GUIDE TO EYEPIECE SELECTION Last updated: 13 April 2001

YOUR ETX and LX GUIDE TO EYEPIECE SELECTION

Making the Right Choice When You Observe....And Buy!

One of the most frequently asked questions of all - particularly from "new" users of ETX and LX 90 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. This guide will help you determine which eyepieces are recommend to supplement your standard equipment that comes with the telescope (normally the 26mm Super Plossl, which still remains one of my favorites).

In addition, there are tried-and-true rules that concern such issues as staying away from "hyped expensive-type" eyepieces when a less-expensive alternative that is just as good can get you by.

Covered in this guide are:

ETX: -60, -70, -90, -125
LX: -90, -200 (8")

You will see references to both field of view and magnifications as determined for MEADE eyepieces; I happen to like the Meade eyepieces 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.

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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 clearly listed 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 lens at the eyepiece of any ETX or LX telescope for two reasons:

1) the barlow provides double 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 add 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.

With the ETX and LX scopes I highly recommend the "short" series of barlows that are out as they - because of their stubby design - allow better alignment with the axial path of light than do the longer ones, since the eyepiece receptacle (holder) of the right angle attachment at the rear of the scope is very short, also, by design.

The barlow essentially allows you to double 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!

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.

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:

ETX 60/70 AT - Focal Length 350mm 
        350mm divided by 26mm  = 13.5x (27x with the barlow)
ETX 90 EC - Focal Length 1250mm
        1250mm divided by 26mm = 48x (96x with barlow)
ETX 125 EC - Focal Legnth 1900mm
        1900mm divided by 26mm = 73x (146x with barlow)
LX 90 EMC - Focal Length 2000mm
        2000mm divided by 26mm = 77x (154x with barlow)

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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 used the entire Series 4000 line of eyepieces offered by Meade since I consider these to be the BEST overall eyepieces on the market, dollar-for-dollar.

Other eyepieces can be "fitted in" 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 (does not have to be exact but if of the Plossl design and using the same number of elements) 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 as Mike Weasner's Mighty ETX site (offering a Product Showcase which does feature eyepieces frequently) and discussion group sites such as the LX 90 site are excellent forums 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 super Plossl eyepieces out there on the market which I have tested and are excellent at about half the price of others more visibly advertised.

As a general rule for the Maksutov and the Schmidt-Cassegrain telescopes such as ours, the ORTHOSCOPIC or PLOSSL designs are the very best and any variation on that theme for your selection. Nearlyl 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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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 that the 26mm Series 4000 Super Plossl is listed with the ETX 90 and 125 and LX 90 for at least one application since that the standard-equipment eyepiece; the 25mm MA Meade is also listed similarly for the ETX 60 and ETX 70 telescopes.

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

Simply find the proper code in the following section listed by the appropriate eyepiece (REMEMBER: even though the focal lengths for the Meade brand eyepieces are given, ANY similarly designed eyepiece of 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.

ETX 60 / 70
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*
CLAY'S 3-in-the-box Selection: 20mm, 6.7mm (or 6.4mm), 4.7mmUWA, barlow

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IN THE FOLLOWING LIST: Eyepiece Size, by mm / MAGNIFICATION / Field of View ( ) / Recommendations

ETX 90
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
9.7mm - 129X / (24') - m2, p2
6.4mm - 195X (15') - m3, x1, p3*
super wide angle plossl eyepieces (6 elements - moderate light transmission) EXPENSIVE:
13.8mm - 91X / (44' arc) - d2, c2, mw, sun1
18.0mm - 69X / (57') - w3, m1, p1
24.5mm - 51X / (1.4 degrees) - w2, d1
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") - 187X / (27' arc) - x2*, p3*
4.7mm (1-1/4") - 266X / (18') - p2, x2
CLAY'S 3-in-the-box Selection: 20mm, 12mm, (or 13.8mm) 6.4mm, barlow

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IN THE FOLLOWING LIST: Eyepiece Size, by mm / MAGNIFICATION / Field of View ( )/ Recommendations

ETX 125
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
12.4mm - 153X / (20') - m2, x1
9.7mm - 196X / (15') - p3*, x2
6.4mm - 297X / (10') - p2, m3
super wide angle plossl eyepieces (6 elements - moderate light transmission) EXPENSIVE:
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
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") - 284X / (17' arc) - p1, p2*, x1, x2*, m3*
4.7mm (1-1/4") - 404X / (12') - p2, x2, m3
CLAY'S 3-in-the-box Selection: 40mm, 15mm (or 13.8mm), 9.7mm, 6.7mmUWA, barlow

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IN THE FOLLOWING LIST: Eyepiece Size, by mm / MAGNIFICATION / Field of View / Recommendations

LX 90
plossl eyepieces (4-element, excellent light transmission) EXCELLENT INVESTMENT:
56mm (2-inch) - 36X / 1.1 degree - w1
40mm - 50X / 52' arc - w2, d1, m1, pw, c1
32mm - 63X / 49' - w3, p1, sun1, mw
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:
13.8mm - 145X / 27' - d3, m2, c2, sun2
18.0mm - 111X / (36') - d2, mw
24.5mm - 82X / (49') -
32mm - (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:
14mm (1-1/4" / 2-inch) - 143X / (35' arc) -
8.8mm (1-1/4" / 2-inch) - 227X / (22') - x2, p2
6.7mm (1-1/4") - 299X / (16') - p3*, m3*, x2*
4.7mm (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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