Accessories
Eyepieces
Here's one area where beginners tend to go overboard. You don't really NEED more than 3 or 4 carefully chosen eyepieces, a barlow, and perhaps a filter or two, but most of us eventually wind up with collections, some of them quite impressive. Eyepieces: The first accessory a newcomer buys is usually a new eyepiece. Below is a guide to various eyepiece designs.
Ramsden and Huygenian are 2-element eyepiece designs. While simple, they exhibit narrow fields of view, have numerous aberrations, and terrible edge correction. Generally supplied only with the least expensive telescopes. While not of much use visually, they make good solar projection eyepieces (i.e. you can risk 'em). About $25-$40.
The Kellner is a three element design that shows an acceptable 40-45degree FOV (field of view), and good correction of spherical and chromatic aberration. Offshoots include the Meade MA, Celestron SMA, and Edumnd RKE. A decent general-purpose eyepiece for the price. About $30-$50.
Orthoscopic eyepieces were once considered the best for general use, but have lost some of their luster compared with newer Plossl designs. Using 4 elements, they are still popular for planetary work. They are well corrected throughout their 45 degree FOV. About $40-100.
The Plossl is the most popular eyepiece design today. Using 4 or 5 elements, they are very well-corrected and have a wider (50-52 degree) FOV than Orthoscopics. However, some models are said to have shorter eye relief than Orthos. These are what are supplied with most telescopes you purchase today. About $50-$150.
Erfles seem to have fallen out of favor these days. Using 6 elements, Erfles throw up a wide 60-65 degree FOV, with perhaps some minor aberrations near the edge. About $75-$150.
Newer designs, primarily from the efforts of TeleVue, are gaining in popularity. These include the 6 element, 67 degree FOV Panoptics (about $200-$400) and the 7-8 element, 82 degree FOV Naglers (about $175-$425). Both series are truly amazing. It is said that once you have looked through a Nagler, nothing else will be good enough for you. As a Nagler owner, I think they might have a point, sparked by the success of the TeleVue eyepieces, the Japanese have gotten into the act. The Meade Super Wides ($140-$300) and Ultra Wides ($170-$300) are virtual clones of the TeleVues. And Pentax's 6-7 element SMC-XL (about $250 each) are thought by some to exceed the performance of the TeleVues, especially at the longer focal lengths. Vixen's Lanthanums ($150-$250) throwout a generous 20 mm of eye relief regardless of focal length, and are a god send to those who must wear glasses while observing.
Barlows
Many observers find a barlow lens to be a valuable accessory. Inserted between the focuser and your eyepiece, a barlow will typically double or triple the magnification of any eyepiece. Thus, for $60-$200, you have effectively doubled the size of your eyepiece collection. Also, a barlow preserves the eye relief of your longer focal length eyepieces, thus reducing the amount of squinting you have to do. Barlows work better with the medium and low power eyepieces. I own a 2.5X and a 5X barlow. Although I don't use them all the time, it is good to have at least one for those "get up close and personel" times.
Filters
For planets, color filters are the best thing to use. An inexpensive four set (green, red, yellow and orange), can be had for $35.00. They help bring out the detail and sharpness in bright objects. A "moon" filter is a must if you enjoy looking at the moon like I do. About $9.95 to $15.00. After the moon gets more the 1/4 full, it becomes to bright to look at for long. If you live in or near city lights you may want to invest in a Broadband Light pollution filter. This will get rid of some of the glaring background lights reflecting from street lights. There are also "nebula" filters to help with those deep sky delights, brings out the details and cuts pollution. For daytime observing you can always get a Sun filter. These will allow you to look directly at the sun with the satisfaction of not having laser surgery. These usally slide on the front of your telescope, reducing the amount of sunlight BEFORE enetering these main objective lens.
Diagonals
Used with Refractors and SCT's these bend the passage of light from the back of your telescope to a 45 or 90* angle, so you can look down instead of trying to look straight through. If the telescope is pointed up, with out a diagonal, you might have to get on your knees on the ground. Most all telescope (except Dob's) come with a diagonal. An L shaped piece that goes between the telescope and your lens. Like all things, you can use the standard 1.25" model($35 - $120) or you can get a "enhanced" reflection, multi coated 2" model for $200 to $300. These can be removed from the scope for terristrial viewing and for photography.
Telrads and Finder Scopes
Heaters/Fans
Focusers
Tripods
Computer Object Locators/Incoders
