Spherical or parabolic?

[FLO]

7 minutes ago, Ben the Ignorant said:

More likely because of the exchange rate; I've bought 100°-110° eyepieces from the UK, 15%-20% less than on the continent. I hope owners of these telescopes will star-test them, possibly Ronchi-test them with pictures and show the result.

I've advised my friends to buy their astro gear from the UK, but they already knew. 😁

Do you not live in the UK? 

[Ben the Ignorant]

No, I assumed everyone had noticed I always quote prices in €. And noticed my english is sometimes flawed. 🤨

[FLO]

Ah. Okay 🙂 

[michael.h.f.wilkinson]

As I do the math, only at F/10 does the difference between parabolic and spherical 130mm mirrors become acceptable (roughly 0.22 wave). At F/ 6.9 this is not yet the case. The difference in curvature as shown in the Ronchi test would be minimal, however. The Faucoult test is a lot more sensitive

[davidc135]

I make the spherical aberration of a 130/900 sphere equal .34 waves;  acceptable at this price although I think the extra performance of 1/6 wave optics would tell in good seeing. One point that hasn't come up is the smoothness or otherwise of the mirror surface and which could be more relevant than whether it's a sphere or not. For example, Ben's Ronchigram appears to show zone(s) or it could just be turbulence as he says. Reducing the Ronchi lines to three will increase sensitivity as would a knife edge test.

David

Edited October 23, 2019 by davidc135

[Ben the Ignorant]

I just tried a star test and a Ronchi test on Capella, wanting to take pictures of them, but heavy haze and clouds interrupted me. The air around big masses of haze and clouds is agitated anyway, so even the so-so brief clearings were not good enough. I'll try again before dawn.

[Ben the Ignorant]

Still no luck with the weather but in the meantime I thought I'd show the thin spider the 130/900 got:

That was before the final tweaking and matte black painting that would make it more difficult to photograph. The cylinder holding everything together is divided in four sections by the vanes so it was more practical to place four collimation screws. They are small but the piles of serrated washers make for a good grip. They are moved by adjacent pairs, by the same angle, of course. So the secondary's motions are left/right, up/down, lined up with the vanes, simpler than the usual triple axis that doesn't match motion in two directions, which is always a two-axis arrangement except in the strange world of telescope collimation.

The cell had those annoying push-pull screws that required a different screwdriver for the collimation and the locking, if I remember well. (Or was it in my brother's Celestron newtonian?)

Anyway, those rubberized tool-less knobs now do the collimation, and the counterpush is provided by tough little springs. To keep them in place while I handle the cell, foam rings are put between the springs and the screws. Removing the cell from the tube is done with those same knobs, couldn't be simpler. The 130/900 is short enough that you can collimate it while you look inside the focuser. The mirror's bevel and edge have been blackened with a felt pen. Light scatter has been suppressed in the bottom of the tube but it still needs a few baffles along its length. The scope is very sharp and that warranted the extra work.

Edited October 25, 2019 by Ben the Ignorant

[Ben the Ignorant]

Can't get away from turbulence so the Capella defocus pictures were totally useless. But the Ronchi patterns are not magnified like the defocus test stars so, by averaging them in your mind you can see the main point in this discussion: the lines are not concave or convex, shapes which appear when the optics' overall curvature is not right. The lines are parallel and only a parabola aimed at a star can do that.

I might try another intrafocal/extrafocal defocus test before dawn, Procyon and Betelgeuse are bright and will be placed well in front of my east window. But, Steve, why don't you pull a couple 130/900's out of their cartons, and do a proper Ronchi and defocus experiment with a proper artificial star? If it's a 9µ the distance needs to be only about 40 meters; my flat doesn't allow more than 7 meters, or the scope would have to have 7 or 8 cm more in backfocus.

Edited October 25, 2019 by Ben the Ignorant

[John]

How do you collimate accurately without a centre spot on the primary ?

[vlaiv]

Question is can we tell the difference between parabolic and spherical mirror at that aperture and F/ratio?

According to telescope-optic.net here is expression for low order spherical aberration of mirror with a given conic constant:

Where K is mirror conic - in case of sphere it will be K=0, and therefore PV error will be:

0.888*130 / 6.9^3 = ~0.35

This is for 550nm, and that is more that 1/4 PV, or about 1/2.85 PV.

From expression it is obvious that parabolic mirror will have 0 low order spherical aberration (K=-1 for paraboloid).

No way that this sort of spherical error could be mistaken for parabolic mirror in a star test. Here is what Aberrator shows for defocus patterns, first the control - parabolic mirror:

Here is what 1/2.85 PV of low order spherical produces:

Star testing should really reveal that level of LSA and image will be soft - such scope is not diffraction limited!

Only "evidence" that I can provide having owned that scope (I don't have it anymore), would be simulated and real Jupiter images.

Aberrator produces this image of Jupiter (from high resolution base image) under ideal conditions (no other aberrations and no atmospheric turbulence) with that level of LSA:

Here is what image with parabolic mirror would look according to Aberrator under ideal circumstances:

This is actual image taken with said scope in real circumstances:

Surely not conclusive test - but I like it

It definitively shows that simulations are quite realistic. I would say that real image maybe, just maybe leans towards parabolic mirror simulation than to spherical one.

[vlaiv]

Btw, 130mm to have 1/4 PV with spherical mirror according to above formula, needs to be at least F/7.84, or about 1020mm FL.

[Ben the Ignorant]

34 minutes ago, John said:

How do you collimate accurately without a centre spot on the primary ?

The way they collimated their scopes for centuries before center spots, with a slight defocus that creates two or three rings around the central dot. This example is from an achromat I centered but it would look the same in a newtonian because a very small defocus pattern makes the secondary shadow disappear.

The fewer the rings, the more a decentering or miscollimation displaces the dot from the center of the pattern. This is more precise and foolproof than center spots, chesires, lasers of whatnot because it measures the relation between the optics and nothing else. The method works for Schmidt-Cassegrains, newtonians, refractors, and maks.

 

[vlaiv]

Just realized that Aberrator has simulated Ronchi test, and that can't really be used to distinguish the two (or maybe it could by someone really experienced), here are sims of parabolic mirror and 0.35 wave spherical :

[Merlin66]

Vlaiv,

The bottom Rochi definitely shows "wavy" lines........

[Cornelius Varley]

8 hours ago, Ben the Ignorant said:

The lines are parallel and only a parabola aimed at a star can do that.

Ben, you are misunderstanding/misrepresenting your results.  Below are two images showing the ronchi tests of a typical spherical and parabolic mirror. The first image "A" shows parallel lines, the other image "B" shows curved (parabolic) lines. Which image shows the results of a spherical mirror ?

Image A

Image B

 

[johninderby]

If you really want to understand optical testing get hold of this book or a similar publication.
Field Guide to Inrerferometric Optical Testing

https://www.amazon.co.uk/Field-Guide-Interferometric-Optical-Testing/dp/0819465100

 

[FLO]

On 26/10/2019 at 01:38, vlaiv said:

Question is can we tell the difference between parabolic and spherical mirror at that aperture and F/ratio?

What he said ^ 🙂

Newtonians with longer focal ratios produce less spherical aberration so a correcting parabolic mirror is less necessary. A spherical mirror can be used instead. This is good because it is easier for the manufacturer to grind/polish a spherical mirror to a high surface accuracy. 

This is why Sky-Watcher’s Explorer 130/130M does not have a parabolic mirror. 

I think the OP’s question was answered some time ago. 

Steve 

[Peter Drew]

When star testing, you are testing the final wavefront which is spherical regardless of the type of optics, hence straight lines when all is good.   🙂

[vlaiv]

There is another way to go about this - doing real test on this scope. It's going to be a bit involved, but we will get detail on that particular sample.

One test that can be done by owners themselves that is a bit involved but not out of the reach of amateurs is Roddier analysis. If you have planetary camera and a bit of planetary imaging skills you can do it.

[Ben the Ignorant]

6 hours ago, Merlin66 said:

The bottom Rochi definitely shows "wavy" lines

That's the sign of zones, not spherical aberration.

Anywhere you look, wavy lines mean zones. German, italian sources, all of them. Look for the wavy pattern on this page:

https://www.otticasanmarco.it/supporto.asp?faq=36

The comment reads: 

Anello rialzato, zona circolare sotto corretta

Meaning "Raised ring, circular undercorrected zone".

The Ronchi tester sold by FLO has the same leaflet, and it says the same, as Steve will confirm if he reads this.

[Cornelius Varley]

The OPs question has already been answered by a retailer and the official UK importer of Skywatcher telescopes. The SW130/900 does have a spherical mirror. Orion Telescopes in the US also retail the same telescope as the Space Probe 130 EQ. In their spec sheet the primary is listed as a spherical mirror. The fact that the primary definitely is spherical does not compromise the optical performance of telescope compared to the 130/650 parabolic version. No more can be gained by simply going around in circles by claims and counter claims.  The thread is now locked.