What does eye relief mean?
Distance from your eye to the glass in the eyepice. Long eye relief = more comfortable view - some eyepieces (eg Orthoscopics - simple designs) require the eye to be very close to the lens and stationary ie short eye relief.
Do I use 2" or 1.25" eyepieces?
Your 12" dob will allow the use of both - it has an adapter as standard.
2" eyepieces are larger/ heavier often more complicated and expensive designs. Once again your Dob will probably ship with a 2" and a couple of 1.25" eyepieces.
What is a Paraccorr
An expensive corrective lens made by Teleview to flatten out the inherent curvuture of the field due to the shape of the mirror. as the mirror is slightly curved, stars in the centre of the field are sharp but start to elongate towards the edge of the field of view - a Paracorr flattens this abberation out.
and a Barlow
A barlow is a lens which multiplies the power of the eyepiece by a factor dependant on the power of the barlow (ep 2x or 3x are common) So, if you're using an eyepiece that yields a 100x magnification...using a 2x barlow will result in a magnification of 200x (easy?)
...and where would I use them?
Probably to push the magnification a bit on Planets
What does 35mm or 4.5mm mean?
These are the focal lengths of the eyepieces and put simply these numbers represent the 'magnification power' of your eyepiece...a bit of calculation is necessary though. To work out the magnification for each eyepiece you must know the focal length of your scope - usually designated as F=1000mm for example. To obtain the magnification for an EP (eyepiece) one simply divides the Telescope focal length by the EP focal length...1000/35 = 28.6x magnification 1000/4.5= 222.2x magn'.
You can see then that the smaller the focal length of the EP - the greater its magnification.
Hope this has helped a little - others will add responses too to answer your other questions I hope
All the best
Have Fun
Doug
