Just be aware that what you’re reading is someone's opinion which may or may not be 100% correct. For instance:-
"A 2x Barlow functions by dividing the focal length of any eyepiece by two"
That is not correct, the functioning of the barlow has nothing to do with the eyepiece its used with. A barlow is a concave tele-negative lens and works by acting on the light cone prior to it reaching the eyepiece, that’s why its inserted before the eyepiece in the light path. A standard design 2X barlow works by halving the angle of incidence of the light cone thereby effectively doubling the focal length of the telescope objective. You could look at the effect of the barlow as halving the focal length of the eyepiece in terms of its effect on magnification, but that’s not how it works. The article correctly states that inserting the barlow earlier in the light path will increase its magnification effect. ie. In a SCT or refractor a 2x barlow inserted before the diagonal effectively becomes a barlow of "about 3X" magnification.
Another example of a clear error:-
Erfles
Invented in the .1940's, the 5 or 6-element Erfle delivers a wide 60 to 70 degree apparent field of view."
Heinrich Erfle designed the type I Erfle whilst working for Zeiss in 1917, for military use. The types II and III Erfles, were designed sometime between 1917 and Erfles death in 1923. The 3 designs were all patented by Zeiss immediately after Erfles death in 1923 so its anybody's guess how this bloke thinks they were invented in the 1940's. Wrong war maybe ?
The article does however contain some helpfull tidbits for beginners, but to be honest I would recommend a good book like Starware by Phil Harrington where you know the content has been properly researched and checked.
Clear Skies
John B
) for a dob.. or what i mean is is there a type of eyepiece that is optically better matched to our dobs? Or should the dob owners just stick with plossls until we can afford naglers?
