Telescope Eyepieces

• telescope eyepieces
• true field of view
• see also:
  • astronomy index and links
  • Astronomy - telescopes

Telescope eyepieces:

• 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
• types:
  • 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 powers.
  • 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 wide-field eyepieces.
  • 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 eyepieces
  • Lanthanum:
    • 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.
  • Ultra-wide:
    • 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 don’t).
    • 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 Naglers
  • 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 70-82deg 
    • 
• 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 power.
• 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 image.
  • 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.
  • also:
    • 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 of eyepiece:
    • 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 magnification.
• 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 eyepiece.
  • 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 the sun)
  • 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 view.
  • 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 5.6-16mm eyepieces.
    • 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 eyepieces.
    • 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 aperture.
• weight:
  • 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 them.
  • 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 most expensive.
  • 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 times!
  • 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 not altered.

True 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

TFOV = (AFOV)/ (Fo/Fe) - note Fo/Fe is the magnification the eyepiece provides

ie. TFOV = AFOV / magnification

Where: Fe = FL of Eyepiece, Fo = FL of OTA

Note: Caution - AFOV is often quite different from the manufactures published specs.

 

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 frequently off.

An alternate (and perhaps more intuitive) formula for the Stellar Drift Method is:

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.