They Just Began Casting the Giant Magellan Telescope’s 5th Mirror. What a Monster Job.

Universe Today ^ | 11/7/17 | Evan Gough

[lefty-lie-spy]

This sucker won’t even start to come online until 2023.

[Telepathic Intruder]

Any imaging success you get from CCD’s will only be improved by focusing incoming light using large mirrors. It makes sense to use both, which is what modern telescopes generally do.

[BwanaNdege]

We looked into making our own 8” mirror so the kids could see Halley’s Comet back in 1986. The basic grinding process was simple. As you got closer to the final surface shape, you had to wet the surface to make it reflective, then look at a sheet of graph paper placed several feet away.

Any mirror distortion was evident in the reflected image.

Once the reflected image was correct, you had to send the mirror off to have it coated.

We ended up buying an 8” silvered mirror and constricting the rest of the telescope.

http://www.scopemaking.net/mirror/mirror.htm

[4Liberty]

Bump!

[dr_lew]

As a physics Ph. D., I’m going to venture to say “No”, as to the general idea, because CCD’s don’t preserve phase information the way mirrors and lenses do.

“Let us now praise optics”

[Comment Not Approved]

My best friend was part owner of a ship repair company with 4 other guys, one of them was into telescopes. They would grind their mirror, not sure of the size, at their meetings pushing and pulling back and forth. Took them awhile.

They used a sonotube?, one of those molds for pouring concrete pilings, as the body and built a Dobsonian? telescope that was very large. You had to stand on a stepladder to see into this thing.

I’ll never forget seeing the Orion Nebula through it. It looked just like the pictures in astronomy class I was taking at the time. I was amazed at the quality of the images, especally as it was a DIY project.

[Fred Hayek]

The CCD’s are chilled to reduce thermal noise. A computer is used to integrate the image - it gets to the point of counting individual photons hitting each pixel.

[mabarker1]

VERY, VERY CAREFULLY with Fine Polish Compound and LOTS of furry Bunny 🐰 wabbits. You hold them by the feet, dip their back into the Compound just a bit and Start Polishing.

[mabarker1]

Very Cool....
Thank You

[BwanaNdege]

Well, you’ve got me motivated now. I’ll have to consider building a telescope with the grandkids.

This time we’ll grind our own mirror! Last time we were in Kenya & the hassles shipping a DIY mirror out to be silvered mandated a ‘store-bought’ mirror.

Sonotubes are a great idea for the larger sizes, perhaps wrapped in a layer of fiberglass cloth & resin to make it water-proof. We used 8” aluminum irrigation pipe, IIRC.

[zeestephen]

Thanks, Doc.

I have always had great difficulty in understanding light, specifically, the particle-wave duality and the multitude of energy levels.

By “phase information” I presume you mean something like infrared, visible, ultraviolet, etc.

My original thought on this subject was, “Why not cover acres of ground with CCDs instead of spending 20 years casting and grinding mirrors?”

I had always assumed the issue was computer processing power for trillions and trillions of pixels.

[minnesota_bound]

960 inches or 24,384mm for the entire telescope.
24,384mm / 25mm = 975x magnification.
I guess we still won’t be seeing Martians.

Calculating Magnification
The magnification is the telescope focal length divided by the eyepiece focal length, in millimeters.

Magnification = Telescope focal length ÷ Eyepiece focal length

For example, if you use a telescope of 1000mm focal length with a 25mm eyepiece, the magnification would be 40x (1000mm ÷ 25 = 40). Doubling the power gives you one-fourth the image brightness and reduces the sharpness by one half.
https://www.telescope.com/Telescope-Power-Magnification/p/99813.uts

[Telepathic Intruder]

But as always, the more photons the better when it comes to imaging dim objects farther out in space than anyone has yet seen. And for that you need more surface area, which means larger mirrors. CCD’s are the modern equivalent of photographic film; they just count photons, and need something to focus them first to form a meaningful picture.

[Chode]

oh YEAH?

[zeestephen]

Re: “CCD’s are the modern equivalent of photographic film; they just count photons, and need something to focus them first to form a meaningful picture.”

Thanks - that's helpful.

Is it possible to mass produce an inexpensive lens for CCDs?

[Telepathic Intruder]

If I knew how to make lenses cheaper, I would patent it before telling you. But I don’t think lenses themselves have changed in the last few hundred years.

[dr_lew]

By “phase information” I presume you mean something like infrared, visible, ultraviolet, etc.

That's not it. "Phase information" charactises "coherence", as a laser, for example, is a coherent beam of light. This is what allows it to produce interference patterns.

You can shine a laser through a lens, and bounce it off mirrors, and this coherence will be preserved, but CCD's require absorption of photons, and so the information thus recorded is reduced to INTENSITY, as opposed to AMPLITUDE, which preserves or carries "phase information". So a surface of CCDs can never duplicate an optical mirror surface.

This is according to my own understanding, and I stand ready to be corrected, ( but it would take some doing! )

[zeestephen]

Thanks. That's helpful.

An earlier Comment said that a CCD was like the raw film in a camera - it just “counts” the photons.

It requires a lens to create a photograph.

So, tomorrow, I will be at You Tube watching educational videos about phase information, interference patterns, and amplitude.

[SunkenCiv]

Thanks LibWhacker. Ping to APoD members.

[Conan the Librarian]

“Dobsonian” -

That is a type of mount for a Newtonian. It was designed by John Dobson, well known amateur astronomer. I met him once at The Orange Blossom Special starparty in Florida. He was interesting, but, a bit of a mooch.

I’ve been using a scope for 45+ years now, and, have never seen any value in grinding my own mirror, when there are so many folks out there that make good ones relatively cheap. It takes all kinds though.

Now building a scope around a mirror is another thing all together. My current scope was hand made by a late member of our club. It’s a fine machine all together, but, the mirrors were bought, not, ground by hand.

All that said, the most important thing is once you get it ready, to go out and use it. Even in the suburbs there are things in the sky to see.