I'm undertaking a project to build a 1.2/1.5m reflector. I've got a couple of old satellite dishes of the aforementioned diameters and plan to turn them into a pair of monster telescopes, starting with the smaller one and progressing to the larger based on how well the first one goes. Obviously, there are many steps to take and obstacles to overcome. After cleaning them up, the areas I'll need help on are:
-Finding a suitable material to coat them with for maximum reflectivity (biggest obstacle)
-Finding the focal point for light (the original receiver [in shot] is at the point to receive radio waves, but the point I've already calculated seems to be the focus of sound and is further out)
-Designing a focus system (probably just moving the eyepiece assembly back and forth)
-Installing a secondary mirror (in Schmit-Cassegrain orientation)
I'm planning on drilling a hole in the centre to insert an eyepiece holder. I've got all the mounting equipment for the back. A GoTo system would come later.
Will, that is some super idea mate and if it all comes together I wish you well indeed, what an effort, My only thing would be the depth of the dishes, I expect it would be of a very short focal lenght, but then again I am probably wrong.
Typically, I have read of glass pushers, that rare breed of amateur telescope makers, who grind their own mirrors from (low expansion glass) optical blanks and then polish them to a parabola cross section with a surface accuracy of a minimum of a ¼ wavelength of light, but usually more like 1/10 lambda.
Typically, the focal ratio (focal length divided by aperture) is between F4 and F8 with F4 being very challenging to get right and F8 more forgiving and producing tighter stars across the whole field of view.
Your dish “looks” more like an F1 or F2 system, based upon the sensor position defined by the 3 struts and I suspect the surface accuracy will be wholly unsuited for optical use?
Sorry to be the bearer of bad news, but that is my understanding and I would strongly recommend that before you invest major amounts of time and money in this project, albeit an exciting one, I’d have a chat to some glass pushers at a local ATM club!
Sounds like trying to push a pile of brown stuff up a hill with a pencil. Are you sure you're not taking the piss?
If you ARE serious, don't waste your time Will, the scope in your avatar will already be giving you a view a million times better than you'll ever get by coating a satellite dish with even the reflectiest reflecto-stuff. Bad surface, bad focal length, bad stability, bad bad bad.........
If you really want to use your dishes for astronomy, get a receiver and try some radio astronomy.
and it looks to have a rather nasty turned down edge.
Will ,makeing your own scope is a wholely rewarding experiance that
you should definately have a go at .Unfortunately with the optics there arn't really many free lunches to be had . If these dishes made good mirrors, cash strapped atm's would of been pinching them of roofs for years,and as far as I know they havn't
Hey Will,
Dont feel too bad mate., I had the same thought, but hadnt gotten around to checking into it yet. I was thinking, that even if it turns out to be half as reflective (or half as accurate as a smaller mirror, it would make up for it with pure grunt Apature). I was also thinking of myself leaning into the dish too much and therefore blocking its light-sucking ability. Unfortunately it wont work, but I believe its good to keep thinking outside the box.
Best thing to do is turn it into a solar reflector or put a usb dongle in front of it and pick up the nieghbours (Or your friends internet 2 miles away). This can easliy be done with a good line of sight.
Like everyone says those dishes are not suitable to be used at the wavelength of light but if you want to do something useful with them why not use them for radio astronomy. Two of them mounted on a suitable tracking device with a suitable antenna at their focus they will give lots of gain at the hydrogen frequency and coupled with modern low noise amplifiers the results can be fed in phase to produce the resolution of a 6 metre or larger dish.
I know an optician in Newcastle who might take on polishing your dishes
Hi Mark,
I know exactly who you have in mind for the job. I dare suggest however, this will slide through to the keeper in most cases and I guess that's how it was intended
Hi Will
I am afraid I will have to agree with those who think that short focal length and inaccuracy of shape would defeat your idea. Some time ago there was a lot of talk about putting a plate of slow hardening resin like polyester resin
on a plate and rotating it on a disk turntable. The theory being that this would form a perfect parabola as the resin set and that this could then be
silvered to produce a workable astro mirror. Has anyone actually heard the results of this theory?
Cheers Peter
The theory being that this would form a perfect parabola as the resin set and that this could then be
silvered to produce a workable astro mirror. Has anyone actually heard the results of this theory?
Cheers Peter
Funny you should mention this. Sorry if this is a bit long winded, but it has to be explained (to a degree / method of madness).
I watched Mythbusters on Foxtel the other night. They were talking about a myth of Archimedes "Death Ray".
I seen the original ep where they tried it, didn't work. Ra ra ra.
But they re-hashed the myth recently, (as it wasn't busted) and that involved heaps of people to debunk it.
One of the selected people from the public were 2 guys, they tried a parabola mirror to focus the sun's energy to create flame.
During the changes in spec's of the myth, the glass based parabola mirror just didn't cut it.
One of the guy's got a theory of making parabola from plaster (pretty sure it was plaster).
They used centrifigul force (as of the above quote) to make a parabola. It didn't serve them well with the effectiveness (for the myth), but the method of madness was there. They only used silver foil/aluminum for the mirror surface.
Unsure of the accuracy, as it was done for TV (fast).
I'll see if I can find a vid of it.
EDIT: http://www.youtube.com/watch?v=T3udgbHjWCs
The guys that create the flame first, is the one's I'm talking about.
This is the first attempt with a standard parabola mirror. The next attempt is the one where they try to make it from plaster.
You will find the parabolas in these reflectors is not nearly accurate enough to be useful as a visual telescope.
I'm not sure what the manufacturing tolerances are for satellite dishes , but if they are parabolas with a few to several mm then I think you'll be lucky. OK for radio and microwaves , but useless for producing a sharp image from visible light where you need at least 1/4wave tolerance , take green light , that's an exceedingly small allowable error from a perfect parabola.
Hi Brett
Thanks for the Mythbuster's video. I really enjoy watching the antics of that crowd but they often raise more questions than they answer.
Cheers Peter
Hi Brett
Thanks for the Mythbuster's video. I really enjoy watching the antics of that crowd but they often raise more questions than they answer.
Cheers Peter
I gave up watching them when I got bored with their rehashes , and most of the things they do have been done properly by real scientists and engineers , and with much more rigor.
I really don't think a lot of what passes for their methodology from an engineering and scientific standpoint, all they produce is pseudo-science and bad science for the masses.
Nor when it comes to shows on SBS , do I think highly of Top Gear (the Brit version, I've not bothered watching the Aus version).
BTW the Greeks more or less invented conics and understood very well the properties of many geometric shapes and deducted by geometric reasoning the ability of parabolas to focus light , I read that somewhere in a university textbook many many many moons ago. So it not unreasonable that they will have invented a kind of heat ray , it wouldn't taken much to set the tar and hemp in enemy ships a fire, or to burn the crews enough to make them retreat.
This will do nicely as a solar barbeque - line it with tinfoil and fasten a grille over the top such that its a bit out of focus (otherwise you'll have a very small but very hot spot).
