5 replies to this topic

[robin_astro]

The fashion for adding fancy multilayer dielectric coatings on mirrors to a gain a few percent increase in reflectance in the visible (an improvement which is impossible to notice in practise)  can have a severe impact on their suitability for science. Look at this 8 inch RC for example, claimed to be "the ideal telescope for photography"

https://www.teleskop...624-mm-OTA.html

 

You would think it might be a nice telescope for spectroscopy but if you zoom in on the reflectance graph you will see the reflectance has already dropped to just 40% by 3900A  and is down to just 20% by 3700A, still well above the Ozone cut off wavelength.

https://www.teleskop...ektrum-1000.jpg

 

Unfortunately few manufacturers reveal these figures and as a result of this marketing hype, finding telescopes that work well in this region of the spectrum is becoming more difficult

 

Cheers

Robin

[yuzameh]

I'm sure most people aren't even aware it is a thing, I certainly didn't until your post.

 

I suppose composite type complex scopes can't be taken apart and taken down to be re-aluminised?  If anyone still does that service.  Last I had that done was in an almost falling down shack of an old 'industrial unti' in outer North London, and that was the nearest despite the significant distance!  Got taken and picked up, no trust for postal service!  Place probably long gone by now.  NB they soaked off the old coating with whatever they use to do that, the mirror was nearly transparent when I got the thing second hand.

[RedLionNJ]

I think the title is somewhat of a misnomer.  Spectroscopy is only one aspect of science carried out using a telescope - there are plenty others for which dielectric coatings would not be a negative.

[pvdv]

Interesting.
I was just looking at this paper (https://arxiv.org/ft...5/0805.1557.pdf - page 15) about GRB 080319B and noticed that Gemini North spectroscopy was in the 4100-6800A range, which led me to wonder which type of coating they used and this paper (https://www.gemini.edu/files/docman/websplash/websplash2004-23/GN_silver_coating.pdf). It turns out they used (at least then) protected silver, which led to a sharp drop in that region as well. Obviously, their goal is to optimized for NIR anyway, so I guess it is no big deal.
Their degradation figures for unprotected coatings are scary though!

[robin_astro]

I think the title is somewhat of a misnomer.  Spectroscopy is only one aspect of science carried out using a telescope - there are plenty others for which dielectric coatings would not be a negative.

The title does not imply a negative impact. There has however been a significant shift in the market towards these coatings in recent years and  has a significant negative impact both for spectroscopy and  U band photometry applications for no significant scientific benefit that I am aware of.  

 

Caveat Emptor

 

Cheers

Robin

[robin_astro]

noticed that Gemini North spectroscopy was in the 4100-6800A range, which led me to wonder which type of coating they used and this paper (https://www.gemini.edu/files/docman/websplash/websplash2004-23/GN_silver_coating.pdf). It turns out they used (at least then) protected silver, which led to a sharp drop in that region as well. Obviously, their goal is to optimized for NIR anyway, so I guess it is no big deal.
 

That is an interesting paper. It seems they are not so worried about the small loss of signal from a few percent drop in reflectivity but the effect on the emissivity, the few percent that is not reflected but emitted as heat adding to the background and reducing the signal/noise.(emissivity, hot matt black surfaces radiate a lot more heat than shiny ones) There a difference from say 2% to 6% makes a big difference.

 

Cheers

Robin

Edited by robin_astro, 30 March 2023 - 06:29 AM.