Metal Mirrors

[rocky]

I have read about the Mercury lens (pretty much vertical use) and

grinding glass for which I doubt my patience will hold and purchase is

not likely. Consequently, I am interested in metal mirrors.

1) Is Aluminum plausible and am thinking silver could be

electroplated to protect the reflective surface?

2) Can a useful mirror of stair-stepped concentric rings (cut into the

Aluminum be made to create the parabola?

3) Wouldn't this cause concentric rings in the image?

4) Which would be better equal treads or equal risers?

5) Alternatively, has anyone endeavored to spin an aluminum (cut to

preliminary specs) mirror and pour either tin (Sn: 231 oC) or Bismuth

(Bi: 276 oC) into the depression and keeping it fluid with flame

until ripples are eliminated then letting it freeze it (possibley

with liquid Nitrogen) to the Al? Ripples have been addressed in

British Columbia.

6) I presume there must be formula to specify rotational speed per

density.

Hopeful, that using an "old" record player or VCR would rotate

sufficiently fast. Can someone address this for me?

I will appreciate constructive suggestions and URLs for direction.

[adamsp123]

problem I see is of accuracy and then expansion. When talking about accuracy at the wavelength of light the slightest expansion or inaccuracy of manufacture will muck up the image.

[acey]

Herschel and other early nineteenth century astronomers made mirrors out of a metal alloy called speculum, first casting then figuring it. The casting process alone wasn't accurate enough, even within the tolerances thought acceptable back then.

The mercury mirror isn't just "pretty much" vertical, it's strictly vertical. And if you go instead for hot molten metal, you'll need a way of ensuring perfect cooling so you don't get any change of shape - accurate to within a fraction of a wavelength of light.

Somebody has tried a turntable as a way of making a paraboloid reflector for a lamp, but I don't think it'd make much of a telescope:

Custom DIY parabolic metal reflector? - CandlePowerForums

[ringz]

University College, London made a new aluminium alloy mirror for the 72" Leviathan telescope at Birr Castle in Ireland. I know they had a great deal of trouble commissioning it but I don't know the reason(s) why.

--

Martyn

[Merlin66]

Alternatives include vacuum deformation of a thin glass sheet. F10 has been achieved successfully.

Doing the same with aluminised mylar doesn't work... believe me I've tried, up to 18"

I've also tried slumped thin glass... accuracy is VERY dependent on the former and still needs final polishing. BTW they wanted $10 to slump the glass and $900 for the former!

[rocky]

AdamSP123: Was your concern of accuracy one of stair-step grinding or

in general?

Aluminum coeff expansion is 23.1 um / m /deg K

Bismuth " " " 13.4 um / m /deg K

Is that really significant while recalling metal expands

more uniformly than glass? Even if the Bismuth comes loose

it could be re-anealed couldn't it?

Acey: Was thinking of using liquid Nitrogen but am not sure it

wouldn't dent the surface first. Do you have experience

in that area?

Thanx 4 furthering my query on turntable. Looks like it

needs slowing to near 20 rpm. Still thinking if the Bismuth

was flowing smoothly it could work like the Mercury in BC.

Checked into CPF forum you suggested, one guy there used

epoxy & fiberglass with very hi success and another suggested

epoxy with UV to freeze it (liked that). Thanx 4 URL!

ringz: Yeh am planning to explore use of airplain aluminum (dural)

as it cuts pretty smooth.

Merlin66: Could you expand further on experiences with mylar?

Found a bunch of 18 & 24 inch glass plates the other day and

nearly dropped my teeth. On further examination the exterior

was flat on the bottom and FL was 19" for 18" plate.

You kinda summed up why I don't wana do glass the vacuum

plating then getting a broken mirror in the return mail and

spending another couple of months making another one.

PS lolrof to your "Scotty" remark.

No one commented on my stair-stepping as opposed to smooth surface.

Am especially interested if anyone used equal tread stepping when

grinding. Really appreciated the kwik responses.

[Merlin66]

PM me for all the details I have on the Mylar mirrors.

basically I had a 18" circular frame with a tensioning ring ( like a kettle drum) and then applied vacuum behind the Aluminised Mylar sheet using a manometer. The creases and the unequal streaching as will as inconsistances in the Mylar never gave a optical surface.

BTW from October 2008 it is illegal to sell Mercury in the UK or EU. Repair shops ( for old barometers etc) are still allow to hold some stock. So bang goes the rotating mercury idea...Health and safety would have canned it anyway!!!!

[adamsp123]

Metal grinding accuracy in general, can it match 1/4 wavelength or better as found with polished glass? I have no idea TBH.

CoE of pyrex less than 5 um/k so a lot less than metals.

I just think we would have seen today more metal mirrors if they had the incredibly fine accuracy required for astronomy.