Last updated on
4/11/
2006

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Eyepieces
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It’s not uncommon for amateur astronomers to invest a lot of money in eyepieces.  The good news is that the 25mm or 26 mm Plössl eyepiece that comes with Celestron or Meade SCTs is usually a decent general-purpose eyepiece when you’re starting out with your scope. You can become absorbed in the process of acquiring a lot of eyepieces, but you only really need to start with a few to cover a wide range of observing situations.  You can think of eyepieces in terms of film photography.  Along with the “standard lens” you need a ”wide-angle lens”, a “telephoto lens”, and a “tele-extender”.  However, the terms used for telescope eyepieces are a little different than those used for film cameras. You need to understand a few of the telescope eyepiece terms at the start, to make sense of what is available, including: Magnification, Field of View, and “Contrast”. Click on the Basics button above for an explanation of these and other eyepiece terms.  If you decide to use 2” diameter lower-power eyepieces for their improved field of view, you will need to replace the standard 1.25” diagonal that’s provided with SCTs, with a 2” diagonal.  Click on the Diagonals button above for more information on the types of diagonals available. Also, when you acquire a number of eyepieces your observing life will become a lot easier if you make them Parfocal - click on the Parfocal button above for an explanation of this.

What Eyepieces You Need: An Over-Simplified Explanation

Put in “camera lens terms”, here is a very over-simplified explanation of the eyepieces you need to start with, to enjoy your observing experience with an SCT:

“Wide-Angle” Eyepiece Many deep sky objects, including a majority of the Messier objects are large; some are substantially larger than the apparent diameter of the moon. They are best viewed at 50X magnification or less.  For an f/10 2000mm 8” SCT this translates to 32mm to 40mm focal-length eyepieces.

“Telephoto” Eyepiece When you view the planets, the moon, and the sun you will immediately want to magnify the view to see more detail. There is a limit on how much you can magnify an image, depending on the viewing conditions (the clarity/turbulence of the sky) and the attributes of your telescope. You can safely magnify 200X to 300X, which for a 2000mm 8” SCT translates to a 10mm to 7mm eyepiece.  But you need good seeing conditions to view a steady image even at 250X, so depending on your local conditions you should consider starting at the 10mm end of this range.

Barlow Lens A Barlow lens fits between the diagonal and eyepiece, and acts much like a “tele-extender” for 35mm cameras.  That is, a 2X Barlow doubles the magnification of your eyepiece (or, it reduces the focal length of the eyepiece in half).  This isn’t free - just like for camera lenses, doubling the magnification cuts the light delivered to your eye (i.e. the speed of the system) in half.  But the Barlow isn’t any worse than switching the eyepiece to one of half the focal length - higher magnification means lower effective image brightness either way. The advantage to a Barlow is that it effectively doubles the number of eyepieces you have. For example, if your scope came with a 26mm and you also have a 40mm eyepiece, with a Barlow you now effectively have a 13mm and 20mm eyepiece as well. Celestron’s 2X Barlow is regarded as a very good one and sells for about $70 - not a bad price for essentially doubling the number of eyepieces you own.
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What Eyepieces You Need: A Better Explanation
The explanation I presented above is the typical one given to beginners but it is actually more over- simplified than it should be; if you’re willing to keep reading, here’s an explanation that is more complicated but will serve you better in the long run (Albert Einstein once noted that you really can’t make something simpler than it actually is):

As I noted above, Deep Sky Objects (DSOs) cover an extremely wide range of sizes, from large
nebulae and galaxies down to tiny nebulae and globular clusters. This means that you need to be able to change the Field of View (FOV) of your scope's optical system to see them all. Since the scope’s primary and secondary mirror are a constant, you change the optical system’s FOV by changing eyepieces, which each have a FOV that when coupled to the scope’s optics provide a combined total system FOV. 

But for a given FOV you want the highest magnification you can, to see the object better.  This is where things start to get a bit complicated. Typically, shorter focal length (higher-magnification) eyepieces have a more narrow FOV, but you can design an eyepiece that has a specific magnification because of its specific focal length, but has a much wider FOV at that focal length. (Unfortunately such designs are - no surprise - more expensive to manufacture.) So what you can do is decide what FOVs you want, and then examine the specifications of various eyepieces to see which ones give you those FOVs with the highest magnification you want to spend money for, at that FOV. Eyepiece manufacturers specify the eyepiece’s  focal length and its Apparent FOV (AFOV) - you need to compute the actual FOV you will get with it in your scope by knowing the focal length of your scope (Actual FOV = Apparent FOV ÷ Magnification). This can be laborious although using a spreadsheet helps, or you can use free software such as What U See - click on the FOV Calculator link above for more information on this software.

Visual Backs
There is one additional consideration involving eyepieces in an SCT. The rear-cell opening in an 8” SCT is 1.5” and this limits the maximum actual field of view you can see without the onset of vignetting, to 1.07°.  SCTs with apertures of 10” or larger have a larger rear-cell opening but they are typically supplied with a visual back that still has a 1.5” opening.  For these scopes you can gain some additional actual usable field of view (or “clear aperture”) by replacing the stock visual back with a larger one such as the Astrophysics “2” adapter for 10” or larger SCTs” that sells for about $60. (Note that if you use the JMI NGF-S electric focuser, there is another option available from JMI that also provides a larger opening, specifically configured for using that focuser.)  However, some caution is in order here. It has traditionally been assumed that manufacturers of SCTs larger than 8”, have supplied a visual back that still has a 1.5” opening in order to retain compatibility with the rest of the rear-cell accessories sold by those manufacturers. But there is some controversy as to whether the use of an after-market visual back with an opening larger than 1.5” is a good thing.  Testing by some experienced amateur observers has indicated that a visual back with an opening larger than 1.5” allows stray light to enter an eyepiece, reducing contrast. They believe that the 1.5” visual back is part of the scope’s overall optical design - it reduces stray light - and that a visual back with an opening larger than 1.5” should only be used with eyepieces that have very wide-field views. They maintain that when using normal (shorter focal-length) eyepieces, contrast is improved by using the visual back with a 1.5” opening.

In summary, it doesn't make sense for someone to tell you what focal length EPs to use on your scope (even though I did that at the beginning of this page - sorry!). You can purchase inexpensive EPs with a more narrow AFOV that will take in a range of DSOs but won’t give you as much magnification at those total-optical-system FOVs, or you can purchase more expensive EPs that have a wide AFOV and will show the same range of DSOs at higher magnifications.  The latter is more satisfying and is usually where amateur observers end up sooner or later (sometimes, expensively, a lot later). As a starting point of comparison, note that I use EPs that give me 0.90°, 0.45°, 0.33°, and 0.19° FOVs on my 8” f/10 SCT - you can see my eyepiece list here. (Note that the 0.90° FOV EP won't take in the largest DSOs, which is why many observers ultimately add a Rich-Field scope to their SCT.)

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Used eyepieces are commonly sold by folks trading up to more expensive ones, and eyepieces don’t wear out like mechanical equipment so a used eyepiece can be a safe purchase. Check the Used Equipment Links page for links to Web sites with classified ads for used eyepieces.  (No one gave me the above FOV explanation when I started out so like many amateur observers I ended up trading EPs to get to where I am now - perhaps you can avoid this step...)
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Rules-of-Thumb and Tips

The whole subject of eyepiece designs can become very involved (much more so than I discussed above), including issues of sharpness, brightness, contrast (especially for planetary viewing), and eye relief as well as the very basic magnification and field of view issues noted above. This translates to a wide variety of eyepieces sold over a wide price range.  There really isn’t a single good Web site reference I can give you to sort it all out but here are a few rules-of-thumb:

  • A good eyepiece can cost from about US$50 to US$500, and you (usually) get what you pay for. Most experienced amateur astronomers would advise you that a few good eyepieces will satisfy you more, in the long run, than a lot of inexpensive ones and this has been my experience as well.
  • Optical design has improved over the years (as you would expect) and newer designs (plössls, masayumas, and Televue’s Panoptics and Naglers) tend to have a wider field of view and often more eye relief than older designs (erfles, kellners, or orthoscopics). Note however that the newer designs provide wider fields of view by using more individual lens elements in the eyepiece, and this tends to reduce contrast which is a disadvantage especially for planetary viewing. So eyepieces that are highly regarded for planetary work are generally not the same as those highly regarded for deep-sky observing. (See below.)
  • So, you need to be careful about eyepiece design attributes when selecting eyepieces for any given scope (focal length).  Reputable eyepiece manufacturers sell eyepieces of a specific design, in a very wide range of focal lengths but that doesn’t mean that you should stay with that design of eyepieces for all your needs. Specific designs are intended for specific purposes and for any specific design, manufacturers sell a wide range of eyepiece focal lengths because there is a wide range of telescope focal lengths so they need a range of products to meet a particular need across that customer base.  You, however, will be best served by choosing different eyepiece designs across the set of eyepieces you purchase.  This can be confusing because specific eyepiece designs are often widely praised, but the praiser usually neglects to mention the purpose for which the eyepiece is so well regarded. (See the next paragraphs.)
  • For general (medium-magnification) use, observers often start with plössl eyepiece designs due to their affordable cost - this is the type of eyepiece typically provided by manufacturers with a new scope. The classic plössl is a 4-element design that is acceptable for general use although it can be improved - the premium eyepiece line sold by some manufacturers (e.g. the Celestron Ultimas) is a masayuma design, which is a 5-element hybrid plössl that offers a wider Field of View at the expense of a little less eye relief.  Sky & Telescope Magazine tested and reviewed “Basic Eyepieces” in the April 1996 issue, and concluded that the Celestron Ultima and Televue plössl eyepieces were the best of the basic eyepieces they tested. Almost a decade later, the general consensus among experienced observers is that this conclusion is still valid.
  • Lower-magnification eyepieces are used for viewing larger objects (e.g. open clusters), locating objects, and general viewing.  Experienced amateur astronomers would advise you that in the long run for these eyepieces you’ll want to switch from the 1.25”-diameter eyepieces that are standard with GoTo scopes, to 2” eyepieces for the dramatically wider field of view that naturally comes with a larger optic. (If you want more field-of-view you need larger-diameter piece of glass.)  This is a somewhat expensive jump - you would need to switch to a 2” diagonal for your rear cell, which will cost from $100 to $200 or more, and the eyepieces themselves are also correspondingly more expensive.  But the difference in what you can see with your scope is definitely worth it (if you have the money).  The “Nagler”-type eyepieces designed by Al Nagler (an extremely well-regarded optical designer formerly with NASA) and sold by his company TeleVue, are widely regarded as among the best you can buy for deep-sky observing at lower magnifications - they have a very wide field of view at any given focal length.  But they are expensive and not everyone can afford them.  The Televue Panoptics and the “super-wide-angle” eyepieces sold by Pentax or Orion (which are Vixen eyepieces), or even Celestron or Meade are a less-expensive choice than Televue Naglers.
  • Higher-magnification eyepieces are used for smaller deep-sky objects (e.g. many globular clusters and remote galaxies) as well as planets. For higher-magnification eyepieces a wide field of view is much less important and in fact such an eyepiece design is often detrimental because of its reduced contrast; at higher magnifications you need contrast more than you need field of view. Many observers believe that the eyepieces made by Pentax or Takahashi provide higher contrast for higher-magnification viewing.  And experienced planetary observers often note that the older, simple four-element orthoscopic eyepiece design provides about the highest contrast for observing planets, having one less element than the 5-element masayuma design of a Takahashi or Celestron Ultima. This is nice because a good orthoscopic eyepiece such as those sold by University Optics, are inexpensive. Brandon eyepieces, which are the ones provided with Questar telescopes, are often regarded as having the very best contrast but they do sell for about four times the price of a good orthoscopic, and curiously I prefer the view I get from my Tak over that of the Brandon I own.  Finally, the TMB Super Monocentric eyepieces are receiving good reviews for high-magnification planetary work - these are premium (in the world of astronomy, “premium” is synonymous with “expensive” <grin>) 3-element EPs with excellent contrast although they do have the more limited eye relief one would expect from a 3-element design. 

    High-contrast, short focal-length eyepieces are usually 1.25” EPs. If you get a 2” diagonal for the 2” low-magnification EPs I discuss above, you may want to consider using the Televue "Equalizer".  It is a solid bronze 1.25”-to-2” adapter which weighs 12 ounces. With this adapter you can place 1.25” EPs in a 2” diagonal and the extra weight of the “Equalizer” helps keep the scope in balance.  (See the Parfocal page for more information.)
  • If you plan to do astrophotography with your scope, one option for mounting 2” eyepieces is to purchase a 2” Flip-Mirror, which would allow you to use 2” eyepieces for visual observing and have that Flip-Mirror available when you need it for astrophotography. Just be aware that many observers believe that a high quality 2” diagonal is important for observing with high-quality eyepieces, and the mirror in the Meade 2” Flip-Mirror probably doesn’t fit the “high-quality” category for those visual observers who spend more than $200 (often way, way more) for an eyepiece.  Click on the Diagonals button at the top of this page for more information.
  • The older kellner design is perfectly fine for a finder or guide scope, and is commonly provided in these.
  • A broad review of eyepiece design and modern offerings was done by the respected amateur observer Ed Ting in the Spring, 1999 issue of Amateur Astronomy magazine. Purchase Issue #21 from them for a copy of this review (unfortunately they don’t have any of their articles on-line).
  • Jay Reynolds Freeman’s Astronomical Telescope Eyepieces - A Discussion for the Beginner goes into somewhat more detail than I have, on the characteristics you need to consider for choosing eyepieces. But note that whereas Jay Reynolds Freeman is an extremely experienced observer (see his Messier Surveys article in the March 2001 issue of Sky & Telescope), you need to know that his opinion that the most expensive modern eyepiece designs are too expensive to be worth the money, are very much at odds with most other (experienced) observers’ opinions, including Ed Ting’s (noted below). Part of the reason he feels this way is that he uses a wide range of telescopes (so he needs to have a wide variety of eyepieces), whereas you only need a few eyepieces if you are only using a single specific scope like a 2000mm f/10 SCT.  For what it’s worth, his conclusion is also interesting in that he is one of the only observers I know of, who owns the $10,000 Astrophysics 10” Maksutof scope and I can’t imagine that he uses $50 eyepieces in it.
  • Excelsis.com is a web site that posts reviews of eyepieces and telescopes (as well as many other items not related to astronomy). The reviewers are folks like you and me who simply hit that Web site and add a review, so you don’t have any way of determining the skill or experience of any particular reviewer and often the reviewers don’t mention what they’re using the eyepiece for. Nevertheless the Excelsis Eyepiece Review section includes most of the eyepieces commercially available and the reviews are worth perusing.  Web sites that have eyepiece reviews by amateur astronomers commonly regarded to be experienced observers, include Ed Ting’s ScopeReviews, Todd Gross’ Reviews, the Cloudy Nights reviews, and Curt Irwin's Affordable Astronomy Equipment Reviews.  (Note that unlike Jay Reynolds Freeman, most of these reviewers assert that the relatively expensive TeleView designs are indeed worth the money for lower-power viewing, if you can afford them.)  Reviews of Takahashi eyepieces are rare for some odd reason, but a good Takahashi comparison review was done by Chris Ellis of the Astronomical Society of Melbourne, Australia. 
  • It is sometimes rumored that the Celestron Ultima line of eyepieces, which are very highly respected, appear to be the Takahashi LE eyepiece line re-branded for Celestron.  This is not true - the Ultimas are actually Baader Eudiascopic eyepieces, re-badged for Celestron (many Celestron accessories, including their 2” diagonal, are manufactured by Baader).  The Takahashi LEs use extra-low-dispersion glass and have substantially more eye relief than the Ultimas. Nevertheless the Ultimas are worth consideration since they are sold at half the price of the corresponding Tak, and are regarded as very good eyepieces.
  • Regardless of which brand of eyepiece you purchase, if you don’t get an eyepiece case to protect them then I recommend the “StarDust” individual poly eyepiece “bottles” sold by Hands On Optics, which are the same really great cases provided by Meade for their Series 4000 eyepieces. Note however that Orion sells some nice “briefcase”-style cases that can hold many eyepieces, for $20 to $30 which is a small price to pay for convenience and protection of what can become an investment of a few hundred dollars over time. The Orion cases work for most 1.25” eyepieces but many 2” eyepieces are too long to allow the case to shut.  So if you use 2” eyepieces you may want to make your own eyepiece case. See the Eyepiece Case page on this site for more details.
  • If you’re interested in a list of the eyepieces I use, click on the My Eyepieces button above.
  • Check the Cleaning tips page for advice on cleaning your eyepieces.

 

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