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Optical Quality
By: Fred B. Rayworth
I've heard it a hundred times. “My mirror
isn't 1/10th wave, it must be junk!” Such statements are common to the
uninformed. For those that are new to the hobby, you probably do not even
know what 1/10th wave means, let alone why that would upset anyone.
Therefore, this article is a discussion on optical quality ratings; what it
really means to the telescope user. I will try not to get too technical with
this explanation. For simplicity, we will be talking about mirrors for
reflecting telescopes but the principles hold true for any optical system
and must include the diagonal mirror, corrector plate, and any other
elements in the light path.
A Newtonian reflector telescope takes light from a star and reflects it back
to an eyepiece after bouncing off a smaller mirror that bends the light so
that the eyepiece will be in a convenient spot. Well… there is a little more
to it! To reflect light to an eyepiece, the mirror must have a concave
reflective surface. This “bowl” shape requires a certain accuracy and
smoothness so that the eyepiece can focus the image.
If a mirror has a perfectly round bowl shape, it is a sphere. In other
words, the curve of the mirror would be a perfect fit for a large ball that
matched the curve radius. If the telescope has a long focal length, beyond a
certain range, the mirror will produce a decent image with an eyepiece.
Unfortunately, with a telescope of 8” or larger, this would be a very long
telescope. An 8” mirror would require a focal length of 80” or more to get a
decent image with a spherical surface. Now just imagine a 16” mirror. It
would have to be over twenty feet long to get an acceptable image with a
spherical surface!
A spherical surface reflects light differently from the center to the edge
of the mirror. On a short focal length mirror, the light at the center of
the mirror focuses at one point, and the light at the edge focuses at a
different point. Spherical aberration is the difference between these two
points and affects how the image focuses. The less the spherical aberration,
the sharper the image will be. So what do we do to eliminate, or minimize
the spherical aberration? There are two choices. One is to make the
telescope long enough where the spherical aberration is so little it does
not affect the image. This maximum aberration is the Rayleigh limit, or ¼
wavelength of light. That is about 1/300,000th of an inch on the mirror's
surface! The difference between the focus of the center and edge of the
mirror many not exceed ¼ wave to get an acceptable (sharp) image. We will
talk about the reality of that a little later.
Now there has to be a way to minimize the spherical aberration of a mirror
if you do not want the telescope to be twenty feet long! The second choice
is to flatten the sphere slightly which turns down the edge and creates a
slight hump in the middle of the mirror (greatly exaggerated description),
an effect that creates a parabolic shape. The term for turning a perfect
sphere into a parabola is “correction.” A fraction expresses correction and
the smaller the fraction, the better the correction of the mirror. In other
words, if a mirror is corrected to 1/10th wave, in theory it is going to
focus sharper than one that is corrected to only ¼ wave.
To complicate the equation, not only does the edge and center of the mirror
have to focus within a certain range, so does everything in-between. A
mathematical formula determines this curve from the outer edge of the mirror
to the center in zones. The mirror will perform better the smoother the
transition is from the center to the edge. A ¼ wave mirror with a smooth
surface (transition between zones) is likely to perform better than a 1/10th
wave mirror with a rough surface. There are also factors like astigmatism
(where the mirror surface is not perfectly round), bad or unfinished surface
smoothness, and zonal defects that do not show up on the regular tests. All
of these factors affect the result at the eyepiece.
So now, someone star tests your mirror and says your telescope optics are
crap. Do they know what they are talking about? This is a very highly
subjective opinion, and I'll explain why.
As with any hobby, there are groups that focus on one subject for whatever
reason. In this case, I am talking about the “opticians” versus the users.
An early example: I attended the first or second Riverside Telescope Maker's
conference in 68 or 69 on Mt. Pinos, in California. There was a modest array
of telescopes situated around the parking lot at the top of the mountain. I
walked around looking at telescopes, comparing my first 8” homemade
reflector to a variety of other sizes. The big discussion that weekend was
optical quality. My mirror (which I still have in storage) is an f/9.44 that
could almost be left at a spherical shape. It is actually slightly
parabolized to correct the spherical aberration, small as it is. I estimated
my correction to be between ¼ and 1/8th wave. My friend had a 12 ½”
telescope that he corrected to 1/27th wave. I compared images of Jupiter in
many different scopes including my friend's and mine. The image was sharp in
every scope I tried. In fact, even at 8,000 feet, I could not tell much
difference in sharpness from one scope to the next, though surface detail
was understandably different depending on the size of the mirror (or lens).
One individual had a 6” reflector that he said he corrected to 1/100th wave.
I seriously doubted that was true and the image through it did not look any
sharper than a brand name scope that we knew had a supposedly “crap” mirror
in it! The owner of said telescope had the “bad mirror” tested and it was
about 1/3 wave! That correction does not even meet the Rayleigh limit, yet I
would have been just as happy with those images! Yeah I know, you optical
snobs, wherever you are, are out there are cringing as you read this!
Second example: I attended an unnamed star party in the nineties. I set up
my 16” homemade scope and had a ball, finding object after object. In this
group were several optical snobs (very similar to the Nagler or Schmidt-Cass
snobs). Their job was to go around, testing scopes and telling you in a
condescending manner what was wrong with your optics. To them your mirror
was crap if you made it yourself, if it was not made by one of the “big
boys,” or was not at least 1/20th wave corrected. I guess these guys were a
boon for telescope manufacturers because a few unsuspecting people were
suckered into thinking the telescope that they've been using for years, was
a piece of junk and they needed to replace it! Of course, my telescope was
not up to snuff, but I already developed my philosophy and will pass that on
to you.
My 16” mirror is far from perfect. I know this, and I accept that. I made it
the best I could for the conditions and circumstances, and though I have
toyed with the idea in the past, I am not going to refigure it. I have had
too many good years with it. I know the defects in my mirror and do not need
an optical snob to tell me what I already know. Not only that, on any given
night, with any other 16” scope, the images I get with my scope are just as
good as any other commercial or homemade scope that I've ever tried.
I have seen great images in scopes known to have bad mirrors and bad images
in scopes known to have super mirrors. A combination of factors makes an
optical system good or bad. Optical quality is only one of them. Sure, you
want the best optical quality you can get (or make) but the reality is that
the only important thing is what you see through that eyepiece. If you are
not happy with the image, it does not matter how good the optics are, you
will never be happy.
The optical snobs have their purpose. They encourage telescope makers,
amateur and professional, to make better optics. However, they can also be a
destructive influence. For example: You are happy with your telescope until
someone says the mirror has a “turned down edge” (or some other optical
“mumbo jumbo” defect). You start looking for things you did not notice
before, look harder at that telescope and wonder what you paid for. You look
through other comparable telescopes and may or may not see the same
problems, but now you have a bug under your shirt that your telescope is a
poor performer. It does not matter that you were happy with it for several
years! Do not fall into this trap!
Our club is fortunate that we do not have any optical snobs that I know of
in our midst. Most of our members seem happy with their scopes and that is a
good thing. We should not worry about 1/10th VS 1/20th wave optics. We
should worry about whether the telescope performs in a satisfying manner!
Does the image focus, does the mount wobble, and do the electronics (if
installed) work correctly?
The whole point of this article is to tell you not to fall into the optical
quality quagmire. If you are going to buy a telescope, look through it. Put
it through its paces and look at different objects. If you are not happy
with the images or operation of the scope, take it back! Once you are
satisfied with it, leave it at that! If someone wants to “test” your optics,
tell them to get bent! If the images just do not look right, you should not
buy the scope!
Now it is time for another dose of reality. The more average the night
conditions, the least variation you will see from one telescope to the next.
What I mean is that take an average night with average seeing conditions.
The sky is relatively clear, turbulence is not too bad, and there is a
slight breeze. There are three telescopes side by side of the same aperture
and focal lengths. One is 1/3 to ¼ wave corrected, one is 1/8th wave
corrected, and one is 1/10th wave corrected, all tested and rated by the
same method. The scopes all point at Jupiter. The chances are you will not
be able to tell the difference between the three scopes! Under perfect
conditions, things may be different, but come on now, how often are
conditions perfect? Maybe they are once a year at best! By the way, I am not
saying you should buy a poorly made cheap mirror to save money!
I have learned over the years that optical quality is a crapshoot on ANY
telescope, from ANY manufacturer! I have seen time and time again that no
matter what kind of quality control a company employs, they still make duds!
If you buy a telescope from a company with a great reputation for optical
quality, you expect your telescope to be a great performer. Well, that may
be true, usually, but when you take it out for first light, there is a
chance you will be disappointed with the images. This problem is true
especially with the higher production telescopes, the most popular brands
where they dedicate less time to the finer elements and more to the whole
system (or the extra doodads).
I compare buying a telescope to buying a guitar. No matter how expensive,
inexpensive, or how good or bad a guitar looks, the sound is what counts.
When you pick that thing up and play it, the money is in the sound: if it
sounds or plays like junk, it is junk! The same holds true for telescopes.
They may look great, be a great brand name, but making good optics is such a
touchy and fickle thing that until you aim it at the sky and take a look,
you are never going to know if it is any good.
The last thing I want to talk about is wave ratings used by commercial
mirror makers and telescope manufacturers. Here is another sales gimmick
they use to sell their product (like the fancy photos on the box taken with
Hubble). One company sells ¼ wave mirrors at a reasonable price. Another
sells 1/20th wave mirrors at almost double the price. Which is the better
deal? There is no simple answer to that because you have to know what that
little fraction REALLY means and I am not talking about the correction as I
discussed earlier. I am talking about HOW THEY DERIVE that fraction in the
first place! There has been much controversy over the years about mirror
correction and wave ratings. There are numerous ways to measure that wave
rating and some of them are almost meaningless when it comes to the image at
the eyepiece. Not all 1/10th wave mirrors are alike. Two 1/10th wave mirrors
may perform different from each other based on how that 1/10th wave
correction was derived! The point is the mirror should be “diffraction
limited.” That means the mirror meets or exceeds the Rayleigh ¼ wave
criteria. That is all. If the mirror is diffraction limited, it should
perform just fine for you. Once again, look through the scope on an average
night. If you like the image, it meets your diffraction limit! If not, take
it back or do not buy it!
What is the bottom line? Do not become hung up on optical quality ratings or
what someone else says about your optics; seeing is believing! |
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