Spherical or parabolic?

[Cornelius Varley]

11 minutes ago, Ben the Ignorant said:

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.

Your own test shows parallel lines, which indicates a spherical surface to the primary mirror.

[Ben the Ignorant]

1 minute ago, Cornelius Varley said:

8 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.

If it was my 130/900 would show spherical aberration but it shows none. Designations don't change what optics do so there has to be a mistake in the apparently outdated letter designation. Wouldn't be the first time, at some point an FPL-51 scope was labelled as FPL-53 by mistake and TS apologized and corrected the error.

[Ben the Ignorant]

Then why are these three parabolic mirrors from the page I already linked showing straight lines? Because professional opticians like Wolfgang Rohr and Carl Zambuto don't know what they're doing?

[Stu]

14 minutes ago, Ben the Ignorant said:

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

I'm sure they don't, so unless there has been a change of spec, it looks like yours is spherical. I wonder if @FLO can answer the question?

[johninderby]

Interesting thread on CN about Ronchi testing.

https://www.cloudynights.com/topic/371739-are-ronchi-eyepieces-useful-for-checking-mirrors/

[Peter Drew]

These don't look like parabolic images gained from basic Ronchi or Foucalt tests. To me they look like null tests where the parabola gives a simulated spherical image by introducing corrective optics, the Dall null test. I used a similar setup on my optical bench when I was a professional telescope maker.     🙂

[j4y7223]

😐🔫

[Cornelius Varley]

22 minutes ago, Ben the Ignorant said:

If it was my 130/900 would show spherical aberration but it shows none. Designations don't change what optics do so there has to be a mistake in the apparently outdated letter designation. Wouldn't be the first time, at some point an FPL-51 scope was labelled as FPL-53 by mistake and TS apologized and corrected the error.

At these sizes the difference in optical performance between spherical and parabolic mirrors is negligible. A good small spherical mirror is easier to make than a similar sized parabolic. This is also why the 130/900 is relatively cheap.

[John]

2 minutes ago, j4y7223 said:

😐🔫

Fair point

To get back to your original question, at the focal ratio that the 130 / 900 is operating at, I think a spherical figure will work fine.

I've used one of these scopes (unmodified) and it performed pretty well for it's aperture on a wide range of targets.

 

[johninderby]

Re: TS...........Wouldn’t be the first time copying product descriptions leads to the wrong specs being shown. 

[Ben the Ignorant]

From the Cloudy Nights link that johninderby posted:

Actually for a parabolic mirror: when tested using a Ronchi grating eyepiece, the image of the lines WILL be straight, as object(star), is at virtually infinite distance, (light is coming in as a cylinder, with parallel sides), so focuses at focal plane, whereas, when using the grating as a bench test, grating and light source, are located at (near), the "Radius of curvature"(2x "focal length"), of the mirror. In this case, the light incident to the mirror, arrives as a diverging cone. Mirror surface modifies this cone.So the returning light beam, forms the reverse identical cone (converging).This cone of light is less steep (1/2) than the cone coming from an infinite light source, hence the image of the lines take on the curved shape of the classic parabola.

 

All telescopes, no matter the complexity and whether aspheric elements are present or not, should present parallel and evenly spaced Ronchi bands on an infinitely distant target. All are interpreted identically, for one is examining only the final wavefront--which is nominally spherical--near the focus.

I'm obviously not removing the mirror when I test my scope, so parallel lines it has to be, as in the photo I took. Turbulent but parallel.

 

[Peter Drew]

It is quite easy to mix up the results of Ronchi testing without knowing how it was performed.  If testing a mirror whilst working it, the usual procedure, the Ronchi screen will be at the radius of curvature and a parabolic mirror will exhibit convex lines. If a parabolic mirror is tested on a star at infinity, the Ronchi screen will produce straight lines if all is well.  I think this is also highlighted in the CN discussion.     🙂

[Alkaid]

I wouldn’t get too hung up on spherical vs parabolic....the small TAL-1 uses a spherical mirror and produces excellent images. 

[Peter Drew]

I also have a TAL-1 telescope with a spherical mirror, the performance is outstanding for the aperture.   🙂

[Chris]

If the aperture is small and the focal length is long as is with the scope you mentioned you can get away with a spherical mirror. As the aperture gets larger and the focal length becomes shorter it becomes important to figure the mirror into a parabola in order to bring all light to a single focus.  E.g. if you had a 6" f/5 scope I think a parabolic mirror would noticeably out perform a spherical one. Or at least this is my educated guess  

[Ben the Ignorant]

From Jean Texereau's How to Make a Telescope:

 

To satisfy the Rayleigh criterion, therefore, the defect on the glass may not exceed 0.14 / 2 micron. That's 0.07 micron.

 

And:

If the f number is large enough, i.e., if the mirror is relatively shallow, the difference between the parabola and sphere becomes very small; in fact the uncorrected spherical mirror may satisfy Rayleigh's rule and give a practically perfect star image.

 

BUT:

The Rayleigh criterion must be qualified for low-contrast images. The situation here is much less favorable than in viewing a star. Francon, studying planetary detail of minimum visible contrast, found that a wavefront of 1/16 lamba begins to be objectionable. This implies a tolerable error in the mirror surface of 0.02 micron. The difficulty here is not in making the mirror to this precision,...

 

So, separating star dots with a mirror deviating from the ideal parabolic shape by 0.07 micron is possible but solar, lunar and planetary detail is not made of bright round disks separated by a hard-contrast black space. It is more subtle, and continuous so it requires a 0.02 micron accuracy mirror, a 3.5 times smaller tolerance.

[Cornelius Varley]

48 minutes ago, Ben the Ignorant said:

From Jean Texereau's How to Make a Telescope:

 

To satisfy the Rayleigh criterion, therefore, the defect on the glass may not exceed 0.14 / 2 micron. That's 0.07 micron.

 

And:

If the f number is large enough, i.e., if the mirror is relatively shallow, the difference between the parabola and sphere becomes very small; in fact the uncorrected spherical mirror may satisfy Rayleigh's rule and give a practically perfect star image.

 

BUT:

The Rayleigh criterion must be qualified for low-contrast images. The situation here is much less favorable than in viewing a star. Francon, studying planetary detail of minimum visible contrast, found that a wavefront of 1/16 lamba begins to be objectionable. This implies a tolerable error in the mirror surface of 0.02 micron. The difficulty here is not in making the mirror to this precision,...

 

So, separating star dots with a mirror deviating from the ideal parabolic shape by 0.07 micron is possible but solar, lunar and planetary detail is not made of bright round disks separated by a hard-contrast black space. It is more subtle, and continuous so it requires a 0.02 micron accuracy mirror, a 3.5 times smaller tolerance.

All very interesting. But can we stay with the OPs question ? The 130/900 has a spherical mirror (this is common knowledge) and there is no noticeable difference in performance between it and the 130/650. You, yourself, have already mentioned that the 130/900 is a good performer.

[johninderby]

Yes the subject of spherical vs parabolic mirrors would make an interesting thread in it’s own right but it is indeed getting away from the OPs question.

[FLO]

On 19/10/2019 at 16:34, 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?

It has a spherical mirror. Some mistakenly think spherical mirrors are inferior but when used with long f-ratios (around f7 or longer) then a spherical mirror is often the better option. This is because Newtonian telescopes with longer focal ratios suffer less from spherical aberrations and it is easier (cheaper) for a manufacturer to grind/polish a spherical mirror to a high optical standard. 

HTH, 

Steve 

[Ben the Ignorant]

I asked this to TS:

Hello.

   Can you please answer a question about the Sky-Watcher 130/900 newton? ( https://www.teleskop-express.de/shop/product_info.php/info/p4778_Skywatcher-Explorer-130---130-900-mm-Newton---optischer-Tubus.html )

   Some people in a forum say it has a spherical mirror but I star-tested it, and the main rings (the outer ring and the ring around the secondary's shadow) have the exact same brightness on both sides of focus, which pleased me a lot when I saw it. No spherical aberration is showing at all.

   With the 10 lines/mm Ronchi I recently bought from you, the lines are totally straight and parallel, which only a parabolic mirror can do when looking at a star at infinity. It was specifically advertized as parabolic when I bought it in 2015, and that's why I chosed it; it proved very sharp on the Moon, more than my Celestron 5. I made a smaller secondary holder and a thinner spider for better contrast.

   Despite that, some are still saying all the 130/900 have spherical mirrors but I believe a spherical mirror couldn't be that sharp and contrasty. So, what do you say?

   Thank you for your attention.

 

And that was their answer:

 

Hello,

it has a parabolic mirror as all newtonians well made. A spherical mirror will introduce spehrical aberration if there is not a corrector plate (schmidt-newtonian or schimidt-cassegrain). as you well noted, you have no spherical aberration due to parabolic mirror

Freundliche Grüße | Kind regards

Riccardo Cappellaro

 

Someone from this forum had asked the same question in the year I bought the 130/900...

...and got this answer:

I have had a reply from Edgar at TS Optics who concedes that the "ff7.9" figure is incorrect and will be changed to F6.9, but  also says "yes it has a parabolic mirror!"

 

Skypoint says the 130/900 is paraboloidal (which is the right term but everybody says parabolic for short)

https://www.skypoint.it/en/reflecting-telescopes/skywatcher/9623-skywatcher-130900-eq2-newtonian-telescope.html

 

And someone from an older thread here has this to say:

Spherical mirrors will work if the focal length is high enough (and has a theoretical Strehl of 0.80 or higher).

A 130 900 will not have 0.80;

100*e^-(2*pi*( (130/2)^4/(2*900)^3/16/0.00056)/3.5)^2 = 68.65

And this is not factoring in obstruction, roughness of the mirror, bad edges and other errors, but it purely the theoreticly possible.

 

A 68% Strehl would not be acceptable, and if Sky-Watcher couldn't make an affordable 130mm f/7 parabolic when they can make affordable 130mm f/5 parabolics which are a bit harder to produce, that would be strange. Not to mention how do they figure the large ones that are well-made and not expensive?

The 130/900 TS sold me clearly has identical diffraction rings on both sides of focus and straight Ronchi lines, and only a parabola can provide that. A slight deviation from the ideal parabola shows in the tests, so no parabolizing at all would show more but it doesn't.

So, Steve, can you ask your provider(s) about that, and can you star-test and Ronchi-test one of your 130/900's to sort this out? You do have Ronchi gratings so that should be easy.

https://www.firstlightoptics.com/specialist/gerd-neumann-ronchi-eyepiece.html

[FLO]

1 hour ago, Ben the Ignorant said:

So, Steve, can you ask your provider(s) about that...

I wasn't aware we were discussing something sold at a German retailer... 

We are a UK retailer and SGL is a predominantly UK forum. I can only speak for what is sold in the UK (we don't pay too much attention to retailers in other countries). 

Having said that, I know German retailers source Sky-Watcher from the same distributor we do (Optical Vision Ltd).  UK retailers don't offer the Explorer 130 OTA but we do offer it on a tripod: 

https://www.firstlightoptics.com/reflectors/skywatcher-explorer-130.html

Also available at your German retailer: 

https://www.teleskop-express.de/shop/product_info.php/info/p1037_Skywatcher-Explorer-130-on-EQ2---130-900mm-Newtonian-Telescope.html

But on their product page they say it has a parabolic mirror. Their price is £25 higher than ours (£155 at FLO, £180 at the German retailer). Their higher price would make sense if they have specified a parabolic mirror but Synta/Sky-Watcher don't normally offer variants (though it is possible) and a parabolic mirror isn't necessary for an f7 reflector. I will ask Optical Vision Ltd. 

HTH, 

Steve 

[Alkaid]

58 minutes ago, Ben the Ignorant said:

 

And someone from an older thread here has this to say:

Spherical mirrors will work if the focal length is high enough (and has a theoretical Strehl of 0.80 or higher).

A 130 900 will not have 0.80;

100*e^-(2*pi*( (130/2)^4/(2*900)^3/16/0.00056)/3.5)^2 = 68.65

And this is not factoring in obstruction, roughness of the mirror, bad edges and other errors, but it purely the theoreticly possible.

 

 

I don't think that's quite right?  A diffraction limited optic must have a minimum strehl of 0.8  (80%) and this is the minimum benchmark for the manufacturer.  A scope delivering 68% or 0.68 would not come to focus??  The 130/900 clearly does actually work and delivers good images, so it definitely has a strehl of at least 0.80.

[FLO]

3 hours ago, Ben the Ignorant said:

So, Steve, can you ask your provider(s) about that...

 

2 hours ago, FLO said:

Their higher price would make sense if they have specified a parabolic mirror but Synta/Sky-Watcher don't normally offer variants (though it is possible) and a parabolic mirror isn't necessary for an f7 reflector. I will ask Optical Vision Ltd. 

Optical Vision Ltd have replied, they say it must be a mistake. "Nothing has changed... it is and has always been a spherical mirror on that f/6.9 model". 

At this point because you are discussing a German retailer, not FLO, I will bow out of the conversation. 

Steve 

[FLO]

19 hours ago, FLO said:

It has a spherical mirror. Some mistakenly think spherical mirrors are inferior but when used with long f-ratios (around f7 or longer) then a spherical mirror is often the better option. This is because Newtonian telescopes with longer focal ratios suffer less from spherical aberrations and it is easier (cheaper) for a manufacturer to grind/polish a spherical mirror to a high optical standard. 

Worth stressing that part I think 🙂 

[Ben the Ignorant]

4 hours ago, FLO said:

Their price is £25 higher than ours (£155 at FLO, £180 at the German retailer). Their higher price would make sense if they have specified a parabolic mirror but Synta/Sky-Watcher don't normally offer variants (though it is possible)

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. 😁