Information for purchasers and users of scopes. By Dr. Stuart Forster
Huygens, Ramsden, Kellner, Nagler’s, Type 1, Type2. AAAAGGGHHH!!!!! This is part of the alphabet soup of eyepiece designs confronting not only newbies, but seasoned observers looking to update their eyepiece collections. Where does one start? Lets first start with the features that we want to maximize in a design to help us choose.
- Eye relief: This is the furthest your eye can be from the eyepiece and still see the entire field. Eyeglass wearers need long eye relief of 20mm or more. Even though non-wearers of glasses can get away with shorter eye relief, it can get uncomfortable and also eyelashes will soil the glass.
- Field of view (FOV): This is measured in degrees. There are actually two fields of view. The field of view (FOV) is the true area of sky you see through the eyepiece. The apparent field of view (AFOV) is the angular area you see with your eye looking through the eyepiece, not the area of sky. The true FOV is calculated by dividing the AFOV by the power (magnification). FOV = AFOV/magnification.
- Exit pupil: This is the diameter of the cylinder of light that exits the eyepiece. The largest most pupils dilate to in younger individuals is 7mm. As we age, our maximum exit pupil decreases to 5mm or less. Having an exit pupil greater than 6-7mm may be a waste. Also, in scopes with central obstructions, such as Schmidt Cassegrains or reflectors, when the exit pupil is greater than 8mm, a central black spot, which is the shadow of the secondary mirror, may obscure central detail.
- Magnification: The maximum power you can expect from a scope is between 50-60x per inch. These powers can only be attained with high quality optics on pristine nights in the Northeast U.S. On average, count on 25x per inch on an typical night, or 1x per mm of aperture. Rarely can a scope be pushed greater than 300x in these parts.
- Area of the eyepiece field of view: The area of sky you view through the eyepiece increases by the square of the increase in FOV. As an example, if you increase the FOV by 50%, the actual increase in area of sky seen through the increases to 225% of the original. i.e.-an eyepiece with a 75 degree AFOV covers an area of sky 2 1/4x as large as an eyepiece with a 50 degree AFOV at the same power.
- Weight: Some of the eyepieces on the market can weigh over 2 ½ pounds. There can be severe balance problems in some scopes when these hand grenade sized eyepieces are swapped in and out.
- Par focal: Many series of eyepieces from the same manufacturer will have their focal planes at the same point so there will be minimal refocusing necessary when swapping. For all practical purposes, focusing between eye pieces is not an issue.
There are some old eyepiece designs that suffer from narrow FOV and short eye relief. These are the Huygens, Ramsden and Kellner. They are designated with the letters H, R, and K or MA. These are the eyepieces normally included with starter scopes. Try and upgrade as soon as possible.
Plossl designs should be considered the best starter eyepieces. They tend to have 50-52 degree FOV, good edge sharpness and good ones can be found starting at $30. The Pro-Optic line from Adorama Camera has gotten good reviews. GTO plossls from Gary Hand of Hands-On Optics are also moderately priced and high quality. A step-up in quality gare the Televue Plossls and the Orion Ultrascopics which are the same as The Celestron Ultimas.
The next step up in quality and price are the Meade Super Wide Angles, the Televue Panoptics, and the Pentax XL seris. These all have AFOV in the 65-67 degree range. The quality is good, but you must be careful of exponential increases in weight that accompany long focal length and long eye relief. These are all great eyepieces if you can afford them.
One step above are the Meade UltraWides and Televue Naglers. These all have 82-82 degree AFOV and are known as portholes into space. There are multiple iterations in the Nagler line, Type 1-Type 5. Overall these are exceptional.The prices range up to $595 for the 31mm Nagler Type 5.
The answer? Go to star parties and try different eyepieces in your scope to see what fits best. Don’t settle for less than a Plossl. Save longer if that’s what it takes to get a quality eyepiece. They last forever.
Dr. Stuart Forster was a long-time member, former president, secretary, contributing author, scope builder, astrophotographer extraordinaire, host to several of the Messier Marathons that marked the beginning of the SAS observing year, multi-lecturer at past meetings and Summer Seminars, and a true amateur astronomer’s astronomer.