Astronomy Hacks

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 eyepieces—41mm, 27mm, and 19mm Panoptics; and Radians in 14mm, 12mm, 10mm, 8mm, and 6mm focal lengths—and 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.

Table 4-3. An eyepiece chart for a 400mm f/5 refractor

	Alone				w/ Powermate
Eyepiece	Mag	Field (°)	Field (')	Pupil(mm)	EFL(mm)	Mag	Field (°)	Field (')	Pupil(mm)
41mm Panoptic	10X	6.6	395.3	8.2	16.4	24X	2.6	158.1	3.3
27mm Panoptic	15X	4.4	262.1	5.4	10.8	37X	1.7	104.9	2.2
19mm Panoptic	21X	3.1	183.1	3.8	7.6	53X	1.2	73.2	1.5
14mm Radian	29X	2.1	123.8	2.8	5.6	71X	0.8	49.5	1.1
12mm Radian	33X	1.8	108.3	2.4	4.8	83X	0.7	43.3	1.0
10mm Radian	40X	1.5	90.2	2.0	4.0	100X	0.6	36.1	0.8
8mm Radian	50X	1.2	71.3	1.6	3.2	125X	0.5	28.5	0.6
6mm Radian	67X	0.9	54.1	1.2	2.4	167X	0.4	21.7	0.5

This chart presents a lot of very useful information in a small space:

• Magnifications are useful to know for obvious reasons.

• The Field (°) columns list the field in degrees, which is useful as a rough indication of how suitable the eyepiece is for observing a given object. For example, if you’re observing a large galaxy, nebula, or star cluster, you probably want a wide field of view. Conversely, if you’re trying to tease out detail in a tiny globular cluster or planetary nebula, you probably want to put some serious power on and narrow the field of view.

• The Field (') columns list the field of view in arcminutes, which is useful for star hopping, estimating separation of close objects, and so on.

• The Pupil columns list the exit pupil provided by the various eyepieces and combinations, which are useful for several reasons. First, human visual acuity peaks between 2mm and 3mm, which is the ideal range for observing any object. Second, an exit pupil of 1.0mm to 0.7mm is the smallest suitable for general observing. Below that, diffraction and floaters (microscopic bits of material in your eye) begin reducing acuity significantly. Third, an exit pupil around 0.5mm is the smallest useful for specialized observing, such as splitting double stars. Finally, some interference filters work best within narrow ranges of exit pupil size.

• The EFL (effective focal length) column lists the equivalent focal length of each eyepiece when used with the Powermate.

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.

Table 4-4. An improved eyepiece chart for a 400mm f/5 refractor

	Alone				w/ Powermate
Eyepiece	Mag	Field (°)	Field (')	Pupil(mm)	EFL(mm)	Mag	Field (°)	Field (')	Pupil(mm)
41mm Panoptic	10X	6.6	395.3	8.2
27mm Panoptic	15X	4.4	262.1	5.4
19mm Panoptic	21X	3.1	183.1	3.8
14mm Radian	29X	2.1	123.8	2.8
12mm Radian	33X	1.8	108.3	2.4	4.8	83X	0.7	43.3	1.0
10mm Radian	40X	1.5	90.2	2.0	4.0	100X	0.6	36.1	0.8
8mm Radian	50X	1.2	71.3	1.6	3.2	125X	0.5	28.5	0.6
6mm Radian	67X	0.9	54.1	1.2	2.4	167X	0.4	21.7	0.5

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.

Table 4-5. An eyepiece chart for a 1,255mm f/5 Dob

	Alone				w/ Powermate
Eyepiece	Mag	Field (°)	Field (')	Pupil(mm)	EFL(mm)	Mag	Field (°)	Field (')	Pupil(mm)
41mm Panoptic	31X	2.1	126	8.2
27mm Panoptic	46X	1.4	83.5	5.4
19mm Panoptic	66X	1	58.3	3.8
14mm Radian	90X	0.7	39.4	2.8
12mm Radian	105X	0.6	34.5	2.4	4.8	261X	0.2	13.8	1.0
10mm Radian	126X	0.5	28.8	2.0	4.0	314x	0.2	11.5	0.8
8mm Radian	157X	0.4	22.7	1.6	3.2	392x	0.2	9.1	0.6
6mm Radian	209X	0.3	17.3	1.2	2.4	523x	0.1	6.9	0.5

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.

Table 4-6. An eyepiece chart for a 2,032mm f/10 SCT

	Alone				w/ Powermate
Eyepiece	Mag	Field (°)	Field (')	Pupil(mm)	EFL(mm)	Mag	ottField (°)	Field (')	Pupil(mm)
41mm Panoptic	50X	1.3	77.8	4.1
27mm Panoptic	75X	0.9	51.6	2.7
19mm Panoptic	107X	0.6	36	1.9
14mm Radian	145X	0.4	24.4	1.4
12mm Radian	169X	0.4	21.3	1.2	4.8	423X	0.1	8.5	0.5
10mm Radian	203X	0.3	17.8	1.0	4.0	508X	0.1	7.1	0.4
8mm Radian	254X	0.2	14	0.8
6mm Radian	339X	0.2	10.7	0.6

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.