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A GUIDE TO PRACTICAL EYEPIECE SELECTION

Making the Right Choice Before you Buy

by: Clay Sherrod

One of the most frequently asked questions of all - particularly from "new" owners of telescopes is "....what other eyepieces do I need?"  Likewise, there are frequently-asked questions regarding the best choice to use for a particular observing application, like Jupiter compared to a faint comet.  Indeed, there are vast differences in the type and design of eyepieces to optimize your viewing enjoyment of nearly all celestial objects.  This Guide will help you determine which eyepieces that I recommend from five decades of experience, to supplement your standard eyepieces that normally come with the telescope (normally the 25mm Plossl, which still remains one of my favorites).

In addition, there are tried-and-true rules that concern such issues as staying away from many "hyped expensive-type" eyepieces when a less-expensive alternative may be just as good or even better for some applications.  I also want to warn everyone against "over-buying" both lots of eyepieces and filters.  The astro want ads are packed full of used ones and the buying of eyepieces can not only be expensive....it can be addictive..

You will see references to both field of view and magnifications in regard to eyepieces; for the beginner I very much favor and recommend the Plossl design as the best quality for the dollar spent.  However, you can easily determine within a few minutes ( ' ) arc for field and a few "power" for magnification by merely looking at the closest FOCAL LENGTH eyepiece that is listed to what you may have that may NOT be a Meade eyepiece.  I have learned through my aging astronomy career that - if two eyepieces, made (or distributed) by different companies are of the SAME design (i.e., 6-element Plossls), then the ACTUAL fields of view will be nearly identical; the only difference might be a "stop" or small diaphragm placed in the barrel to improve contrast and image quality in less expensive eyepieces.

Be very, very wary of such promotional goodies as "120 degree field of view!", "Argon-filled", "purged and sealed", etc.  The gimmicks go on and on.  In fact, those being sold with such claims toward superiority are typically the ones that I intentionally avoid.  They may be good, but if they need unconventional marketing to get the high price they are asking, I never even consider trying or buying them.
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CONCERNING ZOOM EYEPIECES -

I have gone on record many times stating that I do not prefer "zoom" eyepieces and the reasons are stated below; this does NOT mean that they might not be suited for you, particularly if you really enjoy the ease of observing and not fiddling around in the dark looking for things.  But for discriminating views of very subtle detail, the zoom eyepiece falls short....no matter WHAT brand you buy or HOW MUCH money you spend for it. Regardless of the claims.  The shortcomings of zoom eyepieces (from my use of virtually all that are on, or have been on, the market) are:

1) field of view - the zoom inherently provides a more restricted actual field of view at a given focal length (say, zoomed to 8mm) than would a good eyepiece of 8mm focal length;

2) light loss - the zoom requires extra lens elements to accomplish the variance of magnification as well as maintain parfocal (focused the same at all settings) integrity; the extra lenses merely absorb more light through refractive indices and allow less light through than would a simple eyepiece; thus your images will be slightly fainter;

3) contrast - by far and large, the zoom fails on the contrast tests at all magnifications when compared to a simple eyepiece;

4) resolution and spherical aberration - at higher magnifications I have routinely seen very poor resolution on double stars and planets with zooms; in addition, at LOW magnifications, most seem to have inherent spherical aberration: a problem whereby if the CENTER of the field of view is in focus, the edges will be slightly out-of-focus.....if you focus the edges, then you lose focus toward the center; and last but not least:

5) cost - for the price of a quality zoom, a user could buy at least two good quality Plossl eyepieces of his or her choice and obtain the attributes noted above.
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NOW ABOUT BARLOW LENSES -

I am an advocate FOR using a good quality Barlow/telenegative lens at the eyepiece of any telescope for two reasons:

1) the Barlow provides double (or in some cases 1.5x, 2x, 3x, 4x and 5x) the magnification of any eyepiece without robbing you of that precious eye relief (your ability to remain a comfortable distance back away from the front lens);

2) if properly chosen modern Barlow lenses provide excellent optical quality that are equal to the quality of the eyepiece itself; of course, cheap barlows will adversely affect the quality of even the BEST eyepieces;

3) the Barlow puts the eyepiece HIGHER toward the top rear of your telescope, thereby reducing you need to "scrunch" your face down into the back of the scope and against the small finder.

Of course, the Barlow adds either two (achromatic) or three (apochromatic, or "color free") more lenses to an already-taxed optical system. Remember that every piece of glass that you put in the path of oncoming light from your celestial object, the more light that is "lost" by refractive absorption or by simple reflection of the shiny glass surfaces.

The Barlow essentially allows you to double (or multiply) the power of any eyepiece, but you DO NOT reduce the eye relief by the same factor of "2", but only about 20 percent at most. Therefore if you own only the 26mm Plossl eyepiece PLUS a good barlow, you essentially have TWO eyepieces: a 26mm and a 13mm.

It is VERY important when choosing eyepieces to remember that fact if you have - or plan to buy - a good Barlow.  If you already have, say the 26mm and want (or already have) a good Barlow lens, it would be foolish to turn around and order a 13mm or a 12mm, or a 15mm eyepiece, when you WILL HAVE essentially just that combined with the barlow.  Do some math before you buy; one Barlow can save the investment of three or four eyepieces if you are chosing wisely.

A tip: stay away from any amplification in a barlow GREATER than 2.5x, and be cautious with those; the 2x is ideal and most from reputable manufacturers are very good. The 3x and 5x barlows that are so popular right now are the bargains on the "astro flea market" of tomorrow; they simply offer too much amplification and light loss from your image.  EXCEPTION:  Many of the 3x to 5x Telenegatives that are on the market today are not being used visually, but rather in concert with rapid-fire webcams to obtain some of the finest high resolution planetary photography ever made, all in the hands of amateurs.  Using, say a 4x amplifier, at prime focus with no eyepiece of an f/10 telescope converts the system to an f/40 planet killer.

So the idea of a Barlow can greatly help you reduce your selection decision for eyepiece focal lengths and at the same time keep some of that "burning money" in your pocket!
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MAGNIFICATION -

Not to dwell on it here, but the magnification of your telescope and various eyepieces is computed below in a rather "overall" form.   For eyepieces not listed, the "POWER" or magnification of the telescope is determined by taking the FOCAL LENGTH (in millimeters) of the telescope and dividing INTO that focal length the focal length of the EYEPIECE.  The result is exactly the magnification of your scope. As an example with the 26mm (focal length) eyepiece:

Focal Length 350mm
        350mm divided by 26mm  = 13.5x (27x with the 2x Barlow)
Focal Length 1250mm
        1250mm divided by 26mm = 48x (96x with Barlow)
Focal Legnth 1900mm
        1900mm divided by 26mm = 73x (146x with Barlow)
Focal Length 2000mm
        2000mm divided by 26mm = 77x (154x with Barlow)

You can see an interesting situation with the ETX 125 and the f/10 SCT (the last two); both have the same focal length even though one is a 5" and the other is an 8" scope!  This is because of the Maksutov design (ETX 125) having an f/15 FOCAL RATIO and the Schmidt-Cassegrain using the f/10 FOCAL RATIO provide for nearly the same focal length.  Although the "power" is the same in both, the "light gathering" and resolution is of course greater in the larger telescope using the same magnification.
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EYEPIECE SELECTION GUIDE

The following breakdown had to have some limits since there is a plethora of eyepieces flooding the market today. For my guide which follows, I have simply chosen the category of object being viewed with eyepiece options.  My opinion is that it is always better to have THREE eyepieces of excellent quality and performance than a suitcase full of ones that give only marginal viewing.

Other eyepieces can be fit into my charts; for example, the data you need for say a 10mm Plossl would be nearly identical to that for the 9.7mm listed; a 5mm of another brand would fit nicely somewhere in between the 6.7mm and the 4.7mm version. This is assuming, of course,that the eyepiece design is similar to that used in this reference.

Virtually EVERY focal length, from 2mm (can you imagine the eye relief on that one!) to nearly 60mm super wide angle versions are available on nearly every page of popular astronomy magazines.  What do you buy? How do you know if you are getting the best for the dollar?  DON'T GUESS.  ASK...... Or in this case "read."

Popular web sites such the many telescope forums and discussion group sites are excellent platforms for user feedback on particular types and brands of eyepieces.  Star parties or astronomy club observing sessions are an excellent source of information and first-hand opportunities to actually test the performance of particular eyepieces that you might be considering.  NEVER BUY AN EYEPIECE THAT YOU HAVE NOT HEARD OF, and/or one that is not recommend by someone you trust.  There are some "generic" brand eyepieces out there on the market which I have tested and are excellent at about half the price of others more visibly advertised.  But by the same token, I have learned to avoid brands that suddenly surface and appear to be assembled out of someone's garage.

As a general rule for beginners and those who enjoy CASUAL stargazing with thefine APO refractors available today, the Maksutov and the Schmidt-Cassegrain telescopes, the ORTHOSCOPIC or PLOSSL designs are the very best and any variation on that theme for your selection. Nearly all major brands now focus on the Plossl design, with "wide field" and "super-dooper wide field" versions available in some focal lengths.

But remember this rule: the more glass in the eyepiece....the less light your eye gets.  Sometimes simpler is better.
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DOC CLAY'S EYEPIECE SELECTION AND PRACTICAL USE GUIDE -

Following are individual breakdowns on each of the telescope models (ETX 60-70 / ETX 90 / ETX 125 /ETX 125 / LX 90) for eyepiece selection and use. USE THE FOLLOWING INDEX CODE to select and eyepiece for observation or for possible purchase for a particular purpose. You will see these reference codes under the appropriate section for your telescope. If you DO NOT see a code by a particular eyepiece listed in your section, then this means that the EYEPIECE has no suitable application, or that it merely duplicates the performance of another one already listed.

NOTE: AN (" * ") BEFORE A CODE indicates that this selection is recommended when only coupled with a good 2x Barlow lens, and in many cases is preferred over a straight eyepiece with no Barlow!

CODES TO IDENTIFY PROPER EYEPIECES FOR YOUR OBSERVING NEEDS

WIDE FIELD
w1 - lowest possible magnification
w2 - widest practical field @ low magnf.
w3 - BEST RECOMMENDED

DEEP SKY
d1 - low power but enough scale to show large objects
d2 - medium power to enhance object contrast against sky
d3 - higher power for smaller objects (i.e., planetary nebulae)

MOON
mw - allows the entire moon in field
m1 - excellent for low power views
m2 - scan the terminator shadows with this
m3 - ideal for highest power viewing

PLANETS
pw - aesthetic views, planet and its satellites, star fields
p1 - medium power, ideal for tracking Jupiter's moons
p2 - highest power on average steady nights
p3 - highest practical power on the very best nights

DOUBLE STARS                                         
x1 - medium power for most bright doubles within reach
x2 - max power to reach Dawe's limit on nights of very best seeing             
 
COMETS
c1 - widest good field for tail, etc.
c2 - good medium - details in head, nucleus

SUN
sun1 - can get the entire solar disk in for sunspot counts
sun2 - ideal medium for sunspot/flare/granulation details
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Simply find the proper code in the following section listed by the appropriate eyepiece (REMEMBER: even though the focal lengths for the eyepieces are given, ANY similarly designed eyepiece CLOSE to this focal length will provide the same or very similar results!  REMEMBER!! If the CODE has an (" * ") by it, then it means that this eyepiece is selected for use WITH BARLOW!

IN THE FOLLOWING LIST: Eyepiece Size, by mm / MAGNIFICATION / Field of View ( ) for YOUR particular telescope / Recommendations in the above detailed CODE.

COMPACT TELESCOPES (60-80mm)
25mm MA - w2 / d1 / m1 /pw / c1
plossl eyepieces (4-element, excellent light transmission) EXCELLENT INVESTMENT: 56mm (2-inch) - n/a
40mm - n/a
32mm - n/a
26mm - 13.5X / (3.9 degrees) - w2, mw, c1
20mm - 17.5X / (3.0) - w3, d1
15mm - 23.4X / (2.3) - pw
12.4mm - 28.3X / (1.9) - m1
9.7mm - 36.1X / (1.5) - d3, p3*, sun1
6.4mm - 54.7X / (57' arc) -m3, p1, x1*, sun2, c2
super wide angle plossl eyepieces (6 elements - moderate light transmission) EXPENSIVE:
13.8mm - 25.4X / (2.7 degree) (not recommended)
18.0mm - 19.5X / (3.5) (not recommended)
24.5mm - 14.3C / (4.6) (not recommended)
32mm - (2 inch) - n/a
40mm - (2 inch) - n/a
ultra wide angle plossls (8 elements, less light transmission) VERY EXPENSIVE:
14mm (1-1/4" / 2-inch) - not recommended
8.8mm (1-1/4" / 2-inch) - not recommended
6.7mm (1-1/4") - 52.3X / (1.7 degree) - d3, m2, p2, c2, sun2
4.7mm (1-1/4") - 74.5X / (1.2 degree) - m3*, p3*
DOC'S 3-in-the-box Selection: 25mm, 10mm,  Barlow
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IN THE FOLLOWING LIST: Eyepiece Size, by mm / MAGNIFICATION / Field of View ( ) / Recommendations

MEDIUM-SMALL APERTURES (90-105mm)
plossl eyepieces (4-element, excellent light transmission) EXCELLENT INVESTMENT:
56mm (2-inch) - n/a
40mm - 31X / (1.5 degree) - w3
32mm - 39X / (1.4) - w1, mw, c1
26mm - 48X / (1.1) - w2, m1, p1
20mm - 63X / (49' arc) - w3, d2, sun1, pw
15mm - 83X / (37') - c2
12.4mm - 101X / (30') - d3, sun2
10mm - 129X / (24') - m2, p2
5mm - 195X (15') - m3, x1, p3*
super wide angle eyepieces (6+ elements - moderate light transmission) EXPENSIVE:  
19mm - 91X / (44' arc) - d2, c2, mw, sun1
22.0mm - 69X / (57') - w3, m1, p1
27mm - 51X / (1.4 degrees) - w2, d1
35mm - (2 inch) - n/a
40mm - (2 inch) - n/a
ultra wide angle  (8 elements, less light transmission) VERY EXPENSIVE:  Highly recommend Televue Naglers and Panoptics
15mm (1-1/4" / 2-inch) - not recommended
10mm (1-1/4" / 2-inch) - not recommended
7mm (1-1/4") - 187X / (27' arc) - x2*, p3*
5mm (1-1/4") - 266X / (18') - p2, x2
Doc's 3-in-the-box Selection: 32mm, 15mm, (or 13.8mm) 7mm, Barlow
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IN THE FOLLOWING LIST: Eyepiece Size, by mm / MAGNIFICATION / Field of View ( )/ Recommendations

MODERATE APERTURE (125-180mm)
plossl eyepieces (4-element, excellent light transmission) EXCELLENT INVESTMENT:
56mm (2-inch) - n/a
40mm - 48X / (55' arc) - w1, d1, c1, mw
32mm - 59X / (52') - w2
26mm - 73X / (42') - m1, sun1, pw, w3
20mm - 95X / (32') - d2, p1
15mm - 127X / (24') - d3, m2, p1, c2, sun2
12mm - 153X / (20') - m2, x1
10mm - 196X / (15') - p3*, x2
6mm - 297X / (10') - p2, m3
super wide angle plossl eyepieces (6 elements - moderate light transmission) EXPENSIVE:  NOTE: NO advantage to 2" size in this aperture range!
13.8mm - 138X / (29' arc) - d3, m2, p1, c2, sun2
18.0mm - 105X / (38') - p2, x1
24.5mm - 78X / (51') - m1, d2, p1, sun1
32mm - (2 inch) - n/a
35mm - (2 inch) - n/a
ultra wide angle  (8 elements, less light transmission) VERY EXPENSIVE: - Highly recommend Televue Naglers and Panoptics
15mm (1-1/4" / 2-inch) - not recommended
9mm (1-1/4" / 2-inch) - not recommended
6mm (1-1/4") - 284X / (17' arc) - p1, p2*, x1, x2*, m3*
4mm (1-1/4") - 404X / (12') - p2, x2, m3
Doc's 3-in-the-box Selection: 32mm, 15mm (or 13.8mm), 10mm,  Barlow
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IN THE FOLLOWING LIST: Eyepiece Size, by mm / MAGNIFICATION / Field of View / Recommendations

LARGER APERTURES - 200mm and up
plossl eyepieces (4-element, excellent light transmission) EXCELLENT INVESTMENT:  NOTE:  you only need ONE 2-inch eyepiece....LOW power.
56mm (2-inch) - 36X / 1.1 degree - w1
40mm - 50X / 52' arc - w2, d1, m1, pw, c1
32mm - 63X / 49' - w3, p1, sun1, mw  (Televue Plossl)
26mm - 77X / 40' - d2
20mm - 100X / 31' - m2, sun2
15mm - 133X / 23' - x1, c2, d3
12.4mm - 161X / 19' - p2*
9.7mm - 206X / 15' - x2*
6.4mm - 313X / 9' - p2, x2
super wide angle plossl eyepieces (6 elements - moderate light transmission) EXPENSIVE:  Highly recommend Televue Naglers and Panoptics
15mm - 145X / 27' - d3, m2, c2, sun2
18.0mm - 111X / (36') - d2, mw
25mm - 82X / (49') -
35mm - (2 inch) - 63X / (1.5 degree) - w3, c1, m1, pw
40mm - (2 inch) - 50x / (1.7 degree) -
ultra wide angle plossls (8 elements, less light transmission) VERY EXPENSIVE:  Highly recommend Televue Naglers and Panoptics
18mm (1-1/4" / 2-inch) - 143X / (35' arc) -
9mm (1-1/4" / 2-inch) - 227X / (22') - x2, p2
7mm (1-1/4") - 299X / (16') - p3*, m3*, x2*
5mm (1-1/4") - 426X / (11') - m3, p3
CLAY'S 3-in-the-box Selection: 32mm (SWA), 15mm (or 14mm/13.8mm), 6.4mm (or 6.7mmUWA), Barlow
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REGARDING 2-INCH EYEPIECES

Some of the finest eyepieces you will ever use are made in the 2-inch barrel format.  These include the incredible Televue Naglers, Panoptics, Ethos and other such designs, all cutting edge optical technology.  These provide the ultimate in viewing pleasure, like a porthole to space, but do so with a very high price tag.  It is very important to note that there is absolutely NO advantage to the 2-inch design EXCEPT for your very lowest power providing he widest field of view.  There is NO advantage to a high power 2-inch eyepiece.

Thus, when selecting your three- or four-eyepiece set for your telescope, put your MOST MONEY in the lowest power; that is the magnification and eyepiece that you will be using the most and the one that will provide the most exciting and rewarding views.  If your telescope can accept a 2-inch eyepiece (not all can), then by all means invest in ONE low power wide field top-quality eyepiece.  You will be thanking me every time you scan the Milky Way or exploring the Andromeda Galaxy.

Enjoy....and remember:  RESIST....investing in eyepieces is addictive and there is no intervention group as yet.

Doc Clay
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