Truth in advertising....

[Peter Ward]

I've been doing a but of research on mirror coatings lately, spawned in part by the claim below:

"ALL OUR (made in China) RC TRUSS TUBE ASTROGRAPHS INCLUDE...

............Hyperbolic quartz primary and secondary mirrors with 99% reflectivity dielectric coatings in all GSO RC truss tube models!"

I've concluded this is pure crap.

While it is possible to dielectricaly coat primaries, if you have a NASA sized budget...this process distorts the underlying optics and is normally only used on smaller optics where only the central section being used (eg diagonals or flats).

So what do we make of the claim above???

Sure sounds cool.

I strongly suspect these mirrors are simply aluminium coated, with a quartz overcoat.

To be fair, a quartz overcoat does have good dielectric properties....but this is not the same as a dielectric mirror coating!!
(it's a plain mirror with a dielectric over-coat at best)

A small dab of ferric chloride on the mirror will prove my thesis beyond reasonable doubt. It will eat aluminium...but won't hurt a dielectric mirror.

I also suspect not many punters will be soaking their RC mirrors in ferric chloride to see whether I am correct.

P.S.
Consumer law in Australia would likely allow you a full refund if the product was not as claimed.

[Mamba]

Surely your title is an oxymoron.
Mamba

[Shiraz]

Peter, other makers can do >98% reflectivity coatings as a matter of course and at low cost - it seems reasonable that GSO could do the same. Spectrum use IAD to avoid substrate heating/distortion and don't call the coatings "full dielectric", but that becomes a matter of semantics, since they must have a multi-layer dielectric component overlaid on Al. Spectrum make the point that having an Al undercoat is actually an advantage over a full dielectric, since it allows the optics to be recoated.

With CF trusses, quartz substrate and 99% coatings, the GSO scopes seem to have the basis for quite high end optics.

http://www.spectrum-coatings.com/Telescope-Mirror-Prices.htm

[Peter Ward]

They are not, (nor are Spectrum) dielectric, or Bragg type mirrors.

The process is entirely different, and not simply one of semantics.

P.S.
I have other data to show GSO's coatings are not 99%...but that was not the point of my post. I simply wanted to point out some advertisers have clearly misrepresented the product as being something its not.

[Peter.M]

If your consider that the other options in the same price range are non existent. Do their claims really matter. Say their mirrors are 90% reflective (which I doubt) for the price you could get a 16 inch for less than a competitors 10 inch 99% reflective scope.

It reminds me of VW and their economy claims. Does it really matter if they are bogus? Google some real world figures if you really want to know.

If I was buying a scope, the first thing I do is look at images created by one. Not look over the figures

[Shano592]

Personally, I think it is an important distinction. If it is a false claim, then they should be called out on it. If it turns out to be a true claim, then they should be lauded for that, too.

Question is ... who is going to outlay the bling, in order to disprove/prove their claim? AS+T Perhaps? A road test of sorts, maybe.

[Somnium]

i think it really does matter, the information you gather about a product all factor into your decision. i am not saying that these claims are false, but it is dangerous to say that claims don't matter because i don't rely on them. where would you draw the line then? what if the product doesn't look like the picture, does that influence you ... what if they put up images taken from other scopes ?

to be clear, i am not saying that GSO products are bad or that they are lying, but i do think the clams are influential and should represent reality

[Peter.M]

Do you people assay your Gatorade to make sure it has the same salt concentration as your body? Believing someone with vested interests is insanity, not that I am saying GSO are claiming something that is wrong.

Their specs also include a " heavy duty 3 inch focuser" which anyone worth their salt would know is ditched at the get go.

Maybe someone is just a little upset that GSO are competing a little too well.

[SpaceNoob]

I've been impressed with what I've seen come from the GSO RC's, considering the price....

[Peter.M]

The real world images? That's my issue. If you look at an image and it looks ok. But they claim 99% and it is only 98, is the acceptable image any less acceptable?

[rustigsmed]

yes, I also think it does matter. if you are claiming something which states a measurement you should be able to provide atleast some data on how they reached their figure. of course individual build quality there will be some variation.

on their website they only refer to the parabolic mirrors which we know are BK7. these are stated to have a 93% reflectivity with SiO2. of course there is no graphs or the like, that would expect if you were looking at NB filters.

when I purchased my GSO newt, I wasn't particularly looking for reflectivity values; I looked at what photos people had taken with them and I suspect that most other GSO purchasers wouldn't care about reflectivity either.

I don't know if skywatcher (similarly priced) state any figures?

cheers

[Peter.M]

Skywatcher don't make RCs which are the values being disputed.

[Peter Ward]

GSO themselves make no claim about their mirror coatings on their web site.

My guess is an Australia Distributor picked up on a US website which perversely referred to their Quartz overcoat as a "dielectric coating"

Hey presto, we have dielectric mirrors. :lol:

FYI dielectric mirrors have multiple layers that are very hard, and are very resistant to corrosion. There are a number of diagonals on the market using this technology.

Given the expense and other difficulties (they distort the underlying substrate) it would be a world first for GSO to have 8-16" optics coated thus.

For everyone else, their primary mirrors are coated with Aluminium (sometimes enhanced with Silver etc.)

A Quartz overcoat simply protects the Aluminium layer.

[SpaceNoob]

Exactly. I've seen raw subs and fully processed images from these RC's and they've continued to impress me. v1, v2, v3, they're evolving and hopefully improving, meanwhile everything else in this hobby is just a bit stale...

[clive milne]

Thucydides would have (had) much to say about this thread.

[Shiraz]

Well no, I don't think that is correct. High reflectivity dielectric coatings are certainly available at reasonable prices for largish mirrors. As previously linked, Spectrum coatings do what is probably a multi-layer dielectric over Al with >98% reflectivity ($300 for a 16inch mirror). Edmund optics offer a high reflectivity coating (about 98% from their curves) - and they state that they use a multilayer dielectric film on Al. Orion Optics offer a multilayer dielectric stack over Al for 97% reflectivity over a broad spectral range. The Meade UHTC reflective coating uses Al "with a complex stack of multi-layer coatings of titanium dioxide (TiO2) and silicon dioxide (SiO2)". These coatings are a long way from being just a protective layer of quartz - the dielectric stack is a key active element of the reflecting surface. The substrate issue apparently does not apply with ion assisted deposition.

The technology of using multilayer dielectric coatings on Al to get ~98% reflectivity seems to be readily available at affordable prices for mirrors in the amateur class. I have no idea if GSO uses such technology, but if they do, it would be reasonable to describe these as dielectric coated mirrors. In any event, this discussion would be way more meaningful if it was based on data rather than conjecture - do you know of anyone who has done any measurements?

As far as I know, silver is not used to enhance Al - it is used as a higher reflectivity alternative to Al.

[Peter Ward]

No deity needed for the proof here.

Sadly, I suspect the crux of my post has been lost.

The 99% reflectivity is a fairy-floss figure I care not about.

But...... if I paid for a dielectric mirror and didn't get one....well...I'd be really pee'd off.

Dielectric mirrors never need re-coating.

[Peter.M]

So let me get this straight, just so were 100% sure on this. You would be upset, that you need to recoat your GSO mirror at a cost of around $300 plus shipping. After say 10-15 years, as opposed to buying any other RC, which also will need re-coating at the same interval. Even though you saved probably $10k in upfront costs? Am I missing something?

[Capricorn1(Tom)]

Well am I missing something.... how about staying on topic with the post... seems to be going of on irrelevant tangents not associated with the subject matter.

[RobF]

You SCIENTIST you! Is that going to be flame photometry or ISE Peter!?
Direct or indirect? Actually, I'm expecting ICP-MS for all my sodium estimations please.

Err, sorry. Back to mirrors now..... :)

[Shiraz]

re the longevity of multilayer dielectric mirrors with Al base coat, according to Orion Optics:
"there is no reason Hilux coatings will not last 25 years or even longer. ...We have had a small mirror outside in all weathers now since 2003 and if we wipe it carefully, it looks as good as the day we coated it. It has had, rain, snow, hail, everything the elements could throw at it."

I think we might be arguing about angels and pins

[Kunama]

I would think inductively coupled plasma mass spectroscopy would be overkill, salt or no salt.
As for the OP's first post, I think one would be within their rights to expect such a claim to have a basis in fact. But then again I have seen people advertising items made from FPL51 and 53 as CaF2.

[LewisM]

They don't anymore - too much criticism here and elsewhere :) A little bit of BS can only fool a certain type.

Hang on, W.O still claims some of their FPL53 scopes are fluorite... (which they aren't)

[Peter.M]

Sodium method of choice is OES. Call me when you want rare earths or something ;)

[Peter Ward]

:thumbsup:


If it has a Al base coat its not a dielectric mirror. :shrug:

Dielectric mirrors use the interference of light reflected from the different layers of dielectric stack.

If Orion are calling their mirrors dielectric...they are equally guilty of mis-representing their coatings.

Suggest you Google "Bragg mirror" .

P.S.
Just had a look at Orion's HILUX info...while they claim excellent reflectivity, I did not see any bogus
claims about them being dielectric. :thumbsup: