Hack 50. Chart Your Eyepiece Characteristics
Knowing the capabilities of your eyepieces makes it easier to choose the right one. Paste a reference chart to your scope to keep this information at your fingertips. If you have only one scope and a few eyepieces, it's easy enough to remember the magnification, field of view, and other characteristics of each eyepiece. But if you have multiple scopes and many eyepieces, it can be difficult to remember all the details. For example, assume you have three telescopes, an 80mm f/5 [Hack #9] short-tube refractor with a focal length of 400mm, a 10" f/5 Dob with a focal length of 1,255mm, and an 8" f/10 SCT with a focal length of 2,032mm. You also have a very nice selection of Tele Vue eyepieces41mm, 27mm, and 19mm Panoptics; and Radians in 14mm, 12mm, 10mm, 8mm, and 6mm focal lengthsand a 2.5X Powermate, which effectively multiplies the power of any eyepiece it is used with by 2.5X. Your first attempt at an eyepiece chart for the 400mm short-tube refractor might look something like Table 4-3.
This chart presents a lot of very useful information in a small space:
Unfortunately, there are also a few things wrong with this chart. Most obviously, the 41mm and 27mm Panoptics have 2" barrels, while the Powermate accepts only 1.25" eyepieces. Oops. Less obviously, there is some overlap in the combinations. For example, you probably would choose to use the 8mm Radian alone (50X, 1.2° FoV) rather than the 19mm Panoptic with the Powermate (53X, 1.2° FoV). The Radian has much better eye relief than the Panoptic/Powermate combination, and you'll be looking through less glass. Removing the superfluous combinations gives us a chart that looks something like Table 4-4.
You can still use the 19mm Panoptic or the 14mm Radian with the 2.5X Powermate, of course. But having only optimum combinations listed makes it faster to choose the ideal combination. Turning to the 1,255mm f/5 Dob, we once again run the calculations to reflect the higher magnifications and smaller fields with the longer scope.We also considered eliminating one or two combinations. The 6mm Radian with the 2.5X Powermate yields 523X magnification in our 1,255mm Dob. The atmospheric turbulence in our area makes that too much to be usable on any but the best nights. In fact, we can seldom push much higher than 300X, so we considered eliminating the 8mm Radian/Powermate combination as well. We kept both, though, because they are usable on some nights, particularly for Lunar observing. Table 4-5 shows our results for the 1,255mm Dob.
Finally, we recalculate the chart for a 2,032mm 8" f/10 SCT. Although we left the eyepiece/Powermate combinations for the 12mm and 10mm Radians, the truth is that those combinations might actually be useful only once or twice a year, unless you live in an area like southern Florida that has extremely stable air. In practical terms, the Powermate would probably never come out of our eyepiece case. Instead, we'd just use the eyepieces natively.They provide an excellent range of magnifications, fields of view, and exit pupil sizes all by themselves. Table 4-6 shows our results for a 2,032mm SCT.
You can calculate your own charts based on the measured or nominal focal length of your scope(s) and the fields of view of your eyepieces. Once you've created a chart for each of your telescopes, print it out, laminate it, and paste it on the side of the scope near the focuser. You'll always have these important eyepiece characteristics immediately available. |