18 votes
Accepted

Why are segmented mirrors lighter than monolithic mirrors?

A telescope's precision mirror needs to be very large, to collect lots of light, and needs to be very precisely shaped. Holding that proper shape is based on the mirror's ability to resist stress on ...
Darth Pseudonym's user avatar
11 votes
Accepted

Why are telescope mirrors made of glass?

you are partially correct. Glass is used for several reasons. It is a very stable material and will hold its shape well for thousands of years. Glass can also be polished to a high degree of accuracy ...
Natsfan's user avatar
  • 4,494
8 votes
Accepted

Why do telescopes use hexagonal mirror pieces instead of pie slice shaped ones?

The ideal shape for the mirror is round. It's the easiest to make. It's the best-behaved while in use. The hex tiles are already harder. The mirror is a revolution surface generated by a conic curve (...
Florin Andrei's user avatar
8 votes

When did "resilvering" large telescope mirrors actually refer to aluminization, and why was it necessary?

Amateur telescope and mirror maker here. Not sure if I qualify as a "citable source" but anyway, here it is: All metals will eventually tarnish. It may take a long time, but it will happen. The ...
Florin Andrei's user avatar
8 votes

Why are telescope mirrors made of glass?

The accepted answer is correct as far as it goes but to add a bit of why for the claim: jmh> "Glass can also be polished to a high degree of accuracy without having defects" Comes down to glass ...
tomc's user avatar
  • 463
8 votes

How do I calculate my desired distance of mirror from screen?

One time I put a "pinhole" in some paper, it was roughly 5 or 10 mm and roughly round (I think I just pushed a pen through it) placed the paper over a flat household mirror, inclined the ...
uhoh's user avatar
  • 31.1k
8 votes
Accepted

Could two-way mirrors be used to completely eliminate obstruction for a reflector telescope?

You may be confused about how two-way mirrors work. The reflection coefficient is the same for light coming from both directions. You would reflect away most of your starlight.
ProfRob's user avatar
  • 152k
7 votes
Accepted

Why telescope mirror glass shipped as randomly broken chunks of a constant size in cardboard boxes?

The material used for the mirror is indeed E6 Borosilicate manufactured by Ohara corporation as pointed out by @notovny. The fabrication process of a borosilicate glass honeycomb mirror has remained ...
Wilhelmroentgen's user avatar
7 votes
Accepted

Was the use of silver on 'optical' telescope mirrors more common in the past? Why?

Aluminium coating is a relatively recent process - it became available around the 1920s or 1930s. The Hale telescope arrived just in time to take advantage of this new technology. (It requires a ...
Andy's user avatar
  • 2,467
7 votes

Instead of a 4 meter diameter, 50 liter spinning pool of dangerous mercury, why didn't the ILMT just use an ionic liquid? Or gallium?

Ionic liquids are not reflective. An ionic liquid being poured, from )source( They have different properties, but may are toxic, note the nitrile gloves, the researcher doesn't want to get any of ...
James K's user avatar
  • 121k
6 votes

Does the size of the atom limit the focal length of telescopes?

Ok, finally I can explain it. Not sure why it took so long. Bottom line, for visible light atoms don't matter. Let's say the blue side of the visible spectrum of light has a wavelength of 400 nm (...
Florin Andrei's user avatar
6 votes

Why are segmented mirrors lighter than monolithic mirrors?

When creating a mirror, you need to take a block of glass and grind and polish it until it has the desired shape. For telescopes this is typically a parabolic shape. So The thickness of the slab of ...
planetmaker's user avatar
  • 19.4k
6 votes

Instead of a 4 meter diameter, 50 liter spinning pool of dangerous mercury, why didn't the ILMT just use an ionic liquid? Or gallium?

I'll add this to point out that the mercury is managed such that choosing something else might not be worth the bother. The paper by Brajesh Kumar et al. (Monthly Notices of the Royal Astronomical ...
Jon Custer's user avatar
5 votes

Why are telescope mirrors made of glass?

Why are telescope mirrors made of glass? They are not always made of glass. In situations where mass counts and thermal variations can be large, optical telescope mirrors are sometimes made out of ...
uhoh's user avatar
  • 31.1k
4 votes
Accepted

Do primary mirrors in large observatories undergo regular removal and re-coating of the aluminum? Why?

This might not be the type of telescopes your are thinking about, but as IACTs (Imaging Atmospheric Cherencov Telescopes) in the end also measure light in (or near) the optical range, their mirrors ...
samcarter_is_at_topanswers.xyz's user avatar
4 votes

Need advice on mirror-making for a home-made amateur reflector

Here's what I've learned from making telescope mirrors. Start with a mirror that's of a reasonable size and curvature. It is definitely possible to begin with a 12" f/5 mirror, but the problem is you ...
Florin Andrei's user avatar
4 votes

Need advice on mirror-making for a home-made amateur reflector

I second the comment about Texereau -- originally written in French; it's the best, bar none. You won't need anything else. Remember the old adage that is quicker to make a 4" mirror and then an 8" ...
Mick's user avatar
  • 1,546
4 votes
Accepted

How would a flat mirror on the Moon reflect sunlight

The (weird for historical reasons) defintion of magnitude is that a difference of 5 magnitudes corresponds to a factor of 100 in the brightness of the source. So a difference of 3-(-27) = 30 ...
James K's user avatar
  • 121k
4 votes

Is there a reflecting telescope like as a Newtonian telescope but with a negative lens before the diagonal mirror?

Yes! Those with a Coudé focus and others The following this answer to Why aren't reflector telescopes built with an offset secondary mirror? this answer to Do telescopes exist that reflect the ...
uhoh's user avatar
  • 31.1k
3 votes
Accepted

What are some formulas that are associated with the Schmidt corrector?

The specific purpose of a Schmidt Corrector plate is to have an equal but opposite spherical aberration to the primary mirror they compensate for. So any formula you try to come up with will depend on ...
Rory Alsop's user avatar
  • 5,063
3 votes
Accepted

Why are telescope mirrors nearly flat?

The main reason is that the red mirror produces a smaller image in its image plane, which requires a smaller pixel spacing in the detector (or a stronger eyepiece) to get the same resolution. In a ...
Keith McClary's user avatar
3 votes

Why are telescope mirrors nearly flat?

Fast mirrors (low f ratio) such as the red mirror in your illustration, are more subject to aberrations, principally coma. Coma is the effect that circular or point like objects that are off the ...
Dr Chuck's user avatar
  • 4,304
3 votes
Accepted

Can this mirror be cleaned?

The spots could be mold or fungus. If a gentle cleaning with distilled water, a little detergent, and sterile cotton (no abrasives!) fails to remove them, there are several vendors who can strip the ...
Mike G's user avatar
  • 18.7k
3 votes

Source of mirror reflectivity data

The spectral reflectance curve depends on the coatings applied. For 'scopes used by people there is no point in maintaining reflectivity below 450 or above 700 nm. You idea of "visible" ...
Carl Witthoft's user avatar
3 votes
Accepted

When did "resilvering" large telescope mirrors actually refer to aluminization, and why was it necessary?

It's been quite a while with no activity, so I'll post an answer based on my comment. tl;dr: It started in the 1930's but even the Kepler space telescope still used silver! This post in cloudynights....
uhoh's user avatar
  • 31.1k
3 votes

How do tip tilt mirrors correct distortion in adaptive optics?

Not an expert, but offer one solution: Question 1 The adaptive optics correction is accomplished by a tip-tilt mirror and a deformable mirror. Usually the atmospherical wavefront distortion (or say, ...
WDC's user avatar
  • 454
3 votes

JWST mirrors each can be "positioned in tip, tilt, piston, horizontal & vertical decentering and clocking". What does this mean?

Each of the primary mirrors has seven degrees of freedom, not six. It's just the secondary mirror that only has six degrees of freedom. The six degrees of freedom for the eighteen primary mirrors and ...
David Hammen's user avatar
3 votes
Accepted

JWST mirrors each can be "positioned in tip, tilt, piston, horizontal & vertical decentering and clocking". What does this mean?

Can I assume that "piston, horizontal & vertical decentering" correspond to 3D positioning in space and "tip, tilt, and clocking" correspond to attitude? Could someone help ...
BradV's user avatar
  • 747
3 votes

Can telescopes under $200 help you see saturn clearly by eye without editing the image on a computer?

My simple answer to your question is that you're probably going to have to increase your budget to find something worthwhile. But there are some options. For planetary and lunar observing, there's a ...
J.M. Haynes's user avatar
  • 1,058
3 votes

Why is plasma used to deposit aluminum on the VLT's four large mirrors?

As shown in an ESO image, the mirrors are coated in a giant plasma sputter system, where ions produced in a (presumably argon) plasma are accelerated and strike the pure aluminum, freeing neutral ...
Jon Custer's user avatar

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