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Mirror re-coating

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BillW's picture
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Mirror re-coating

Hi,

After sitting in a box for well over 10 years I decided it is time to examine the possibilities of using a mirror that I was given before it hit the skip! Sadly I didn't get to the rest of the hardware before that particular skip had been taken away...

It is a 16 inch f2 from either a Schmidt or a fast Cassegrain astrograph. So it's a deep mirror. It was in pretty poor shape when I got it but yesterday I stripped of the old coating and the surface, visually it looks good. I'll need to coble together some test gear to see what the figure is really like.

The big chip was done during the disassembly I was told but is on the back of the mirror. I've ground out some of the sharp looking fractures and hopefully it won't be an issue. This thing is 3 inches thick and weighs a hefty 20.5kg!

So the question is, has anyone had a mirror re-coated by Orion Optics? These guys are currently first choice as I can drive to them the stay local. Any comments, honest user reviews are always the best source of info...

Cheers,

Bill.

spurrp's picture
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Orion Optics

Bill, I asked Orion Optics to regrind an 8.5" mirror (I'd had the home made Newtonian on its end for a while and the diagonal fell onto the main mirror - an expensive mistake - I won't be doing that again!) and then re-silver. The accuracy claims were very good, although I've no means to prove it subsequently.

However, I found the people there very offhand, arrogant even. That was c. 6 years ago so they might have changed out.

Your mirror looks (and weighs) amazing! By the way when my mirror came back it was only half the original thickness!

Good luck. Paul

Andy Wilson's picture
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Secondary mirror recoating

I had a secondary mirror recoated by them about 3 years ago. They did a good job of it, though of course this was much simpler than a heavy curved mirror.

Andy

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Hi-Lux coatings

Hi Bill,

I had a Hi-Lux coating put on my 12 inch mirror by Orion Optics a few years ago and it is still in good condition.  I haven't examined it recent for pinholes but it still looks uniformly bright despite several years outside in the observatory. So I'm happy with it.

One thing that may be an issue is the reflectivity in the UV and IR, outside the 'visible' range.  When doing spectroscopy I get the impression I'm not seeing as far into the UV or IR ends as I did when using just a plane old aluminium coating.  I haven't quantified this and it's not really a significant problem but maybe something to investigate?  I did contact Orion Optics to ask what the reflectivity was beyond their specified range of 400-700nm but the customer service person didn't seem to understand the question(!).  

Mike.

Robin Leadbeater's picture
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High reflectivity coatings

There have indeed been various discussions on the ARAS spectroscopy forum about some modern coatings (on mirrors, lenses and camera windows) which while very efficient in the visible, drop off rapidly in the UV, for example this plot for the GSO RC telescope which effectively cuts off steeply at 4000A

https://www.spectro-aras.com/forum/viewtopic.php?f=45&t=2398&p=13202#p13470

This has been a big issue for the UVEX spectrograph, summed up by  Christian Buil in the same thread

"What always worries me is to see reflectivity of 99%, via dielectric treatments. These things are really our enemies in spectrography when we want to go to slightly extreme wavelengths. Besides that 99%, it is only for a narrow area of ​​the spectrum - it is misleading. Long live the coating with a single protective layer, even if we are at 96% reflection in the visible."
Robin Leadbeater's picture
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high reflectivity coatings
mike harlow's picture
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"Enhanced" coatings

Thanks for the info Robin, very interesting.

Looks like going back to silver is the best option for the UV!!!

Mike.

BillW's picture
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Hi,

Hi,

Thanks all for the various comments. I don't think I'll be opting for any fancy coatings as I don't know how good the mirror actually is nor how I'll be able to fully exploit such a fast mirror without other aberrations being corrected.

Even if the figure isn't that good, plain aluminium will do to let me play around with secondary optics with at least usable results.

Might be the worlds shortest dobsonian... ; - ))

I'll keep you posted!

cheers,

Bill.

Xilman's picture
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No secondary?

 Have you thought about mounting a camera at the prime focus and doing away with the secondary altogether?

No worries about light-loss from multiple reflections.

Might not work, but if you are experimenting anyway, perhaps this might be worth a try.

A R Pratt's picture
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No secondary?

Good suggestion. Along that line, have a look at:

https://britastro.org/sites/default/files/10-KONRAD_a%20portable%202016.pdf

and results from its use: (11 MB PDF)

https://www.iota-es.de/JOA/joa2017_2.pdf

Cheers,

Alex.

BillW's picture
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Hi,

Hi,

ahhh,  I meant "secondary" as in some sort of corrector not a secondary mirror...  Even a "perfect" mirror will exhibit some aberrations at f2. Something to stretch it out to f4 maybe, a barlow lens basically, then the camera. I've got boxes of lenses to play with that I've accumulated over the years. However I have a small fibre fed Littrow spectroscope I built a while back sitting in a drawer. It needs a bit of tlc but offers interesting possibilities. Ultimately the mirror might just become a flux collector rather than a "telescope", no need to worry about imaging quality then. The next problem is a suitable mount, it is a chunky piece of glass...

It's all good fun!

Bill.

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Recoating

Very interesting topic, I’m wondering if Orion is the only company who reposts mirrors in the U.K. or do anyone know of another vendor.

Grant Privett's picture
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Other Companies

I had a couple of mirrors done by VCSM last winter/spring during the covid surge. I couldn't call them speedy, but one of the mirrors had unusual coatings which took a while to remove.

New coatings look fine. Images seem clean.

They are on: http://www.scientificmirrors.co.uk/

Americo Watkins's picture
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Recoating

Daryl, I use Galve Optics https://www.galvoptics.co.uk at Basildon, Essex and have found them ok. About a 3 week turn around.  I drop off and collect the mirrors.

Eric

BillW's picture
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Hi, Grant/Eric,

Hi, Grant/Eric,

thanks for the comments. I've used Galvoptics before but they, like Scientific Mirrors are a bit too far south for an easy drive. I'm a regular visitor to Manchester so Orion is not to much further down the M6. I'm always nervous when it comes to shipping big optics. Couriers will promise the earth but "...it fell off the forklift..." has happened to me before!

cheers,

Bill.

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Mirror coating

Thanks for the recommendations I’ll look them up