Last updated: 3 November 2000

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From:	brians@mdbs.com (Brian Straight)

A Rant About Aperture

Let me begin by saying that the views expressed here are very much my
own personal opinions, supported by my own experiences using a variety
of telescopes.  I know that many have had different experiences, and I
respect other points of view.  However, this is a subjective rant on the
subject of aperture.

Go to any astronomy mail group, and you'll see variations on the theme
of 'aperture is everything.'  You'll read posts from people who say that
they could see vast differences between an 8 inch and 10 inch scope
"'The images are so much brighter and sharper...").  And you'll also see
posts from disappointed individuals.  These folks are usually new to
astronomy. They've seen the pictures in magazines, and they've seen the
images on the manufacturer's boxes.  And a surprising number of them
have thousands of dollars to plonk down on 12 inch LX200s and the like. 
And what do they ee?  -- fuzzy, monochromatic clouds instead of swirling
galaxies and bright, coloful nebulae.  If you read no further in this
rant that this, I would simply say--unless you are preapared to invest
in a scope with more than 20 inches of aperture, you will NOT see any
color in deep sky objects.  Even a 20 inch scope will show only the
faintest hint of it in a bright object like the Orion Nebula.

I would argue that, for the general, amateur observer, the telescope
companies greatly oversell the concept of aperture.

It is undeniably true that the bigger the aperture, the more light a
scope collects and the brighter the image.  For example, a five inch
scope collects more than twice as much light as a three and a half inch
scope; an eight inch scope collects over twice the light of a five inch
scope.  Sounds vast, doesn't it?

But what does it mean in terms of magnitude?

An ETX90 will reach magnitude 11.7; a 125, magnitude 12.5, and an 8 inch
scope, magnitude 14.0.  In other words, you gain about a magnitude at
each aperture step.  Visually, that's not a great deal--think about the
difference between and mag 6 star and a mag 7 star through your
scope--not really a huge difference is it? Throw in some skyglow, and
you'll find that sometimes larger apertures are swamped by the
background light, producing lower-contrast images than smaller apertures
under the same conditions.

I'm blessed with good vision (20/10), and, although I'm in my 40s, my
optometrist tells me my eyes open surprisingly wide in the dark; I
rountinely see 9 or 10 stars in the Pleides with my naked eye.  At a
recent outing of our local astronomy group, I got to compare the same
objects in a number of different scopes.

Here are my notes on one object::

Ring Nebula:

ETX 125  -- clearly visible.  Ring structure apparent in 26mm eyepiece.
Some texture visible in the ring.

6 inch refractor (f/6):  clearly visible, though rather small and
condensed in the 26mm eyepiece. Did not seem as bright or well-resolved
as in the ETX 125, which is surprising.   Ring structure clearly
visible.  Glimpsed some texture in the ring itself.

8 inch SCT:  somewhat brighter than the refractor; and it seemed to me
to be very similar to the ETX 125.  Perhaps a little brighter, but not
by much. Image scale was better than the refractor with a 26mm eyepiece
due to longer FL.  Could see ring structure a little more clearly.
Texture remained about the same.

16 inch SCT:  not appreciably brighter than the 8 inch, but a larger
image scale (about 2X the size--this probably accounts for much of the
difference in brightness)..  Texture not as visible as in the 8 inch;
contrast seems lower (due to skyglow?).

The point is--I didn't get my socks knocked off by the 16 incher.  The
image was larger and a little brighter than the ETX 125, but would I pay
$14,000 for the difference?  Absolutely not!

Manufacturers also tell you that with larger aperture comes more
resolution. An ETX 90 will resolve stars 1.3 arcseconds apart; a 125,
0.9 arcseconds; an 8 inch scope, .56 arcseconds.  In all, less than an
arcsecond separates them all.  Where I live in Indiana, the seeing is
rarely better than 2 arcseconds anyway, so that larger aperture doesn't
really give me that much advantage. I suspect most people living in the
"moister" parts of the world will have the same experience--it's amazing
how water vapor mucks things up!   Also note that alignment of optics is
very important.  MAK designs like the ETX are nearly always in perfect
collimation.  SCT's will probably need frequent "tweaking" to maintain
collimation and performance.  A small, well-collimated scope, will
always outperform a larger scope which is imperfectly collimated.

In addition, larger scopes are much more susceptible to air turbulence.
Research done by Purdue University indicates that an 8 inch scope is
about the maximum that can be usefully used on most nights here in
Indiana. Unstable air currents negate the advantage of aperture beyond
tihis size.

What do I conclude?

You'll find some advantages to a bigger scope, escpecially if it's on a
good, permanent mount.  But don't expect to see huge differences in the
images in your eyepiece.  By the time you subtract seeing conditions and
collimation factors, you might just find that your small scope does a
better job of showing you the sky than that monster dob the telescope
company is trying to sell you.

Clear skies!

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Copyright ©2000 Michael L. Weasner / etx@me.com
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