The focal length of the eyepiece f'OK (mm) uniquely associated with the magnification of the telescopeG = f'about/fOK, where f'about - the focal length of the telescope lens in the focuser the eyepiece. Thus the choice of focal set of eyepieces for a telescope is the choice of a reasonable set of increases this tool.
The lower limit of a reasonable range of magnifications ("ravnopravki increase") is determined by the fact that the diameter of the exit pupil at lower magnification becomes more the observer's eye pupil.
Since pupil diameter in the dark is 5-7 mm, Gis equal.Trac.about/6 - ravnoznachnosti increase.
The upper bound of rational gains ("allow increase") linked to the influence of diffraction phenomena (with increasing zoom reduces the size of the exit pupil of the telescope). Empirically found that the highest resolution is achieved when the exit pupil of the order of 0.7 mm and further increase does not lead already to a better conspicuity of details.
For observing close double stars and for the alignment telescope is often convenient to use and large increase in 2D-3D. This is especially true for people with disabilities.
For convenience of selection of the offer the following table (XLS, 10Kb). It allows you to quickly select a set of eyepieces for your telescope.
Necessary in the red cell to insert the focal distance and the diameter of the lens of your telescope.
It should be noted that for telescopes of small aperture (>F/9) the table can recommend eyepieces with great focal distance. For example, for D=90 F=1250 "ravnoznachnosti eyepiece" should be fokinym a distance of 83 mm, These eyepieces do not happen.
Abouticno, the longer eyepieces is 40-50 mm. So you have to limit their appetites and possible visible fields of view of such telescopes.
And "rapid" Newton (F/5) for over major increases are recommended for eyepieces with a focal length of 1-3 mm. That means that you must think about purchasing the Barlow lenses.
The Barlow lens is often effective to overcome some of the difficulties in the use of eyepieces, especially short throw (they have usually a small removal of the eyeball) and can replace one or two of korotkoplodnyh. For example, a set of eyepieces 15 and 5 mm with 2x Barlow lens gets the virtual "Deposit" in the form of 7.5 mm and 2.5 mm eyepieces. The removal of the pupil will remain.
But there are limitations. If 2x Barlow lenses work quite well, 3 or more make a measurable distortion in the image.
I hope that this article will help you understand which set of eyepieces to collect.