black2 to slip into the back of the mirror baffle up to the step. That reduces reflections a lot. And even though most of the light is cut off by the collimator lens holder the reflection back into the baffle tube does not seem to be an issue.

An old barlow body just fits in the Vixen screwed on back adapter with the 1.25" eyepiece part unscrewed from the adapter. With a bit of masking tape to make a tight fit.
Another barlow bit screws on to the front of the barlow body. I moved the first PST collimating lens into the front piece of barlow. And I can just get focus, at the floppy mirror end of adjustment though. Rod turned the front to have a slope to the outside to deflect light into the blackened rear telescope body. And a baffle inside was turned down to a knife edge. I had a look at space telescope baffles where they point forward at an angle, before the secondary, so there is a wedge shaped light dump formed with the inside of the tube. So all knige edge baffles have had an inward slope front, or back facing the reflective etalon, turned in where possible. And all baffles in the main barlow adapter were turned down to knife edges with sloping light dumps. Simple ribbed sections reflect too much light.

The front collimator lens holder is still in the etalon but Black2 painted to the front.

A 15mm baffle has been placed into the front of the PST block and Black2 painted along with some of the rear collimator lens holder.

This means the light is constrained to that which fits into the side of the PST prism.

I removed the side of the PST block.

The prism had some of the grease on it so needed cleaning. It has the standard off axis position all are reported to have.

The inside of the box has been black flocked where there are transmission spots out of the prism. From the front beam or smaller reflections back of the blocking filter.

I made a little 'post box hole' out of flocking to go between the prism and the entrance to the eyepiece tube but there was no obvious stray light. Shining a torch into the new front baffle shows a bit of light off the prism sides and the ribbing in the eyepiece entrance, original with no ERF.

Looking down on the blocking filter you can see a series of thin lit edges of the baffles but no obvious off axis light.

RodAstro has re-jigged his Omega Bobs filters. Gone back to a F30 configuration, 155mm F15 stopped down.
A UV-IR 2", the Omega blocker, and the two Omega 20mm 1.5nm in the diagonal holder.
The problems he had with reflections in the collimator set up have gone.

He observed at the same time as me and comparing what we could see later had a similar view. He seemed to be able to see more details over a bright sun spot region. Some variation in brightness across the image as cosmetic seconds. Just move image about a bit.

Hopefully we can have a side by side comparision soon.

At the moment I prefer my 25mm Clave' Plossl as the image is brighter though smaller than 16mm FF or a 12mm Clave'. The larger lenses may mean less scatter? I have blackened the lens edges.

I need to make a permanent ERF holder and proper hood. One idea is to put a stop at the mirror radius of curvature to remove spherical abberation. We are only using a part of the full aperture.
With the Lunt ERF moving the image just out of the field leaves a small amount of field light with a central feature, I assume some dispersion light getting through.

Andrew.

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Re: PST Mod Reflections back off the Blocking Filter

Post by marktownley » Thu Mar 21, 2019 8:32 pm

Good to hear you are progressing well. Good problem solving to get rid of the reflections.
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