- focal length:
- determines final magnification of system
- the smaller the focal length, the greater the magnification =
primary objective focal length / eyepiece focal length
- remember maximum useable magnification is determined by both
aperture of telescope and seeing conditions, thus:
- in excellent seeing conditions with high quality optics, max.
magnification may be 50x per inch aperture
- in average seeing conditions, max. magnification is 25x per
inch aperture (ie. 1x per mm aperture)
- in daytime seeing conditions, max. magnification is ~75x
(hence solar scopes rarely need to be > 90mm aperture)
- barrel size:
- 2" is best but expensive, 1.25" is most common, 1"
is usually only for cheap beginner's scopes
- Huygen & Ramsden types:
- The most simple and somewhat primitive design available. These
are usually the low quality kinds that come with many telescopes
& are the poorest quality
- Kellner ($110 for 30 & 40mm)
- Economical, general purpose eyepieces that deliver a
moderately wide field of view, fairly short eye relief and are
best suited for low to medium magnifications.
- Kellner and RKE (Edmund Scientific's patented
modification of Kellner) are a three-element design that produce
images in a 40-degree field of view, with some chromatic
aberration. They have good eye relief. Kellner eyepieces work
best in long focal length telescopes. They are a OK balance
between performance and economy, but you will usually be better
off with a Plossl.
- Plossl ($55):
- have a four-element or five-element design, with a 50-degree
field of view. They have good eye relief (except for 10 mm and
shorter lenses). They work best in the 15- to 30-mm size. The
quality is good, especially for planetary viewing. They have
some astigmatism, especially at the edge of the field.
- Sharp images across the field. Good color and field of view.
Performs well at higher magnifications. Eye relief is OK.
- Twenty years ago, these were considered "luxury"
eyepieces; today they are normal general-purpose eyepieces.
- Super Plossl:
- Excellent image sharpness, minimum distortions, wide fields of
view, and OK eye relief. Performs well at all magnifications.
- This is 5-6 element design with field of view 55-60° and
better view quality than typical Plössl. It has very good eye
relief that is recommended for observations at medium to high
- Erfle 63deg ($150)
- were invented during World War II. They have a five-element
design and a wide, 60-degree field of view. They suffer from
ghost images and astigmatism, which makes them unsuitable for
planetary viewing. Improvements on the Erfle design are called
- Orthoscopic 43deg ($130)
- were invented by Ernst Abbe in 1880. They have four elements
and a 45-degree apparent
field of view, which is somewhat narrow. The optical design
gives a crisp view, has a good eye relief, and is considered
excellent for planetary viewing.
- 2" Konig 70deg 25-40mm ($300-$500)
- Nagler (82deg):
- introduced in 1982, advertised as "like taking a
spacewalk." They have a seven-element design with an
incredible 82-degree field of view. The original series came in
2-inch barrel size only, and were heavy -- up to 2 pounds (1 kg)
-- and expensive and eye relief only 7-10mm for high power ones.
- Nagler eyepiece complemented fast Dobsonians and Newtonians by
removing eyepiece induced astigmatism.
- Series 4 - Experience of designing the
Radian eyepieces was applied to the development of the type 4
Nagler to series to give more contrast, reduced pincushion, more
true field and longer eye-relief (17mm) in 12mm, 17mm and 22mm
focal lengths. with added eye-relief the click stop instadjust
eyeguard helps maintain proper eye placement. 8 element design;
- Series 5 - 6 element design - whilst the 1
1/4" 16mm is only 0.4lb and has eye relief of 10mm, the
massive 2" 31mm is 2.2lbs with eye relief of 19mm;
- Series 6 introduced in 2002 - 7 element design with exotic
materials are more compact, lighter and have better contrast
with 12mm eye relief for 5-9mm focal lengths but are a more
expensive series. Still has some pincushioning;
- Panoptic (68deg 15-35mm focal lengths):
- 10mm eye relief for the 15mm; has some pincushioning;
- Radian (60deg, 20mm eye relief 3-18mm focal lengths):
- By optimising the design with fully coated exotic glass, Tele
Vue achieved their goal of full field sharpness with true
orthoscopic linearitiy, highest contrast for critical planetary
viewing, and compact size
- The size and weight blend of these Radian eypieces make them
preferable for small scopes and bino-viewers, and have the added
convenience of being parfocal with other 1 ¼ inch Tele Vue
- Very modern optical design by Vixen ("LV") with a
Lanthanum optical glass used. It has an extra long eye relief
20mm for all sizes, comfortable observation even with glasses,
excellent field of view virtually eliminating ghosting and
flare, recommendable for observations at medium or high powers.
- not as bright and lack the contrast of orthoscopics and Radian
and so not quite as good for planetary detail.
- This are very modern variations of Erfles and Naglers
("UWA" & "SWA"). Various improved
designs incorporating 6 to 8 lens elements boast apparent fields
up to 90° — so wide you have to move your eye around to take
in the whole panorama (which some people like and others
- Light transmission is slightly diminished because of the
additional lens elements, but otherwise the image quality in
these eyepieces is very high. So, too, can be their price.
- 14mm Meade UWA has flatter field and much less pincushioning
than a 13mm series 6 Nagler but is very much bigger and heavier,
has less contrast and has a greater central kidney bean or
darkening effect which, although not very troublesome for night
viewing, is a real bother during the daytime.
- Takahashi LE eyepieces in 1999, were brighter, more
contrasty and gave better resolution than Televue's Plossl or
- buy the best you can afford although can easily buy them later
- apparent viewing angle or field of view (AFOV):
- this refers to the angle of view of the eyepiece in terms of the
light coming from the telescope
- the true field of view (TFOV)
is dependent on the telescope magnification and the eyepiece
apparent field of view
- normal viewing angle is about 43deg
- "wide angle" usually have viewing angle of 63deg
- "extra wide angle" usually have viewing angle of
- area of the eyepiece field of view:
- the area of sky one views through the eyepiece increases by the square
of the increase in FOV. As an example, if one increases the FOV by 50%,
the actual increase in area of sky seen through the increases to 225% of
the original. i.e.- an eyepiece with a 75 degree AFOV covers an area of
sky 2 1/4x as large as an eyepiece with a 50 degree AFOV at the same
- eye relief:
- The eye relief is usually defined as the distance from the vertex of
the eye lens to where the exit pupil is formed. The eyepiece acts like a
camera lens, and images the entrance pupil (the telescope's objective)
at the exit pupil.
- eyerelief is a function of the entire optical system and is not fixed
for any particular eyepiece.
- when wearing glasses, it may not be possible to get close enough to
see the full field of view and the percent fiel of view in such cases
depends on the eye relief of the eyepiece and its physical design but is
often only 33-40% but may be 60% with the better ones and even 90-100%
with ones such as Televue's Radian eyepieces.
- for high power view, it may not be necessary to see the full field of
view anyway as you are often only interested in the central planet
- for attaching a camera with camera lens, long eye relief is desirable
to minimise vignetting as this is minimised by having the camera lens
diaphragm as close as possible to the exit pupil of the eyepiece.
- You can use the simple formula:
- 1/eyepiece f.l. = 1/scope f.l + 1/image distance
- The formula is useful to calculate the GROWTH in eye relief (image
distance minus eyepiece focal length) as eyepiece focal lengths and
object distances vary.
- For most systems, where the eyepiece is a small fraction of the
telescope focal length, the growth is insignificant.
Note that long focal length eyepieces will show more significant
movement than short focal length eyepieces.
- Let's use a worst case scenario:
- a 50mm Plossl eyepiece with a 500mm scope:
- 1/50=1/500+1/x => .02=.002+1/x => 1/x=.02-.002=.018
=> x=55.6 => Growth=55.6-50=5.6
- a 50mm Plossl eyepiece with a 1000mm scope:
- 1/50=1/1000+1/x => .02=.001+1/x => 1/x=.02-.001=.019
=> x=52.6 => Growth=52.6-50=2.6
- It's pretty tough to notice a 3mm difference in an eyepiece
with approximately 35mm of eye-relief. Note also that if the
object distance is long (at infinity) the image distance equals
the eyepiece focal length.
- However, this formula does NOT work with a Barlow since a Barlow
dramatically shortens the object distance (entrance pupil) throwing
eye relief way out, for long focal length eyepieces.
- 1/f = 1/e + 1/( f + F )
- where f = focal length of eyepiece lens; e = exit pupil distance;
F = focal length of the primary.
- A quick pass through this formula with real values shows that e ~=
f but e is slightly smaller. Now, of course, almost all practical
eyepieces are multiple lens assemblies. This means that the
effective principle plane for the exiting end of the eyepiece could
be extended out into the space behind the eyepiece. The position of
the exit pupil behind the eyepiece would then be e as calculated
from the formula plus the distance that the principle plane was
extended beyond the eyepieces exit lens. For a smaller focal length
eyepiece, moving the effective principle plane away from the
eyepiece is a method to keep good eye relief and still have shorter
effective focal lengths. How is this done, you might say? Lets us
look at the effect of using a Barlow. Remember, the exit pupil is
still where the image of the primary forms behind the eyepiece. The
Barlow does two things. First it effectively increases the focal
length of the primary, giving us more magnification for a given
eyepiece. It also modifies the effective optical distance that the
eyepiece sees to the primary. Still, the 1/( f + Fm ) term is quite
small compared to the 1/f. This means that the eyepiece still has
close to the same exit pupil distance. The addition of a Barlow lens
has made a composite lens system that has an apparent principle
plane moved outwards from the exit lens but the assembly has a
decreased effective focal length. If you must use your glasses, the
lanthanum series eyepieces that Orion sells is your easiest option.
The other option would be to take a number of 30-35mm eyepieces and
make custom Barlow setups for different equivalent focal lengths.
This is essentially what the lanthanum eyepieces are.
- The back focal length of the eyepiece is fixed for any eyepiece
and is the minimum eyerelief possible with it unless it is modified
with a negative barlow lens. This effect is what makes the Nagler
type eyepiece possible which can have a backfocus of 2 or 3 times
the efl of the eyepiece. A bonus to eyepiece designers is that the
curvature of field may be removed with no detrimental effect -
especially on astigmatism - with the barlow/eyepiece combination
optimised as one unit.
- approx. eye relief for various eyepiece designs as % of focal length
- Huygen's 30%; Kellner, Ramsted, Orthoscopic, Plossl 80%; Erfle
35%; Nagler 2 = ~10mm; Lanthanum = ~20mm;
- higher power and cheaper eyepieces (even Super Plossl) tend to have poor eye relief of
only a few mm
- some old eyepiece designs that suffer from narrow FOV and short
eye relief . These are the Huygens, Ramsden and Kellner. They are
designated with the letters H, R, and K or MA. These are the
eyepieces normally included with starter scopes. Try and upgrade as
soon as possible.
- some, such as Tele Vue's Radian series and Vixen have 20mm eye relief
- shorter focal length usually means shorter eye relief--that is, you
need to crunch your eyelashes right up against the eyepiece to see the
whole field of view.
- this is one of the main reasons for using a Barlow lens, it allows use
of eyepieces with larger eye relief to achieve the same
- exit pupil:
- exit pupil = eyepiece focal length / telescope focal ratio =
scope aperture in mm / magnification
- this is the diameter of the cylinder of light that exits the
- the exit pupil is the image of the objective as formed by the
eyepiece behind the eyepiece and can be located easily by placing a
white card behind the eyepiece and moving it until this forms a
minimum diameter. Best if the scope is pointed at a blue sky (not
- field of view doesn't enter this calculation at all. Where it does
sort of comes into play is in determining eye relief which is
basically the measurement of how far back from the end of the
eyepiece all of those cylinders intersect to form a common disk.
When you put your eye at that spot, all of the exit pupils enter
your eye at the same time and you see the full view. If you are
slightly inside, outside, or off-center from that point you will
only catch some of the exit pupil cylinders and get a truncated
- the largest most pupils dilate to in dark adapted eyes in younger individuals
is 7mm. As we age, our maximum exit pupil decreases to 5mm or less.
Having an exit pupil greater than 6-7mm may be a waste.
- THUS, best range of exit pupil for dark adapted eyes is 0.5 to
6mm with best contrast usually at 1-3mm.
- thus for a f/5.6 telescope, eyepiece focal length range for an
exit pupil 0.5 to 5mm = 2.8mm to 28mm with best contrast with
- thus for a f/10 telescope, eyepiece focal length range for an exit
pupil 0.5 to 5mm = 5mm to 50mm with best contrast with 10-30mm
- why is 50x magnification per inch aperture the max. magnification
possible assuming perfect seeing and optics?
- 1” = 25.4 mm => 25.4 / 50 x = 0.5 mm exit pupil
- the maximum focal length eyepiece you can reasonably use on an
obstructed light path scope (e.g. newt, mac/newt, cass, sct, etc.)
depends largely on the f-ratio of the primary mirror (and somewhat
on the %obstruction by the secondary).
- with a non-obstructed 'scope using a larger exit pupil can
have its advantages. Although it won't make the image any
brighter, it will enlarge the region in which you can take in
the whole field of view comfortably both in terms of width and
depth, so your eye-placement doesn't have to be as exact.
However with an obstructed light path 'scope the exit pupil has
a dark central region where the secondary casts it's shadow. A
longer focal length eyepiece means a larger central shadow. If
the shadow gets too big, then you get a floating dark spot in
the center of the view.
- for terrestrial daytime viewing, the pupil will be much smaller
say 3mm, and thus for an f/5.6 scope, max. eyepiece may be 16mm or
less and thus lowest visual magnification may be 8x per inch
- some of the eyepieces on the market can weigh over 2 ½ pounds.
There can be severe balance problems in some scopes when these hand
grenade sized eyepieces are swapped in and out.
- par focal:
- many series of eyepieces from the same manufacturer will have
their focal planes at the same point so there will be minimal
refocusing necessary when swapping
- try getting:
- ideally, an eyepiece should have:
- flat field of view so that everything is in focus
- excellent contrast - loss of contrast is what makes fine
planetary detail and faint deep sky objects hard to see
- if you wear glasses, then good eye relief
- 3 to 5 elements
- filter thread
- starter eyepieces:
- Plossl or Super Plossl designs should be considered the best
starter eyepieces. They tend to have 50-52 degree FOV, good edge
sharpness but still poor eye relief with high power eyepieces.
- save longer if that's what it takes to get a quality eyepiece.
They last forever.
- next step up in quality:
- Meade Super Wide Angles, the Televue Panoptics, and the Pentax
XL seris. These all have AFOV in the 65-67 degree range. The
quality is good, but one must be careful of exponential
increases in weight that accompany long focal length and long
eye relief. These are all great eyepieces if you can afford
- if u is rich then consider:
- Meade UltraWides and Televue Naglers. These all have 82-82
degree AFOV and are known as portholes into space. There are
multiple iterations in the Nagler line, Type 1-Type 5. Overall
these are exceptional. The prices range up to $US595 for the
31mm Nagler Type 5
- for planetary or double star viewing, consider a Televue
Radian 3-4mm with its high contrast and long eye relief, but if
money is an issue, consider either an orthoscopic for its
contrast or a Vixen Lanthanum for its eye relief.
- for deep sky viewing consider a Televue Nagler with its wide
field of view.
- the newer series 6 Televue Naglers are more compact, lighter
and offer more contrast than earlier series and will double up
for planetary viewing as well as deep sky viewing, but are the
- a wide angular view low power eyepiece eg. 25mm
- a high quality orthoscopic high power eyepiece eg. 9mm
- a Barlow lens to act as a 2x magnifier, allowing maintenance of
eye relief at the expense of contrast
- consider special eyepieces designed for digital astrophotography
such as the MaxView40
- Barlow lens:
- increase magnification usually 1.5 to 3x and thus allow longer focal
length eyepiece and thus better eye relief to attain same high power but
at cost of some loss of contrast.
- Televue Paracorr lens:
- used in similar way to a barlow lens but corrects the inherent defect
of coma in a parabolic mirror especially if fast f/ratio mirror.
- uses 2 multi-coated, high index doublets, is completely color-free,
center and edge, and installs like a Barlow. Coma is corrected so well,
the diffraction limited field area of an f/4.5 Dob/Newt is increased 36
- cannot use a barlow as well though as may hit the lens.
- may need to be "tuned" by rotating its top ring to adjust
eyepiece-Paracorr lens distance to optimum.
- can be used for both 2" and 1.25" eyepieces.
- erecting diagnonals:
- whilst these erect the upside down image the left-right reversal is
field of view (TFOV):
Methods of Calculating the TFOV of an eyepiece:
Note: TFOV = True Field of View, AFOV = Apparent Field of View
1: The Apparent FOV Method
2: The Stellar Drift Method
TFOV = (AFOV)/ (Fo/Fe) - note Fo/Fe is the magnification the eyepiece
ie. TFOV = AFOV / magnification
Where: Fe = FL of Eyepiece, Fo = FL of OTA
Note: Caution - AFOV is often quite different from the manufactures
TFOV = 15.04*Ts*cos(d)
Where: d = stellar declination, Ts = time in seconds for a star to cross
the center of the FOV
Note - This is the most accurate method of calculating the actual true FOV
because it deals with what is actually observed through the eyepiece unlike
the other two methods which depend on manufactures specs, such as FL which are
An alternate (and perhaps more intuitive) formula for the Stellar Drift
TFOV = (Ts/86,400)*(360)
Where: Ts = The transit of the star in seconds, 86,400 = number of seconds
in a day, 360 = number of degrees in a circle.
Note - the star should lie within a few degrees of the celestial equator
for this to be accurate, the closer the better.
3: The Field Stop Method
TFOV = (FSD / Fo) X 180 / Pi
Where: Fo = FL of OTA, FSD = Field Stop Diameter, Pi = 3.14159... (note:
180/pi is the degrees in a radian)
Note: Caution - some field stops (particularly the more expensive wide
field designs) can only be determined by dismantling the eyepiece because the
field stop may be inside the elements. In some cases the field stop may be the
barrel itself. In 1.25 inch eyepiece the maximum field stop is around 27-28mm,
in a 2" eyepiece, the maximum field stop is around 46-47mm.
4: The Star Chart Method
Use a good star chart which utilizes either (or both) an accurate scale of
measurement in degrees and minutes of an arc or a measuring tool. One example,
though there are many, is Uranometria 2000 which contains clear acetate sheets
with scales measured in degrees of arc. Select a well populated star field
near the celestial equator. The belt of Orion is a good choice. With the clock
drive off, find and position two stars at the opposite edges of the
eyepiece’s field of view, on or very near a diameter line that bisects the
field of view. You will need to pick two stars that are plotted on the star
chart of your choice. For Uranometria, any two stars magnitude 8 or better
will do nicely. Having found two such stars and having positioned them as
instructed, you now know that these two stars frame your TFOV. That is, the
distance between these two stars, measured in degrees and minutes of an arc,
is the TFOV for that eyepiece in your telescope. Returning to your star chart,
find these two stars. Using the measurement scale or tool provided, read the
distance between those two stars. That is the TFOV.
Using a camera lens as a visual telescope:
- there are two main issues here:
- you need a camera lens mount adapter to 1.25" eyepiece adapter to
enable use of eyepieces
- the short backfocus of most camera lenses makes it difficult to focus
at infinity, especially when attempting to use a right angle adapter
that would make its use more ergonomic.
- you can buy camera lens to 1.25" eyepiece adapters (eg. Hutech)
- Perrti describes his technique of unscrewing the lens out of a 1.25"
Barlow lens and screwing this lens into the "telescope" side of a
90 deg diagonal
- Perrti has had excellent results with a Sigma 300mm f/2.8 and a Canon
400mm f/2.8 EF II lens, even up to 400x magnifications.