Telescope mirrors?

[Keith 66]

Just seen some old Mirrors in my mates yard, hes having a retirement clear out, Three Circular mirrors 6", 10" & 18" diameter Glass is 1 1/4" thick on the small one 1 1/2" thick on the others. Doesnt appear to be much curvature to them, They are silvered on the back but need re doing. He thought they might have come from an old telescope.
Any idea what these might have come out off? Are they of any use to anyone? I can get some pics next time im up there but they wont show much more than the description!

 

[Arc Tourist]

Check to see if there are any lables on them, and also look around the edge for information that may be scratched into it. If they are reflector telescopic mirrors, you might post an inquiry on the "Stargazers Lounge" website here in the UK, as there are most likely some amateur telescope makers that would be interested. If they are dirty " DO NOT " wipe them clean, just rinse them off if necessary and let air dry.

 

[Misterg]

They are silvered on the back but need re doing.

If they're telescope mirrors, they'll be silvered on the concave face (the front). There won't be much curvature on them - maybe 6 - 10mm or so, across the diameter. I would expect them to be quite sought after if they are telescope mirrors.

 

[Keith 66]

Thanks gents, will have another look in next few days & take a straight edge with me to check curvature, absolutely filthy & covered in thick dust, one definately silvered on front so could well be.

 

[voipio]

If the glass pieces are silvered, then the chances are they've been ground to shape. An 18" reflector telescope would be quite a beast. The 1 1/2" thickness on the largest mirror might be a bit too thin to keep the optical curvature from being excessively affected by gravity, as it is tilted. A 12" mirror telescope is usually something that is getting on for being a two man lift. A good mirror is figured to ~1/10 of the wavelength of light. There's a fascinating video on YouTube of the (ex Rev) John Dobson, making a telescope mirror.

 

[blackolive (Ste)]

The maker usually marks the focal ratio on them somewhere with a pencil or chinagraph marker. Also if the are commercially made a grade may be marked on them which determines the accuracy of the parabolic surface. If not you would have to measure the focal length optically. A longer focal length gives more magnification shorter focal length wider field.

I did some telescope making years ago and found out that the best conditions for seeing stars and planets coincided with it generally being dark, cold, and after beer o clock

 

[Keith 66]

Right I have been up & had a shufti, They are silvered on the front face, laying a straight edge carefully across the front & sighting under it against the sun there is a very faint concave surface aparent I have not tried to clean them. Nothing written on the back or edges that i can see save that some arrows possibly broad arrows surface point to the front, So they are optical mirrors out of something, Rangefinder? Telescope?
Bad news is the big one has a nasty chip out of one edge that extends into the mirrored surface by about 1.5 ", the medium sized one has a couple of smaller chips. Photo below. Im curious more than anything, I suppose the chipped ones could be reduced in size or masked?

 

[voipio]

Those metallised surfaces look pretty far gone. They would need to be chemically removed and recoated, with optical surface checking and repolishing performed before the new metallised surface is applied. The chips can be left as is, and possibly painted with blackboard paint, to reduce their impact on image quality.

A commonly used focal length for large-ish telescope mirrors is 80" which, for an 18" mirror diameter, would give a saggita (distance from straight edge across the mirror to the "dip" in the middle) of around 0.22", so around 5.7mm. If you're barely seeing daylight between mirror and ruler, that would suggest a much, much larger focal length, that would probably be impractical for a telescope.

 

[Seadog]

The blanks might still be useful however. They can be reground.

 

[daedalusminos]

The blanks might still be useful however. They can be reground.

And they're probably good quality low CTE glass

 

[fizzy]

If the glass pieces are silvered, then the chances are they've been ground to shape. An 18" reflector telescope would be quite a beast. The 1 1/2" thickness on the largest mirror might be a bit too thin to keep the optical curvature from being excessively affected by gravity, as it is tilted. A 12" mirror telescope is usually something that is getting on for being a two man lift. A good mirror is figured to ~1/10 of the wavelength of light. There's a fascinating video on YouTube of the (ex Rev) John Dobson, making a telescope mirror.

I have made a few telescope and laser mirrors in the past - life is too short now

 

[Misterg]

Interesting...

The usual reflecting telescope (Newtonian) would only need one mirror of that sort of size - 3 telescopes?

Also, the shallow curvature of the large mirror suggests a much longer focal length that would be useful for a Newtonian...

Cassegrain designs (and their variants) could use a longer focal length, but again, only one big mirror - and it would have a hole in the middle.

The Tri-Schiefspiegler design uses 3 different sized mirrors, but the middle one is slightly convex. It would be an incredibly niche design if that's what they are. Is there any hope of finding anything more about their history?

(Another one who has made a telescope mirror in the past )

 

[Keith 66]

I asked my mate where he got them & he said only that they came in the scrap. He is nearly 80 & has been in the scrap game since he was 15, I suspect if there were any mounts or tubes they would have been stripped & run in donkeys years ago. I looked up the Tri-schiefspiegler Telescope & the earliest were built in the 1950's by Anton Kutter. The odds on a set of mirrors from one of them turning up in a scrapyard in essex are astronomical! (no pun intended)
Surely gotta be from something else or more than one.
But as my mate said "Everything comes in the scrap if you wait long enough"!

 

[Stueeee]

Could they be something like this? https://www.ebay.co.uk/itm/32596785...iOKkaXAGSpy_KrinpmMt1vR-9azx04O0aAiQJEALw_wcB
If they were marine items, the whole lamp would have had plenty of brass/bronze which would just leave the mirror once the metal was stripped off.

 

[Misterg]

Could they be something like this?

I thought about lamp reflectors, etc, but they're usually much more deeply curved than these parts seem to be. If that ~450mm diameter mirror has a sag of (say) 0.5mm in the middle ('barely visible'?), that gives a radius of curvature of about 50m and a focal length of about 25m - to get a parallel searchlight beam, the lamp needs to be positioned at roughly the focal length away from the mirror, so in my mind, at least, I don't think it's for that sort of thing, but happy to be proved wrong.

An interesting puzzle...

 

[Keith 66]

We had a searchlight down the yacht club, the mirror was quite deeply curved, silvered or chromed brass, with big ventilation slots, It had to be metal & thin to radiate the intense heat from the big light bulb, larger ones used carbon arcs.
I have been looking at videos on you tube on telescope grinding & aluminizing mirrors & to be honest the levels of precision required are simply mind boggling. A field i certainly havent got the time to explore. I have contacted the local astronomy club, If i can find someone or a group who can use them they will be welcome.
This vid shows aTrischiefspiegler telescope being dismantled to show the layout,

, Its unlikely the came from one of these it would have been huge,
Be interesting to see what they think!

 

[fizzy]

The 12" mirror I ground had a focal length of f8 - the curve was very slight so could be from 3 different long focus Newtonians. Remember the longer the focal length the easier it is to get high accuracy and also better for planetary work etc.