Eyepieces
The finder eyepiece, or the planetary one?
These two sit at opposite ends of an eyepiece case and are used for opposite jobs. Twenty-five millimetres is what an object is found with; four millimetres is what it is examined with, on the nights the atmosphere allows. Comparing them is really comparing two roles.
| Compared on | Plössl 25 mm | 4 mm aspheric |
|---|---|---|
| Focal length | 25 mm | 4 mm |
| Apparent field | 50 degrees, as the maker publishes it | 62 degrees, as the maker states it |
| Eye relief | Rubber, against side light | Not published by the maker. |
| Barrel size | 1.25 in with a filter thread | 1.25 in, with an M28.5 × 0.6 filter thread named by the maker |
| Optical design | Plössl, four elements | Aspheric |
| Coatings | Coated lenses, as the maker words it | Fully coated, as the maker words it |
| Weight | Not published by the maker. | About 42 g, given by the maker in ounces |
| Supplied accessories | Not published by the maker. | Two dust caps |
A tick means one listing published something the other did not, or published a better figure. It does not mean the product is better, and the column is not a score out of anything: a maker who says less loses lines here without losing a comparison. The two verdicts below are where the page is settled.
The one used every night. A 25 mm Plössl gives the widest true field either of these can, which is what an object is found with before anything else happens. Its coating is described with the weakest of the conventional labels, and its weight is not given.
The one used on the good nights. Four millimetres gives high magnification on a modest telescope, and 62 degrees is a wider window than a Plössl offers. Whether it is comfortable depends on eye relief, which is not published — and at this focal length that is the figure that decides it.
The low-power one. Nothing is examined at high magnification until it has been found at low magnification, and a wide true field is what makes finding possible.
Less often than most observers expect. On a 650 mm telescope it gives about 160x, and the atmosphere supports that on a minority of nights.
At the same focal length, yes. Between these two the 4 mm shows far less sky than the 25 mm despite its wider apparent field, because true field is apparent field divided by magnification.
25 mm focal length · 50° apparent field · four elements
A 25 mm Plössl with a 50-degree apparent field, coated optics, a metal body, rubber eyecups and a filter thread.
Best for: The finding eyepiece — the one that stays in the focuser most of the evening
≈ £32 / €34
4 mm focal length · 62° apparent field · 42 g
A 4 mm aspheric eyepiece with a 62-degree apparent field, fully coated, in a 51 mm aluminium barrel with an M28.5 × 0.6 filter thread.
Best for: Lunar and planetary detail on a telescope with a long focal length
≈ £18 / €20
Any figure written into the body of this page was true on the day it was written. The amount shown next to each instrument is requested from the retailer as the page loads, and it is that amount, not the text, that stands.
