Strehl ratio – what does it really tell you?

[Guest Gneiss]

Strehl ratio is becoming yet another one of those figures that is freely banded about in advertisements. But what does it really tell you?

It is in simple terms the ratio between the amount of light brought to focus at a specific point (the Airy disc) and what should be there in a theoretical perfect system. Therefore the best it can be is 1, which is sometimes expressed as 100.

So the higher the number the better the system, all very simple…

Or is it?

Let me give you a telescope that will take the world by storm; I’ll call it the ‘big shiny one’, because it will be big and shiny.

First I pick the optics and let’s say TMB do a nice big lens with a Strehl ratio of 0.98 a nice big figure to go with my big lens.

Next the tube, well I called it big and shiny so big and shiny it will be. So I have my tube chromed and polished to a highly reflective finish inside and out.

Baffles? I don’t need them, they look ugly and people might think they were stopping down my nice big lens.

Finally my focuser, this will of course be big and shiny but I’ll make it in three segments so it will fold away neatly just like the sea captains scopes from the good old days.

My design complete, I take the prototype to the optics lab and test it out. Sure enough my tests show that .98 of the light ends up just where it should be and I go to market with my world beating design.

“THE WORLD BEATING BIG SHINY TELESCOPE STUNNING OPTICS Strehl ratio 98!!!!!!!!!”

Shortly afterwards I read the eagerly awaited first review…

“The images were dire, we couldn’t even see the Moon”

“the focuser was atrocious with at least 5 degrees of image shift”

“Don’t touch the big shiny telescope with a bargepole”

OK, so this is somewhat satirical and the big shiny telescope would never actually make it to the market. But it highlights the fact that you shouldn’t base your buying decision on such figures alone. In the example above a well-designed scope with a Strehl ratio .85 would actually perform considerably better.

Back in the real world, would a scope with a scope with a Strehl ratio of .92 be any better than the scope with a Strehl ratio of .85? The answer is potentially it could be, but equally it may not be…

Any optical system is only as good as the weakest link and not everything can be determined from the figures quoted in the advertisements. The best optics in the world placed in a poorly designed tube with a wobbly focuser will not make a great scope!

The message is simple, look at the whole picture and if possible look through a sample of the scope - if you like what you see then these figures become irrelevant.

One of the best views I have ever seen of Saturn through any small amateur telescope were those produced by a humble 6” f/8 Newtonian. I have no idea what the Strehl ratio is or the wavefront error and nor do I care, because the views of Saturn on one night of perfect seeing will stay with me for a lifetime and that is what amateur astronomy is all about!

[Kaptain Klevtsov]

One of the best views I have ever seen of Saturn through any small amateur telescope were those produced by a humble 6” f/8 Newtonian. I have no idea what the Strehl ratio is or the wavefront error and nor do I care, because the views of Saturn on one night of perfect seeing will stay with me for a lifetime and that is what amateur astronomy is all about!

So you've finished then, well done.

I'll get my coat.

Captain Chaos

[Astroman]

This is a great post! An excellent example of over-technical-izing the purchase of a telescope. It falls in the ralm of audiophiles searching for that extra 0.01% Total Harmonic Distortion they will never actually detect.

Conversely, for those of us that design and build telescopes, knowing we have a really fine piece of glass (of Strehl ration >0.8), it gives us something of a barometer for the success or failure of our design. If we can't see the Moon after hours of toil, we know we really screwed up something. If we've done due diligence in design and craftsmanship, we'll be assured of a great instrument.

My point I guess is the same as Gneiss'; don't be overwhelmed by minutae. Take in the whole picture.

[Kitsunegari]

 

 

 

 

Here is my "big silver shiny zero baffle scope" that i point at the sun and get the images below.. .         250 years ago people literally mounted  huge optics to a roof, tied a string to it, and literally tied the eyepiece to a string dangling from said roof optic. and brought it to their eye.  No tube. No baffles. Nothing but 20 feet between them and the optic.     You would be pretty surprised how good a 4000mm focal length optic pointed at the moon looks through an eyepiece attached to a string......

 

Strehl ratio is the division line factor of  "are you ever gonna take a picture or just use a 25mm eyepiece."               

 

 

 

 

 

 

[John]

This thread is 14 years old !

I'm amazed that the mount, and EQ5 clone, handles such a large, long, heavy optical tube

Amazing images though !

 

Edited November 29, 2020 by John

[Kitsunegari]

4 minutes ago, John said:

This thread is 14 years old !

I'm amazed that the mount, and EQ5 clone, handles such a large, long, heavy optical tube

 

I love looking at old posts and reviving them!  Just seeing  the stargate telescopes by explore scientific really shows not much is needed other than the optics!

 

 

 

I do not have any problems with the mount unless the wind kicks up to about 20mph,  the entire scope weighs about 11kg.  

 

Edited November 29, 2020 by Kitsunegari

[John]

Well, for nighttime observing I've found that light shrouds are needed otherwise the wonderful optics show lowered contrast due to ambient light intrusion.

The solar image you post are H-alpha I believe - what H-a filter do you use to get them ?

 

[John]

Also, what has this to do with strehl ratio ?

 

[Kitsunegari]

3 minutes ago, John said:

Well, for nighttime observing I've found that light shrouds are needed otherwise the wonderful optics show lowered contrast due to ambient light intrusion.

The solar image you post are H-alpha I believe - what H-a filter do you use to get them ?

 

Works great for night time too    

 

 

 

The first image is h-alpha,  the second is calcium k-line.

 

For the first image its a modified Lunt LS50C etalon that i put in a shroud for internal use. (normally is intended for external)

 

That particular calcium filter is something that I developed and cannot be purchased (was my own development).    Its called a Skybender.

[Kitsunegari]

6 minutes ago, John said:

Also, what has this to do with strehl ratio ?

 

Strehl ratio changes with color.  

 

Usually telescopes are rated for one color of light,  typically green.             The hard truth here, is that a triplet / quadruplet telescope that was designed for visual use at 450nm -680nm  maybe absolute garbage at 390nm.         (very important to solar observation)

 

Spectrally a little different for mirrors because they perform the most  amazing at blue light,   however spherochromatism is going to make or break your enjoyment with imaging and high magnification....            

 

[Kitsunegari]

And yea, People really did use a string as a focuser...  

The Era of Long Telescopes | SpringerLink

[markse68]

32 minutes ago, Kitsunegari said:

And yea, People really did use a string as a focuser...  

The Era of Long Telescopes | SpringerLink

Fascinating stuff! Not very practical though! In my new oldest book (270yrs!) the author uses a 126foot telescope of Huygens! I think it must be of similar design to your image. I’m wondering what S and N are in the image? Some sort of inertial dampers? Your scope looks amazing too!

Mark

[Ags]

I think S and N are a counterweight for the objective lens.

[John]

8 hours ago, Kitsunegari said:

Strehl ratio changes with color.  

 

Usually telescopes are rated for one color of light,  typically green.             The hard truth here, is that a triplet / quadruplet telescope that was designed for visual use at 450nm -680nm  maybe absolute garbage at 390nm.         (very important to solar observation)

 

Spectrally a little different for mirrors because they perform the most  amazing at blue light,   however spherochromatism is going to make or break your enjoyment with imaging and high magnification....            

 

Thanks for explaining that.

The test on my 130mm refractor was at 532nm which is green I believe.

Polystrehl ratings are talked about quite a lot these days but you hardly ever see test reports that cover this.

I have seen drawings of those string tubed refractors before. It would be fun to make one if the space was available !

That is a very unusual looking scope you have. It reminds me of a design that Istar used a few years back:

 

 

Edited November 29, 2020 by John

[markse68]

3 hours ago, Ags said:

I think S and N are a counterweight for the objective lens.

I think you’re right Ags - N to balance the objective and s to balance the eye lens about the pivot Q. I think the little dog leg L on the end of the objective beam is to compensate for the string sag and keep the objective pointing down a little from the line of tension? And does it also keep the objective cell from tipping sideways in the neat looking ball joint M? Wonder why they used a ball joint though?

Mark

[Ken82]

As mentioned previously strehl ratios are mostly measured in the green spectrum for visual use.

From what i have learnt above 90 strehl is really fine for astrophotography.

I have seen some certificates from LZOS scopes with strehl of 98% in green and 86% in blue.

I believe a strehl of 0.8 is diffraction limited so any aberration error will be smaller than airy disk. And of course strehl plays no role in chromatic aberration!

Ken 

 

 

[Kitsunegari]

4 hours ago, Ken82 said:

As mentioned previously strehl ratios are mostly measured in the green spectrum for visual use.

From what i have learnt above 90 strehl is really fine for astrophotography.

I have seen some certificates from LZOS scopes with strehl of 98% in green and 86% in blue.

I believe a strehl of 0.8 is diffraction limited so any aberration error will be smaller than airy disk. And of course strehl plays no role in chromatic aberration!

Ken 

 

 

If i am not mistaken ,  LZOS built  the best lens ever manufactured outside of a government agency.      I would love to see what adding LIDAR could do for a cnc milling/polishing process.    Probably would never need an interferometer report....

 

Especially after the magic formula was cracked   How One Mathematician Solved a 2,000-Year-Old Camera Lens Problem (popularmechanics.com)

 

 

[John]

LZOS objectives often show as a touch better than the manufacturers supplied tests, when they are tested independently. The manufacturers test is just to demonstrate that they have met the minimum standards required I believe.

This is the specification that APM agreed with LZOS for the supply of objectives:

APMApo-Linsen-Spezifikationen.pdf

Edited November 29, 2020 by John