Spherical Mirror

[Stu1smartcookie]

Hi , My question today is about scopes with Spherical Mirrors . There still seems to be scopes out there that have these . Can anyone explain why they are not suitable for scopes ?The point is , they must have been ok at some point otherwise they would not have been produced . The answer will hopefully be of use to others just entering into the hobby .

Stu 

[andrew s]

Spherical mirrors suffer from spherical aberation which limits their ability to form a sharp image. However, if they have a large enough focal ratio they are very close enough in shape to a parabolic mirror and are diffraction limited.

They are much simpler to make than parabolic mirrors.

Regards Andrew 

Edited February 19, 2021 by andrew s

[Zermelo]

Spherical mirrors don't even focus rays from the target object (at infinity) to a single focus.
Parabolic mirrors do focus such rays correctly, but don't focus rays from off-axis targets perfectly (hence: "coma").

Spherical mirrors are used - successfully - in catadioptric designs that combine the spherical mirror with a correcting 'plate' (large lens) to correct for the spherical aberration. They are also used - largely unsuccessfully - in "Bird-Jones" variant Newtonian reflectors, where a barlow lens built into the focuser attempts to correct for the aberration. I think it's the second of these two uses where you may have picked up on some lack of enthusiasm.

[Waddensky]

Here's an exaggerated illustation of the differences. As Andrew says, a spherical mirror is not able to focus all light in one point, creating a blurry image as a result. This may or may not be a problem depending on the mirror diameter and magnification.

 

Image source

Edited February 19, 2021 by Waddensky

[Stu1smartcookie]

9 minutes ago, andrew s said:

They are much simpler to make than parabolic mirrors.

Which presumably makes them cheaper 

[NGC 1502]

As above, focal ratio determines how useful a spherical mirror is. Examples-

A 6” F4 spherical would be very poor.

A 6” F12 spherical would be fine, but much harder to mount effectively.

[andrew s]

Just now, Stu1smartcookie said:

Which presumably makes them cheaper 

Yes. Regards Andrew 

[Tiny Clanger]

2 minutes ago, Zermelo said:

....  you may have picked up on some lack of enthusiasm.

English understatement at it's finest ! 🙂

Heather

[Stu1smartcookie]

Brilliant answers ( i never expected less)  . I have seen a fair few examples of short Focal Ratio scopes on various sites . Its a fair bet that these scopes are bought by first time buyers , as , they usually look like great value . 

[Seelive]

For Newtonians the ideal primary mirror profile to minimise abberations is parabolic but, as the focal ratio of the primary mirror increases, the 'abberational' (if that's a word) difference between parabolic and spherical mirrors diminishes. Many years ago I believe the 1/4 wave (visually) accepted cut-off was about F8 but these days I suspect it will be claimed to be much higher.

[Alien 13]

Spherical mirrors do have some plus points as previously mentioned, they are relatively easy to figure to a high accuracy and combined in a scope that can correct distortions like the Maks/SCT etc that use additional optics then all is good. The parabolic mirror is not without its own issues too so is not often used in large professional scopes.

Alan

[michael.h.f.wilkinson]

26 minutes ago, Stu1smartcookie said:

Brilliant answers ( i never expected less)  . I have seen a fair few examples of short Focal Ratio scopes on various sites . Its a fair bet that these scopes are bought by first time buyers , as , they usually look like great value . 

Sometimes these fast Newtonians with spherical mirrors are intended as "rich-field" telescope. Thus, they are intended for low magnification, wide-field views, where the coma of a parabolic mirror might actually be more troublesome than the spherical aberration of a spherical one.

[Zermelo]

24 minutes ago, NGC 1502 said:

focal ratio determines how useful a spherical mirror is

Yes, as a third use case I should have included the very slow spherical reflector, which as you say may perform perfectly acceptably.  Going back a few years, I think there were more of these about, when manufacturing processes weren't so good and amateurs' expectations were perhaps less exacting (and certainly lacking the modern prevalence of high-quality amateur astrophotography).

30 minutes ago, NGC 1502 said:

A 6” F12 spherical would be fine, but much harder to mount effectively

Agreed on both counts - put it on an EQ3/2 and it would sail you to China and back 

[Stu1smartcookie]

2 minutes ago, michael.h.f.wilkinson said:

Sometimes these fast Newtonians with spherical mirrors are intended as "rich-field" telescope. Thus, they are intended for low magnification, wide-field views, where the coma of a parabolic mirror might actually be more troublesome than the spherical aberration of a spherical one.

So,  an F5 scope ( 150 /750) spherical mirror newt  would be considered as a wide field scope . Only suitable to use with high focal length EP's ?

[Stu1smartcookie]

7 minutes ago, Zermelo said:

Yes, as a third use case I should have included the very slow spherical reflector, which as you say may perform perfectly acceptably.  Going back a few years, I think there were more of these about, when manufacturing processes weren't so good and amateurs' expectations were perhaps less exacting (and certainly lacking the modern prevalence of high-quality amateur astrophotography).

Maybe they were more popular due to costs .. lets face it , although costs are rising now , Astronomy was an even more expensive hobby for much more basic equipment .

[michael.h.f.wilkinson]

11 minutes ago, Stu1smartcookie said:

So,  an F5 scope ( 150 /750) spherical mirror newt  would be considered as a wide field scope . Only suitable to use with high focal length EP's ?

More like a 4.5" F/4.3 Newtonian. I have one that works happily with a 20 mm Plössl, or a 24 mm 68° Maxvision, but not so much with a 4mm planetary EP. 

[Zermelo]

It would have a wide field because of the F-ratio, but a spherical mirror would produce a lot of distortion on a scope that fast, irrespective of the eyepiece.

Faster scopes (ignore the spherical/parabolic distinction for this point) tend to need eyepieces that are better "corrected" (whatever the eyepiece focal length) because the light cone is converging more steeply.

 

[Space Hopper]

No doubt. My old OO 12"F4 surprised me once when i put my illuminated reticule 26mm (basic) eyepiece in it once.

The coma was that bad i laughed out loud.

Yet with Tv Panoptics and Naglers i'd hardly even noticed it before. I never even needed a Paracorr with it.

[Stu1smartcookie]

So do coma correctors come into play with these scopes ? 

[barkis]

Producing a Paraboloid can be a time consuming task, particularly if it is required to be a fast photographic objective. It is the main reason these mirrors are pricier.     A spherical mirror is no pushover to fashion either though, and not be taken lightly, the same care and attention has to be applied to it's creation, not having to figure it does not mean it will be a doddle to produce.

Ron.

 

[andrew s]

25 minutes ago, Stu1smartcookie said:

So do coma correctors come into play with these scopes ? 

Yes, that's exactly what they are for. Regards Andrew