And what Eyepiece would you like with that?

By Lesa Moore, 1994

Having recently taken the plunge and deciding on which new telescope to buy, I was then confronted with the next most difficult decision - which eyepieces? I looked up my old magazines. The American "Astronomy" did an article in June 1993 and our Australian "Sky & Space" covered eyepieces in their April 1994 issue.

From my limited personal experience, I’ve found that my favourite eyepiece, a 16mm Konig, is somewhat of a novelty (and almost unprocurable). It is a medium-cost wide-field type. With the new telescope being double the focal length of my existing one, I realised that I would have to choose something else for low-power observation. So I tried to sort out what was available, and what would best suit my needs.

For the benefit of those who are just starting out, I recommend you begin with just one good eyepiece which will yield about 50x magnification. To determine the magnification, you divide the focal length of the primary (ie telescope) by the focal length of the eyepiece (which is always written on the eyepiece). For example, if your telescope has a focal length of one metre (for a simple refractor or reflector, this is a direct measurment of the length of the light path from the main mirror or lens to the eyepiece) and you want 50x magnification, you would need a 20mm eyepiece. 1000mm/20mm = 50x. This level of magnification is suitable for beginners, as it is not too difficult to locate your objects. When you feel satisfied that you are ready for a closer look, then progress to shorter eyepieces to give higher magnifications.

Eyepieces also come in different diameters (for some odd reason, the focal length is given in millimetres, but the diameter is in inches): .965” (sometimes loosely called 1”), 1.25” and 2”. The most popular is the 1.25” size, to facilitate wider fields of view. The 2” are for the real enthusiasts (very expensive, and you need a violin-case to carry them around!)

There are many different types of eyepieces with various numbers of lenses used in combination to reduce certain aberrations (optical errors). I won’t go into great detail here (refer to the above articles for more information) but I have compiled a table which summarises their designs and advantages. From top to bottom, we have progressively more complex, and generally more expensive types.

In the table, apparent field is the field of view in degrees (the bigger the better). Many designs are versions of original concepts. Some are only made by specific manufacturers as shown.

Focal ratio can be relevant as some eyepieces work best at longer focal ratios. This is the ratio of the focal length of the telescope to the diameter of the primary. Eg, a scope with a one metre focal length and 200mm diameter would be f/5 (1000mm/200mm = 5). Small numbers (5 or less) are termed “fast” and big numbers (6 or more) are “slow”. Eye relief refers to how far away your eye can be and still see the whole field of view.

For the curious among you, I decided to purchase a 35mm Celestron Ultima (modified Plossl) for its medium-wide field, reasonable cost, and general purpose viewing.

[Editor's Comment: As a supplement to Lesa's review on eyepieces I thought I would add definitions of a few basic terms.]

Definitions

Apparent Field: Angular size of the light cone that is permitted to pass through the eyepieces by its optical and mechanical parts.

The true field refers to the angular area of sky seen with a given telescope. The true field is equal to the apparent field divided by the magnification. Using the example above, a 20mm eyepiece will yield a magnification of 50X with a telescope of focal length 1000mm, then the true field with an eyepiece of apparent field 50º will be 1º. However if a 20mm eyepiece with an apparent field of 80º was used with the same scope then the true field would be 1.6º (80/50).. In general a very wide field view is important for deep sky observing, especially objects of large angular size. I personally however find the "looking through a window" impression the wide field eyepieces give rewarding for all observing.

Entrance pupil is another name for the primary or objective of the telescope. The cylinder of light that actually leaves the eyepiece is the exit pupil. The exit pupil can be calculated by dividing the diameter of the primary (objective) by the magnification. Thus a 6" (150mm) thelescope operating at 50X would have an exit pupil of 3mm. It must be noted that the maximum diameter of the dark adapted pupil is about 7mm thus having an eyepiece with an exit pupil greater than 7mm can actually reduce the amount you see. An example would be observing at magnification of 50X with the society 17.5" Dobsonian.

 Design  Elements   App. Field   Brand/Distributor  Comments
 Huygens 2 25-40  Hard to get (Tasco)  Suitable for Solar projection 
 Ramsden 2 30-40  Hard to Get  Similar to Huygens
 Kellner 3 35-45  Various  Best with F6 or slower
 SMA* 3 40-45  Celestron  All purpose
 RKE* 3 40-45  Edmund Scientific  All Purpose
 Plossl 4-5 40-52  Various  Best for F5-F6
 Orthoscopic 4 40-50  Most  All Purpose
 Modified Plossl  4-5 50-54  Celestron Ultima  Better All Purpose
       Meade Super Plossl   
       Orion Ultrascopic  
 Lanthanum LV  5 45-50  Vixen / Celestron  Good eye relief
 Konig 3-4 60-70  University Optics  Economical wide field
 Erfle 4-6 50-70  Parkes, Orion  Better at or above F5
 Super Wide 6-7 65-70  Tele Vue / Meade  If you cant afford a Nagler
       TV Panoptic  Better than wide field
 Nagler I/II 7-8 80-82  Tele Vue  The BEST
       Meade Ultra Wide  

 

*=Modified Kellner