159 replies to this topic

[Not Here Anymore]

We go round and round on this issue and I don't know why, the empirical answer is right in front of you.

 

Astrophysics Stowaway:

 

Original approximate f/5 - fluorite

2nd Run approximately f/7 -  FPL 53

 

So even Roland, when making the more challenging optical configuration used Fluorite for the faster optic.

 

Of course all the "failed scientists", and that certain group of converts, are going to run in and go off in all directions (Abbe this and mating element that  - blah blah blah) trying to negate this difference, but it speaks for itself.

 

Carry on.

[MooEy]

And then you have Tak, which every one of their ED scopes are superior and improved over their fluorites...

[Not Here Anymore]

Yes, with TWO ED elements.

[Suavi]

Yes, with TWO ED elements.

Why not two fluorite elements then, or at least one, to avoid compromising the performance?

[Jeff B]

You know, everybody here obsesses over the ED element.  However, OEMs like Roland and Yuri have been shouting for years from the roof tops that "It's the mating glass stoopid".  Yet, I see scant discussion here concerning the quality of the mating glass....except by the OEMs.  That is what seems to concern them the most.  So why do we ignore that?  To me it's like ignoring the pointer dog and blasting away at a different bush.  

 

Jeff

[MooEy]

Yes, with TWO ED elements.

Why does it matter? There is not a single fluorite that is better than that scope

[Not Here Anymore]

Why does it matter? There is not a single fluorite that is better than that scope

Thank you for this post.  It is very enlightening and I think really helps to drive home the underlying context for many of the threads on this subject matter, as well as many others on CN. We all learn together.

[Element79]

Why does it matter? There is not a single fluorite that is better than that scope

 

Errr... the Zeiss APQ series is renown for being the best telescopes available to humans and it has a Fluorite lens.

[Not Here Anymore]

Errr... the Zeiss APQ series is renown for being the best telescopes available to humans and it has a Fluorite lens.

Understand, I was being facetious. When you read comments like his you realize you are wasting your time.

[Element79]

Understand, I was being facetious. When you read comments like his you realize you are wasting your time.

 

My bad.  Sometimes I think that reading this forum is wasting my time but hey, here I am.

[Suavi]

Errr... the Zeiss APQ series is renown for being the best telescopes available to humans and it has a Fluorite lens.

I thought the best telescope available to humans used to be Hubble 

[Alan French]

The best telescopes known to amateur astronomers have a thin aluminum coating supported by glass, diameters considerably exceeding the largest apochromats, and are at their best under dark, steady skies.

 

Clear skies, Alan

[vahe]

I thought the best telescope available to humans used to be Hubble 

 

The optical figure on the original Hubble primary mirror would be totally unacceptable on any amateur telescope.

.

Vahe

[Suavi]

The optical figure on the original Hubble primary mirror would be totally unacceptable on any amateur telescope.

.

Vahe

But it produces the best images 

[vahe]

But it produces the best images 

FYI,

.

https://www.nasa.gov...les-mirror-flaw

.

Vahe

[gnowellsct]

FYI,
.
https://www.nasa.gov...les-mirror-flaw
.
Vahe

Yeah but Vahe some companies like optical craftsman turned out mirrors like that for two decades. Not EVERY mirror. But enough to undermine the credibility of the brand. Coulter optics were/are a frequent target of refiguring.

[dothead]

the original Hubble primary mirror would be totally unacceptable on any amateur telescope.

as are all very fast paraboloid mirrors. The same as the Hubble needed an optical corrector, one needs corrector lenses ( = coma corrector) to yield an acceptable image at the eyepiece with an F/4 mirror.

Edited by dothead, 04 June 2018 - 02:37 PM.

[Alan French]

as are all very fast paraboloid mirrors. The same as the Hubble needed an optical corrector, one needs corrector lenses ( = coma corrector) to yield an acceptable image at the eyepiece with an F/4 mirror.

The Hubble is not a Newtonian. It's a Ritchey-Chrétien and both mirrors are hyperbolic. Because of an error in setting up the null test the curve on the primary was wrong. Corrective optics were needed because of that mistake, not because the optical design required them.

 

Clear skies, Alan

[gnowellsct]

The Hubble is not a Newtonian. It's a Ritchey-Chrétien and both mirrors are hyperbolic. Because of an error in setting up the null test the curve on the primary was wrong. Corrective optics were needed because of that mistake, not because the optical design required them.

 

Clear skies, Alan

I fear for the Webb telescope.  If that one screws up, it's dead....unless maybe they can come up with some drones to do repairs and fixes?

[BGazing]

I fear for the Webb telescope.  If that one screws up, it's dead....unless maybe they can come up with some drones to do repairs and fixes?

L2 is not close enough to be able to do real-time instantaneous correction if something goes wrong. So they better multiple-check those mirrors on the ground...if they don't know how they can always ask at CN.

[gnowellsct]

L2 is not close enough to be able to do real-time instantaneous correction if something goes wrong. So they better multiple-check those mirrors on the ground...if they don't know how they can always ask at CN.

I guess Perkin Elmer didn't really have a good excuse handy for that one.  And so they perished.

[Element79]

I fear for the Webb telescope.  If that one screws up, it's dead....unless maybe they can come up with some drones to do repairs and fixes?

 

This month's Sky & Tel had an short article that said that the Webb's launch will now be in 2020 rather than 2019 because extensive testing has lasted longer than expected.  So let's hope that they get it right this time!

Edited by Element79, 05 June 2018 - 12:02 AM.

[dothead]

The Hubble is not a Newtonian. It's a Ritchey-Chrétien and both mirrors are hyperbolic. Because of an error in setting up the null test the curve on the primary was wrong. Corrective optics were needed because of that mistake, not because the optical design required them.

 

Clear skies, Alan

But to yield an abberation-free image, even R-Cs need image correctors - namely a field flattener.

And for fast newtonians to yield an abberation-free image, correctors are needed - namely a coma corrector.

Edited by dothead, 04 June 2018 - 08:39 PM.

[Alan French]

But to yield an abberation-free image, even R-Cs need image correctors - namely a field flattener.

And for fast newtonians to yield an abberation-free image, correctors are needed - namely a coma corrector.

I find no evidence the Hubble uses any optics in addition to the hyperbolic primary and secondary.

 

The optics designed to correct the spherical aberration of the system (COSTAR) used two mirrors. COSTAR was later removed and the instruments themselves include optics to correct the spherical aberration of the main telescope.

 

The Hubble was designed to provide imaging from 100 to 2500nm, which may have precluded the use of refractive optics in the main telescope's light path.

 

Clear skies, Alan

[Jeff B]

As Hubble has no fluorite or FPL-53 in the light path, there's nothing to talk about.  So boring really. 

Edited by Jeff B, 05 June 2018 - 09:49 AM.