Originally posted by aaronLiu
Hi guys...
Someone I know is about to make a telescope purchase... and was wonder what are all those extra eye pieces that come as accessories do...
Some of them have a wider degree of "Apparant Field".
Are all these extra accessories needed? Or does the standard set already include most of the stuff that one needs...
Could someone enlighten me? 
Thanks
There's really no such thing as a 'standard' set of eyepieces, unless a telescope is bundled with a few eyepieces.
It largely depends on what you will be observing, if you are planning to be a general observer, that is a bit of planetary, lunar, some deep sky (given the light pollution in SE Asia as awful) etc then the normal Plossel occulars with a 50 degree apparent field of view will be fine.
General recommendations (based on 30 years ATM experience, 35+ years observing).
Orthoscopics (Orth/Ortho) - Best occular design for serious solar and planetary observing. Excellet for lunar work too. Narrow field of view. Typically around 40 degree apparent field of view.
Kellners (KE) - Good general purpose occulars that aren't expensive, narrow field of view that isn't always good for larger objects, slight colour fringing can be apparent. Apparent field from 35-42 degrees typical.
Plossl - Pretty much the defacto standard occular these days. 50 degree apparent field of view and reasonably good coma and colour correction.
Konig - Hard to find, but with superior performance to most plossl designs. 60-65 degree apparent field of view with low coma and excellent fringe colour correction. Not popular these days as the ultrawides from the likes of Meade and teleview dominate the market. My preferred occular for general observing.
Ultrawides. Large, heavy, expensive boat anchors that give lovely views but at great expense. Frankly not worth the money in 1.25" fittings as you really need 2" or larger occular diameters to appreciate these occulars. Apparent fields of view from 70-90+ degrees.
Symetrical Ramsden (SR) - Old old design, bloody awful and come standard with most cheap telescopes. Horrible colour fringing and optical distortion. Old high quality Ramsdens are very good though but have a very narrow field of view.
Huygenian, Huygenian Ramsden (H, HR). Again an old old design, bloody horrid to be frank, again shipped with cheap telescopes.
There are some specialised occular designs such as solid ramsdens that are excellent for planetary work (rare, expensive).
What the 'apparent field of view' actually means.
An eyepiece (ocular) has like any other lens an apparent field of vie that is determined by the field stop. The telescope optical system (lens, mirrors) also have and field of view and to calculate the actual field of view seen through the eyepiece use the following formulae.
Step 1
Calculate eyepiece magnification. As Roy's pointed out the formula is Telescope focal length in mm / eyepiece focal length in mm.
eg: 1000mm telescope fl / 20mm eyepiece = 50x magnification
Step 2
To calculate actual field of view at the eyepeice in arc minutes.
(Eyepiece apparent field of view / eyepiece magnification.) x 60
eg: 50x magnification with 40 degree apparent field of view eyepiece.
(40 / 50 ) x 60 = 48 arc minutes
50x magnification with 82 degree apparent field of view
(82 / 50) x 60 = 98.4 arc minutes or 1 degree 38.4 arc minutes.
To calculate the field of view in degrees leave of the x 60 multiplication
Unless you are doing purely solar or planetary observing avoid eyepieces in the 0.960 inch size, go for a scope with a 1 1/4" (1.25") eyepiece size as it's far better from not only an observing standpoint but also in terms of choice of eyepieces.
