Vixen VMC110 PST mod

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Vixen VMC110 PST mod

Post by AndiesHandyHandies »

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 »

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 »

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 »

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 »

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 »

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 »

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 »

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 »

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 »

Hi

Some pictures of the mod.

Deleted for space issue.

Andrew
Last edited by AndiesHandyHandies on Fri Mar 13, 2020 10:06 am, edited 2 times in total.


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Re: Vixen VMC110 PST mod

Post by marktownley »

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


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Re: Vixen VMC110 PST mod

Post by AndiesHandyHandies »

Hi

I am confused by the image inclusion here.

The baffle over the secondary:
The baffle over the secondary:
The baffle over the secondary:
P1050426.JPG (1.06 MiB) Viewed 8354 times
The baffle from the front.
The baffle over the secondary:
The baffle over the secondary:
P1050426.JPG (1.06 MiB) Viewed 8354 times

The baffle refitted with Sika sealant.
The baffle from the front.
The baffle from the front.
P1050427.JPG (1.02 MiB) Viewed 8354 times
A baffle for the primary baffle made out of a cheap Barlow nosepiece from Rods bits box.
The baffle refitted with Sika sealant.
The baffle refitted with Sika sealant.
P1050433.JPG (1.02 MiB) Viewed 8354 times
Mounted as far forward as needed to mask the stray skylight between the secondary and primary baffle.
A baffle for the primary baffle made out of a cheap Barlow nosepiece from Rods bits box.
A baffle for the primary baffle made out of a cheap Barlow nosepiece from Rods bits box.
P1050438.JPG (1.07 MiB) Viewed 8354 times
I took a punt on reaching focus with the PST Collimator lens moved forward in this arrangement:
Mounted as far forward as needed to mask the stray skylight between the secondary and primary baffle.
Mounted as far forward as needed to mask the stray skylight between the secondary and primary baffle.
P1050437.JPG (1.02 MiB) Viewed 8354 times
The unit screwed back into the Vixen:
I took a punt on reaching focus with the PST Collimator lens moved forward in this arrangement:
I took a punt on reaching focus with the PST Collimator lens moved forward in this arrangement:
P1050439.JPG (1.08 MiB) Viewed 8354 times
Attachments
The unit screwed back into the Vixen:
The unit screwed back into the Vixen:
P1050440.JPG (1.02 MiB) Viewed 8354 times


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Re: Vixen VMC110 PST mod

Post by AndiesHandyHandies »

Hi

The first picture is duplicated somehow but they are all there with the captions displaced underneath.

Andrew


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Re: Vixen VMC110 PST mod

Post by marktownley »

Ahhh, the pics make things much easier, interesting mod indeed.


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Re: Vixen VMC110 PST mod

Post by AndiesHandyHandies »

Hi

First time copied the first picture a few times, and perhaps twice the second or one 'in the system'.

Had two days out in the sun with the PST 'Combat' etalon.
Followed by the Re-focusing lens in a Celestron Moon filter in a BF 48mm to 28.5mm adapter infront of a diagonal.
The diagonal contains one of Omega Bobs 1.5A H-alphas as the Comb Blocker on a tilter.

Bobs filter has one slight off axis ghost reflection. Rod rotates a double diagonal set to get out of the way.

There are various edge reflections, the filter is unmasked in a step with 3 washers on screws to retain.

After various trials I found tilting one way as far as possible to get rid of artefacts best.
Setting the PST to where I normally have the ring so I have a darkish patch, off-centre unfortunately.
Tune the other tilter to the PST sweet spot.

Tried a polariser after the Collimator lens and a circular polariser on the eyepiece.
Does reduce the glare but dims the prominences as well.

The best result is with a 17mm old Moon filter? nose after Bobs filter.
A Chinese Moon filter body with 20mm aperture on a 2" to T2 holder in the back of the diagonal. And the same in the base of the eyepiece.

I could see the prominence in Gong on the right top, I think.

Looked like an onion to me, bulbous base narrowing to the top.
As good as it will do I think.

Rod said Bobs filter is good for prominences as it lets more light through than the PST Comb Filter.

I will try a few photos of the set up later.

Andrew.


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Re: Vixen VMC110 PST mod

Post by AndiesHandyHandies »

Hi

With the moved forward primary to reach focus even with the PST Collimating lens moved forward will result in some primary vignetting.

The best case is for the Lunt 110mm DERF to be in a 100mm diameter aperture cell.

Using Mel Bertles Newtonian secondary field illuminator calculator:

Mirror 100mm
FL 1035mm
Secondary 200mm inside focus
Secondary 20mm
CCD 8mm
-0.250 Magnitude down at edge,

So not too bad.

Need to draw out actual mirror and secondary positions to estimate effective aperture.

Andrew.


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Re: Vixen VMC110 PST mod

Post by AndiesHandyHandies »

Hi

Graphed out the PST, assuming 18mm clear aperture AKA Daystar and 2mm round 10mm Sun for prominences.

The Moon filter front which turned up from somewhere at 16mm was exactly right to go after Bobs Comb Filter so explains why it nicely darkened the field. Need my two Chinese cheap Moon filter bodies stopped down to 15mm and 14mm fo a nice set of refractor like baffles after Bobs filter.

I need to get some nice person to centre the PST etalon as where it is at the moment off axis will be very vignetted.

I used Mel Bartles Newtonian Diagonal calculator and for a 8mm focus its 0.25 mag down at 4mm out. And that assumed 20mm for the PST aperture.

Andrew.


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Re: Vixen VMC110 PST mod

Post by marktownley »

I know from experience it's a lot of fun doing a project like this!


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Re: Vixen VMC110 PST mod

Post by AndiesHandyHandies »

Hi

We had a nice sunny day yesterday.

So I had the Vixen mod out.

Last time I tried various options including polarisers to try to improve the images.
As I was out over the day I had to move the diagonal round for a best field and there did seem to be some benefit from 'clocking' the PST etalon with the Bobs filter in Rods diagonal mod as a comb filter.

I may have to put the Vixen in rings rather than the bolt on Vixen wedge so I can clock it with the diagonal and keep a comfortable angle.

I bodged my Bobs filter 1 into a plastic Celestron/Skywatcher Moon filter. Left it loose and miss threaded it on to tilt it, one benefit of plastic.

I seem to have a good filter as no reflections in the filter and even illumination. It seems to work OK as a Comb Filter. It was nice to see the whole Sun with prominences all round it. My 25mm Clave' seems best still.

I bodged Bobs filter 2 into a Moon filter as well and put in the eyepiece. I dimmed the prominences and made the field black. I could see more detail on the surface and the current bright area on the bottom left for me.

Once Rod has modded a diagonal for me it will be nice to compare it with his 60mm Daystar mod.

I tried out my 80mm F5 and 90mm F6.7 refractors. I had to mess about with a 2x ED Barlow nosepiece and a 2x ES Tele-extender. I got a Sun in both but had to use offset as a few reflections in the field. The Suns were still smaller than the Vixen. The proms looked sharp but small. Probably the collimator lens not in optimum position.

Try the 127mm next time.

Andrew


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Re: Vixen VMC110 PST mod

Post by AndiesHandyHandies »

Hi

Waiting for AstroRod to modify a diagonal to hold my Omega Bobs filter. As the blocker for the PST

Bodging it in a Celestron Moon filter, loose or missed screwed in.

Photo before found position. Panasonic GX80. Micro4/3. The background looks OK to me. Is it?

Image

Finally chanced on right tilt and could see surface detail and two small prominences bottom 7am, on Friday.

Andrew.
Attachments
P1090003.JPG
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Re: Vixen VMC110 PST mod

Post by marktownley »

Background seems nice and dark. Right hand side of disk seems out of focus compared to the left?


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Re: Vixen VMC110 PST mod

Post by AndiesHandyHandies »

Hi

The T2 nosepiece for the eyepiece / camera has an undercut and is in a brass compression ring so wobbles. Need to sort out a better fitting arrangement.

First image with todays nice prom. Less detail than visual.

Image

Andrew
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Re: Vixen VMC110 PST mod

Post by marktownley »

That's a good capture of that prom!


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