Expert: Paul Wagner - 7/14/2005

Question
I have several questions, but will pester you with only a couple here. It seems that the refractors, for their apertures, deliver the best magnification. But since light-gathering power is of paramount importance, I have trouble understanding how a 4-inch lens makes possible as much power as an 8-inch mirror (give or take, I assume). Can't the larger mirror gather more light? Then why is an AP 4" as good as or better than a Meade 8" on bottom-line magnification?

Also, my understanding of how light-gathering power translates to a larger, and not just brighter, image is vague at best. This question also applies to astrophotography, I think. The image can be magnified more due to more light gathered over the elapsed time of a photo - or to the stacking of several images. Right? Wrong? I appreciate any explanation you can give to help me understand this basic data.

Thank you,

Tom Griffin
San Antonio, TX

Answer
Dear Tom:

The first part is easy.  Usable magnification on a scope is primarily a function of the quality of the optics and the interaction with the atmosphere.  Yes, aperture plays a role, but here's how it all fits together:

The quality of your optics depends on the finished optical surfaces.  A reflector has only two surfaces, but those surfaces have a 100% impact on the quality of the image.  If the mirror is slight out of true, or if it is not completely adjusted to ambient temperature, every light ray that strikes it will be bent out of focus. It is a direct relationship. And most reflectors are in the f/6 to f/8 range, introducing the possibility of spherical aberration.  

A refractor has four such surfaces...but since they only refract the light, the surface finish isn't quite so critical.  The impact on image quality is less. The real bending of the light is done by the meat of the lens. In other words--a refractor lens that is slightly out of true will have less impact on the quality of the image, because it only bends the light 5% out of true...and with the focal ratios of f/12 or f/15...the surfaces are very easy to get right.  

And I mentioned the effect of air.  A reflector is open to the air, and tube currents often have a negative effect on image quality.  A Refractor is a sealed tube, so there are no such air currents.  

Finally, a reflector has that secondary mirror which introduces diffraction into the image--concentrating about 10% of the light outside the primary image focus.  

Given all of this, the refractor can almost always provide higher power per inch of aperture than a reflector.  Can a 4" refractor match an 8" reflector for power?  Maybe.  But under good calm skies, a really good reflector, maximized for power (rather than a larger field of view) will still give a better image.  If the reflector has a long focal length ( f/10 or above, reducing the chances for spherical aberration) and a small diagonal mirror ( reducing the diffraction from that obstuction) it will give pretty darn good images.  Of course, it will be long, big, and hard to use.  But so would an 8 inch refractor!

So what other factors are involved?  There two other factors that affect this question.  One is simple.  The larger the mirror ( or lens ) the more light will be gathered.  I would choose an 8" reflector over a 4" refractor every time, because it collects 16 times more light.  When you are looking at dim fuzzy things, magnification doesn't matter as much as being able to see the dim stuff to begin with.  For planetary work, brightness isn't an issue.  For everything else in the sky, it is.  Go big when you can.

And the other element is more complicated.  The resolving power of a telescope--the ability to distinguish fine detail, is directly related to the aperture of the primary.  The bigger the aperture, the better the definition.  What this means is that a 4 inch scope will be able to magnify its optical image to 200X, for example...but you still won't be able to see details below about 1.25 arc seconds.  An 8" scope will be able to see down to about .65 arc seconds...because it has that larger primary mirror. Admittedly, unless you are really maxing out the power on the scope, these are pretty fine details--and usually won't get noticed much under less than perfect sky conditions. Here's a nice link:

http://coolcosmos.ipac.caltech.edu/cosmic_classroom/cosmic_reference/resolution....

Finally, image size isn't related to mirror size or light gathering power.  Image size is simply how much the telescope magnifies...but it can magnify a really crummy image, and all you will get is a very large blur.  The best telescope would gather a huge amount of light, so everything would be bright.  It would have perfect optics with a very large primary, so that you can see the finest detail.  And it would fit in your pocket, so that you could take it anywhere.  Please let me know if you find one of those!  

Those astrophotographs are made over many hours, and simply add the photons until the image is bright enough for us to see it clearly.  An amateur scope will never show the brightness or detail of such image because they aren't big enough, and your eye doesn't add photons to images over time.

Hope that helps.

Paul Wagner