1. A search for eyepiece fundamentals gave me the following address:
http://www.rocketroberts.com/astro/eyepiece_basics.htm Also a search for eyepiece designs gave me the following address:
http://www.quadibloc.com/science/opt04.htm Two important eyepiece parameters are focal length expressed in millimetres & aparent field of view (AFOV) expressed in degrees.
The barrell diameter (1.25" or 2") determines the maximum true field of view (TFOV) an eyepiece can have with a given telescope. Typically the 1.25" eyepiece can have focal lengths from 2mm to 40mm. 2" eyepieces are typically used for lower powers with wide apparent fields but focal lengths can vary from 6mm Ethos to 55mm Plossl in the Tele Vue range.
2. Eye relief is the point at which your eye is located away from the eyepiece eye lens to take in the image fully. Long eye relief is typically 20mm or more and is suitable for people who observe with glasses on. 20mm eye relief means your eyeball is 20mm from the eye lens of the eyepiece. If you wear glasses for astigmatism then you may want to observe with glasses on especially if the astigmatism is severe. If no astigmatism is present in your eye but are long or short sighted then you can adjust the focus for your eye by moving the telescope focuser in or out. Note another observer will have to re-focus.
Therefore in answer to your question, you will use both 1.25" and 2" eyepieces depending on what magnification and field of view you want. Magnification is simply telescope focal length divided by eyepiece focal length. True field in its simplest form is eyepiece AFOV divided by magnification although I prefer another formula that involves the field stop diameter of the eyepiece which I wont go into now.
3. Your 12" will probably be f5 (focal ratio = focal length of telescope divided by diameter of primary mirror). All parabolic mirros have coma which causes stars to appear as tear drops at the edge of field. A Paracorr sharpens the edge stars by reducing the coma significantly. Some people are annoyed by coma and some are not. My advice is get a quality eyepice first so that you only see coma and nothing else and then decide if it bothers you or not. Poor eyepieces have field curvature and astigmatism than makes the edge stars out of focus and/or look like seagulls. Some lesser quality eyepiece perform poorly at f5 but are very good at f10.
4. A Barlow lens magnifies the image by the Barlow factor. eg. A 2x Barlow makes your effective telescope focal length twice as long OR your eyepiece half as long whichever you prefer (same result). A Powermate is a complex Barlow that retains certain eyepiece parameters and is available in higher multiplying factors, (can also be used for imaging).
5. 35mm could refer to eyepiece focal length, film format, anything else that is 35mm long. Lets say 4.5 mm is an exit pupil number (could be focal length too). Exit pupil is eyepiece focal length divided by telescope focal ratio. It is the diameter of the light circle leaving the eyepiece, ie. what your eye sees. Your pupil can expand to 5mm to 8mm depending on age and if the exit pupil is larger than your pupil diameter you are not taking in all the light available.
6. Filters are available in various colours and are used mainly for observing planets. There are also wide and narrow bandpass filters which let through specific wavelengths that most nebulae emit and thereby enhancing the contrast significantly.
Hope this all helps.
