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Two questions about telescopes.

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#1 omnicide

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Posted 05 December 2020 - 02:41 AM

The first question is what kind of mirror do reflector telescopes use? are they one of those fancy dielectric mirrors or just the regular ones you see around your house? maybe a different type. The second question is about aperture, if I have a 130mm aperture refractor and a 130mm reflector does the reflector has a smaller "real aperture" because of the small secondary mirror is blocking the objective lens? Thanks :) 



#2 TOMDEY

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Posted 05 December 2020 - 03:26 AM

The telescope mirrors are (should be) like a hundred or thousand times more accurately shaped than household mirrors, and the shiny coatings are on the front "outside" surface, rather than the back. Reflectors pass slightly less light than the unobstructed refractor, but the difference is insignificant, unless the obstruction is rather huge. There's a lot on pickayune detail and smaller effects that we constantly banter about here on CN... the two that I mentioned above are by far the biggest dominant ones. You can pretty much ignore the other stuff as arcane, and not get into trouble enjoying the hobby.

 

Avoid "cleaning" your telescope mirrors and lenses, which often makes them worse or damaged. Get a good scope (not a big scope, just a good scope) and just enjoy using it! One of the biggest and most common mistakes that beginners suffer is cleaning and aligning their new and used scopes... pretty much by trial and error.    Tom


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#3 ngc7319_20

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Posted 05 December 2020 - 04:46 AM

Telescope mirrors are typically a highly precise and polished piece of glass (or similar material) with a thin coating of reflective aluminum on the face.  Often the aluminum will have a clear coating of a hard material over it like SiO or MgFl.  Sometimes silver or gold is used instead of aluminum.  Yes, all dielectric coatings are sometimes used instead, but this is most often on small flat mirrors due to the higher cost.

 

Yes, the "effective" aperture or collecting area of a reflector is usually slightly less than implied by the diameter due to the secondary mirror.  Also, it is fairly easy to make a lens with 99% transmission, but difficult (expensive) to make a mirror system with such high reflectivity.  Basic aluminum coatings have around 89% to 93% reflectivity, or about 78% to 85% for two reflections. Enhanced aluminum coatings can run up to about 99% per surface.


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#4 aeajr

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Posted 05 December 2020 - 05:41 AM

Telescope mirrors are typically a highly precise and polished piece of glass (or similar material) with a thin coating of reflective aluminum on the face. Often the aluminum will have a clear coating of a hard material over it like SiO or MgFl. Sometimes silver or gold is used instead of aluminum. Yes, all dielectric coatings are sometimes used instead, but this is most often on small flat mirrors due to the higher cost.

Yes, the "effective" aperture or collecting area of a reflector is usually slightly less than implied by the diameter due to the secondary mirror. Also, it is fairly easy to make a lens with 99% transmission, but difficult (expensive) to make a mirror system with such high reflectivity. Basic aluminum coatings have around 89% to 93% reflectivity, or about 78% to 85% for two reflections. Enhanced aluminum coatings can run up to about 99% per surface.


Well said.


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