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.