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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