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Parabolic or spherical mirror, how can you tell?

moriniboy

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ronin

ronin

Posted
Posted

Don't think there is an easy way just by looking at one.

The difference is actually minutely small.

Think you would have to feed well collimated light on to the mirror then analyse (compare) the resultant image created. There are tests for image quality, usually interferance based. Look up testing a home made mirror would be a good start. However a parabolic mirror is better then a spherical, but better then a parabolic is a hyperbolic. Meaning that a parabolic will not give an ideal image.

But as to looking at a mirror with just a set of eyeballs, I do not think you can tell.

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NGC 1502

NGC 1502

Posted
Posted

Looked on Wiki and its not very obvious (to me) can anyone explain in simple laymans terms.

Thanks

Looking at a mirror will not reveal anything other than the state of the aluminium coating, not whether it is spherical or parabolic, or how accurate the optical surface is.

If you are happy with the views the mirror gives is what really matters.

Small long focus spherical mirrors can be fine, as the difference between spherical and parabolic is too small to matter.  As the aperture increases, and the if focal ratio is shorter, it needs to be parabolic to give satisfactory results.

A star test will reveal most of what the majority of us need to know about mirror quality, and best to remember that perfection is unattainable, and not necessary.  If the optics are cooled to ambient and collimated, a decent eyepiece in the focuser, then the state of the atmosphere is the important factor by far, and has the biggest impact on how good the views are.

Regards, Ed.

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Tiki

Tiki

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It might be helpful to look at the  'Optical Testing' section about 2/3 of the way down this page https://stellafane.org/tm/atm/index.html.

The difference between parabolic and spherical mirrors  are so small that diffraction tests are the simplest way of inferring the true shape of a mirror.

A parabolic mirror will reflect incoming parallel rays of light (eg. from a star) to a point whereas the reflected rays in a spherical mirror will be a bit spread out (spherical aberration).

I hope this helps.

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moriniboy

moriniboy

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The mirror in question is a 150mm F9 from the 1980's, it needs re-coating but I was thinking if its spherical then it may not have been worth doing.

The only marking on it is the name J Sewill, Google suggests they were from Liverpool but in the late 1800's so I think there is no connection.

Seems to star test ok though!

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Scott

Scott

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At the risk of stating the obvious, spherical is the same arc all the way where as a parabola varies so that the arc near the edge is different that the centre

post-20406-0-21001700-1404679398.jpg

edit:- Of course, the difference will be so minute that it would be hard to measure easily I would imagine ?

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Astrobits

Astrobits

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The difference between a parabolic and spherical  150mm  f/9 mirror is approx 0.45 lambda ( wavelengths of light ). As this is in the centre of the mirror which will be obscured by the secondary, the rest of the mirror will be not far off 1/4 wave if it is spherical. This will be a perfectly useable mirror whether it is spherical or parabolic.

Nigel

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Astrobits

Astrobits

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Posted

Sure you can get away with a spherical 22" mirror........provided it is f/21 to reach 1/4 wave on the wavefront!!!!!!!  You might be able to get away with something a little bit shorter, say f/20 or 19 if you are not too fussy.

Definitely need that parachute.

:grin: :grin: :grin:

Nigel

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