Hi there, welcome to Part three of the video
series, “Telescope Basics.” My name is David Fuller, host of the “Eyes
on the Sky” weekly stargazing series that can be found at eyes on the sky
dot com. In this video, I will explain the basics of
standard eyepieces used in amateur astronomy telescopes. The first thing to know about eyepieces, is
that your telescope is only as good as the eyepieces you use with it, though you don't
need to spend a lot of money to get decent views, either They come
in three barrel diameters, even if one of them is largely not manufactured anymore:
.965 inch, 1.25 inches and 2 inches. You can easily use adapters to fit smaller
eyepieces into larger focusers, but it's a bit more difficult to go the other way, though
.965” adapters for 1.25” eyepieces do exist.. So why do we have different eyepieces and
different barrel diameters for them? Well, in the prior video we learned about
focal ratio. Shorter focal ratios allow telescopes to be
shorter. What that also does though, is make the angle
of the light cone much steeper, so larger eyepieces are needed, otherwise part of the
light cone vignettes, or gets cut off, usually due to focuser tube
diameters. Vignetting is not a good thing, because it
effectively reduces the aperture of your telescope, and the larger your aperture, the fainter
objects you can see. No sense in reducing aperture unnecessarily! ["Billionaire" Character] “That would be
dumb.” So the .965” eyepieces were used in the
long focal length telescopes, because they would not vignette the shallow light cone. They could also be simpler designs. Christian Huygens developed one of the first
true eyepieces that used two lenses to focus the telescope's light. A simple design, they also have a narrow field
of view, often just 35 or 40 degrees. These are often designated with an “H”
on the top or side. A similar 2-lens design is the Ramsden, usually
signified by an “R” or “SR” on the eyepiece. Because of the simplistic two-lens focusing
set up, they are adequate for short focal lengths in long f-ratio
telescopes. When made well and paired with longer focal
length telescopes, they can perform fairly well,
but most current offerings are done inexpensively and placed with shorter f-ratio scopes, which
means they often do not provide good views and produce optical aberrations. A somewhat better design is the 3-element
Kellner eyepiece. Offering a slightly wider apparent field of
view of 45 degrees, it also does a better job
of correcting some of those optical aberrations of slightly faster telescope designs. It is usually designated by a “K” on eyepieces. The Edmund Scientific
company took that design, flipped it upside-down, and called it an “RKE”. Meade did something similar terming it a “Modified
Achromat” or “MA.” Though simple in design and inexpensive to
make, they do perform adequately in f/8 instruments or longer, though the apparent field of view
is somewhat restricted. But the real breakthrough is the four-element
Plossl, today's workhorse, standard “good” eyepiece. With a 52 degrees apparent field of view,
and good optical correction for aberrations, even in somewhat
faster telescopes, these are an eyepiece that can be used in most any telescope and it will
work fairly well. They are not terribly expensive, and in mid-to-longer
focal lengths also offer good eye relief. Wait, eye-WHAT? Yeah, eye relief. That is the distance you can place your eye
away from the lens and see the full field of view. And the problem with all
of the eyepieces above is that the shorter the focal length, the shorter the eye relief
is, until you get situations like this. If you wear glasses, this can be problematic. More exotic types of eyepieces have been developed,
with 5, 6 and sometimes 9 or more lenses (occasionally making for very heavy and LARGE eyepieces!)
that can be used in very fast telescope, have very good eye relief, and with enormous apparent
fields of view – but correspondingly, they also cost quite
a bit more too. One thing to look for in lesser eyepieces
is to be sure that all of the lens elements are coated with anti-reflection coatings – at
minimum, a layer of coatings, and ideally, every lens surface will be multi-coated. Expensive eyepieces will have this standard,
the lower cost the eyepiece, the less likely all lens elements
– and all sides of lenses! - will have them. Without them, it does lower light throughput,
hurting what you can see. This Huygens eyepiece has no lens coating
at all, and it came with a short focal length
f/4 telescope; that's an inexpensive, but poor, combination. Keep that in mind when shopping for telescopes;
many starter telescopes are sold with quality Plossl eyepieces,
but longer focal length ones will perform adequately with Kellners too. So those are the basics of eyepieces. Now in part four of this video series, let's
take some of the knowledge from these previous videos to understand apparent and
telescopic field of view in the next video. And also, would you like to see what's up
in the sky this week? Look for the weekly sky videos at eyes on
the sky dot com. Thanks for watching; keep your eyes on the
sky and your outdoor lights aimed down, so we can all see, what's up.
