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Alaness

29" Acrylic mirror as a super cheap primary

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I've been looking at some of these very large parabolic mirrors on eBay and it's got me thinking - How well would one of those serve as a cheap, lightweight primary mirror?

Cheep_Dob.jpg

The one I'm looking at is 93% reflective and has a 550mm focal length - so not exactly competing with a glass mirror, but it's 29 inches! The light gathering power could be immense, at very little cost. So what I'm asking is could this work?

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It probably could work, all sorts of materials have been used for mirrors, even mirrors of about 60% or even less reflectivity have been used (in older telescopes)

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I'm sure it would 'work' but I wouldn't expect too much as far as image quality. I'm guessing the accuracy is lacking!

I'm not too sure about the thermal stability either.

Typed by me on my fone, using fumms... Excuse eny speling errurs.

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This mirror is designed to concentrate the energy from the Sun for a solar energy project. While it is shaped as a parabola to bring the light rays to a focus point , the chances of it having the optical tolerance needed for to prove an effective imaging mirror are slim to none.

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As Dirk says, an energy collector NOT an optical instrument.

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Besides, at 29" F=550 mm we have F/0.747, which may be a tad fast even for TV EPs ;). As said, the optical figure needed for solar heating is way lower than for imaging.

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Expansion and contraction with temperature changes would be a nightmare - even if the figure was good

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Give it a shot, if it doesn't work you'll still have the beginnings of a death ray! :D

Or, if you use it in the UK, it might make quite a pleasant hand-warmer.

James

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No, no, no! In the UK it points upwards to gather precipitation!

Yep - you then call it "a pond" :smiley:

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I've been looking at some of these very large parabolic mirrors on eBay and it's got me thinking - How well would one of those serve as a cheap, lightweight primary mirror? The one I'm looking at is 93% reflective and has a 550mm focal length - so not exactly competing with a glass mirror, but it's 29 inches! The light gathering power could be immense, at very little cost. So what I'm asking is could this work?

I like it - looks fun and it's called a 'flux collector' :cool: Enjoyed the watery quips. Built one similar long ago with a partial vacuum behind an aluminised mylar - published in S&T at the time - my worked at f/0.25 to f/50 [the focal length could changed] projected a huge solar image at long range thus http://home.freeuk.c....gavin/flux.htm :rolleyes: Edited by nytecam
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Isn't a water butt the large container used for gathering rain water? My Aunty and uncle have them for watering the garden.....

Any way it seems a very good idea but wouldn't the whole bigger the mirror the thicker n more optically challenged it would become come into play? I thought I read somewhere it took a year to make hubbles mirrors wrong the first time...

Sent from my iPhone using Tapatalk

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Some really good points here. I think I'll just stop being lazy and learn how to grind a decent mirror aha.

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I'm sure one of einsteins myriad quotes was "never stop questioning " if we don't play with these gizmos we will never know mate . And good choice mr wilkinson :)

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It might well make an all sky camera. :smiley:

Hi Peter - you're the only optician I know here - perhaps you could answer my query? Can a parabolic mirror [as described] form a sharp image on axis? I'm under the impression the image scale is set by the focal length of the objective and in this case the focal length [and image scale] from the edge of the mirror is longer than from the centre of the mirror - any thoughts? Edited by nytecam

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Nytecam. I was an astro-engineer, not an optician, however I worked very closely with several who were and at least a bit rubbed off. I've no experience of using such a fast mirror other than for an all sky camera. As far as I am aware, a good paraboloid will form a good image on axis, the question is, how far off axis will this good image be retained. Coma, which starts to become very obvious at F4 becomes increasingly worse as the F number reduces, I would expect this to be the limiting factor for a good paraboloid let alone the figure of the component you specify. :smiley:

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I would be surprised if this mirror produced an image at all. Glass mirrors are made to an accuracy of fractions of a micron ( 1 micron = 1/1000 mm ). I would not expect this mirror to be more accurate than 1/100mm, ie tens or hundreds of wavelengths off paraboloid. I used to work in the Plastics Research of ICI on, among other things, Perspex and have blown domes such as this. The mirror also looks very thin and Perspex/Plexiglass is not as rigid as glass so I would expect it to flex easily under it's own weight. Further, it is probably aluminised on the backside where the coating can be protected with a substantial overcoating hence the light will be going through the sheet twice.

Peter, I don't think that it would work as an all sky camera either as you need a convex surface not a concave and as I mention above, I expect the Aluminium coating to be overcoated with some opaque paint.

If you have one for it's intended purpose then by all means try it out for image quality but I wouldn't spend any money buying one just for astronomy.

Nigel

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Hi Astrobits (Nigel) . Just for clarity on very wide angle & all sky imaging a concave mirror can (& was ) used in the Henyey-Greenstein Camera created in the States in the 1940s. Peter was correct in that concave can be used (in addition to the more common convex approach) .This PDF has two excellent photos on good scale of the concave kit http://oralhistories.library.caltech.edu/51/1/OH_Greenstein_J.pdf . Images of the Milky Way taken with the camera are found here http://adsabs.harvard.edu/full/1951ApJ...113..222O as it was used by Osterbrook & Sharpless in their research.

For those that would like to 'play' with a concave mirror for imaging very wide angles have a look here http://user.tninet.se/~sdt522w/Gofo_astro_1_2.htm - From Gote Flodqvists webpages . :smiley: .Can't beat a bit of optical fun :grin: .Bring back the Exchange & Mart , H W English & Military Surplus!!

Len E

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Hi Len,

Thanks for that. I wasn't aware that concave mirrors would work, I thought that the image of the camera would be quite big.

Nigel

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