USER OPINIONS PAGE
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From: firstname.lastname@example.org (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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