Chapter 4 - Accessories
Eyepieces are a critical part of your telescope system. A poor eyepiece can seriously limit the performance of a good mirror or objective lens. And just as corrective optics solved many of the problems with the Hubble Space Telescope, certain eyepieces actually compensate for defects in the other optics of a telescope. While good eyepieces won't make a silk purse out of a sow's ear, a few eyepieces are designed to minimize the effects of common problems, such as coma. They do work, so long as the defects in the telescope optics are not too severe. Combine good telescope optics with topnotch eyepieces and you'll see everything there is to see within the size limits of your equipment.
There's been a lot of discussion about the newer high-resolution designs that have hit the market primarily during the last ten years. To those contemplating the purchase of these eyepieces, this can be a critical issue. With prices of even the most basic eyepieces almost always in excess of $50 and the high-tech versions well over $100, you can't afford to waste your money on a less-than-great product!
While you shouldn't look for the very cheapest eyepieces, you don't always have to spend top dollar either. The old Kellner, Orthoscopic, and Erfle eyepiece designs are perfectly adequate. The Orthoscopic is probably the best of these three for general use. Avoid any eyepieces identified as "MA" (modified achromat) or "H" (Huygenian) - these are obsolete designs with narrow fields and poorer definition that are generally included with department store telescopes in the .965" size format.
Some time back, there was an interesting article on eyepieces in Guidestar - the newsletter of the Houston Astronomical Society. The author, Steve Linscott, compared the performances of several different eyepieces to be used for lunar and planetary observing. In referring to the Nagler eyepieces, he makes the point that:
"...some dealer ads would lead you to believe that they are the best eyepieces, PERIOD! Such claims have no meaning unless the telescope and intended use are specified."
In his comparison, he found that the Takahashi brand Orthoscopics out-resolved the Naglers of almost identical focal lengths. He later tested the Takahashi 18mm Orthoscopic against a Brandon 16mm Ortho and a Clave 16mm Plossl using both the Moon and Saturn. There was no detectable difference between these three. He then re-tested the Nagler 16mm Type 2 and found that it was slightly inferior to the three other eyepieces.
Bear in mind that this test involved only resolution on solar system objects. The author notes that for wide-field deep sky work, the Naglers are unsurpassed. (Presumably, this would apply also to some other wide angle eyepiece designs.) This underscores the need to consider the intended use for any eyepiece before you buy it. Another factor is the quality of the eyepieces tested. A Brandon Orthoscopic or Clave Plossl cost more than any other brand of the same design. Sometimes you can find the other brands starting as low as $50. But the Brandons and Claves are such superbly crafted oculars with such good resolution that many owners of these eyepieces - especially those into planetary observation - wouldn't trade them for anything now on the market.
Moreover, if you read Jean Texereau's authoritative book How To Make A telescope, you'll find this comment:
"Only for certain studies with so-called richest field telescopes which use a maximum exit pupil (6-7mm) and usually a beam of very low f ratio (f/4 or f/5) does one need a highly corrected wide-angle eyepiece. For any other work, eyepieces of this complexity are unjustified."
Despite the opinions of Texereau and others, many SCT owners are opting for the new wide-field eyepiece designs. Certainly, this makes sense. SCTs inherently have narrower fields than fast Newtonians, so a wide-field ocular is especially welcome.
What makes any pro-and-con discussion interesting is that it's pretty much a no-win argument. For every point of view, there seems to be someone else who has experienced just the opposite. In contrast to those who favor the "old reliable" designs, consider the comments by the Rev. Robert Evans recounting his discovery of Supernova SN 1990M:
"...at the beginning of June a new eyepiece arrived from Dr. Carl Pennypacker as a helpful gesture in the common cause - a 4.8mm Nagler. This eyepiece would provide just that extra bit of resolution more than the 6mm Plossl I had been using regularly for some years. The wider field would be an extra help. ... It was with this new eyepiece that SN 1990M was discovered, and in a location that tested the resolving power of the telescope."
That quote didn't come from an ad by Tele Vue, which makes the Nagler. As anyone knows who has compared a short focus Orthoscopic or Plossl versus a comparably-sized new wide-field ocular, the high-tech job wins hands down in terms of field size and contrast. This conclusion seems to stand up regardless of the size or configuration of the telescope used.
The article in Guidestar was based on tests using an 8" f/6 Newtonian, which is close to being a rich field ("fast") scope. The new-design oculars offer very wide fields, but some do this at the expense of edge-of-field sharpness. This shows up especially on fast telescopes; for such instruments, check to make sure the eyepiece offers both wide field and good edge definition. If you're in doubt, contact the supplier. Several LCAS people have been getting impressive wide-angle views with the University Wide Scan eyepieces, but I was concerned about the edge-of-field issue with my f/5.6 scope. At Astrofest, I spoke with one of the people from University Optics regarding these oculars. Their typically straightforward response was that there is some loss of definition at the edges in fast systems. But these are excellent eyepieces otherwise; slower optical systems (such as SCTs) encounter no problems at all.
An added consideration is that due to their wider apparent fields, the new designs encourage the use of higher magnifications. This in itself contributes to a darker background and the ability to grasp slightly fainter magnitudes. A generation ago, it would be unheard of to routinely use eyepieces with focal lengths under 10mm. Yet today, that's exactly what many observers are doing! This option was alluded to by Bradley E. Schaefer writing in the November 1989 issue of Sky & Telescope. Referring to the various ways to see fainter magnitudes through your telescope, he said:
"For noticeable changes, you can do only three things. First, get a bigger telescope. Second, find darker skies. Third, increase your magnification."
There are those of us who have reached the maximum reasonable sizes for our telescopes. The big light buckets may be prohibitive in terms of portability, and the larger SCTs may be out of the question due to cost. The "eyepiece solution" offers better viewing with our present instruments. And they're a good investment because they'll do a fine job in whatever instruments we may eventually acquire. Of course, one other rule remains: if the seeing conditions aren't very good, then high magnification from any eyepiece is a waste.
Another advantage of the newer eyepiece designs is that they're highly corrected in order to provide sharper images across the entire field. A side benefit of this is that for those of us with fast telescopes (focal ratios shorter than f/6), these eyepieces tend to somewhat compensate for the coma that's inherent in these systems. There is a noticeable difference going from an Orthoscopic to a Plossl, and a still greater improvement going to a new design, such as a Tele Vue Wide Field.
You may have seen ads for "zoom" eyepieces, which offer a range of focal lengths from the same ocular. That type has been around for awhile. About twenty years ago I bought a variable power Orthoscopic eyepiece. What a waste of money! The concept sounded good, but the images were never really sharp at any magnification. It was necessary to take the eyepiece out of the scope to change magnifications anyway, because you had to hold the eyepiece barrel in one hand while turning the adjusting ring. (On cold nights you just about needed a channel-lock wrench to turn the adjusting ring!)
If you have your eye on a new eyepiece, the full range of Tele Vue, Meade, and Celestron products is usually available from most suppliers. Once in awhile, University Optics will be out of stock on a popular eyepiece. Orion is regularly out of stock on their newest lines of eyepieces; in the case of the MegaVista ocular, they were completely out for over four months!
Despite conflicting opinions on the various eyepieces, there is one view that's unanimous: regardless of which eyepiece design you choose, you will always have to pay more for higher quality. But it is worth it. Once you start using a really good eyepiece, your lesser eyepieces generally will lay around and collect dust. Nonetheless, be aware that you can survive very nicely without one of the expensive ultra-wide-field oculars. Several years ago, I felt the really high-tech jobs were not worth the considerable extra cost; the early versions of the larger focal length Naglers with their long eye relief, "dark spot" and huge size were awkward. However, opinions are subject to change. The Nagler Type 2 is much improved; it and the counterpart Meade Ultra Wide Angle have become the wide field eyepieces. If you're economy-minded, there are oculars that offer fields a bit narrower than those of the monster eyepieces, but at costs well under the $200+ figure. Virtually all of the newest products use high-refractive index glass and advanced coating methods. Together with well-designed lens systems, these features provide impressive views. Even oculars that are merely modifications of older designs, such as the Meade Super Plossl and Orion Ultrascopic, benefit from the newer technology - the images are brilliant and sharp right to the edge of the field. I include myself among the many observers who are sold on the new wide-field oculars. What turned us on to these expensive pieces of glass? Generally, it was because we've had a chance to look through several different types. That'll do it every time!
2. Barlow Lenses
One common accessory is the Barlow lens (sometimes called a "telenegative amplifier"). This is a plano-convex lens - actually a pair of lenses to make it achromatic - that extends the cone of light to multiply the power of an eyepiece by a certain factor, usually doubling or tripling the magnification. It does this by effectively increasing the focal length of the telescope objective, and the longer the focal length, the greater the magnification will be with any given eyepiece. Here's how it works:
Many department store telescopes include Barlows in order to be able to advertise those insanely high magnifications. I would urge anyone with one of these telescopes to throw away the highest magnification eyepiece and keep the low power eyepiece and the Barlow, which will provide more than enough magnification for a telescope of that size. (The ideal, of course, is to replace all those cheap eyepieces and get a couple of good ones; that'll make a big difference even on department store telescopes.)
An analysis shows that some of the new high-tech eyepiece designs achieve their results by simply incorporating a Barlow lens into the barrel. That's a practical concept that works very well...but it's hardly exotic. Assuming you have a good longer focal length eyepiece to begin with, it often makes sense to employ a Barlow to increase your magnification, rather than spend even more money for an additional eyepiece. The older Barlows degrade the overall performance of the system somewhat more than do the newer Barlows. The latest versions use enhanced lens coatings, plus a third element to improve the correction of the system. There is one possible hitch, however. If you use one of the new Barlows with a high-tech eyepiece that employs anywhere up to eight optical elements, your total eyepiece system now has as many as eleven elements. In the absence of any hard data on this, I suspect that so many lenses would just have to prove detrimental to light transmission. In fact, the Tele Vue catalog doesn't mention using their Barlows in conjunction with Naglers - only with their Plossl and Wide Field oculars.
A Barlow offers the best performance at a single optimal position in the lens system. This means that a fixed Barlow (which is presumably optimized) might be preferable to a variable power Barlow. However, there's some disagreement on this, with the opposing view pointing out that an adjustable Barlow allows the user to set the lens for the best position with each different eyepiece used. If you have an adjustable Barlow, it's wise to experiment to locate the ideal position for the lenses. As an example, the images from my variable power Goodwin Barlow seem to be brighter and have better contrast when set near the 3x position, rather than in a lower power setting.
To compute the optical characteristics of a telescope with a Barlow, the following formulas apply:
- F - focal length of objective
- f - focal length of the Barlow lens
- E - apparent focal length, objective lens or mirror plus Barlow
- M - magnification factor of the Barlow
- d - distance of the Barlow inside the original focal plane
- D - distance of the Barlow inside the new focal plane
E = F*f / (f-d),
M = E / F = f / (f-d),
D = f *(M-1).
The following illustrates the different magnification factors for an eyepiece brought to best focus using a Barlow. These are approximate examples using the Goodwin Barlow; other brands may produce different results depending on the focal point of the Barlow lens.
Filters perform the same function as they do in photography - suppressing or enhancing certain details of the image as captured by the telescope. While they'll help you see things better, they cannot show you more than what the telescope itself is capable of delivering. But they are considered essential accessories by most experienced observers. There are three basic types of filters - colored, nebula (sometimes called a "light pollution filter"), and solar. The first two are mounted in threaded rings so that they can screw into the barrel of an eyepiece. Today, most eyepieces are threaded to accept standard filters, either in the 11/4" or 2" formats. A solar filter completely covers the skyward end of a telescope so that only a small fraction of the Sun's light enters the optical system.
By filtering out certain wavelengths of light, this type enhances some details, primarily when observing the Moon and planets. The different colors are designated by their Wratten classifications - the same as photographic filters. The good ones are made of dyed optical glass and are overcoated to maximize transmission of details; avoid plastic filters, which seriously degrade the quality of the image. The following is a list of the most commonly used colored filters, with the Wratten numbers and their usual applications.
|#8||light yellow||Enhances Martian cloud details|
|#11||yellow-green|| Increases contrast on Martian features and cloud
|#12||yellow|| Reduces scattered blue light; useful for observing
and Venus during daylight hours; increases lunar contrast
|#15||dark yellow||Increases contrast on Martian polar caps and white clouds|
|#21||orange|| Enhances structure in cloud bands of Jupiter and Saturn;|
increases lunar contrast; helps observing seasonal changes
|#23A||light red|| Increases contrast on martian maria; shows details in
on Jupiter; enhances daytime observation of Venus
|#25||red|| Most important filter for observing martian detail, melt |
bands, polar caps, and maria; decreases skyglow; brings out
detail in Jupiter's cloud belts and helps observing transits of
the Galilean moons
|#38A||dark blue||Increases contrast of Jupiter's clouds and great red spot|
|#47||violet||Enhances clouds on Jupiter and Venus|
|#56||green|| Helps observing yellow dust storms on Mars; enhances|
contrast on Moon and Saturn
|#80A||medium blue|| Shows off upper atmosphere martian clouds; brings
subtle details on Jupiter and Saturn, especially light zones;
improves lunar contrast
|#82A||light blue|| Shows blue clearings on Mars and light spots in
|-||Neutral density|| These filters simply darken bright objects and reduce
for better resolution of details. Used primarily on the Moon,
but can be stacked with other filters for further darkening.
Identified by density (darkness), for example ND13, ND25,
and ND50, with the higher number being darker. These are
often included with department store telescopes as "Moon
Filter rings are normally also threaded on the inside circumference so that they can be stacked - screwed together and mounted in an eyepiece to make use of the features of various colors. For example, Venus is extremely bright and appears low in the sky, so it tends to "dance around" in the atmosphere as you look at it in a telescope. To get a better look at details and steady the image, it's helpful to stack a neutral density filter with a #12, #21 or #25.
Polarizing filters are in a different class, since they're not a colored filter, as such. Normally, these filters are adjustable so that you can select the amount of light you want to have transmitted to the eyepiece. By reducing glare they can improve the contrast and detail of bright objects such as the Moon, Venus, Jupiter, Saturn and bright stars, without altering the colors of these objects.
You're probably not going to run out and buy all the different colors, so which filters should you get? A lot depends on your particular interests, but in order of usefulness, I'd list #25 red, #21 orange, and #80A medium blue, then perhaps a neutral density or polarizing filter for darkening the Moon and Venus. If you're at a group observing session, you might want to borrow different filters from other observers and experiment to see which ones you find most helpful. The results of using filters often are quite subtle, but they do work.
4. Nebula Filters
The term "nebula filter" is a generic description of a variety of filters that block certain discrete frequencies of light to enable the observing of nebulae even where light pollution is a problem. But this technology also produces filters that are intended for observing comets ("Swan band" filters) and some purportedly to enhance the light emitted by galaxies. Instead of being a piece of glass that's dyed a certain color, these filters use thin film interference technology to discriminate between discrete bandwidths of light, measured in Angstroms. This technology actually spans a wide range. On the low-end are light pollution rejection (LPR) filters that admit light across a broad range and stop only those bandwidths emitted by streetlights; these are referred to as "broadband" filters. The light that the filter admits is in the "passband". A true nebula filter stops more light than it admits, so these are designated as "narrowband" filters. While colored filters sell for about $12.00 apiece, many newcomers to the hobby are astounded to find that the nebula filter that looks like a piece of tinted glass the size of a quarter starts at about $75.00 a pop! In fact, the same concept is applied to hydrogen alpha filters intended to reveal solar flares. Some of these are so sensitive that they must be heated to keep the passband within a fraction of an angstrom of the correct wavelength. It's not uncommon for these filters to cost over $2000.00!
For our purposes here, we'll concentrate on those filters intended for observing nebulosity and identify each by its capabilities. That's not such an easy task, as you'll see.
The LPR (broadband) filter approaches the problem from the angle of rejecting light emitted by mercury or sodium vapor lights. From this standpoint, it darkens the sky background and may actually make it easier for you to see certain bright galaxies, as well as nebulae. Though it's good for general applications in areas of moderate light pollution, the LPR filter becomes overwhelmed in locations where there's severe light pollution. On the other end, it won't help you see things better from a truly dark site. But within its working range, it can prove very helpful and is what most newcomers to astronomy acquire. Each supplier has its own brand name for this type of filter. Some examples: Lumicon Deep Sky, University Mark V Photo-Visual, Meade Nebula, Parks LPB, Orion Skyglow, and Celestron LPR.
The true nebula filter (narrowband) is able to do its thing because nebulae emit their visible light in very narrow wavelengths. Note that these filters won't work on reflection nebulosity, such as that involved in the Pleiades cluster, because this type is only reflecting starlight, which covers a very broad range of visible light frequencies. One supplier (Lumicon) has a trio of filters for specific purposes:
Ultra High Contrast - provides high contrast on planetary and other emission nebulae.
Oxygen III - has higher contrast than the UHC above, but in a narrower range - primarily that of planetary nebulae which emit much of their radiation in the band of doubly and triply ionized oxygen.
Hydrogen Beta - the most discriminating filter - intended specifically for the California, Horsehead, and other extremely faint nebulae with hydrogen beta emission.
These filters often improve the view even at dark sites, because most such sites aren't really dark - there's natural skyglow (neutral oxygen) and residual glow from far-off towns, especially in the winter when snow adds to the reflection of light. During times of peak solar activity, there may be airglow from atmospheric gases excited by gamma rays from the Sun. While Lumicon filters are generally regarded as the best in the field, other firms also produce narrowband filters at somewhat lower cost, such as Orion's Ultrablock.
Which filter is best? In all cases, the filters are identified as either broadband or narrowband. Most experienced deep sky observers opt for narrowband filters because they provide the greatest overall improvement. In fact, some nebulae look considerably different when viewed through a very narrow narrowband filter, such as the Oxygen III. The following chart shows the wavelengths of light (in nanometers) that are allowed to pass by some of the filters for which data is available from the manufacturer. Note that light pollution occupies a range primarily in the center of the chart; in all cases, the filters do not pass any light in this range. Actually, light pollution somewhat spills over into other frequencies, so the more discriminating narrowband filters provide a darker sky background. Of course, this is done at the cost of transmitting light only from those particular nebulae that emit radiation in the very narrow ranges of these filters. There is no clear consensus among experienced observers, but many feel that a general purpose narrowband filter such as the Orion Ultrablock or Lumicon UHC is probably the best choice for the first-time purchaser of a nebula filter, especially in view of the amount of light pollution with which most of us have to contend.
5. Solar Filters
The projection method is a common means of observing the Sun - using the eyepiece to project the Sun's image onto a white screen. This works very well for group observing, since several people can view the image simultaneously. However, a larger scope gathers so much light focused through the optics that the eyepiece heats up to the point where it becomes too hot to touch. This can cause irreparable harm if the adhesive that binds the lenses is caused to melt. The effects would be seen as bubbles and/or discoloration between the lens elements of the eyepiece. To avoid ruining good optics, I used a cheap eyepiece that I bought for $5.00 several years ago. In addition, projection doesn't show as detailed an image as can be seen with direct viewing. With this in mind, I opted for a solar filter for direct viewing, which would also permit the use of better eyepieces. This filter is not the type that used to be sold for mounting on eyepieces. That type is extremely dangerous because it heats up just like the eyepiece; if it were to shatter from the heat, the observer would be blinded. The correct type of filter covers the skyward end of the telescope so less than 1% of the Sun's light enters the optical system.
There are two standard types of filter material. The more durable, and more expensive, is a glass filter that has been coated with special materials to reduce the Sun's brightness. These are available in three variations: normal, extra durable, and lower density for astrophotography. The alternative is aluminized mylar, but not the cheap type that you normally buy off rolls for use as a craft or decorating material. If you've ever looked at the roll-type material, you've noted that the coating is fairly thin and uneven. You have to use several layers to achieve sufficient darkening. Moreover, the cheap stuff is easy to scratch. For the sake of protecting your vision, it's important that there be no pinholes or thin spots through which the blinding solar light can enter your eyepiece. The optical-quality type has a uniform coating of aluminum; when mounted properly, this coating is protected between two layers of mylar. The glass filters give a yellow-orange solar image, while mylar gives a light blue image. Although both filters are equally safe, the coated mylar removes more of the infra-red radiation, thus causing the blue cast. A #15 yellow or #21 amber filter added on the eyepiece will give a more natural rendering. The aluminized mylar offers the greatest amount of material for the money. One advantage of glass is that it might induce slightly less image distortion, though I haven't noted any problems that are obviously attributable to the mylar.
The material comes as two pieces of mylar. While I selected the 6"x6" size from the catalog, each piece as delivered was actually over ten inches square. For adequate darkening, two pieces must be used together with the aluminized (shiney) sides facing each other; this protects the aluminized layers from getting scratched and from atmospheric contaminants. The bonus of the extra material allows for any mistakes that might ruin a piece of mylar and/or provide leftovers for a camera or spotting scope filter.
Mounting the Filter
Both the glass and mylar filters come with cells or they can be purchased separately for custom mounting. The Sun's image is so bright that, especially on larger telescopes, you don't need to use full aperture. Many users make an off-axis diaphragm, sometimes referred to as an "aperture mask", in which the hole (diaphragm) is covered by the mylar or glass filter.
The above illustration shows the filter mounted on an off-axis diaphragm for use with a reflecting telescope, either a Newtonian or Schmidt-Cassegrain. An off-axis diaphragm has the added benefit of sharpening the image by bypassing the secondary mirror. On a refractor, you can also use a diaphragm, except that the hole would be centered on the primary lens, since there's no secondary obstruction. Just make sure the mask is securely fastened to the scope so it can't be accidentally knocked off while observing!
The cost comparison below shows the alternatives (exclusive of shipping or tax) for a diaphragm on a 10" scope.
|Mylar in cell||4"||$79|
|Glass in cell----------||-||-|
For my 10" Newtonian, I made an aperture mask with a diaphragm slightly smaller than four inches. I purchased the mylar filter material from Roger W. Tuthill, Inc. The prices shown for the glass filters are for the Thousand Oaks brand. Full aperture filters would obviously cost much more.
The mylar is tough, but very flimsy, which means a pair of sharp scissors is needed to cut it. The instructions caution against stretching the mylar to remove wrinkles. Specifically, we are warned: "Do not try to stretch it flat, as the molecules in the material become disoriented and the result is a less superior image as far as contrast is concerned." It's hard not to give into the temptation to make the filter lie perfectly smooth and taut! The instructions stipulate that the filter must be mounted using double-faced tape, since glue might outgas, thus damaging the aluminum coating. I found that once the mylar hits the double-faced tape, it really sticks fast.
For maximum contrast, all stray light should be eliminated from inside the body of the telescope. The background is very bright in a strut-tube scope, so a shroud of black fabric or plastic should be used to cover the body of the telescope. While this may not eliminate all the stray light, it will improve the contrast significantly. Even more important than a shroud over the telescope body, it's necessary while observing to shield your face from the sunlight, which comes at your eyes from the side and makes it very difficult to see the solar image in the eyepiece.
One final note: Nebula filters do their job by restricting certain frequencies of light. The object you are observing appears brighter because the background becomes darker, while the light from the nebula is allowed to pass through. In effect, the view is improved by virtue of an improvement in contrast. But in reality, even the nebula is slightly dimmer, because in fact, there is less light passed to your eye. The effect is so minute that it isn't a problem with larger telescopes, but on small telescopes, you may see no improvement whatsoever, or a faint nebula may disappear altogether when using a narrowband filter. The telescope itself must gather enough light to compensate for the loss of light caused by the filter. Personal experience suggests that a narrowband filter works best on telescopes of 6-inches or larger, although I have successfully used an Oxygen III filter when observing bright emission nebulae (such as M42 in Orion) with a 4-inch refractor. For scopes smaller than 6-inches, a broadband filter will be more useful, and in scopes of 3-inches or less only a broadband filter will somewhat improve the view, and only on the brightest nebulae.