Vixen VMC110 PST mod

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AndiesHandyHandies
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Vixen VMC110 PST mod

Post by AndiesHandyHandies » Wed May 22, 2019 7:19 am

Hi

I bought this rather than a hard to find now big F10 100mm refractor for a first PST Mod as its easy to store and move about.
It was a bare dealer return? at a good price.

The good thing is learning about all the baffling needed to achieve a black field.

Being a commercial scope the secondary and primary baffles are limited for a decent field but let lots of off-axis stray light in which is a big problem for a solar mod.

I have been using a bodged up dew shield made out of a roll of foam underlay, nice gold foil finish, and a few cardboard baffles. The next project is for a hard dew shield with a stop at the front at the primary centre of curvature to take spherical abberation off that away and to hold a Lunt 110mm ERF, though I went from that to a Baader 35nm initially because of the dispersed red light from the glass. The 35nm gave me a black field but a back reflection.

The corrector lenses in the secondary did not have blackened edges so Rod made me a baffle to hide the edges, sloped to direct reflected rays into the telescope side. Black2 coated.

He had a bit from an old barlow which just fitted over the front of the primary baffle. It was grub screwed on. I observed the size of the bright core on its front. Fortuitously there was a little ridge there so easy to get Rod to turn it down to size. Because of the small PST blocker I am observing priminences off axis so it stops unwanted light going in to the gubbins. The front was angled to reflect dispersed light out of the front aperture. Also Black2ed.

I had to use the emery paper tube, Black2ed twice with 50% thinned paint, in the primary baffle.

The front of the PST to Vixen T2 adapter has the front angled to disperse reflected light into the Vixen back. The back of the centre of the mirror Black2ed and what I can get at through the two eyepiece holes in the back.

It was a bit of a risk buying the Vixen because of back focus need for the PST. But moving the PST collimator lens to just behind the mirror has just reached focus.

I am now using a Omega Bob 40nm blocker in the freed up collimator holder in the front of the PST. Needs a of shimming doing. Bobs 40nm ERF does not have the back reflection issues of the Baader 35nm and it blocks the far IR as well.

I looked at space telescope baffles. After the secondary they have vanes coming out at 45degrees forming a little light dump with the wall. So the useless ribbed baffles in the Barlows have been turned down to have vee shaped light dumps behind foward facing wedged baffles. One faces the PST etalon to deal with the bright reflection off it people complain about. All Black2ed.

I have had a delrin baffle put in the entrance to the PST box sized to have a beam fit into the prism face, flocked the inside. I just need another favour to get a baffle put in the blocking filter holder as I can see light off its ribbed surface and from the edges of the prism.

Be interesting to go back to using the original PST tube.

I now have a black field with the Sun just outside the field. And no pesky reflections.

Comparing the Vixen with the Meade yesterday, Mark T said the Meade would be better. Seeing not good.

Obviously the smaller size aperture shows.

And the less sharp image as its a cheap Mak.

I could go down to a 19mm EFF Kunming eyepiece on a 1035mm fl.

With the 127mm I use a 25mm Clave on 1900mm fl. With Beloptic UV-IR coated KG3.

Rod found me an Ioptron mini tower which it could go on when away from my garden pier. And kindly fettled out the slop due to poor design.

I think I need another cheap PST with naff comb blocking filter for a stage 2 mod I have in mind.

PST finder and Mod on the one mount.

Andrew

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Re: Vixen VMC110 PST mod

Post by marktownley » Wed May 22, 2019 8:12 pm

Sounds like you've been busy! Post up some pictures of all of this when you get chance, I know people would be interested to see them.
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Re: Vixen VMC110 PST mod

Post by AndiesHandyHandies » Mon May 27, 2019 1:33 pm

Thermal calculations.

Starting with a proposed second space solar telescope:

https://www.spiedigitallibrary.org/conf ... full?SSO=1

Calculate the thermal inputs and absorbtion for the Vixen:

Primary 110mm

Area = 0.055 x 0.055 x 3.142 = 0.0095 Mx2

Taking 1000w per Mx2 on earths surface

Watts = 1000 x 0.0095 = 9.5W

Taking 6% absorbtion as used above

Absorbed Watts = 9.5 x 0.06 = 0.57

Transmitted about 9W

Secondary:

Absorbed Watts = 9 x 0.06 = 0.54W

Two lenses? 1W?

Total absorbed 1.5W.

Transmitted 8.5W

UV-IR filter.

Reflected? 4.5W

Absorbed 4 x 0.06 = 0.24W

Transmitted 3.76W

Bobs internal ERF 40nm:

Reflected 90%

Reflected 3.75 x 0.9 = 3.375W

Transmitted 3.76 - 3.375 = 0.385W

Absorbed 0.385 x 0.06 = 0.023W

Transmitted on 0.385 - 0.023 = 0.362W

PST F-P Etalon 400A bandwidth in.

Comb = 1A

Combs every 10A

PST reflects 90%

Reflection 0.362 x 0.9 = 0.326W

Transmitted = 0.362 - 0.326 = 0.036W

400 / 10 = 40 Combs One Comb selected by Comb filter.

Comb filter transmits

0.036 / 40 = 0.0009W

Sun diameter in PST 4mm

0.0009 / ( 0.004 x 0.004 x 3.142 ) = 0.0009 / 0.00005 = 180W / Mx2 1/5th suns brightness at earths surface.

Heat flow basics

http://www.civil.uwaterloo.ca/beg/arch2 ... basics.pdf

Using these figures I am going to calculate the temperature difference at equilibrium from ambient to dissipate the absorbed watts on the secondary mirror. The space telescope has a criteria of 40C at the secondary mirror, and the only place I can sensibly cool.

The incoming absorbtion on the secondary is 1.5W

50% is reflected off the UV-IR filter behind the collimator lens (In the post)
and 45% off the Bobs 40nm internal ERF infront of the PST
and 4.5% off the PST etalon.

So absorbation is essentially 1.5 x 2 = 3W

Taking from the table in open air of conductance of 17 W / Mx2 x K

We can derive the temperature difference K to dissipate 3W for a 40mm diameter secondary mirror and corrector lenses.

K = 1
Mx2 = 0.02 x 0.02 x 3.142 = 0.00126
17W x 0.00126 = 0.0214W

K = 3 / 0.0214 = 140K

I assumed the two lenses absorbed the same as the mirror each.

Mirror alone is 0.5W x 2 = 1W Which will be similar for MAK180 as no lenses.

K = 1/0.0214 = 46.7K

Use a fan to double the heat flow then 46.7K / 2 = 23.4K which on a 20C day gives 43.4K only just over space telescope secondary temperature design target.

We will see what temperature measurements show.

Andrew.

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Re: Vixen VMC110 PST mod

Post by AndiesHandyHandies » Sun Jul 07, 2019 8:40 pm

Hi

I have been thinking about moving the PST Collimating lens forward to get 200mm inside the objectives primary focus.

I have found one reference to Collimators and thats for lasers and singlet lenses. There it say the beam diverges as from memory 1/Theta.

I have used a white screen to move from the moved Collimator and the beam does seem to diverge.

Could you add a weak positive lens to offset the divergence?

Andrew.

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Re: Vixen VMC110 PST mod

Post by marktownley » Mon Jul 08, 2019 5:20 am

If you can reach focus where it currently is why do you want to move it again?
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Re: Vixen VMC110 PST mod

Post by AndiesHandyHandies » Tue Jul 09, 2019 8:34 pm

Hi Mark,

I have a focus. Compared to AstroRods bigger Zeiss it seems OK, dimmer and less contrast, in light of the smaller aperture and secondary obstruction.

But having moved the collimating lens about 70mm forward of where it was will I be losing light and/or sharpness from the divergence of the 'collimated beam' ?

Andrew.

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Re: Vixen VMC110 PST mod

Post by AndiesHandyHandies » Thu Jul 25, 2019 4:32 pm

Hi

I tried spacing the black box further back with T2 extension tubes but outside of focus range then.
So I must be near to the 200mm backfocus requirement.

I have put a 20mm washer in the back of the main baffle tube to see if makes any difference to incoming off-axis sunlight glare or reflections back off the Collimator lens, BoBs ERF or the etalon. And taken out the Black2ed emery.

PST Mod 2

I replaced the PST black box with a diagonal, with a Bobs H-alpha 1.5A set filter on a tilter in it to use as the Comb blocking filter. And used various bits to get the eyepiece field stop 200mm back.

The was better in terms of light scatter as no pentaprism.

The sweet spot was there but as you scanned across visibility came and went due to variations in Bobs filter.
RodAstro said he had similar effects in his Quark 60DS mod.

More details in my 90mm TS Triplet Post.

Andrew.

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Re: Vixen VMC110 PST mod

Post by AndiesHandyHandies » Fri Aug 16, 2019 7:39 am

Hi

Had a session though windy and almost no proms so nothing to see really.

When AstroRod borrowed my laser pointed remote thermometer his highest reading from the 6" Zeiss with internal ERFs was 24C looking into the eyepiece. He bought a UV meter and that showed lower readings up the eyepiece than ambient.

I have a handrail with blue paint on the side. In the shade 20C and sunlit 26C. My blue towel was hottest at 30C.

Took the cosmetic cap of the secondary, showing bright alloy back, and that was 21C. Waving at black secondary and vanes, main mirror, root of baffle tube, down the side port at moved Collimator lens and Baader 1.25" 35nm, outside of PST got readings arond 21C. Pointing up the eyepiece 24C.

So no problems.

Someone did stick a baffle on the front of the secondary with a strip of double sided tape and after 4 hours on a hot day there was an expensive sounding tinkly sound as it fell off. Now glued on with Sika sealent. Sticky and rubbery. I had a stress toy they make out of two squares of MDF with a fillet joining which you can bend double at will. I was playing with it at HOEAS club and a freind who maintained lorries said the Sika rep came in and showed them how to glue lorry panels they had to remove to get at a fault on with it, so pretty strong.

Andrew

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Re: Vixen VMC110 PST mod

Post by marktownley » Fri Aug 16, 2019 8:34 am

Some pictures of all this as you're going along would make the dialogue much easier to follow...
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Re: Vixen VMC110 PST mod

Post by AndiesHandyHandies » Mon Aug 19, 2019 3:52 pm

Hi

Some pictures of the mod.

The baffle over the secondary:
P1050426.JPG
P1050426.JPG (1.06 MiB) Viewed 40 times
It seems to repeat this image even though the attachment names are different. I used Cut and Pase which gave me a little box underneath where I said paste inline where I was. ???

As you can see the edges of the correction lenses are not blackened.
The first try to attach it was double sided tape which failed after 4 hours on a hot day. Poor contact.
Also commercial Maksutovs are not well baffled to increase the claimed fully illuminated field. Look down the focusser to the edge and you can see sky between the secondary baffle and the primary baffle.

The baffle from the front.
P1050426.JPG
P1050426.JPG (1.06 MiB) Viewed 40 times
The front has a knife edge pointing forward with a slope on the front to direct reflections off the lenses, etalon or filters to one side. Inside it has knife edge ribbing as well.

Refitted with Sika sealent.
P1050426.JPG
P1050426.JPG (1.06 MiB) Viewed 40 times
A baffle for the primary baffle made out of a cheap Barlow nosepiece from Rods bits box.
P1050426.JPG
P1050426.JPG (1.06 MiB) Viewed 40 times
This was first attached as was and the suns image centred on it. There was a little ridge in the right place so it was turned out just inside it. And turned to have a sloping out front.

Mounted as far forward as needed to mask the stray skylight between the secondary and primary baffle.

P1050426.JPG
P1050426.JPG (1.06 MiB) Viewed 40 times
I took a punt on reaching focus with the PST Collimator lens moved forward in this arrangement:
P1050426.JPG
P1050426.JPG (1.06 MiB) Viewed 40 times
On the left in shiny black is the Vixen T2 to telescope port adapter. The AOK Swiss front PST to T2 adapter screws onto this.
The matt black part sticking out on the right is the front of a barlow body. Its a push fit into the Vixen adpater with a bit of masking tape.
The barlow had a useless ribbed baffle behind the lens, like the inside of the PST tube below the Comb Filter. Rod turned it down to have a 20mm knife edge pointing forward for the Suns image and one pointing backward for the bright reflection off the Etalon. So there is a beam dump on the outside. Between the two knife edges its turned down with a slope up to the knife edge to direct stray light into the Black2 walls.

Although the Omega Bobs 40nm ERF works up to 5" I have placed a Baader 28.5mm 35nm filter just behind the moved PST Collimator lens to protect it. And to reflect the majority of the energy back through the collimator lens so it expands back out roughly the way it came in safely. Offset a bit as its tilted. Fortunately it has enough thread on the front when tilted for the piece holding the Collimator lens to screw on securely.

The PST Collimator lens is mounted into another Barlow front using a plastic Celestron/Skywatcher Moon filter body inside. There is a 20mm stop infront of the lens with a sloping front directing stray light into the back of the Vixen, with Black2 paint on it.

The unit screwed back into the Vixen:
P1050426.JPG
P1050426.JPG (1.06 MiB) Viewed 40 times
At the moment I have a 20mm rubber washer fund on a local car park stuffed up the back of the primary baffle as an additional baffle. Before I had a piece of Black2ed 180 grit emery paper in the baffle back up to a step inside. Not sure which is best. Black2 50/50 with water shaken up in a NT mini Jam jar. Several coats.

See my PST Black box mod for baffling needed there.

Andrew
Attachments
P1050427.JPG
P1050427.JPG (1.02 MiB) Viewed 38 times

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Re: Vixen VMC110 PST mod

Post by marktownley » Tue Aug 20, 2019 6:34 am

Like the added pics, but they're all the same apart from the last one! tee hee hee! :D
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