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Stu Forster’s Scopes Page

Information for purchasers and users of scopes. By Dr. Stuart Forster

Eyepiece Types Finder Scopes Scope Links

Eyepiece Basics

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.

1. Eye relief

This is the furthest ones 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.

2. 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 one sees through the eyepiece. The apparent field of view (AFOV) is the angular area one sees with his 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.

3. 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.

4. Magnification

The maximum power one 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. figure 25x per inch on an average night, or 1x per mm of aperture. Rarely can a scope be pushed greater than 300x in these parts.

5. Area of the eyepiece field of view

The area of sky one views through the eyepiece increases by the square of the increase in FOV. As an example, if one increases 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.

6. 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.

7. 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.

Eyepiece Types

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 in quality gets one to 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 one 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.

Finder Scopes

The best telescope in the world is relatively useless if you can’t find anything with it. Telescopes were traditionally equipped with a finder scope, which was nothing more than a small, wide field refractor scope with crosshairs. Finder scopes come in two styles. The first is the straight through variety which completely inverts the image. One must turn his or her finder chart upside down to get properly oriented with this type of scope. On the plus side, you can site down the main scope tube, which facilitates initial aiming. Another school of amateurs prefer the right angle finder, which keeps the field in correct orientation. One negative is the small amount of light lost secondary to the extra reflections in the erecting prism. One additional option in these scopes is an LED illuminated reticule, which makes the crosshairs easier to see.

There are four common size finder scopes found either as original equipment, or as add-ons.

5×24, 6×30, 8×50, 9×60

The 5×24 does not have an achromatic objective, and is subject to chromatic aberration (color fringing). To combat this effect, there is a field stop (ring of plastic) midway down the inside of the finder which blocks half the light collected by the lens. It basically is no better than the unaided eye.

The other three finders provide at least a 5mm exit pupil to aid in utilizing the entire field of view. If possible, aim for a 50mm or larger finder. Also, look for a mount with six, rather than 3 adjustment screws. For easy storage, a dovetail mounting system is also nice.

The other style of finder is the unit or 1x finder scope. These are basically heads up display (HUD) units that project either a red dot, or series of circular reticules on an optical window, giving the appearance of the pattern being in the sky. All are used with both eyes open. The only disadvantage of these units as a group is the difficulty of use in light polluted skies.

The oldest and most popular of these units is the Telrad. This unit projects ½, 1, and 2 degree circles on the optical window. The advantage of the Telrad over the other units is the use of standard AA batteries and the fact that there are atlases dedicated to its reticule pattern. It’s only disadvantage is its weight of 11 ounces.

The Rigel Quickfinder is the second popular unit. It weighs only 3.3 ounces and stand taller than the Telrad. It is excellent for small refractors and small catadioptric scopes (Meade ETX), as it does not through of the scope’s balance. It also has adjustable brightness, as does the Telrad. In addition it has a pulsar unit, which blinks the LED on and off , allowing easier visualization of dim objects.

The third unit power finder is the Orion E-Z Finder. This is another lightweight unit. It is a modified Daisy BB gun scope. It projects only a red dot and not a reticule. It is a little expensive in my opinion.

My recommendation? Telrad for main finder for beginners. Once you gain experience and start star-hopping, an 8×50 or 9×60.

Scope Links

Here are some links for informative sites on what 2 Buy.

Astronomics www.astronomics.com
Orion Telescope and binocular www.telescope.com
Astromart www.astromart.com
Purchasing Amateur Telescopes FAQ’s
Kevin’s Astronuts Homepage


First appearance: January 23rd, 2011. Page subject to updates.