The eyepieces coming with a telescope, if any, are often of bad quality
and not well suited in their range of focal length.
For beginners we recommend for parallactical mounted telescopes at least 4 eyepieces of the Ploessl- or Super-Ploessl type. For Dobsons you should use about 4 eyepieces also, but better wide angle types, because a wider field of view for this type of telescope makes it easier to find objects and to keep them within the field of view.
The focal length can be derived as follows:
- Shortest focal length: according to the useful maximum magnification – see basic formaulas
- Longest focal length: according to the useful minimum magnification – see basic formulas
- In-between at nearly same factors of focal length 2 additional eyepieces
If the shortest focal length is lower than about 4mm, then you should
think about using a Barlow lens – but a good one, as any additional optical
element is leading to a degradation of the optical system. Exception from this
basic rule are optical systems, which make use of additional lenses to achieve
a better corrected field, minimized chromatic errors, wider field of etc.
Example: refracting telescope, 100mm objective diameter, 800mm focal length
Shortest focal length:
Useful maximum magnification: about 200-times, according to 5mm eyepiece focal length
1. Eyepiece: orthoscopic or Ploessl type, 5mm (may be 4mm)
Longest focal length:
Useful minimum magnification: aperture ratio F/8, therefore about 50mm at 6.25mm exit pupil.
2. Eyepiece: If you can use 2”-eyepieces, do so. Otherwise use a Ploessl at about 50mm focal length, at least 40mm.
In-between focal lengths:
3. Eyepiece about 10mm (= 5mm times 2)
4. Eyepiece about 20mm (= 5mm times 2 times 2)
That is your basic set.
Try it for a time, your need for extension will come automatically. This may be additional eyepieces at short focal lengths to take the best one according to weather conditions for planetary observations, or a really high-priced wide angle eyepiece to have the impression of walking through the space, etc.