The Eyepiece is half the Telescope


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The following introduction was delivered by Jim Foy before Bryan’s presentation:

Tonight’s speaker is our own Bryan Stone.

Bryan has a passion for eyepieces and will be sharing this with us tonight.

Bryan has owned dozens of eyepieces.  He once sold a Sirius 10mm Plossl on Astromart for $200.

If you and Bryan both owned a 13 mm ethos, his would be better.  If you swapped, his would still be better.

When Edmund Hillary reached the summit of Mount Everest, he found Bryan’s missing eyepiece cap.

One of Bryans sketches won a deep sky imaging contest.

Bryan has won the Astronomical league lifetime achievement award… Twice….

Stephen Hawking has a tattoo that says, “Bryan”

He is the most interesting astronomer in the Universe


“The eyepiece is half the telescope so use a good one.  The eyepiece takes the image produced by the objective lens or mirror and magnifies it and projects it at infinity into your eye.” – Al Nagler


Intrinsic Characteristics

Barrel size: 0.965, 1.25, 2” – Cheap department store scopes

Focal length

Apparent field of view (FOV)

Field stop: Determines FOV

  • Max 27mm for 1.25”
  • Max 46mm for 2”

Eye Relief: Need 17020mm for eyeglasses

Mechanical quality

Fit & Finish

Optical Quality – coatings, polish

Edge performance in fast scopes, Ex. Bad=Erfle, Good=Nagler

Size, Weight


Filter threading – made not standardized

Barlow friendliness

Telescope Dependent Characteristics

Magnification = Focal Length (FL) scope / FL of eyepiece

Total / Actual FOV = Apparent FOV / Magnification

Edge performance

Exit pupil – Eyepiece FL / Telescope focal ration, i.e. telescope Focal Ration (FR) = FL / Objective Diameter

Eyepiece Categories

Eyepieces to Avoid

Older designs: Ransden, Huygens

Surplus Erfles

Eyepiece giving too high/ low power

Eyepieces giving too large an exit pupil

Standard Field Eyepeices

Inexpensive: RKE, Orthoscopic, generic plossls

Premium: Manocentrics, Brandons, French Clave Plossls, Televue plossls

Wide Field Eyepeices

Inexpensive: Erfle, Knig, not good in Fast scopes. In fast scopes Erfle = Awful

Premium: Many great ones.  All $$$

Axiom: In general you can have 2 out of 3 of the categories above in an eyepiece but not all three:

  • Inexpensive
  • Wide field
  • Good edge performance

Eyepiece sets for you and your Telescope

3 Key Questions

  1. Focuser Size
  2. Do you need glasses to correct astigmatism? If so need 18mm plus eye relief
  3. Driven or undriven scope? If undriven, wide FOV desirable.

Low Power

You need a wide field low power finder eyepiece.  For 2’ focuser many choices with a field stop close to 46mm.  BUT: Do not exceed a 7mm exit pupil in a reflector.

For 1.25” focuser you want an eyepiece with a 27mm field stop. Ex: 32mm Plossl or 24 mm panoptic.  24 pan shows the same amount of sky but with more magnification.

Medium Power

This is where all the fun is, invest the most $$ here. If you only have one premium eyepiece, this should e it.  Ideally you want an eyepiece with 1.8 – 2.2 mm exit pupil with a wide FOV.

Many eyeglass wearers do not need to use glasses below a certain exit pupil because you are using the smallest and est part of your eyes’ retina.  2mm would be fine without glasses for many observers.

A carefully chose medium power eyepiece can be used with a good 2x barlow for a high power option.

High Power

Is limited by telescope quality, arpeture, and seeing conditions. Goal is resolution, seeing fine detail on moon or a planet or splitting a close double star

Myth: 50x / inch, theoretical.  Ignore for now

Real World: One arc second resolution in most parts of the United States is pretty good on most  nights.  Massachusetts is not the Florida keys. Best human eyes can resolve  one arc minute (60 seconds) so 60x is all that’s needed to match the resolution limit of the telescope and seeing conditions on most nights. To avoid eyestrain triple 60x to get 180x200x practical high power.  Lots of ways to get to 180x200x. Ask Bryan – he loves this stuff.

Gap Fillers

Caution: My 11mm Nagler has the same size field stop as a 18mm plossl. I go right from low power to my 11mm Nagler.  0.75 degree FOV is big enough for medium power option.  There is no gap to fill.

Good Inexpensive eyepieces under $100

For outreach, newbies, budget conscious. Key: There are stars and dogs in most eyepiece lines.

Inexpensive Plossls ~ $40-$50

Sterling, Sirius, GSO 32, 25mm are respectable and okay with glasses. Avoid 1.25” 40mm and shorter focal lengths  – very poor eye relief) . Avoid 40mm 1.25 inch because 32mm 1.25 inch has same true FOV.

Expanse Clones, aka Synta Widefields

9MM IS GREAT!! Star of the line.

6mm is very good, avoid 15, 20mm.  Poor edge performance less than F/10

Don’t buy from Orion ($60).  Agena Astron $38, OWL $35 with free shipping. These re “Poo man’s Naglers”. I recommend starlings and 9mm expanse clones very highly.

GSO SV 30mm 2” < $100

Very good at F/6

Farily good even at F/5

85% of the performance of the 27mm Panoptic at 20$ of the price

Used Eyepieces

Cloudynights Classifieds, Astromart

Konig / Barlow Strategy

Barlows improve Konig performance, but magnifications change. Ex. 16mm Konig with 1.8x televue barlow is close in performance to a 9mm Nagler.

Use a slow telescope F/10 -> F/15

They are very easy on inexpensive eyepieces.


Standard length is better than “Shorties”

  • Better image quality
  • Less vignetting
  • Less excessive eye relieve with long focal length eyepieces

Barlows increase eye relief at a rate proportional to the eyepieces focal length.

Begginner Kit alternative to commercial eyepiece kits

30-32mm plossl

9mm syntra widefield from agena or OWL

80A Filter

Standard length 1.8-2x Barlow | Not “Shortie”

Plano pluck foam pistol case

$10-15 at sporting goods store / Wal-Mart

Home made red flashlight – use red nail polish on lens


Book: Turn Left at Orion



EP: Eyepiece

FR: Focal Ratio

FL: Focal Length

FOV: Field of View

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