Ideal eyepiece collection

Since I have decided to pursue my astronomy hobby again, one of the things that I need to do is to update my equipment arsenal. Having just bought a telescope (a superb 105mm f/12.4 refractor) and having completed a 6" f/5 mirror (and I have also started grinding a 12" f/5 mirror), I decided that now would be a good time to augment my eyepieces. While a telescope gets most attention in astronomy, an eyepiece is also a very important part. A bad eyepiece can ruin a good telescope. Upgrading the eyepiece to a better one may be the easiest (and probably the cheapest) way to enhance the "output" of a telescope. Thinking along these lines, I decided to upgrade my eyepieces.


Before moving further, the first step was to take stock of what I have. All in all, I have three eyepieces currently --- a 10mm Plossl, a 20mm wide field and a 40mm Plossl. All three have a 1.25" diameter barrel. The 10mm is a standard Plossl with approx 52degrees Apparent Field of View (AFOV for short), the 20mm eyepiece has 65degrees AFOV and the 40mm Plossl has a 45degrees AFOV. when used with my long focus refractor, these serve as a low power (32.5X, 1.6degrees TFOV), medium power (65X, 1.0degrees TFOV) and a high power (130X, 0.4degrees TFOV). Although having three powers is considered as ideal, the quality of the 10mm eyepiece leaves a lot to be desired. This eyepiece was received as a freebie along with some ATM components that I purchased a while back. I would certainly like to replace this eyepiece. And with the advent of some decent quality low-cost ultra-super-ultimate-enhanced wide-angle eyepieces (eyepieces probably use more adjectives to describe them than any other object in the Universe), I decided it was time to upgrade. So in this article I am setting out to describe what should be my ideal eyepiece collection. I will try to have a broad range of powers, wide and flat fields, and a suitability for my long focus refractor as well as for the short-focus scope that I am trying to build. The eyepieces must also serve any future scopes (most likely a grab-and-go scope and/or a large-aperture dobsonian) that I might buy.


In order to decide the extreme range of powers, I had to do some basic calculations of exit-pupil and maximum magnifications. Most of my observations are done from a partly dark suburban park like area so I doubt that my eye-pupil size exceeds 6mm. Further since I plan to complete my short focus 6" f/5 scope eventually, the eyepiece that gives lowest possible power and widest possible true fields with these scopes would be essential. For a scope having f/5 as focal ratio and 6mm exit pupil, the lowest power I can use is 25X. Accurate measurements indicate that the mirror has a focal length of about 775mm (30.5"). An eyepiece having 31mm focal-length would give the lowest possible useful magnification on this scope while allowing me to use a full 6mm eye-pupil. With the refractor, the minimum useful magnification is only 18X. Since the focal length of this scope is 1300mm, that translates to an eyepiece of approx 74mm. The thing to note here is that the long focus refractor is not designed to provide wide field views so I doubt I will ever buy an eyepiece of 74mm or even anything having more than 50mm focal length. So for the lowest possible magnifications, the shorty scope can use an eyepiece with not more than 31mm focal length while the refractor not more than 74mm.


On the other hand, for the highest possible magnifications, for the shorty, I would consider 30X/inch as the maximum useful (note the word "useful" here, not "theoretical") magnification since this scope is primarily to be used as wide field rather than a high powered scope. That converts to about 180X of useful magnification. With its' 775mm focal length, an eyepiece of 4.3mm would be required to do the trick. For the refractor,a magnification of 50X/inch is probably the upper limit. Considering 50x/inch of aperture as the upper limit, it would mean a maximum magnification of 205X (4.1" x 50 = 205X). With its 1300mm focal length, a 6.3mm eyepiece would give the required magnification.


Below is a table that lists the minimum, moderate and maximum magnifications for both the scopes. I am defining moderate magnifications as half the maximum magnifications (i.e. 25X/inch for the 'fractor and 15X/inch for the shorty).
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Scope Minimum Moderate Maximum
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6" f/5 31X (31mm) 90X (8.6mm) 180X (4.3mm)
4" f/12.4 18X (74mm) 103X (13mm) 205X (6.3mm)
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Now my goal is to find a range of eyepieces that allow me to use the maximum possible range of magnifications while providing with the widest possible fields and a flat-field. I will also try to lookup particular brand reviews on Cloudy-nights and other websites and try to narrow down my selection.


Starting on the highest end, having a 6-7mm eyepiece would be a good start. There are reasons I do not want to buy a 4.3mm eyepiece ---- Firstly, eyepieces having focal lengths less than 6mm usually have extremely short eye-reliefs. Secondly, it is very difficult to eliminate most aberrations from an eyepiece having such short focal lengths. Finally, considering that there are very few nights which offer the best "seeing", I suppose using a slightly lower powered eyepiece in conjunction with a 1.5X barlow lens may be more desirable. That would allow me to use somewhat less magnifications when the seeing is normal (i.e. not so good) while still allowing a high magnification for those "perfect" nights. Thus, rather than using an eyepiece specially for this highest possible magnification for the perfect night, I would prefer to use a more "general" eyepice along with a 1.5X - 2X barlow. For e.g. using 6.5mm eyepiece would give a magnification of 119X for the shorty scope and 200X for the refractor. Using this eyepeice in combination with a 1.5X barlow lens would give me 179X on the shorty. Another reason for the same is that a 4.3mm eyepiece would be almost useless with the refractor as it would mean a magnification of 302X (73X/inch of aperture) which would be beyond what that instrument can handle. Since this eyepiece is to be used for high powers, having very wide fields is not a condition. A wide field would be nice to have but it is not essential. Thus, buying an eyepiece of 6-7mm focal length with 60-80degrees AFOV and using it with a 1.5X barlow lens would be a good start on the high power end.


On the lower end, a 74mm eyepiece is out of question. Not only would an eyepiece with that kind of focal length be extremely rare, it is likely to be expensive also. Further, since the reason I purchased the refractor is not for wide-sky views but rather for planetary studies which require moderate to high powers. An eyepiece of around 40-45mm focal length would be ideal as it would give a low power on the refractor and while the exit-pupil on the shorty would exceed 6mm (8.8mm to 7.8mm), I could still use it when I am in a dark location. In addition using the 1.5X barlow would give me


For the intermediate powers, I am planning to get use eyepieces. I plan to keep the 20mm eyepiece that I have. It will give me 65X of the refractor and 39X on the shorty. With the 1.5X barlow, I would get 98X and 58X. In the table below, I am listing various commonly used eyepieces and the magnification/TFOV that I can get when used with the shorty and the 'fractor.

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Eyepiece 6" f/5 shorty 105mm f/12.4 refractor Remarks
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6.5mm 80deg AFOV 119X/0.67 179X/0.45 200X/0.40 -------
10mm 52deg AFOV 78X/0.67 116X/0.45 130X/0.40 195X/0.27
13mm 80deg AFOV 60X/1.34 90X/0.90 100X/0.80 150X/0.53
16mm 80deg AFOV 48X/1.65 73X/1.10 81X/0.99 122X/0.66
20mm 67deg AFOV 39X/1.73 58X/1.15 65X/1.03 98X/0.69
25mm 80deg AFOV 31X/2.58 47X/1.72 52X/1.54 78X/1.03
30mm 80deg AFOV 26X/3.10 39X/2.06 43X/1.85 65X/1.23
40mm 44deg AFOV -------- 29X/1.51 32X/1.35 49X/0.90
1.5X barlow lens
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This is the ideal list of eyepieces that I would like to have. Since I already own three eyepieces, now the task before me is to read through reviews and select eyepieces that match or closely match the above criteria. I realize that the tables above are not displaying properly. I will try to find out a way so that the tables look as they were intended to.