More Jupiter action.
Over the years Jupiter's Great Red Spot has varied dramatically in color, size, and longitude. Although the spot's present tan color is very pale, it is well delineated within the South Equatorial Belt. Viewing with a 12 1/2-inch Newtonian last September, Sky & Telescope associate editor Alan MacRobert likened the spot's appearance to that of an eye - complete with eyeliner. Observing with a 3.5-inch Maksutov the same night, I had a difficult time discerning any color within the spot, but the planet's North Equatorial Belt displayed a subtle rusty brown shading.
Regardless of what size of telescope you have, take advantage of the remainder of this remarkable Jupiter season to enjoy the splendors of the giant planet's various spots, belts, and zones.
To Wear Glasses or Not?
Whether observing subtle detail in the cloudtops of Jupiter or nebulosity at the very edge of visibility, telescope users are always seeking the optimum view. Prescription eyeglasses are for many observers a link in this optical chain. Unfortunately, they are too often the weakest link. Eyeglasses often prevent positioning the eye close enough to an eyepiece to take in the whole field of view. Worse still, most eyeglass lenses also show the wear and tear of daily use, which degrades the view. It's little wonder so many bespectacled telescope users try to minimize the amount of observing they do with their glasses on.
Observers who are only near- or farsighted can remove their glasses and simply refocus the telescope to compensate. Those with prescriptions that correct for astigmatism are less fortunate. Their choice consists of either wearing the glasses or living with an astigmatic view of the universe. There are circumstances, however, in which the eye's astigmatism is completely unnoticeable while observing with a telescope. As the magnification increases and the exit-pupil diameter shrinks, astigmatism becomes less objectionable. For critical observing it would be helpful to know whether astigmatism is impairing the telescopic view - even if it is not obviously degrading the image.
Arizona optical designer Richard Buchroeder has devised a simple equation that helps observers with astigmatism decide when they should wear their glasses: A [less than or equal to] 1/[-square root of D]. To use Buchroeder's formula all you need to know is the amount of "cylinder" (in diopters) in your eyeglass prescription. Once you drop this into the formula as the value for D, the answer, A, yields the maximum diameter of the exit pupil (in millimeters) before astigmatism exceeds 1/4-wave - the acknowledged limit for telescope work. For example, if your prescription is for 1.5 diopters of astigmatism, the maximum exit pupil that permits 1/4-wave performance is 1/[-square root of 1.5], or 0.8mm. Since a telescope's exit pupil is equal to the aperture divided by the magnification, a 6-inch (152-mm) scope must operate at a magnification of 191x or greater to have an exit pupil smaller than 0.8mm.
Before you start trimming down your eyepiece collection to reflect this criterion, take heed of Buchroeder's caution: "Note that at telescope powers lower than approximately 50x per inch of aperture, the overall magnification is insufficient to readily see flaws as small as 1/4-wave. Of course, noncritical viewing can be enjoyed even with clearly visible astigmatism."
Seeking Gyulbudaghian's Nebula
Deep-sky observers with a taste for the exotic will enjoy hunting down Gyulbudaghian's Nebula in Cepheus. It is one of a mere handful of nebulae that appear to change with time, Hubble's Variable Nebula (NGC 2261) in Monoceros being the best-known example. This object's unusual nature was first noted by Armenian astronomer Armen L. Gyulbudaghian in 1976.
Information on this nebula is difficult to come by. None of the amateur observing guides mentions it, and of the popular atlases only the recently published Millennium Star Atlas plots its position. Nevertheless, while browsing through Vicker's Deep Space CCD Atlas, Maryland amateur Roy Diffrient, stumbled across an image of this object and wondered if anyone had ever attempted a visual observation. Armed with a variety of telescopes, Kenneth Drake and a group of fellow observers took up the challenge at Fort Griffin State Park, about 100 miles west of Fort Worth, Texas. Using a 24-inch telescope at 259x, Drake was able to see the nebula as a "small fuzz" With higher magnification, Drake noted that "it appeared round, just under 10 arcseconds, with a very abrupt border." Subsequently, the observers were able to see the object with an 18-inch Newtonian.
Gyulbudaghian's Nebula is at right ascension [20.sup.h][45.9.sup.m], declination +67 [degrees] [58.sup.m] (equinox 2000.0), near the Cepheus-Draco border, about 1 [degree] north of 4 Cephei.
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|Title Annotation:||views of the planet|
|Publication:||Sky & Telescope|
|Date:||Jan 1, 1999|
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