What is Spot Size?

[rgm]

I am looking at both the classic achromatic and R30 scopes. The R30 has a smaller spot size. What is this, and does it make a difference in the image the scope produces?

[mikey cee]

That's a damn good question! Come to think of it we could have a thread devoted to interpreting interferograms, ronchigrams, RMS, P to V etc. etc. I wager most like myself haven't a clue to what half of the bad optical aberrations look like. Probably because I've never owned a dud. I'd love to see illustrations with explanations of spot sizes too. Show me a spot chart with those colored lines on a graph and I might as well look at Chinese script!! How about it Mike and Ales or anyone else in the know? Mike

[criverside]

I have always assumed when its mentioned on this site, its a reference to the "airy disk" and its apparent size. Hope that helps.

Craig

[criverside]

You also asked how it effects the Image, that can be more subjective. With things like what percentage of light falls within the airy disk ( spot ) and the remainder falling outside. depends on magnification, brightness of object and how sensitive the viewer is to Chromatic Aberration. Small spot size is one if a few things that have to be weighed, though it does sound nice. I have not seen any specs, it could be a big plus even if the other specs are only average

Craig

[rgm]

I must admit that for someone who has been in the hobby for 40 years, I have little working knowledge on optical testing and what the actually results mean at the eyepiece. In my ignorance, I am assuming that a smaller spot size means a smaller and tighter star image. For me, that is a plus. Hope to learn more.

[mikey cee]

I've always thought of myself as being the smartest rock in the pile. ;D I've always thought that the spot size illustrated how tight the red, green and blue CA was overlaid on the airy disc at the optical axis and at different distances out to the edge of field. Cigar?? How do you determine the strehl of a lens from am interferogram ? What is the difference between a stated strehl and the average strehl? Mike

[Mike]

There are a few good books to pick up if you really want teach yourself about optics. One really good one, in fact one of the best I've seen come along is "Telescopes, Eyepieces and Astrographs" written by Smith, Ceragioli and Berry. It's more than reading a bunch of dry optics. It's a great referrence book. I pick it up often. Any body interested in how their scope works, obviously more than just simple lens or mirror principles, will benefit from this book. Obviously, a smaller spot size is better. An example we use is that the R30 has a 30% smaller spot size than a classic achromatic lens of the same size and F ratio. Easy. The lens has the ability to focus light rays in a tight group. However, what most do not show is how the spot grows as you move away from that sweet spot in the center. A good example is the common Schmidt Cassegrain. I have a standard Celestron C14. The spot size is quite small when taken from the center of the optics. Start to move away from that center even a small amount and the spot expands quickly (focus is not as tight). Celestron sticks a field flattner in the optical train in their Edge HD series and that spot stays small out to a much greater disance (40mm). Long story short, I would like to see spot diagrams that show the spot from the center and how it progresses as you move outward.
The Strehl ratio is an expression of optical quality. Under perfect conditions, a telescope's objective focuses light from a distant star to a spot called the Airy disc. This disc has some definite, positive size, not because the telescope isn't perfectly made, but because of the very nature of light. The brightest part of the Airy disc is at its center. The more light the optics bring to the center of the Airy disc, the sharper the image is. The Strehl ratio is the amount of light that the optics put in the center of the Airy disc, divided by the amount of light that would have been put there, if the optics were perfect. A Strehl ratio of 1.0 signifies perfect optics. So we're all tickled if the Strehl ratio is .98 or .99 So, if someone says you have a lens that has a Strehl of .98 you understand that 98 percent of the theoretical maximum amount of light is going where it should go. And 2 percent of that light is going into the surrounding rings (of the airy disk) and contributing to a reduction in contrast. It is the measure of the fractional drop in the peak of the Airy disk. It's really pretty simple. An important factor in judging lens quality is surface roughness. With knife edge and Ronchi testing surface roughness can be judged by a skilled worker making visual estimates according to his experience. But with interferometry, roughness can actually be measured. Interferometry is a powerful tool in determining what constitutes the quality of an optic. Analyzing an optic by measuring a large number of data points results in a measurement that contains the data necessary to calculate the Strehl ratio and the impact of roughness on the overall performance of an optic. The great value of the Strehl ratio is that it expresses an intuitive and memorable figure: 1.0 is perfection, .95 is extremely good, .9 is good, .85 is OK and so on.
I've been in optics and the production of optics most of my life. I hesitate to post this kind of information because I end up in an exercise in semantics with some self proclaimed optical engineer. So don't bother please. I do however appreciate that some just want to educate themselves a little and I hope this helps.

[mikey cee]

Mike I see most if not all strehls stated as "strehl xx" but Istar states it as "average strehl xx". Is one statement say in green light and the other an average among RGB light together? Hell I'm just guessing. Mike

[Mike]

Average Strehl ratio is taking several readings across the lens and then averaging to get the final figure. It's a little more complicated than this as there is some math involved but you get the idea. If you didn't average several readings you could use just the one that was the highest.