It looks like a good focus if not long. But does it follow y=ax^2 as I understand it? If not a parabola, does it still focus as much energy as a true parabola. Why would the compressed air automatically make a parabola? Truncated version. Full length. Mikek

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Is this a parabola?

Nov 14, 2019 45 Replies

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amdx 6 years ago

OK, I reread what I wrote and you could infer that I thought I could pull the vacuum and then remove it and the parabola would stay. What I meant was, I could pull the vacuum and be done, no need to put fiberglas on the back side or use some other method to stabilize it. My thinking at this point is a 12" round pan with the Mylar pulled taut over the opening and sealed-glued-clamped at the outer rim of the pan. Then pull a vacuum as a test of the idea. I don't know enough about telescope mirrors to know if the Parabola shape is extremely important or if other shapes can be dealt with. Temperature changes could be adjusted for with a second vacuum tank to increase the vacuum or a slow vacuum release valve, all fine adjustments. It's mostly pie in the sky thinking, but if I get enough positive feedback making it look possible, I'd pass it along to some telescope forum. It is probably an old discarded idea that's been recycled many times, but I never heard of it. I have recently watched several videos of guys grinding mirrors, a very laborious process, but pretty neat. Mikek

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amdx 6 years ago

Ya, I've been looking at rolls, precisely because of the wrinkle from being folded. I noted some seems shinier than others. Mikek

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Rick C 6 years ago

That is still saying exactly the same thing as before. You mean pull the v acuum and then maintain the vacuum, not "be done".

Of course if you want to maintain a partial vacuum one way or the other it will retain the shape. I don't understand what you are unsure of to need t o even ask this question. Not trying to be rude, I just don't know what yo u are unclear about.

The parabola is the only shape that will focus arbitrarily close to a point . If you want to see starts as points you need the parabola. If you are n ot so concerned about the resolving power of the scope, you can use a spher ical shape.

Assuming you even have a parabolic shape. No one has provided any convinci ng evidence this will produce a parabola. If you can live with an arbitrar ily long focal length, you can get as close to a parabola as you wish by th e fact that towards the center of a paraboloid the shape approximates a sph ere.

Much like ham radio, I'm sure there aren't many ideas that are actually new . It also would be far from an optimal surface, even if it ideally would p roduce a paraboloid, any variation in the plastic thickness will cause dist ortions and celestial telescopes typically want to have high resolving powe r which might be very suboptimal with such a scope.

Rick C. --- Get 1,000 miles of free Supercharging --- Tesla referral code - https://ts.la/richard11209

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amdx 6 years ago

No, I meant, I would be done! I pulled the vacuum, there are no leaks in my pan. I'm done I don't need to fiberglas the back of the mylar, I'm done. I don't need to find any other way to make my mylar stay in position with another material, I'm done. And with that we are done.

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Jasen Betts 6 years ago

no.

yeah helium is going to leak in (but there's not much of that in the atmosphere), also old mylar helium baloons seem to contain gasses other than helium so I guess other gasses can diffuse in too.

It's probably not a perfect whatever shape it actually approximates. Maybe it could be tuned using hot air to soften the membrane where needed.

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Jasen Betts 6 years ago

see 6:10 in the video. it seems to have a resolving power of about 1 degree.

When I tried casting out nines I made a hash of it.

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