Telescopes Using Mirrors – Glass Appears To Be Always Used?
When constructing the mirrored surfaces for telescopes, I've read of the painstaking efforts involved in producing the glass to exacting dimensions/surface roughness, and then depositing the reflective coating on the surface. Manufacturer's are at/near the limits on the size of glass that can be produced, resulting in some other approaches used to work around the limitations...such at the telescope that uses multiple hexagonal mirrors (name?) instead of a single large mirror.
My question is (and it's probably obvious, but I'm not seeing it), why must glass be used as the substrate? Could not many other materials be used (which don't have the manufacturing limitions that glass possesses), and then the surface mirrored? I would think telescopes of huge appatures could be produced, without the tremendous time and expense of working with glass. Again, I'm sure I'm missing the obvious reason(s) that's not done! I appreciate your time for reading/responding!
Possibly related posts: (automatically generated)
- Related posts on Always
- I always took the road less tr… | Lean Religion
- The Marriage Ref you always wanted. | Internet Marketing Tips
- Why do you women always want to lose weight? | Weight Loss For ...
- Related posts on Appears
- Microsoft's antitrust retort: Just what is it accusing Google of ...
- I Can't Connect To Halo1 Servers? « sey7
- How to Use Coupon Code in Store « Apple Bazaar Blog
- Related posts on Glass
- Witness: Glass 'everywhere' in NYC atrium collapse (AP) | enUws
- Striking Emerald Glass Exterior Design an Across Hotel in Beijing ...
- Glass Bead Kits » Glass Foil | Tower of Charms, Fashion Stuff
 |
US $2.95
The type of glass that’s used for the mirror substrates in the large telescopes is much more thermally stable than many other materials. The segmented mirrors in the Keck telescopes in Hawaii are some of the largest in the world.
The carbon silicate that glass, well, IS, is the best known substrate for admitting,without filtering or refracting, light sources to the observer.
I freely admit that I am no Expert, but I have yet to hear tell of a substrate that combines rigidity(i.e. molecules are relatively stable, and static) with transparency as effectively as glass.
Over a century has shown that glass is by far the best material to use. In some telescopes nowadays, quartz and other materials are used, but glass is much more easily polished to the degree of perfection required for telescope optics than any other material.
It’s pretty simple actually. Glass used in optical instruments has been formulated to be thermally stable. Other materials like ceramics and metals are too susceptable to temperature gradients and will expand and contract with temperature changes. Optical instruments like reflecting telescopes require thermal stability to reduce or eliminate minute distortion of star images resulting from temperature changes. This is especially important when doing astrometric measurements and astro-imaging.
Good question !!
Indeed other substrates besides glass have been used in the past. Before the glass coated with aluminum combination became the norm, a metal called “speculum” was the universal substrate. This metal was a hard alloy of copper, tin and bismuth. Really a form of bronze whose composition depended very much on the individual maker to achieve the best reflecting surface. Folks like Herschel and Lord Rosse did lots of experiments to come up with the best combinations. But they all had one fatal flaw: They tarnished very quickly and required a complete resurfacing of the mirror.
Later, Silver on glass was tried and achieved great results, better than the ones now achieved with aluminum since silver is much more reflective than aluminum. But after only 6 months or so the silver tarnished to the point that reflectivity could be down to less than 50% or so. The current method of Coating a Pyrex glass blank with aluminum and covering that with a clear oxide coating to keep it from tarnishing seems to be the best solution all around. While there are some ceramics available with slightly better characteristics, such as Zerodur, they are prohibitively expensive and don’t really justify the extra cost when compared to the advantages gained.
Adolph
nylons
shoes
hair accessories
blower dryer
curling iron
BTBAM is actually a GOOD Deathcore band. Damn, that’s a good feat.
no way!!!!!!!! that is sooo cool!!!! were you at the 9:30 club Wednesday, or was this a while ago?
this song will live on long after Satriani goes away. A gem of a song.
used audacity to record mp3 clips playing on my laptop-now editing for Rock Opera sound track and putting a kid in charge!
forgot how good this song actually was.
Globally Proven System Used By The Funniest Pro Comedians Since 2001. Substantial Results Provided On Product Page. High End, High Quality, Low Refund Product. Perfect For Serious Niche Affiliates:
e ba y man 100$ to 250$……. a good one
The new building certainly mirrors one of Zionism’s key objectives: driving out the Arabs and replacing them with newly minted Israelis from around the world. Museum of Tolerance Desecrates Graves by John Taylor Rabbi Marvin Hier’s Los Angeles-based Museum of Tolerance, part of the Simon Wiesenthal Center, is building a $150 million [...]
I find having your mind on heaven makes you more active in life!
It's not christians on drugs and drink escaping reality!
96 minutes