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Are spherical mirrors THAT bad?

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#1 Emirhan_05

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Posted 21 January 2021 - 01:20 PM

hey all,

 

I have recently learned that my telescope, despite being promised a parabolic mirror, has a spherical mirror. Is it actually as bad as the rumors are or should it work just fine? I'm not planning to do any type of astrophotography, some pictures of the moon perhaps, but that's it. I'll use it for observing bright DSOs and planetary viewing. I have ordered high-quality eyepieces that'll replace the poor ones that came with the telescope so I shouldn't worry about the moon and other objects looking like a rainbow anymore I think. What are your thoughts on this?

 

specs are:

mirror diameter: 130mm / 4,5 in

focal ratio: F/5

tube length: 650 mm

 

yours sincerely,

emirhan



#2 sg6

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Posted 21 January 2021 - 01:28 PM

If spherical and 130mm and f/5 it will be poor, probably awful.

Equally at 130mm and f/5 I would have expected it to be parabolic to some extent - I say extent as the normal route to a parabolic is to start with a spherical then "parabolize" the mirror.

 

Which actual scope and why do you say "spherical" ?



#3 pyrasanth

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Posted 21 January 2021 - 01:28 PM

The center of the mirror may well be okay however the further out toward the edges you will get a lot of distortion as a spherical mirror cant bring light to a sharp focus- hence, as an example, the SCT which uses a corrector plate to compensate for a spherical mirror.

 

The moon is quite big so you may only have a small area of the moon that is in sharp focus. Good eyepieces won't especially help as you are really magnifying the error in your mirror which has little to do with the eyepiece quality- higher quality eyepieces are normally nicer from the pespective of good correction of the field but in your case you can't make a silk purse out of a sow's ear.

 

I suggest you give it a try- you may get lucky and be happy with the results however in all consideration a good parabolic mirror is not that expensive in sensible apertures so you may well consider an upgrade being worthwhile.



#4 Emirhan_05

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Posted 21 January 2021 - 01:31 PM

If spherical and 130mm and f/5 it will be poor, probably awful.

Equally at 130mm and f/5 I would have expected it to be parabolic to some extent - I say extent as the normal route to a parabolic is to start with a spherical then "parabolize" the mirror.

 

Which actual scope and why do you say "spherical" ?

I have a celestron astromaster 130EQ, but I'm not sure tbh. Lots of people saying it's parabolic, lots of people saying it's spherical. It seems that even Celestron doesn't know anymore.



#5 Dave Mitsky

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Posted 21 January 2021 - 01:32 PM

How did you learn that the mirror was spherical?  Most 130mm f/5 Newtonians have parabolic mirrors.


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#6 Emirhan_05

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Posted 21 January 2021 - 01:33 PM

How did you learn that the mirror was spherical?  Most 130mm f/5 Newtonians have parabolic mirrors.

Just people online arguing whether my scope is good or not.

It's a Celestron Astromaster 130EQ.



#7 Jon Isaacs

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Posted 21 January 2021 - 01:44 PM

Hi:

 

A 130 mm F/5 spherical mirror will have about 0.9 waves of spherical aberration. This is not good. A diffraction limited mirror is considered to be 0.25 waves and the minimum acceptable.

 

The center of the mirror may well be okay however the further out toward the edges

 

 

Spherical aberration doesn't work that way. Light from the entire mirror forms the image at each point so the aberration exists at every point, center included. 

 

A parabola focuses every ray of light in the center of the field to a point, a sphere does not. The difference is called spherical aberration.

 

One can test for spherical aberration with a star test.

 

At this point, it is what it is. Use it enjoy it..

 

Jon


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#8 sg6

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Posted 21 January 2021 - 01:46 PM

OK, found a "useful" review, and it seems you could end up with either Spherical, Parabolic, something between.

 

Seemingly as part of an effort to reduce manufacturing costs and maximize profit, Celestron is putting spherical primary mirrors in at least some AstroMaster 130EQ units. These mirrors cannot focus light correctly (a proper Newtonian telescope uses a parabolic mirror), and make the telescope nearly unusable at high magnifications.

 

The bulk of the AstroMaster 130 scopes I’ve seen have had acceptable primary mirrors – usually not quite parabolic but not quite spherical either – but a few have had spherical primaries that made for mushy views. I’ve also seen a few with great optics.

Came from Telescopicwatch

So it may be chance, or whoever is the retailer knows the state of the mirror.

 

Have a read of the artical anyway, but they don't rate the scope.

 

Celestron used to say they only used parabolic mirrors but since they became Synta all that seems to have changed and been dropped. If they have put an f/5 spherical in there do not expect great things. Sorry.



#9 ngc7319_20

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Posted 21 January 2021 - 02:02 PM

Time to do a star test... Let scope cool outside for hour.  Bright star overhead with 5mm (+/-) eyepiece.  Or Polaris if you have trouble tracking.  What do star images look like just inside best focus?  Just outside best focus?  Make some rough drawings.


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#10 kathyastro

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Posted 21 January 2021 - 02:25 PM

There's nothing inherently wrong with a spherical mirror.  There are thousands of happy Mak and SCT owners out there, who love their spherical-mirror scopes.  What matters is how well it is corrected.  The typical SCT mirror is ground to about f/2, so there is nothing inherently wrong with a fast spherical mirror either.

 

Bird-Jones scopes get a bad reputation because they are relatively inexpensive to make.  So a manufacturer looking to churn out junky scopes is going to choose that design to save money.  It doesn't mean that all Bird-Jones scopes are junky.  Just the junky ones are.


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#11 Jon Isaacs

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Posted 21 January 2021 - 02:36 PM

There's nothing inherently wrong with a spherical mirror.  There are thousands of happy Mak and SCT owners out there, who love their spherical-mirror scopes.  What matters is how well it is corrected.  The typical SCT mirror is ground to about f/2, so there is nothing inherently wrong with a fast spherical mirror either.

 

Bird-Jones scopes get a bad reputation because they are relatively inexpensive to make.  So a manufacturer looking to churn out junky scopes is going to choose that design to save money.  It doesn't mean that all Bird-Jones scopes are junky.  Just the junky ones are.

This is not a Jones-Bird scope.  This is a standard Newtonin with a 130mm F/5 Mirror.  Some of these are apparently spherical.  

 

SCTs and Maks have spherical mirrors but they have full aperture correctors that correct for the spherical aberration.  Without the correctors, they are very poor performers. A standard Newtonian does not have a corrector and therefore needs a parabolic mirror.

 

If the mirror is small enough and with a sufficiently long focal ratio, a spherical mirror can be essentially parabolic, the difference is so small, it doesn't matter.

 

The equation looks like this:

 

Lambda = 22.55 x D (inches) /FocalRatio3

 

The standard 4.5 inch F/8 Newtonian has a spherical mirror which calculates to 1/5 wave. These can provide very good images.  

 

Jon


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#12 Jon Isaacs

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Posted 21 January 2021 - 02:44 PM

Time to do a star test... Let scope cool outside for hour.  Bright star overhead with 5mm (+/-) eyepiece.  Or Polaris if you have trouble tracking.  What do star images look like just inside best focus?  Just outside best focus?  Make some rough drawings.

 

:waytogo:

 

emirhan

 

This is very good, you will learn how to evaluate your mirror and determine exactly what you do have. 

 

ngc7319_20 is very knowledgeable, he will be able to help you figure this out.

 

As an aside, I think the most famous case of spherical aberration is the Hubble Space Telescope.  The mirror was made incorrectly but a corrector was fabricated and the problem solved. 

 

Jon



#13 kathyastro

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Posted 21 January 2021 - 04:07 PM

This is not a Jones-Bird scope.  This is a standard Newtonin with a 130mm F/5 Mirror.  Some of these are apparently spherical. 

 

Ah, OK.  Thanks.  Yeah, that's not good.
 


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#14 sevenofnine

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Posted 21 January 2021 - 09:19 PM

My first scope (1975) was an old 2.5 inch Tasco refractor on a EQ mount with wooden tripod legs. The achro lens was so bad that every star was a kaleidoscope of color. But I saw Jupiter, Mars, Venus, Saturn and the Moon. I even did Solar projection to view Sun spots and an eclipse. All done with those horrible eyepieces with and H on them lol.gif  And I loved that scope. It's still in the family somewhere. The point is enjoy what scope you've got. It's way better than what Galileo had. Good luck with your new scope and have fun!


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#15 jgraham

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Posted 21 January 2021 - 11:04 PM

Ugh, that’s a shame if it turns out to be spherical. However, if you are happy with the image, then all is well. I don’t think that I’ve every heard of a 5” f/5 with a spherical mirror. The mirror in my Tasco 11TR is a 4.5” f/8 and it produces a fine image.

 

Curiouser and curiouser...



#16 Jon Isaacs

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Posted 22 January 2021 - 05:22 AM

Ugh, that’s a shame if it turns out to be spherical. However, if you are happy with the image, then all is well. I don’t think that I’ve every heard of a 5” f/5 with a spherical mirror. The mirror in my Tasco 11TR is a 4.5” f/8 and it produces a fine image.

 

Curiouser and curiouser...

130mm F/5s with spherical mirrors are out there. There are even 100mm F/4s with spherical mirrors.  Some of the inexpensive 100mm f/4 from companies like Bushnell and Tasco have spherical mirrors. I had the Bushnell Astroscan clone, it had a spherical mirror.  

 

The wave error is a function of the cube of the focal ratio so an 4.5 inch F/8 like your Tasco 11TR is about 1/5th wave, whereas the 4.5 inch F/4 Astroscan clone is about 1.6 waves.

 

The Orion 130mm F/6.9 Space Probe 130 has a spherical mirror.  That's about 1/3 wave.  This should not be confused with the 130mm F/5 SpaceProbe 130ST which has a parabolic mirror.  1/3 wave isn't great but it's definitely useable.

 

It should be noted that these numbers assume a perfect sphere.

 

Jon


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#17 Tony Flanders

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Posted 22 January 2021 - 05:48 AM

Rather than fret about whether a mirror is nominally spherical or parabaloid, I think it makes a whole lot more sense to evaluate each mirror on its own merits. A batch of mirrors that was meant to be paraboloid may well include some lemons. Mirrors can be over-parabolized as well as under-parabolized, and there's no shortage of other possible flaws besides spherical aberration. If you're really skilled at star testing, you may be able to tease out the individual aberrations. But for most of us, all that really matters is whether it produces sharp, contrasty images.

 

I have seen some 130-mm f/5 scopes with truly awful mirrors as well as some with superb mirrors.


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#18 JoshUrban

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Posted 22 January 2021 - 08:21 AM

I also vote for a "test drive!"  If you LIKE the views, then great!  I've had memorable views through all sorts of fancy, top-dollar gear, as well as through cheap binoculars and entry-level telescopes.  I think I've actually had more fun with my brother's old starter DJ turntable and busted speakers than my current audiophile rig and tube amp, if we're using different examples of "better" is relative.  

 

  Give it a shot, and see what's up.  You can always snag a Takahashi later ;)  


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#19 starcanoe

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Posted 22 January 2021 - 09:13 AM

A 5 inch f5 spherical mirror MIGHT work okay at the lowest possible power. For that matter it might actually be BETTER at lowest possible power than a parabolic mirror because that mirror would have some serious coma on the outer edge of the field. I have a 5 inch f5 spherical mirror pulled from one of those Bird-Jones junker scope that I intend to try out for that very thing....a low power only scope on the cheap...



#20 KBHornblower

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Posted 22 January 2021 - 10:43 AM

A 5 inch f5 spherical mirror MIGHT work okay at the lowest possible power. For that matter it might actually be BETTER at lowest possible power than a parabolic mirror because that mirror would have some serious coma on the outer edge of the field. I have a 5 inch f5 spherical mirror pulled from one of those Bird-Jones junker scope that I intend to try out for that very thing....a low power only scope on the cheap...

If I am not mistaken a fast conventional Newtonian with a spherical mirror will not be coma free.  To eliminate the coma it would be necessary to use an oversized mirror and an aperture stop of the desired diameter at the center of curvature.  This is what I have seen called a "lensless Schmidt", and was written up as an easy instrument for wide-field photography in which the spherical aberration was not excessive.

 

For a simple Newtonian the question is whether leaving the mirror spherical reduces coma significantly, if at all.  I do not have the mathematical knowhow to calculate the image patterns, and would welcome any link to a reliable source of such information.



#21 JohnnyBGood

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Posted 22 January 2021 - 02:28 PM

Here's my take on it:

 

I have a Meade Polaris 130 f/5 scope. Some people claim the mirror is spherical. The back of the box it came in said it was a parabolic mirror. Doing my best amateur efforts at a star test it looks like a parabolic mirror with something like a half wave of spherical aberration. In other words, I think it's parabolic but not an especially well-figured parabolic. I'm guessing that's why Meade and Celestron give such conflicted answers: they decided customers would be happier if they expected a spherical mirror and instead got something that was parabolic than if they expected a parabolic mirror and got one with lots of spherical aberration. Or because both brands sell (or have sold) 130mm "parabolic" scopes and 130mm Bird-Jones scopes with spherical mirrors customer service doesn't know the difference between the different scopes and thus gives muddled responses.

 

Is it bad? Heck, no, or at least not to me. My Polaris 130 is my favorite and most used telescope. It excels at low and medium power viewing. High power views are somewhat limited, it's extremely hard to focus the scope above 160x or so. I got a helical microfocuser that allows me to get satisfactory focus up to about 200x or so. But the vast majority of the viewing I do is at low and medium power so it doesn't matter. I find Ploessl eyepieces to be much more enjoyable to use than the MA eyepeices it came with. Most of my observing with the scope has been done with a 26mm Cassini Erfle and a 7-21 Celestron zoom. I do NOT like how the scope performs with my "red line" expanse clones--those should be reserved for "slower" telescopes. But I have spent many, many nights happily sitting in the dark with my 130mm f/5 scope regardless of what kind of mirror it has.

 

If you like your scope then don't worry about what others say about it.

 

In any case, there's probably no point in trying to buy replacement mirrors that may or may not be any better than what have. Better to enjoy the scope for what it is and save your money for when you feel the urge to upsize to a larger scope.


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