# Tag Info

10

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 without having defects. Another major reason is the expansion properties are very favorable as well. Glass doesn't corrode and is easier to mold into shape ...

8

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 (circle, parabola, hyperbola, ellipse), which needs to be machined with a precision greater than 0.1 microns. That's extremely difficult already with a round ...

8

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 process is not entirely chemical always. Sometimes it's purely mechanical (abrasion). Other times it's in between. Surface phenomena are complex. Even gold-coated ...

7

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 reasonably good vacuum to work, which probably explains why it took a while to come along.) Before that - around the mid-1800s - various chemical "silvering" ...

6

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 (round number for simplicity). Let's assume the distance between two atoms in glass is 0.2 nm. The thing is, light is a wave. Anything smaller than about 1/4 of a ...

6

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 almost unchanged since the first one, an 8.4m LBT mirror, was cast in 1997 (here is the article of the casting). Quoting the article: E6 is a good match to our ...

6

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 being amorphous instead of polycrystalline as metal is. When a material is polycrystalline its atoms have preferred (stronger) connections with some of its ...

5

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 silicon carbide instead. From this answer to How are space telescopes stabilised to a perfect standstill?: Silicon carbide is a very popular material in newer ...

4

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 are of similar build. The important arrays (H.E.S.S., VERITAS and MAGIC) have, at least partially, mirrors made of glass or with a glass surfaces, coated on the ...

4

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 have to handle not just the higher volume of work, but also the learning process in a field that's completely new to you. Begin with making a 6" f/8 mirror. ...

4

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" mirror than it is to make an 8" mirror first. Don't bother with borosilicate. You should be able to get mirror kits and all the abrasives that you need online. ...

3

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 the primary. In fact the information on SCT's shows how it is made using physical forms, not using formulae. From revolvy.com: A thin glass disk with a ...

3

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 Newtonian telescope the focal length cannot be too small: (Image Krishnavedala) Also, a more ideal shape is a paraboloid (not a sphere). Spherical mirrors are ...

3

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.com says: Aluminizing mirrors was developed by John Strong, in the early 1930's. One can read about it in his book Procedures in Experimental Physics From ...

3

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 optical axis do not form uniform circular images, blurred in a coneshape image. It is possible to correct for this using additional optical components, but this ...

3

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 magnitudes is a difference of $10^{12}$ in brightness. The surface brightness of the mirror and the sun is effectively equal, since the mirror reflects 100% of the ...

3

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 old aluminum coating and apply a new one. If you don't want to go that far, you can probably find a buyer who would. Criterion scopes have a considerable fan ...

3

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, the phase screen) looks like this: Note its ramp-like shape. In other word, the 2nd and 3rd Zernike terms are large. The deformable mirror has limited ...

2

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 understand how the clutch operates, etc. There are many reasons why a telescope does not provide a satisfactory image. Perhaps the expectations were too high. ...

2

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 via Astronomy in high school. Went to the University of Iowa, started in Physics and Astronomy, but my math skills were not up to the task, so went to Radio-TV-...

2

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 light at the center, and thus a different focal point. The image above, from Wikipedia's "Schmidt corrector plate" webpage, shows an exaggerated corrector; they ...

2

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 astigmatism. This makes for relatively long telescopes, particularly with large primary mirrors. The sct design is similar, in that it uses a corrector plate, but this (...

2

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 contacted one of the authors of the article for the source of the theoretical data who sent me a scan of the pages from R. N. Wilson's "Reflecting Telescope Optics II",...

2

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 does not limit the focal length of a telescope. Halfway through this answer I further explain why the size of an atom isn't relevant and that instead we can ...

2

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 house your telescope in and you also get a larger field of view. For example, the VLT telescopes at Paranal have a diameter of 8.2-m and a focal length of just ...

1

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 manner, causing the diffusion issue posted in this question. Before addressing this issue, let me briefly describe how adaptive optics works in practice. In a ...

1

Just for fun here's the diffraction pattern from Hubble's circular aperture and 4-vane secondary support. Most of the time the concentric rings are not visible because images are broadband and they get washed out, but they are here because the exposure is through a narrow-band filter. From What is the cause of all of these sharp, concentric rings around ...

1

Astigmatism. No way to eliminate in SCT. Reduce it with proper fixing of every optical element. Astigmatism is only eliminated with 3-mirror scheme. Curvature of field. No way to eliminate in SCT. You are forced to use so-called "field flatteners". If kept in good range, does not hurt, if you are not going to do astrophotography (which with SCT nobody does)....

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