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Smokey 45 Degree Diagonal Mirror?


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I'm puzzled by the brand-new GSO diagonal I bought - Quartz Dielectric 2", 99% reflectivity.

The mirror looks smokey if you look at it with the light behind you. Get it at a certain angle and its very smokey indeed. This "smokey film" does not clean off however, and is perfectly consistent - not patchy in any way.

However when allowing light to pass through at the (normal) 45 degrees, there is no sign of this smokiness. It looks absolutely fine.

Is this mirror meant to be like this? Is it how it is made? Or is it defective? I don't want to send it back if this is just a characteristic of its silvering or the dielectric overcoat...

Can anyone help?

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Hmmm... I just had a look through my WO CF Dielectric to see if I could see the same. At first I thought I'd found it - then realised it was a shadow change on the wall. I'ts hard to get an angle on it cos it's 45 deg's and you can only see straight into it (maybe a deg or three either side) without taking apart.

It sounds like part of the coating - but you can allways send an email question to GSO ......

Cheers :)

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Yup - you're spot on Polar - I just did that and got a bit more of an angle on it. It's crystal clear and consistent right accoss the surface - I couldn't detect any "smokey'ness" anywhere on it.

Hopefully someone with the same diag will confirm it for you.

ATB :)

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Its probably the coating - some Maks show that on their mirrors.

How do you get to see the effect?

Does the 180 Skymax show it somehow?

Jon Issacs over on cloudy nights pointed out that dielectric coatings depend on the angle of incidence and an industrial web site I just looked at said that customers specify the angle of incidence they want the mirror to work at...

Maybe that's all it is and I don't need to panic maybe?

Just when I think I've cracked this optics business, there's something new to learn :)...

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I cant see it with the Mak 180 but the Nexstar shows it quite clearly as a kind of oily film when its placed near a tungsten light. I also saw it years ago on a Nikon mirror in a camera and the dealer told me it was the coatings that Nikon used.

I have a Revelation 2" diagonal which is made by GSO as far as I know - its a dielectric one at any rate with a stated 99% reflectivity.

Just for you :) I took mine to bits just now to have a quick look and lo' - yes it shows (at some incidences and under a tungsten bulb) a kind of haziness - as if the mirror had a light coating of something - almost milky colored. At its normal incidence its not there at all as far as I can see.

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Actually mine is the Revelation one too (I just said GSO since I figured maybe more people had the GSO branded version) - and yes, it's that milkyness that I am referring to.

Glad to learn that mine is functioning normally.

Reading up on how dielectrics work was a bit of an eye-opener. I had no idea that these things were so sophisticated... :)

Thanks very much indeed - most grateful for your help & reassurance.

Edited by great_bear
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Just come indoors.

First light with "ol' smokey" was wonderful!

Best views of Jupiter yet!

Did my new Diag make a difference?

-well it did where it counts. Much darker sky and (seemingly) less flaring.

- and the nice thing about a Diagonal like that - like almost no other kit - you can keep this one for a lifetime without having to "upgrade"!

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Just come indoors.

First light with "ol' smokey" was wonderful!

Best views of Jupiter yet!

Did my new Diag make a difference?

-well it did where it counts. Much darker sky and (seemingly) less flaring.

- and the nice thing about a Diagonal like that - like almost no other kit - you can keep this one for a lifetime without having to "upgrade"!

Until they introduce coatings with 110% reflectivity :) :)

Edited by haitch
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