Mirrors for Telescopes

[Sconesbie]

Hi. Just something I have been thinking about.

If someone was making a home made telescope, can they use a normal everyday mirror for the primary? If not, why?

I'm not making one and no one I know is, I'm just curious as to the logistics and practicalities of it.


Regards
Scott

[glend]

What do you consider a normal everyday mirror? Not a bathroom mirror i hope? Most reflector scopes use a parabolic shaped mirror to focus the light into a cone.

[JA]

Hi Scott

As Glen mentioned the shape needs to be parabolic (usually) and also the bathroom mirror is back silvered /aluminised. You want it front silvered /reflective coated to avoid creating double/multiple images.

As an aside and just thinking out aloud- How bad would a GIANT (think metres) mirror made up of tiny hex front silvered tiles be? It would be good to know the allowable/accepted curvature error in a large ground mirror and extrapolate that to something manageable say 3m or so....

Best
JA

[OzEclipse]

An interferometer, the Narrabri Stellar interferometer (https://en.wikipedia.org/wiki/Narrabri_Stellar_Intensity_Interfer ometer)that operated from 1963 to 1974 was constructed of many 15 inch polygon cut mirrors.

Modern professional large telescopes are being made as multi-segment mirrors with the segments driven to provide adaptive optics - optics that adapt to atmospheric changes.

You would be hard pressed to align all the mirror tile segments to the required accuracy. All the tiles would have to have their curved surfaces sit on the theoreticalperfect curved surface with errors less than than 100 nanometers just to perform like a mediocre mirror.

Joe

[glend]

The James Webb Telescope (Hubble Replacement), is made of mirror tiles. So its very feasible. I believe with the James Webb, that the individual tiles can be moved to alter shape.

[JA]

Curved - Ok there goes easy DIY :D



And then there's that :D

Thanks for quantifying that Joe and also to Glen for your response :thumbsup:

Best
JA

[Wavytone]

Hi Scott,

You should do some reading to find out the accuracy required for a converging optical beam to produce a good image - which is a quarter of the wavelength of the light - 200 nanometers over the entire surface of the wavefront.

For a lens this translates to an accuracy required around 100nm on each surface, and for a mirror 50nm because when light is reflected the wavefront error is double the surface error.

The reason the image in an ordinary bathroom mirror looks fine is that when you look at the reflected scene the aperture stop is the iris in your eye, typically 2-3mm indoors. Mirrors are typically flat to a fraction of a wavelength over that dimension, so the reflected image looks ok, though large scale defects are clearly visible in most domestic glassover the surface if you look at it edge-on. This is also why making glass with small ripples on the surface is also sufficient to break up an image, useful for privacy.

But for a telescope to form a sharp image the aperture stop is the mirror or lens and the wavefront error must also be within a quarter wavelength - over the entire wavefront. This means a mirror must be uniformly parabolic within 50nm over the whole surface.

Get yourself a micrometer and start figuring out how small 50nm actually is. Then measure some bits of glass around home and you will discover how inaccurate they are.

As for your idea of mirror tiles - it's not new - what you will end up with is a solar barbecue (small) or a solar furnace (large). Nothing wrong with a solar BBQ - an f/1 dish 90cm across is ample to grill sausages or a billy for tea.

There are also solar power plants. The key point with all of them though is that all they are doing is directing energy onto a receiver - there is no attempt nor need to form a good image of the sun.

What is more remarkable IMHO is that it is possible to grind and polish good telescope mirrors at home with no more than two disks of glass, abrasives and pitch, and a polishing agent (rouge or cerium oxide) - plus a great deal of elbow grease.

I'd also suggest you try making a small mirror 50-100mm across, to find out what is involved. Then you may gain a real appreciation as to why most amateurs now buy them.

[Sconesbie]

This is interesting reading...for me anyway. I'm not wanting to make one but just curious as to know if it would work or not and why. There have been some good responses.

Sometimes I think of weird things and the "I wonder" comes out.

[JA]

That's the best way to be :thumbsup:

Best
JA

[astronobob]

Basically Scott,

1) as mentioned, a household or even shop mirrors are only glass panels with a coating on the back, that would produce doubled reflections for astro, and why astro mirrors have coatings on the front !

2) again, household mirror surfaces & all mirror surfaces in general are very very uneven, both front & back surfaces, the front surface introduces bad reflections, & the rear surface which the coating is on, and not a great coating either, only surprisingly reflecting 50 - 75% of light - who'd of thunk :P
Anyway, but if thinking of them 'magnifying circular mirrors' for girls make-up etc, Lol , then they too are very very irregular on their surfaces compared to astro mirror surfaces, ( I know, I tried one when was young :rofl: - Imagine roller skating across a cow-grid, Lol . Honestly :-)
Astro mirrors are also polished smoothed to only a few millionths of an inch and we all know that they need to be 'shaped' accordingly, each with a certain central depth, ( Sagittal Depth) for the Focal Length to be reached & Parabolized to certain tolerances depending on size, and speed = F/Ratio :thumbsup::thumbsup:

Hope this also helps,

[gaseous]

Extremely informative and interesting answers guys, you've increased my very limited mirror knowledge about tenfold - thanks!