Eyepices Question

[ElPelado]

Hi again. i have an other question now: i know there are eyepicies with different diammeters, 1.25" and 0.9" are the most common i think. my telescope has a 1.25" hole for the eyepice, but i can also put the 0.9" ones that came with the telescope. someone told me that i should buy 1.25" because those are better.
Can someone told me what's the diference between the diameter of the eyepice???
Thanks.

[t00fri]

Hi again. i have an other question now: i know there are eyepicies with different diammeters, 1.25" and 0.9" are the most common i think. my telescope has a 1.25" hole for the eyepice, but i can also put the 0.9" ones that came with the telescope. someone told me that i should buy 1.25" because those are better.
Can someone told me what's the diference between the diameter of the eyepice???
Thanks.

ElPelado,

the standard eyepiece diameters for 'more serious' telescopes are 1.25 " and 2" (for deep sky wide field). The 0.9" is a size that came originally from Japan and is still to be found in cheaper telescopes (department stores) like yours.

In principle, you could have excellent eyepieces for any size.

Yet, if you want to cover a very large field, you obviously need a large diameter, like 2"! In general, the optical design of an eyepiece is characterized by the name of its inventor, like

Huygens (cheapest, non-achromatic, 45deg),
Ramsden (similar, but exists also with one achromatic lens),
Kellner (one acromatic lense, field lens non-achromatic)
Othoscopic (fully acromatic, 4 lenses)
Ploessl (very good symmetrical design, 4 lenses, 50 deg),
Erfle (5-6 lenses, wide field ~65 deg),
Nagler (super expensive 82 deg field)...
...

just to name a few. The prices vary tremendously among the different kinds.

The 0.9" ones are typically Huygens, Ramsden or perhaps Kellner designs.

The field that is specified for an eyepiece is the so-called apparent field of view. You must divide it by your magnification to get the true field.

Suppose you have an eyepiece with 50degs apparent field and 100 times magnification then the full moon (30 mins= 1/2 deg) just fits into your view...

Bye Fridger

[ElPelado]

thanks for the info fridger, but you didnt answer my question. i want to know whats the difference between them. just the field of view??

[guest]

ElPelado:

The size refers to the diameter of the barrel, and correspondingly (in principle) to the diameter of the field stop (the opening at the focal plane), so that fatter eyepiece let a broader light cone through. As said, any class of eyepiece can be good or bad, cheap or expensive. However, the 0.9 are mostly poor quality. I recommend you stick to the 1.25 class. You can find reasonable Ploessls for 20-30 us dollars.

Cheers,

Guillermo Abramson