Hot answers tagged

10 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 ...
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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 (...
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 ...
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 ...
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 ...
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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 (...
6 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 ...
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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 ...
  • 30.4k
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 ...
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 ...
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" ...
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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 ...
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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 ...
  • 4,986
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 ...
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 ...
  • 4,065
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 ...
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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" ...
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....
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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, ...
  • 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 ...
  • 28.6k
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 ...
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2 votes

Where do I find a better mirror?

Well, if you buy a car and then you go "this thing doesn't drive well, where do I find a better engine" - then folks are going to ask you questions such as: have you driven stick shift before, do you ...
2 votes

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

I apologize for the length of this answer, but this is a complicated subject to address. Thank you all. I owned a camera store for many years, and have 60+ years in photo. I got my start in photo ...
2 votes

What are the aberrations of an SCT? And how can they be eliminated?

The primary aberration present in a Schmidt–Cassegrain telescope is spherical, due to the primary mirror being spherical in shape; that causes light at the edges to have a different path length than ...
  • 2,527
2 votes
Accepted

Why aren't corrector plates aligned with the center of curvature in an SCT?

In a schmidt camera, the corrector is placed at the centre of curvature, which is at a distance of twice the focal length from the primary mirror, and minimizes aberrations such as coma and ...
  • 4,065
2 votes

Source of mirror reflectivity data

I came across an article in the NOAO 2018 October Newsletter which discussed recoating of the 4-meter Blanco primary in Chile. This contained before, after and theoretical reflectivity data. I ...
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2 votes

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

Does the size of the atom limit the focal length of telescopes? Does the size of the atom place a theoretical limit on a telescope's focal length (and thus, resolution)? No, the size of an atom ...
  • 2,527
2 votes

Why are larger hex mirrors round and the smaller aren't?

Start by drawing the largest inscribable circle (circle completely contained within the mirror assembly). Then draw, e.g., the smallest superscribing circle (completely contains all the mirrors), ...
2 votes

Why are telescope mirrors nearly flat?

Most modern telescopes do have very small f-ratios. i.e. The focal length is not a large multiple of the mirror diameter. The main advantage of this is that you don't have to build a massive dome to ...
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1 vote
Accepted

Does a continuous deformable mirror cause diffuse reflection?

Given your reply it seems your definition of diffusion is slightly different from my sense. But the phenomenon is true that at segmentation locations wavefronts are being deflected not in a uniform ...
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