Your Eyepiece Collection - Field of View
A new telescope normally comes with at least one eyepiece (or "ocular"), sometimes two. The eyepieces that come with most telescopes are generally adequate at best. Moreover, you soon find that one or two eyepieces don't quite give the range of magnification that you want. So what is the right combination to have?
Referring to the advantages of having a good selection of eyepieces, Paul Hyndman, an amateur astronomer from New Hampshire, wrote: "Not unlike a golfer's clubs, together they form your arsenal. Some may get in more 'play time' than others, but it's the power of their combined versatility that brings fullness to the game!"
So let's consider the philosophy behind building a collection of eyepieces that will serve you well over many years. To some extent, it depends on your telescope design and observing preferences. While it also depends on what you can afford, eyepieces are usable on any telescope you own now or may ever own in the future, so they are a good long-term investment. I refer to this as an eyepiece "collection" because few observers have a matched "set". Most prefer a pretty eclectic mix of different brands and designs.
Field of View
The maximum true field of view (TFOV) depends on the focal ratio of the telescope, and on the design of the eyepiece and its focal length. The faster the scope (short focal ratio), the wider the TFOV. If you imagine a little circle in the sky, the TFOV would be the angle the pupil of your eye would sweep out if you looked to one edge of the circle, then move your eye to the opposite side. If something in the telescope's optical path restricts the view of the sky, then the TFOV could be less. With an eyepiece of 32mm focal length or greater, you need a 2" barrel in order to get the maximum TFOV; a 1.25" format eyepiece restricts the field due to the narrowness of its barrel.
The apparent field of view (AFOV) is the field of view advertised by eyepiece suppliers. It's the angle your eye sweeps out when you look at one edge of the field in the eyepiece and then look to the opposite edge. To determine the true field, you can get pretty close by taking the apparent field of view and dividing it by the magnification that eyepiece gives in your telescope. Thus if a particular design provides a 50o AFOV, an eyepiece in that series with a focal length of 25mm will give a larger TFOV than one with a focal length of 10mm, even though their apparent fields are identical.
Although a wide field of view makes the eyepiece more comfortable to use and makes it easier to locate objects, some designs distort the shape of objects toward the edge of the field. In effect, your usable field becomes somewhat less than the advertised AFOV. To improve edge of field performance, many eyepiece designers use a field stop - a sort of collar around the inside circumference of the eyepiece barrel. This cuts off the edge of the field where the image is distorted. A fast telescope (focal ratio shorter than f/6) is tough on eyepieces - only the better oculars are able to minimize edge of field distortion that is common on such instruments. The Tele Vue "Paracorr" add-on helps lessen the effects of coma, which is a deformation of the image away from the optical axis of a fast telescope.
If your target is pretty small, like a planet or double star, then you really don't need to see a lot of empty space around the object. You want the object as large as you can get it, so you can sacrifice field of view in favor of magnification. The times when you can use very high magnification are limited by the steadiness of the atmosphere, but on that rare night with great seeing, you'd want high power in order to get a good look at the moon or planets. Bear in mind that your ability to use high magnification also depends on the quality of the optics in the telescope itself and the steadiness of your mounting. It's not uncommon for dedicated planetary and double star observers to own a bunch of eyepieces with short focal lengths of 8mm, 6mm, 5mm, 4mm, and even 3mm. I was curious about this apparent overkill until one such observer reminded me that in the short focal lengths, even one millimeter makes a significant difference in magnification. In a 1200mm focal length telescope, a 5mm eyepiece gives you 240x, while a 4mm jumps to 300x. (Telescope focal length in millimeters divided by eyepiece focal length equals magnification.)
The eyepiece that you will likely use most regularly is something in the middle range of magnification. This allows a decent view of solar system objects when seeing conditions are not superb; also, many deep sky objects take this magnification range very well. Medium magnification tends to darken the sky background a bit to increase contrast and help objects stand out better than they do in lower power oculars.
Low power is good for sky sweeping, especially when you're searching for an object and need as wide a field as possible. And some objects are so large that in order to see them well, you need a low power, wide TFOV. That's the case with many large nebulae such as the Rosette and Veil. The largest open star clusters, such as the Pleiades, Beehive, and Perseus Double Cluster are much better at low power. I also find a large galaxy, such as M31, more impressive at low power. Comet and supernova hunters typically gravitate toward the low power end for their sweeping. And a leisurely low magnification trip along the Milky Way is awesome!
A Range of Eyepieces
So what are the ranges of eyepiece focal lengths that are generally useful? Others may have different opinions, but I'd consider about 20mm on up as low power, 10mm to 20mm as medium, and shorter than 10mm as high magnification. With something in each of those ranges, you'll be nominally equipped. Many observers have two or more oculars within each of the magnification ranges. Of course, in many respects this is all relative. It also depends on whether you own more than one telescope, and the design parameters and focal length of each instrument.
Steve Coe, a well-known amateur astronomer in Phoenix, suggests the following. "To begin with get three eyepieces. They should provide powers that are: 40X to 60X, then approximately 100X, the last should provide between 150X and 200X. These three eyepieces will provide a good magnification for whatever you are viewing. If you have the money, get one more eyepiece that provides around 300X. Those four eyepieces will cover most every eventuality." Other observers approach it more systematically. One planned his complement of eyepieces so as to provide roughly 5, 10, 15, 20, 30, 40 and 50 power per inch of aperture. Another has a rule of thumb that dictates a jump of about 1.4x between eyepieces in increasing order of magnification, though he admits to regularly violating his own rule.
Past newsletter articles have addressed various eyepiece designs and the virtues and vices of each. We won't dwell on that here. But the range of focal lengths is affected by unique aspects of different eyepieces. Here's an example. A 26mm Plossl (50o AFOV) gives about 55x with a .90o TFOV in my 10-inch Newtonian, while a 19mm Panoptic (65o AFOV) gives 75x and a .86 TFOV. That's almost half again as much power with about the same TFOV. As a result, I seldom use the Plossl, and the Panoptic has become my main "low power" workhorse, even though it falls in the "mid range" cited above.
Admittedly, the Panoptic costs three times as much as the average Plossl. It's a truism that a relatively small increase in image quality ends up costing a lot more money. Is it worth it? That's up to you. If you feel you can afford some of the premium wide field designs, then that may shift your preferred magnification range and possibly affect the number of eyepieces you'll want. The highest quality eyepieces give you the best image your scope can deliver by not introducing any distortions of their own. Some experienced observers say that a good eyepiece should, in effect, "get out of the way" in terms of the overall view.
Moreover, it seems to me that the better eyepieces actually tend to somewhat diminish minor optical effects such as the chromatic aberration typical in achromatic refractors or the coma exhibited in fast Newtonian systems. But even the most expensive eyepiece cannot correct substantial defects in telescope optics, such as astigmatism or serious chromatic aberration.
Veteran planetary observers often insist that the best high-power eyepieces are those with the least number of lens elements, such as the Orthoscopic and Plossl. The fewer the lenses, the brighter and sharper will be the images, they point out. That's probably true. One of the best eyepieces I ever had in terms of image quality was a 5mm Orthoscopic. But the short eye relief and peephole-sized eye lens were annoying.
One way to expand your eyepiece collection is with a Barlow lens. Some people have a couple of them with different magnification factors, say 2x and 3x. One Barlow with three different focal length eyepieces gives you six different magnifications for less than the cost of one really good eyepiece. While a good Barlow does not cause a deterioration of the image, I've found they provide sharper and brighter images when used with simpler eyepieces such as a Plossl. Today's Barlows use anywhere from two to four lens elements to provide highly corrected images. When used with a complex eyepiece design such as a Nagler, Radian, or Pentax XL, photons may have to fight their way through ten or eleven lens elements before reaching your eye.
Another way to add to your collection is by purchasing used eyepieces. Others may be upgrading their own collections or disposing of an eyepiece that they don't use very often, and selling at a price well below the new cost. One issue here is the matter of lens coatings, which have improved over the years, with a resulting enhancement in contrast and with less ghosting and light scattering. But if it's a good eyepiece and the price is right, an older lens coating shouldn't be a deterrent.
You need not spend a fortune in order to acquire a useful assortment of oculars. And whatever is in my eyepiece case may not be what you will want in your eyepiece case. So do some planning to meet your needs and have fun expanding your eyepiece collection!Published in the February 2003 issue of the NightTimes