Dish antenna as parabolic mirror for OPTICAL telescope?

  • Can I use a chrome painted dish TV antenna as a parabolic reflector for my optical telescope? Given that glass mirrors are quite expensive, I am thinking of using a Dish TV antenna and coating its reflecting surface with chrome paint to give it a mirror finish. But I couldn't find any instance of it online. I did find several examples of people using a dish antenna for making an amateur radio telescope.

    I was wondering if there is a reason why we can't use it in optical telescope.

    You might look at something that's been painted with chrome paint and see if you can see anything.

    chrome paint? That must be some good paint.

    Not a scope, but I've heard they make a decent focusing element for a solar cooker.

    It might work as a solar oven, but a satellite dish is not going to make an optical image, ever, not even close. The active surfaces in an optical telescope need to be machined with a precision of 1 / 10000 of a mm or better. This will never happen with a regular satellite dish made for microwaves.

  • It's not possible I'm afraid.

    Optical wavelengths (light) are typically of a wavelength under a micron, and an optical surface needs to be accurate to this level or better to be useful.

    Radio wavelengths are typically 10-20cm or longer, and an adequate reflector can be made with surfaces accurate to a few centimetres (at a guess as I don't know the exact tolerance).

    Also - I once experimented using a frequently mentioned substitute, a convex shaving mirror(!) as a reflecting telescope. Even that was so poor it was unusable at the lowest possible magnification.

    (But it is possible to make your own reflector out of reasonably stable glass - it just won't be as big as a satellite dish! Search around for "Amateur telescope making".)

    You could place your antenna flat on the ground, spin it up, and pour in several quarts of Mercury to generate a decent mirror. You'll also need a big flat mirror to make the thing look at any piece of the sky except directly overhead.

    An extra bit of detail: - many commercial satellite antennas are in Ku band (but not all, you'd need to check), if so the wavelength range is 1.67 - 2 cm.

  • I would have added this as a comment (not enough rep yet, I'm afraid)...
    To elaborate on Andy's answer, the first reason is that the surface of the satellite dishes are too coarse to form any kind of image. Polished optical surfaces are smoothed to a polish (generally much smoother than one would achieve in polishing a car, though).

    Other problems come from the precision of the form - the paraboloid of the satellite dish may have errors on the order of millimeters, because radio waves (which satellites use) don't care about that much error. In fact, satellite dishes are EXACTLY the same as radio telescopes, except the receiver cone is designed for frequencies used by communication satellites instead of frequencies of interest to astronomy.

    Just for your interest, old satellite dishes ARE well-suited to function as rough solar collectors (for a solar oven) or as an acoustic telescope (position a microphone or speaker where the receiver cone normally lies)!

  • I think you should try. Sattelite dishes are made to collect very weak singals. And dish need to be aligned in millimeter range. And they are factory made with accuracy.
    start with 2ft parapolic offset dish which is much flat.
    i will try this also. In place of lnb you can fit mirror and it will reflect image to center of dish surface. Make a hole in center of dish surface and fit the eye peice lense to see.

    It's not going to work, at all, period. A barely acceptable optical telescope uses mirrors machined to a precision of 0.1 microns - that's 1 / 10000 of a milimeter. With a satellite dish you'd be very lucky to get within 1 mm of precison. Source: I make telescopes and telescope mirrors.

    1/4 wavelength accuracy is about the minimum acceptable accuracy for an imaging mirror. For 400 nm light, That's "circular to within 100 nm". You don't get that by stamping steel.

    Lotta people use old tv dishes as radio telescopes. Never done it, but I hear it can be a rewarding hobby.

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Content dated before 7/24/2021 11:53 AM