Spherical or parabolic?

[j4y7223]

Hello, quick question, does anyone know if the Sky-Watcher 130M × 900 has got a Spherical or parabolic mirror? also, with a small aperture, does it really matter?

Edited October 19, 2019 by j4y7223

[Cornelius Varley]

2 minutes ago, j4y7223 said:

Hello, quick question, does anyone know if the Sky-Watcher 130M × 900 has got a Spherical or parabolic mirror? also, with a small aperture, does it really matter?

The 130x900 has a spherical mirror. At apertures less than 6" the difference between parabolic and spherical mirrors is minimal.

[j4y7223]

Thank you, Peter. 

[Ben the Ignorant]

Sorry to contradict but I own a 130/900 and it is parabolic.

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p4778_Skywatcher-Explorer-130---130-900-mm-Newton---optischer-Tubus.html

I would never have bought one otherwise, and upon receiving it I did a star test which was one of the very best I've ever seen in any scope of any type. I still remember the orange star's diffraction patterns that were a mirror image of each other on either side of focus.

Edited October 19, 2019 by Ben the Ignorant

[Ben the Ignorant]

1 hour ago, j4y7223 said:

Spherical or parabolic mirror? also, with a small aperture, does it really matter?

Small or large, the spherical aberration will be the same for a given f/number. A spherical 130mm f/5 has the same aberration as a spherical 300 f/5, but fortunately Sky-Watcher seems to have stopped making spherical newtonians. The info about them has become too easy to find with the Internet, so the old claim that spherical mirrors of a certain f/ratio or a certain diameter are acceptable doesn't catch on anymore.

Edited October 19, 2019 by Ben the Ignorant

[j4y7223]

Hi Ben the Ignorant, looked everywhere for a spec list like that, I suppose that settles it, thank you for the link.

[Ben the Ignorant]

1 minute ago, j4y7223 said:

Hi Ben the Ignorant, looked everywhere for a spec list like that, I suppose that settles it, thank you for the link.

Beware that very old 130/900's might be spherical, but those of recent years are parabolic. The parabola is just a requirement of the newtonian, like having salt in the kitchen, if it's not there the whole kitchen is unusable.

[j4y7223]

1 minute ago, Ben the Ignorant said:

Beware that very old 130/900's might be spherical, but those of recent years are parabolic. The parabola is just a requirement of the newtonian, like having salt in the kitchen, if it's not there the whole kitchen is unusable.

Appreciate your advice, Thank you.

[Cornelius Varley]

2 hours ago, Ben the Ignorant said:

Sorry to contradict but I own a 130/900 and it is parabolic.

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p4778_Skywatcher-Explorer-130---130-900-mm-Newton---optischer-Tubus.html

I would never have bought one otherwise, and upon receiving it I did a star test which was one of the very best I've ever seen in any scope of any type. I still remember the orange star's diffraction patterns that were a mirror image of each other on either side of focus.

I don't think so. The 130x900 does have a spherical primary mirror https://www.firstlightoptics.com/blog/skywatcher-explorer-130-vs-130p.html . The Skywatcher reflectors with the thicker spider veins (130x900) have spherical mirrors.

[Peter Drew]

As Peter has said, the difference between a spherical mirror and a parabolic one at those specs is minimal. In fact you are more likely to get a good sphere than a good parabola on an entry level telescope.  😀

 

[Stu]

3 hours ago, Ben the Ignorant said:

Beware that very old 130/900's might be spherical, but those of recent years are parabolic. The parabola is just a requirement of the newtonian, like having salt in the kitchen, if it's not there the whole kitchen is unusable.

The 130M is spherical. You seemed to link to a different model.

[Ben the Ignorant]

According to the FLO blog page, 130M means M for motorized, 130 without additional letter is the f/5, but the 130P with a P should mean parabolic. There seems to be some confusion here. It was always my understanding that the P in 130P meant "parabolic", and the 130/900 I bought IS parabolic.

Here is a reminder of what a spherical mirror's Ronchi pattern looks like:

The lines of a spherical newtonian are not parallel, they form either a crescent or a barrel. And so they are thicker in some places and thinner in some places.

This is my 130/900 Ronchi (10 lines/mm) pattern, sorry but complete cloud cover so I used a very remote road lamp as an artificial star, and ground turbulence was horrible.

The unevenness is the lines is not due to zones or whatnot, just turbulence. What matters in this discussion about spherical aberration is the lines are parallel, no crescent, no barrel. They have the same width throughout despite the troubled atmosphere.

2 hours ago, Cornelius Varley said:

The Skywatcher reflectors with the thicker spider veins (130x900) have spherical mirrors.

My Sky-Watcher had the thicker spider vanes that I amputated and replaced, but it is a parabolic, the Ronchi test shows it.

I tried to photograph the defocus test but air conditions are too bad. Visually the defocus test is excellent, intrafocal and extrafocal figures are the same. It looks like the descriptions of these telescopes need to be checked.

[Peter Drew]

A small aperture mirror at F10 spherical will be equivalent to a 1/4 wave parabolic one. I don't think a Ronchi test would be sensitive enough to show much difference. A Foucalt test would be preferable as a sphere completely nulls out when it's a good one.   😀

[Stu]

1 hour ago, Ben the Ignorant said:

130M means M for motorized,

It may well do, but everything I read says that scopes with 130M as the description have spherical mirrors. In fact if it hasn't got a P after it then I strongly suspect it is spherical.

[Sporadic Dobstronomer]

The difference between a sphere and a paraboloid is given by r^4 / 8.R^3 where r is the radius of the mirror and R is the radius of curvature

for 130/900 this works out as 0.7 wave so it does matter which shape the mirror is

[Ben the Ignorant]

15 hours ago, Peter Drew said:

A small aperture mirror at F10 spherical will be equivalent to a 1/4 wave parabolic one. I don't think a Ronchi test would be sensitive enough to show much difference. A Foucalt test would be preferable as a sphere completely nulls out when it's a good one. 

This is not an f/10 mirror but an f/7, it's not a small aperture, small would be 114m or 76mm, and small diameters don't allow spherical aberration in newtonians like they don't allow it in refractors and catadioptrics. Aberration is aberration, nothing hides it when it's there. If hiding it instead of correcting was possible the manufacturers and our wallets would have an easier job.

If a Ronchi test is not sensitive enough, why are optics labs using it?

https://astro-foren.de/index.php?thread/14862-zambuto-enjoy-your-mirror/

Which they do in addition to the Foucault test, which tells the same result.

The "good" sphere "nulling out" means nothing, when a spherical shape is there instead of a parabolic one, the spherical aberration shows, the image is junk. If it didn't all our gear and the lab equipment could go the trash can. And the Hubble space telescope wouldn't have needed a billion-dollar repair.

Edited October 20, 2019 by Ben the Ignorant

[Cornelius Varley]

16 hours ago, Ben the Ignorant said:

Sorry to contradict but I own a 130/900 and it is parabolic.

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p4778_Skywatcher-Explorer-130---130-900-mm-Newton---optischer-Tubus.html

I would never have bought one otherwise, and upon receiving it I did a star test which was one of the very best I've ever seen in any scope of any type. I still remember the orange star's diffraction patterns that were a mirror image of each other on either side of focus.

One more way to check that a Skywatcher reflector has a parabolic or spherical primary is to look for the P in the designation ie SW Heritage 130P, SW Explorer 130P, Explorer 150P etc. All of the Explorer P and Skyliner P range have the thin, 0.5mm, spider vanes and a parabolic primary . The 130M/900, on the other hand, has a thick cast secondary holder which also combines the spider vanes. The 130M/900 also has a spherical, not parabolic. mirror. The difference in performance between a spherical and parabolic mirror at this size is minimal.

[Cornelius Varley]

13 hours ago, Ben the Ignorant said:

The lines of a spherical newtonian are not parallel, they form either a crescent or a barrel. And so they are thicker in some places and thinner in some places.

It is my understanding that a spherical mirror will produce straight lines in a Ronchi test ie a uniformity of curvature.

http://www.atm-workshop.com/ronchi-test.html

[barkis]

A paraboloid has a curve with differing focii from it's centre out to it'edge zone. A Ronchi screen will show curved shadows.
If the surface is a perfect sphere, those lines will be perpendicular.  If you were to intercept a pinhead light source with a knife edge at the radius of curvature of a spherical mirror, It would darken completely all over, an indication that every part of it's surface comes to the same focus.
In a fast Newt, a spherical mirror will perform poorly. Some telescopes are artificially equipped to extend their focal length to accommodate 
a spherical objective.
 

[Ben the Ignorant]

4 hours ago, Cornelius Varley said:

One more way to check that a Skywatcher reflector has a parabolic or spherical primary is to look for the P in the designation ie SW Heritage 130P, SW Explorer 130P

All the Sky-Watcher 130 in TS' page are parabolic, regardless of designation:

https://www.teleskop-express.de/shop/index.php/cat/c59_Newton-Reflektoren.html/page/2

They specifically say these are not spherical because sphericals are and outdated idea that disappointed stargazers.

[johninderby]

I have noticed that some of the Skywatcher 130 are described as speherical but all the 130p are described as parabolic. Could there be two versions of the 130?
 

Edit. Ahhh here’s the answer.

https://www.firstlightoptics.com/blog/skywatcher-explorer-130-vs-130p.html

Edited October 20, 2019 by johninderby

[Ben the Ignorant]

2 hours ago, Cornelius Varley said:

It is my understanding that a spherical mirror will produce straight lines in a Ronchi test ie a uniformity of curvature.

http://www.atm-workshop.com/ronchi-test.html

The Ronchi test is simply watching an artificial or natural star through the telescope, with the Ronchi grating placed in the focuser as an ordinary eyepiece. What this guy shows is a convoluted method that few people could use because it involves removing the mirror from the telescope, and not testing the secondary, which could make or break the wavefront. He then uses the Ronchi grating as a Foucault apparatus would be used, and claims the light source can be a slit when it HAS TO BE a dot, like a star.

This is a Ronchi tester:

Simply put it in the focuser and observe the lines. No one removes their mirrors from the scope to make a Ronchi test. That would be terribly inconvenient.

[JamesF]

I'm certain that at some point there were two different 130 newtonians produced by Skywatcher at the same time, with different focal lengths.  I think there was one with a spherical mirror with a 900mm focal length and one with a parabolic mirror with a 650mm focal length.

I've no idea if that is still the case though.

James

[Cornelius Varley]

5 minutes ago, JamesF said:

I'm certain that at some point there were two different 130 newtonians produced by Skywatcher at the same time, with different focal lengths.  I think there was one with a spherical mirror with a 900mm focal length and one with a parabolic mirror with a 650mm focal length.

I've no idea if that is still the case though.

James

This is still the case.

[Ben the Ignorant]

15 minutes ago, johninderby said:

I have noticed that some of the Skywatcher 130 are described as speherical but all the 130p are described as parabolic. Could there be two versions of the 130?
 

Edit. Ahhh here’s the answer.

https://www.firstlightoptics.com/blog/skywatcher-explorer-130-vs-130p.html

Sky-Watcher surely doesn't make different newtonians for British or German outlets.