Information & Opinion

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WHY A 6"F/15 REFRACTOR

Why a 6-inch F/15 Refractor? From my years of experience as an astronomer who has used a variety of telescopes of different types along with using very rare large apertures of 8-12 inches. I have found the 6 inch F/15 refracting telescope is the optimum design for lunar, planetary, and solar observing. The telescope’s size makes it the best instrument in my opinion for advanced observers to use and simple enough for elementary observers to use for their viewing. An observer can master the telescope system with relatively few instructions, and yet the system is large enough to provide optimum viewing in city astmospheric conditions. Furthermore for more advanced observing the 6" F/15 refractor with it’s accessories provides opportunities for photographic and spectroscopic work as well. Observatories and educational institutions usually have found the 6-inch F/15 refractor to be the finest instrument for lunar, planetary, and solar observing. For those wanting to build a 6" F/15 refracting telescope system for their observing should obtain a copy of "Telescope Making44" and read the article entitled "Constructing a Large Brass Refractor" by Larry Myers. Telescope Making was a publication of Kalmbach Publishing Company (Astronomy Magazine)
GEORGE H. LUTZ OBSERVATORY, MOUNTAINGATE, CA.

THE TELESCOPE'S F/number

The f/number (focal ratio) of any telescope tells two things about the telescope and that is it's intended purpose and it's photographic performance. The brightness of a star depends on the telescope's aperture not the telescope's f/number. All telescopes of the same aperture at any magnification will show the same visual brightness. There are many that insist their long focal ratio telescope gets higher contrast, this is not correct. A refracting telescope does have more contrast than any other because of it's optical system not because of the f/ratio. You can see that when you are comparing very well made and very well corrected refractors, you will see there is no gain in contrast regardless of the f/ratio of each telescope. Even if a reflecting telescope is well made and has the same size secondary mirror obstruction as another, it will have the same contrast regardless of the f/number of each telescope. All of the confusion and there is much on this issue, is because of the photographic use of the f/number. A faster f/ratio does mean brighter images on film but not in a telescope. Aperture not f/ratio is the important factor with a telescope. Some photographers have a great deal of trouble with this concept. The f/number of any objective lens or a mirror of a telescope has nothing to do with the visual brightness of an image, the bigger the aperture the better.
GEORGE H. LUTZ OBSERVATORY, MOUNTAINGATE, CA.

TESTING THE OPTICS OF YOUR TELESCOPE

You will gain experience in using your telescope especially if you direct your attention to the main points of quality. You will find in time that you will become critical of imperfections of your telescope, casual or inherent. When you test your telescope's optics, you should be careful and pay close attention to your own eyesight and be guided by proper directions and only then will you have a good idea of the quality of your telescope. Remember that no telescope will give the most perfect image. Your telescope is good when you can see fine definition and not to worry about the out of focus star image. If there seems to be a problem with the telescope because of the star image, you can not say what the problem is until you make an examination of the out-of-focus image on a night fine enough to allow you to make a star test at high power, unless the fault is because of a astigmatism, maladjustment, or centering of the optics and on a good night these can be seen at focus.
GERGE H. LUTZ OBSERVATORY, MOUNTAINGATE, CA.

BRANDON OCULARS

Brandon is a brand name of VernonScope, and is applied to a variety of products, including a well-established line of eyepieces. Several of the short- to medium focal-length Brandon eyepieces are advertised as orthoscopics, yet they appear upon disassembly to resemble Plossls. They use four lenses in an unsymmetrical duplet formulation designed by Chester Brandon, one of the designers of the top-secret Norden bombsight, which played an important role in World War II.

Brandon eyepieces are a popular choice of professional astronomers worldwide. VernonScope estimates that over one million Brandons in various incarnations and focal lengths have been produced since their introduction in the early 1940s. Designed for medium to high power use with any telescope at focal ratios down to f/4, they are very sharp at the center (although somewhat less so at the edges) and ideally suited to lunar, planetary, star cluster, and binary star observing. They have excellent color correction and very low astigmatism, with very minor field curvature and ghosting. They are fully coated, but not multicoated, in order to reduce a phenomenon known as "narrow angle light scatter" that is sometimes seen in multicoated optics. This scatter shows as a loss of contrast and subtle details within bright objects such as Venus, Mars, and Jupiter. It also shows as a significant difference in the background darkness immediately surrounding a bright object, which is especially important for double-star observing. Brandons are accordingly noted for their exceptional contrast and extremely dark background, two reasons why Questar has been using them in their ultra-premium Maksutov-Cassegrain scopes since 1971. They have moderately wide apparent fields of view, of about 45°.

Brandons are threaded for Vernonscope filters only. The use of standard eyepiece color or nebula filters requires adapter #3405. They have rubber eyecups that roll down for eyeglass use, although their eye relief is quite short in the shorter focal lengths. Brandons are the sharpest design currently available in a 1.25" barrel size for eyepiece projection photography.
GEORGE H. LUTZ OBSERVATORY, MOUNTAINGATE, CA.