Telescope mountings

Huygens (H): Ancient technology, as in over 300 years old. Apparent field-35 degrees Eye relief 30% of focal length

Kellner(K)/Achromatic Ramsden (AR)/Modified Achromatic (MA): What comes with most telescopes. Inexpensive. 40 degree apparent field of view, eye relief 80% of FL

Orthoscopic (Or): An improvement over above. Apparent field 45 degrees, eye relief 80% of FL

Plossl (PL): Eyepiece of choice for many due to reasonable cost, excellent performance. Apparent field 50 degrees, eye relief 80% of FL

Erfle (Er): A good low power, low cost 2" eyepiece. Apparent field 65 degrees, eye relief 35% of FL.

Nagler Type 2: Modern technology. Expensive. Very wide field but not for eyeglass wearers. Apparent field 82 degrees, eye relief only 10mm.

Lanthanum: Made for eyeglass wearers. Average FOV, generous eye relief. Apparent field 50 degrees, eye relief 20 mm

Konig: The authors choice. Generous FOV (65 degree) and eye relief. A true wide field eyepiece at reasonable prices. Find them at University Optics.

Reflectors - Refractors - Binoculars - Schmidt Cas- Mountings - Eyepieces - Filters - Goto Telescopes
The Barlow Lens: The Barlow lens is a concave lens that fits in front of the eyepiece. It enlarges the image and in effect increases both focal length and focal ratio. The amount of magnification depends on how far the lens is placed from the eyepiece. A 2X Barlow doubles the effective power of an eyepiece without affecting the eye relief.		The eyepiece does the job we all think of when we think of telescopes; it magnifies. But there is more to the story than that. Chances are, the eyepiece that comes with the telescope you purchase is not the very best. That's ok, because part of the fun of owning a telescope is constantly doing things that make it better. Upgrading your eyepieces is easily the best way to improve a telescope. But which eyepiece is best? Now it gets tricky. The best eyepiece is the one that matches your telescope with your eye, and the type of observing you like to do most. Lets look at some eyepiece facts. Eyepieces come in 3 barrel sizes: 24.5 mm (0.965 inch) for smaller Japanese telescopes 1.25 inches (32 mm) Standard on most scopes 2 inches (50.8mm) for larger scopes Many telescopes have 2 inch focusers and an adapter for 1.25 inch eyepieces. There are few options for owners of telescopes with the smaller Japanese eyepieces, except to purchase a hybrid diagonal that converts it to use of 1.25" eyepieces. The longer the focal length of the eyepiece, expressed in milliliters, the lower the magnification. To find the magnification of the system, divide the focal length of the eyepiece into the focal length of the telescope. A 6 inch F-8 reflector has a focal length of 48 inches, or 1219.2 mm. (48 X 25.4mm). A 25mm eyepiece will give power of 48.7. A 12.5mm eyepiece will double the power. As the power is increased, the field of view, the amount of sky we see in the eyepiece, shrinks. It is easier to find objects in the sky when the field of view is as wide as possible, so always use low power when hunting.	Huygens (H): Ancient technology, as in over 300 years old. Apparent field-35 degrees Eye relief 30% of focal length Kellner(K)/Achromatic Ramsden (AR)/Modified Achromatic (MA): What comes with most telescopes. Inexpensive. 40 degree apparent field of view, eye relief 80% of FL Orthoscopic (Or): An improvement over above. Apparent field 45 degrees, eye relief 80% of FL Plossl (PL): Eyepiece of choice for many due to reasonable cost, excellent performance. Apparent field 50 degrees, eye relief 80% of FL Erfle (Er): A good low power, low cost 2" eyepiece. Apparent field 65 degrees, eye relief 35% of FL. Nagler Type 2: Modern technology. Expensive. Very wide field but not for eyeglass wearers. Apparent field 82 degrees, eye relief only 10mm. Lanthanum: Made for eyeglass wearers. Average FOV, generous eye relief. Apparent field 50 degrees, eye relief 20 mm Konig: The authors choice. Generous FOV (65 degree) and eye relief. A true wide field eyepiece at reasonable prices. Find them at University Optics.

	When it comes to field of view, not all eyepieces are created equal. Each design has an "apparent" FOV. The eyepieces supplied with new telescopes usually have a low apparent field, around 40 degrees, while some wide-field types have as much as 80 degrees. Apparent field is not the amount of sky visible in the eyepiece. That is found by dividing the apparent field by the power. The 25mm eyepiece in the example above will have a FOV of .82 degrees, while the 12.5mm will show only .41 degree, or less than the diameter of the moon. However, if a 12.5mm wide-field with 80 degree afov is substituted, we get the increased magnification but with a nice .82 degrees of sky visible. There is another consideration related to eyepiece design that is important. Eye relief is the distance from the eyepiece to where the image is formed for your eye. This also decreases as power goes up. With inexpensive, factory included eyepieces, the image at high power forms so close to the eyepiece it becomes difficult to view. And if you wear glasses, you may not even be able to see the image. A good way to judge eye relief by looking at an eyepiece is to look at the diameter of the glass closest to the eye, or eye lens. If it is small, then so is the eye relief.