How fast does gravity travel???

[Justin Passing]

They figured it to the accuracy they wanted according to the gizmo used - the problem was an error in producing the gizmo. An incorrectly sized or installed spacer. My recollection is that sizing was the problem. Confusion when the design was moved to actual manufacture. They corrected Hubble eventually by adding something similar to a schmidt camera corrector plate but designed for that specific job. It worked because the main mirror was incredibly accurately out which allowed them to design it to suite.

A parabola is nothing remotely like a sphere and with shorter and shorter focal lengths and increasing diameters the differences get even larger.

There are also various designs of cassegrain type telescopes that use curvatures other than parabolic. This one for instance

Ritchey–Chrétien telescope - Wikipedia

en.wikipedia.org

Lenses and more mirrors might get added. Null testing is still done the same way - optics in the test kit added to cause it to null when tested just as a spherical mirror does but only for images at it's centre of curvature. At all other distances spherical aberration is added. The curves correct that, This is for the main mirror. There are various ideas on dealing with the others. Other abberations can also be minimised by use of certain curves eg the RC design, Coma is a particular problem.

The Web is actually a 4 mirror off axis system. All mirror means no chromatic aberration problems. One it seems is flat.

https://www.researchgate.net/figure/Schematic-Layout-of-the-James-Webb-Space-Telescope-Design_fig1_241308855

The paper on that page gives more explanations,

That's not right about the Hubble repairs,,,

I think we all know a parabola and a sphere are different - no idea why you bother to type stuff like that in every post.


You say there are 4 mirrors - well this is one of the more "coo" illustrations:

 

[ajohn]

You say there are 4 mirrors - well this is one of the more "coo" illustrations:

Those are the main features of the optics. The ones that correct the image. There will be others maybe even one to extend the focal length dramatically.

The testing error that led to Hubble mirror fiasco

NASA has established how a mirror aboard its $1.5 billion Hubble Telescope came to be the wrong shape. The agency said last week that errors in a test instrument apparently led Perkin-Elmer, which fabricated the optics, to finish the 2.4-metre primary mirror of the Hubble Space Telescope...

www.newscientist.com

 

[RandomGrinch]

Just to add...
A little anecdote from someone I once worked with, who was involved with Hubble since 1976.

Hubble Space Telescope Cameras | Cardiff Astronomical Society

Hubble Space Telescope Cameras | Cardiff Astronomical Society Cardiff Astronomical Society has been running for 40 years. Most members are from South East Wales and we have a purpose built observatory 8M x 3.5M with a permanently fixed 11in Celestron Schmidt Cassigrain telescope. Membership is...

www.cardiff-astronomical-society.co.uk

I can truly imaging banging my head on a brick wall, when 14 years of my work may have been lost due to faulty optics

 

[ajohn]

A little anecdote from someone I once worked with, who was involved with Hubble since 1976

LOL It's a good job it was so accurately inaccurate. It meant they could ray trace the lot again to design a corrector, fit that and find it works. I like the blokes use of the word compromise. High end astro scopes of size are all about that - trading of abberations for the most precise results which means getting all of the light from a star into as small as possible spot at the focus and over a wide a field as possible. A parabolic mirror system only works perfectly directly on axis. Quality falls of rapidly off axis - hence the RC design, worse on axis performance but much much much better off axis.

 

[bernardgreen]

""When that happens, they create a flash of light and a powerful shockwave ripples across the Universe. It makes everything in the Universe wobble, including, imperceptibly, the atoms inside each one of us.
The shockwave, called a gravitational wave, distorts space. When it is detected on Earth, the new telescope scrambles into action to find the exact location of the flash.""

Neutron stars: New telescope detects dead suns colliding

Light from the crashes is only visible for a couple of nights.

www.bbc.co.uk

also from same source

Neutron stars are so heavy that a small teaspoon of their material weighs four billion tonnes.

 

[ajohn]

the new telescope scrambles into action to find the exact location of the flash.""

They have used the same idea looking for super novas for a long time. Amateurs report them sometimes.

Super nova have been thought to be the source of heavier elements. Colliding neutron stars now??? TV show things that suggest they can resolve each one in a pair - not so sure about that.