June-July-August cAlendAr Summer 2007ISSue 78

BPAA NewsletterBattle Point Astronomical Association, Bainbridge Island, WA

(Unless otherwise noted, all events are at the Edwin Ritchie Observatory, Battle Point Park)

JuneJune 3: John Rudolph Memorial Planetarium Fund

Kiwanis Brunch, Wing PointJune 6: 7 p.m. BPAA Board Meeting June 8: Last-quarter Moon June 9: 8:00 p.m. Planetarium Show: “Seeing Galaxies: the

Milky Way, Andromeda & Others” and Star Party June 13: 7 p.m. Members Meeting: guest speaker Alice

Few of the Tacoma Astronomical Society dis-cusses the NASA/JPL Night Sky Program

June 14: New MoonJune 19: Pluto at oppositionJune 21: 7 p.m. “Fabrication and Testing of Optics at the

University of Arizona,” talk by Martin Valente, College of Optical Sciences, U. of Arizona (article p. 4); Summer Solstice 11:06 a.m. PDT

June 22: First-quarter MoonJune 30: Full Moon

JulyJuly 1: John Rudolph Memorial Planetarium Fund

Kiwanis Brunch, Wing PointJuly 4: Grand Old Fourth in Winslow July 5: 7 p.m. BPAA Board Meeting July 7: 8:00 p.m. Planetarium Show: “The Glorious

Globular Clusters of Summer” and Star Party July 7: Earth at aphelion, 1.017 AU from Sun;

Last-quarter MoonJuly 9–16: 2007 Shingletown Star Party

www.shingletownstarparty.net July 11: 7 p.m. Members Meeting (subject to cancellation;

check bpaa@yahoogroups.com)July 12–14: Table Mountain Star Party www.tmspa.com July 14: New MoonJuly 22: First-quarter Moon July 29: Full Moon

AugustAugust 1: 7 p.m. BPAA Board Meeting August 3–4: ALCon Expo 2007, Portland, Oregon

www.alconexpo.com August 4: 8:00 p.m. Planetarium Show: “How to Observe

August 28th’s Total Lunar Eclipse” and Star Party

August 5: John Rudolph Memorial Planetarium Fund Kiwanis Brunch, Wing Point; Last-quarter Moon

August 8: 7 p.m. Members Meeting (subject to cancellation; check bpaa@yahoogroups.com)

August 8–12: Mt. Bachelor Star Party www.mbsp.org August 10: Deadline for submissions to Fall issue of

BPAA NewsletterAugust 12: Perseids Meteor Shower peak; New Moon August 16–19: Oregon Star Party www.oregonstarparty.orgAugust 20: First-quarter Moon August 28: Total Lunar Eclipse; Full Moon

Selecting Eyepieces Doug Tanaka, page 5

�BPAA newsletter summer �007cAlendAr notes

I expected to open these notes by whining about the weather. Surely this has been one of the worst fall and winter seasons on record for viewing

the night sky. But suddenly, as I write this, things are clearing up. Spring has arrived. Those involved with the planetarium shows are doing wonderful work, but it will be nice to step outdoors and look at the real thing.

Summer will follow: time for the major regional star parties. The Table Mountain Star Party is July 12–14. Register at www.tmspa.com. The Mt. Bachelor Star Party (www.mbsp.org) will be closely followed by the Oregon Star Party (www.oregonstarparty.org). MBSP is August 8–12 and OSP is August 16–19. These dates will allow diehard enthusiasts to go directly from one to the other for almost two weeks of viewing in those very dark central and eastern Oregon skies. An addition to the list this year is the Shingletown Star Party on July 9–6. It is located in northern California near Mt. Lassen Volcanic National Park and is becoming increasingly popular with star party enthusiasts. Check it out at www.shingletownstarparty.net.

Another event of interest in the region is ALCon Expo 2007 on August 3–4, the annual convention of the Astronomical League. It is in Portland this year, a west coast venue for the first time in several years. It will include outstanding speakers and astronomy-related exhibits. More information is in the March 2007 issue of the Reflector and online at www.alconexpo.com.

The March issue of the Reflector (pp. 6–7) also contains a nice tribute to the National Park Service for its work in preserving dark skies. The article credits the Night Sky Team at the NPS with stimulating both discussion and action aimed at protecting and restoring nighttime resources. You can find out more about the efforts of the NPS at www2.nature.nps.gov/air and by attending the Oregon Star Party. Kevin Poe, a Dark Sky ranger at Bryce National Park, is one of the featured speakers this year. Harry and I met Ranger Poe in 2005 during a volunteer gig in Bryce’s astronomy program. (We reported on our experience in the Spring 2006 issue of the BPAA Newsletter, pp. 5–6.) Ranger Poe is a dynamic speaker and his presentation on preservation of dark skies is visually stunning and highly motivating.

A reminder: if you haven’t yet attended one of the brunches at Wing Point benefiting the John Rudolph Memorial Planetarium Fund, you should. The food is excellent and $5 of each brunch goes to the Planetarium Fund. Call 842-2688 for reservations and ask for the Kiwanis Brunch at Wing Point.

In June, Saturn and Venus will align nicely with a crescent moon at dusk on the 17th, 18th and 19th. On June 30 at dusk Saturn and Venus will be just 2/3˚ apart. Look for Saturn and Venus again with the crescent Moon on July 15–17, with Regulus added to the mix. In August we on the west coast will be treated to a total eclipse of the Moon. Totality will last 90 minutes. Look for updates on the eclipse on our Web site and our email group. The Planetarium presentation on August 4 will highlight the eclipse.

Note that in June, July and August the planetarium shows and the beginner sessions for our local star parties will begin at 8:00 p.m. And remember that any member at any time who

is planning to observe can invite others to join in by sending an email to bpaa@yahoogroups.com. To join our email group, send an email with your name to bpaa-owner@yahoogroups.com and we can enroll you. If you want to also have web access to the messages and files, you can join the Yahoogroups by clicking the register link for new users on http://groups.yahoo.com/, and request to join our group on this page: http://groups.yahoo.com/group/bpaa/. The system will send us a message, and we’ll approve your request after we verify your membership.

Diane ColvinBPAA Events Director (dtcolvin@comcast.net)

Crescent Moon and Venus

Natural color view of Saturn, composed from a series of pictures taken by the Cassini spacecraft. (courtesy NASA)

�BPAA newsletter summer �007

part. The length of these tubes controls the location of the focal plane and thus determines whether

In BrIef

President’s MessageHarry Colvin

Winter is finally over; we have even had several clear sky nights. If you are an astronomer in

the Pacific Northwest the urge to get out and do some observing is almost unbearable. As the summer approaches we can look forward to some great evenings under the stars and the long awaited major star parties.

The BPAA and the Ritchie Observatory this summer will be buzzing with activity as Doug Tanaka and his team of elves finish the 20” telescope. The scope will be a thing of beauty: It saw first light May 29 (see article below). Nancy Cooper and Jim Vaughan’s astro and robot camps are scheduled for July and August and just as last year, should be big successes. Also upcoming are the 4th of July parade, the planetarium shows by Cathy Koehler and Paul Below, and several professional astronomy lectures. Check out the calendar in this newsletter for all the dates and any events I have omitted.

The Table Mountain Star Party has limits on the number of attendees and it fills up rather fast. So if you wish to attend this one best register now. The other major event is the Oregon Star Party held east of Prineville, Oregon. This event has plenty of room with no limits on the number or registrants. The drive is a long one but the skies are some of the darkest in our

region. Another major astronomy event in our region this summer is the American Astronomy League annual meeting held this year in Portland on August 3-4.

Okay, so we can look forward to a summer of viewing. What do you do next? Much depends on your level of experience. For those of you who are just beginning I recommend spending the first few months learning the sky with a pair of binoculars. Learn how to read star charts. Go to BPAA star parties and look through other members’ telescopes. Once you have mastered the sky a bit, check out one of the BPAA telescopes and try your hand at finding a few Messier objects.

For those of you who already own telescopes, know the sky, and are looking for projects I recommend setting some observing goals. Maybe try completing all the summer Messier Objects by September. For the more advanced amateurs with a telescope 10 inches or larger the Herschel 400 list waits for you. Then if you really want an adventure try imaging sky objects with a CCD camera or digital SLR. Be warned however the learning curve is rather steep, although the rewards are worth it and can lead you into the realm of amateur/professional research collaborations. I have been invited to give a presentation on the role of amateurs in astronomy research at the OSP this summer. I will be describing how amateurs are performing light curves on cataclysmic variable star systems, discovering exo-planets, and discovering supernovae.

Before I close I wish to announce that the BPAA has seen a 29 % increase in membership since December of 2006. If you are not involved consider becoming active. There is plenty of work, and fun, for everyone.

Twenty-Inch Telescope Sees First LightMalcolm Saunders

Virtually all the wood cutting and gluing for the new 20 inch Dobsonian is finished. Soon we

will be moving on to sanding and varnishing. If you have been interested in helping with the project, but have not felt able to do the woodworking, consider helping with this: it takes place in small stages and lends itself to lots of people doing small portions.

The two photographs show a critical stage in the telescope building project. The truss tubes hold the upper part of the telescope to the lower

Computer model of the new 20" telescope. (model by Malcolm Saunders)

�BPAA newsletter summer �007

Big Time OpticsMac Gardiner

Many of you will

remember that, at one time, we had a huge crate in the foyer of the Observatory, four feet on a side, and about three feet high. We temporarily used it to hold news items, announcements, etc. It held a weird, unique mirror that had cost millions of dollars to shape and polish…a gift from Boeing, who had used it for a terminated “Star Wars” project. Though terribly expensive to form, an Off Axis Parabola Reflector has a very small “niche requirement” and had little market value. We hesitated to use the mirror, as it would have to be recut for astronomical use, and we would have had to build a new observatory to house it! We also hated to even touch that beautiful aspherical surface that had taken so much effort to produce.

Opportunity showed up at the Optical Sciences lab at the Arizona College of Optical Sciences. Marty Valenti, the manager of the Optical Shops, saw the

need for such a mirror in his development program. He didn’t have the resources to buy or build one. What he did have was a beautiful On Axis Parabola 27” mirror of exceptional quality, for which they currently had no need. On this basis an even trade was made, and a truck carried the Arizona mirror up to us, and returned with the Boeing Off Axis reflector.

A short while ago, I was in Tucson with family, and contacted Marty. He and Michael Miller showed us through the labs where huge mirrors are cast, finished, and tested. We saw elements of the test setup that was used to test a huge optical flat for deviations from the true flatness of around 3 nanometers, less than 1/100 the wavelength of blue light. An off axis parabolic mirror is required to collimate the interferometeric light source exit field, and it should “cover” the object being tested. If not, a stitching technique is required, which is time consuming and inaccurate. The Boeing mirror was used in this test and proved successful. Documentation of the test program was given me, and is available. The dimensions of everything used in this, and other tests, are large, and eight story open-bay rooms are required to permit keeping the test objects horizontal.

The mirrors that are made here are parabolic 8.4 meter diameter reflectors, using a rotating mold to cause the optical surface of the liquid glass to form a parabolic shape. This saves tons of glass, years of grinding, and lessens the possibility of thermal cracking. It appears that most of the very large mirrors, now existing, in construction or planned, are built here. The whole facility is fascinating, out of this world, and very satisfying.

Martin has agreed to give a lecture to us on this subject June 21st. Don’t miss it!

any given eyepiece can bring the sky into focus. The most reliable way to do determine the correct length is by a real trial, focusing the telescope on a star. This is always done before the telescope is completed. A temporary fixture is put together with scrap lumber and cabinetmaker’s clamps to hold the telescope components in alignment while the test is run.

We had a good clear night on May 29th and did the test. The first photo shows a laser colimator in use aligning the optics. The second photo shows Harry Colvin and Doug Tanaka (hidden behind his hand at the eyepiece) focusing on a star. This is the moment of first light for the new 20 inch scope!

1.58 meter OAP in counterweighted cell. photo courtesy Arizona College of Optical Sciences

�BPAA newsletter summer �007ArtIcles

SELECTING EYEPIECESDoug Tanaka

You’ve just bought your first telescope. It came with a complimentary eyepiece, but now you’d like to

buy a few more. Orthoscopics, Kellners, Plossls, Erfles, Konigs, Naglers…the choice can be overwhelming. Aside from the telescope itself, eyepieces are an amateur astronomer’s most important pieces of equipment; the cost of a set of premium eyepieces can equal or surpass that of a telescope. This article should make it easier for the novice eyepiece purchaser to select their first set.

Exit Pupils

While it’s possible to choose eyepieces based on the magnifications they will produce on a given telescope, I base my choice on the size of the exit pupil produced by a

particular eyepiece combined with a particular telescope. When eyepiece and telescope are focused, the collected light forms an “exit pupil”—a small circle that hovers just above the eyepiece. To look through the telescope you place the pupil of your eye where this circle comes into focus. If you place an opaque piece of glass or plastic where the exit pupil is focused you can see the tiny image and measure the circle with calipers. The diameter usually ranges from about 7mm down to .5mm or even less.

For most people the daytime pupil size of their eye is about 2mm. Dark-adapted eyes are dilated 2–4 times. In children and young adults the dark-adapted pupil can enlarge to 7mm or more. As we get older our pupils are less able to enlarge, with a lower limit of about 5mm for those 60 years of age and older. Since the exit pupil hovers above the eyepiece and we need to place our own pupil at that location to see the image, we can make the most efficient use of the gathered light if the size of the exit pupil is the same as, or smaller than, our own dark-adapted pupil size. If we try to look at an exit pupil that is larger than our own pupil, the extra light is blocked by our iris and never reaches our cornea. In other words, if you are 60 years old with pupils that can only dilate to 5mm and you try to look at a 7mm exit pupil, you will be losing 2mm worth of light. Likewise, if you are young with a 7mm pupil, you can’t fully use an 8mm exit pupil.

Theoretically, refractor owners are not limited by exit pupil size and can use any size they wish. The

problem is that the image will eventually become overly bright and magnification would decrease until they were comparable to binoculars, though they would still be able to see a very wide swath of sky. This is not the case with reflecting telescope because the central obstruction (the secondary mirror) would start to become visible once the exit pupil reached a diameter of about 10mm.

To simplify: The first few eyepieces that you buy should have an eyepiece/telescope combination that produces an exit pupil 7mm or less in diameter. If you are middle-aged the exit pupil should be 6mm or less, and by age 60 you should shoot for 5mm.

Calculating Exit PupilsWhen you bought your

telescope you probably noticed that there were two main numbers in its specifications, the diameter of the objective (given in either inches or millimeters) and its focal ratio (f/4.5, f/7, etc). To find out what size exit pupil you’ll get with a given eyepiece you divide the focal length of the eyepiece by the telescope’s focal ratio. For example, an eyepiece with a focal length of 18mm combined with a telescope with a focal ratio of f/6 will produce an exit pupil of 3mm (18/6=3). Now, suppose you’re middle-aged (with a maximum pupil size of 6mm), just bought a telescope with a focal ratio of f/5 and want the largest useable exit pupil. In that case you just multiply your telescope’s f/ratio (f/5) by the desired exit pupil (6mm) and you find you need to buy an eyepiece with a focal length of 30mm.

You might wonder why anyone would want to buy an eyepiece that produced a smaller exit pupil, and the answer is that magnification increases as the size of the exit pupil decreases. (see sidebar)

All you really need to know is that exit pupils in the 5-7mm range correlate with low magnification and wide fields of view, exit

Photo by Steve Jurvetson

Calculate magnification by converting the size of your telescope’s objective into millimeters, multiplying that by your telescope’s focal ratio to get the focal length of your telescope, and dividing that by the focal length of the eyepiece. To convert inches into millimeters multiply the number of inches by 25.4.

Using a 5", f6 telescope as an example, the 5" converts to 127mm {5 x 25.4 = 127} and multiplying that by the f/ratio we get a focal length of 762mm {127 x 6 = 762}. Then to find out what magnification you would get if you inserted a 14mm eyepiece you would divide the 762mm focal length of the telescope by 14. 762/14 = 54.4 so you would end up with a magnification of about 54x.

While this is a very handy way to figure out the exact magnification you will get, you can see that it’s easier to find what eyepiece best suits your purposes by using exit pupil size.

�BPAA newsletter summer �007pupils in the 2–4mm range correlate with medium magnification suitable for viewing extended objects like galaxies and nebulae, and exit pupils 1mm or less correlate with high magnifications suitable for planetary viewing or looking at fine detail on the moon.

When we put the same eyepiece into telescopes that are drastically different in size but have the same focal ratio, the magnifications produced will be different but the exit pupils produced will be the same. For example, a 24", f/5 Dobsonian telescope with a 10mm eyepiece will give a magnification of about 305x, while putting the same eyepiece in an f/5 refractor with an objective size of 3" will only give a magnification of 38x. Both examples produce exit pupils of 2mm.

The 2mm Exit PupilIt is relatively easy to calculate

the ideal low-powered eyepiece based on your own maximum pupil size and tailored for your specific telescope focal ratio and maximum pupil diameter. But medium-powered eyepieces produce exit pupils that range from 2–4mm in size. Choose one that produces a 2mm exit pupil. This exit pupil most closely matches the resolution of the human eye: about 60 arc-seconds. The resolution of a telescope is 4.55 arc-seconds divided by its aperture in inches. For optimum resolution we need to find the magnification that will produce an image of 60 arc-seconds, which computes to roughly 13x per inch of aperture. This will produce an exit pupil of 1/13 inch, close to 2mm. You’ll find that a 2mm exit pupil will show the most detail in extended objects like galaxies and nebulae. While it’s true that the largest exit pupils produce the brightest images and, one would think, be the best for deep-sky observing, the background sky will be equally as bright and you’ll find that the overall image will be lacking in contrast, especially in light-polluted areas. With a 2mm exit pupil the overall image (the background sky and the object itself ) will be equally darkened but the size of the image will be increased, and one of the properties of the human eye is that it’s easier to see faint things if they are large. At this point you can continue to decrease exit pupil size (increase magnification) to see if more detail can be drawn out. There are many diffuse objects that look better at higher magnifications but in general the 2mm exit pupil will show the most detail.

This will only optimize detail for extended, diffuse objects like galaxies and nebulae. It does not apply to bright objects like stars and planets. Since stars are points of light, they don’t appear to change in brightness

as exit pupils get smaller. However, the background sky will get increasingly darker as exit pupils decrease, making it easier to see faint stars. This is valuable to observers who are attempting to split very faint double stars, for example, but not something that most novices will be concerned with. For now it’s enough to know that a 2mm exit pupil is an excellent all-around size for most observers and will be a favored eyepiece. Subjectively, I find that a 2mm exit pupil is “cozy”—comfortable to use for a long period, producing the least amount of eye strain. My first telescope had an aperture of 90mm with a focal ratio of 13.9. I soon found that the eyepiece I used the most had a focal length of 24mm. I assumed that this was due to the eyepiece itself. Then I built a Dobsonian telescope with an aperture of 12.5" and a focal ratio of 6. While my “favorite” eyepiece gave nice images in this telescope, it was no longer the one that I used the most. My new “favorite” was an eyepiece with a focal length of 12mm.

It wasn’t until I learned about the 2mm exit pupil and its relation to the resolution of the human eye that I began to see the connection. Both combinations gave exit pupils around 2mm. At the time I was involved with various observing groups on the internet. The question “What is your favorite eyepiece?” would often come up. Many replied by specifying which eyepiece they liked the best, citing the brand and focal length, but the most informative replies also specified the type of telescope they used it in. After reading numerous threads (and an almost equal number of arguments!), I began to realize that if a chart were made showing which exit pupil was the “favorite,” the bell curve would be centered very close to 2mm.

If you are buying just one eyepiece, or a couple of inexpensive eyepieces and one premium eyepiece you should get the best eyepiece you can afford that will produce a 2mm exit pupil. It’s likely to become your workhorse eyepiece.

High-Powered EyepiecesOn the surface your choice of a

high-powered eyepiece might seem straightforward. You want an exit pupil around 1mm, so you take your telescope’s focal ratio and buy an eyepiece with the same focal length. But your choice needs to take into consideration the area you live in, the quality of your telescope, and some eyepieces to avoid.

High-powered eyepieces are good for observing planets, where you want the highest magnification possible without the image becoming blurry. And this is

Andromeda Galaxy (NASA-MFSC)

Saturn (NASA-MFSC)

7BPAA newsletter summer �007the catch. At the highest magnifications it’s not just the image of the planet that gets magnified but everything else in the observing chain—the atmosphere above you, aberrations in your telescope’s mirror or lens, and even the quality of the eyepiece itself.

Let’s first consider the atmosphere. On some nights the stars twinkle and on some nights they are steady points of light. What causes this is air movement miles above you, usually associated with the jet stream, the narrow band of fast-moving air that moves from west to east. To understand how this affects seeing conditions, imagine you are lying at the bottom of a swimming pool wearing a diving mask, looking up at the sky. If the wind is creating ripples on the surface of the pool you can’t see clearly. If there is no wind, the surface of the pool is smooth and you will be able to see perfectly. So it is with the atmosphere.There are two jet streams, the polar, between latitudes

30–70 degrees and the subtropical, between latitudes 20–50 degrees. (These are mirrored in the Southern Hemisphere for a total of four jet streams.) For observers

in the US the primary “twinkler” of stars is the polar jet stream. This stream takes a meandering path, like an out-of-control water hose with a high pressure nozzle. The area this stream can influence extends from Northern Canada to most of the US. It does not distribute its time equally across its range but resides mainly along the US/Canadian border (around latitude 50) with occasional forays to the north and south.

If you live in the north you should consider purchasing an eyepiece that produces an exit pupil closer to 1mm and if you live to the south you will be more likely to be able to use an eyepiece that produces a .5mm exit pupil. This is because exit pupils are closely tied to magnification and when we talk about the smallest useable exit pupil we can just as well be talking about the maximum useable magnification.

Department store claims of 1,000x to the contrary, very few amateur telescopes are capable of achieving decent resolution at even 1/3 this magnification and probably much less. Empirical evidence has shown that a good rule of thumb is to multiply your telescope’s aperture (in inches) by 60x to find its maximum magnification under ideal conditions. Using our 5", f/6 telescope as an example, the maximum useable

magnification under ideal conditions would be 300x (5 x 60 = 300). If you recall, the focal length of this telescope is 762mm, so to find out what size eyepiece would give us this magnification we just divide 762/300 to get an eyepiece with a 2.54mm focal length. This is a very short eyepiece focal length and in this f/6 telescope would give an exit pupil of 0.42mm. But bear in mind that all conditions must be perfect for this to be useable and realistically the minimum useable exit pupil is closer to 0.5mm.

The quality of your telescope and eyepiece places a limit on how small an exit pupil you can reasonably use. As another rough rule of thumb, the more expensive your optics, the closer you can go toward a .5mm exit pupil.

Another factor to consider when choosing a high-powered eyepiece is the amount of eye relief. This is the distance your eye needs to be from the eyepiece in order to see the image. Plossls are widely used eyepieces because they are relatively easy to make, give a decent field of view, and produce a very flat and accurate image, with very little distortion, but their downside is that eye relief decreases in direct proportion to their focal length. Down to about 17mm they are very nice, but by 10mm they start becoming slightly uncomfortable to use and by 7mm are almost impossible to use. At the shortest focal lengths you’ll need to hold your eye so close to the eyepiece your eyelashes will brush against the glass leaving oil and/or mascara streaks on the glass. If you want good eye relief with a high-powered eyepiece you will need to spend money on one of the premium designs, or use what’s called a Barlow lens.

This is a negative lens that fits between the focuser and an eyepiece. Technically it lengthens the focal length of your telescope, but for the purposes of eyepiece selection we can also think of it as shortening the focal length of an eyepiece. Barlow lenses can be 2x, 2.5x, 3x, etc. but the most common is a 2x Barlow. When used with an eyepiece a 2x Barlow will effectively cut the focal length of the eyepiece in half. A 14mm eyepiece will give the same magnification (and produce the same size exit pupil) as a 7mm eyepiece. I highly recommend a Barlow lens because it’s like doubling the number of eyepieces you own. Another big advantage is that it will retain the eye relief of whatever eyepiece it is combined with. A 14mm eyepiece acting as a 7mm eyepiece in a 2x Barlow will have the same eye relief as the same 14mm eyepiece used alone.

If you have purchased a low-powered eyepiece that produces, say, a 6mm exit pupil and another eyepiece that produces a 2mm exit pupil, the addition of a 2x

Jet Stream in Canada (NASA-JSC-ES&IA)

�BPAA newsletter summer �007Barlow to your collection will give you the equivalent of a 6mm exit pupil, a 3mm exit pupil, a 2mm exit pupil and a 1mm exit pupil, which should meet nearly all observing needs.

Your Barlow lens needs to accommodate both 2" and 1.25" eyepiece barrels if your low-powered eyepiece has a 2" barrel. The two common sizes for eyepiece barrels are 1.25" and 2". Very inexpensive (read: cheap) telescopes sometimes come with .965" eyepieces and very high-end research telescopes can accommodate very large barrels of 5" or larger, but for our purposes we’ll limit the discussion to the two common sizes.

The primary difference between eyepieces with 1.25" and a 2" barrels is the size of their respective field stops, the internal ring which defines how wide the circular image appears to be. This is also known as the Apparent Field of View (AFOV). In 1.25" eyepieces field stop diameters cannot get larger than 27mm and in 2" eyepieces the field stop diameter cannot be larger than 46mm.

The design of the eyepiece is the main factor in calculating its AFOV, with Plossls having AFOV’s around 50–55 degrees, wide-field eyepieces 60–70

degrees, and super wide-fields 80 degrees or more. To get a rough idea of how these AFOV’s compare with each other, the view through a Plossl eyepiece would be similar to looking

through a cardboard toilet paper tube cut to 1 3/4" in length, the view through a wide-field eyepiece would be like looking through the same roll cut to 3/4" and the view through a super wide-field would be like having it cut to 1/4".

The AFOV is an attribute of the eyepiece by itself, and when combined with a particular telescope will produce what is called the True Field of View (TFOV), the actual area of the sky contained within the image. You can calculate the TFOV you’ll see with a particular eyepiece/telescope combination by dividing the AFOV of the eyepiece by the magnification produced.

Where the field stop comes into play is that it sets a physical limit on the TFOV you will be able to see and also limits how long the focal length of an eyepiece can be. For 1.25" eyepieces that limit is about 32mm and for 2" eyepieces that limit is about 55mm.

It would be an easy mistake for the novice eyepiece buyer to purchase a 40mm Plossl eyepiece thinking you’ll see a wider area of the sky than a 32mm Plossl, but closer examination will show that the 32mm Plossl

has an advertised AFOV of about 50 degrees while the 40mm Plossl has an AFOV of only 40 degrees. You will see about the same area of the sky but the 32mm eyepiece will have a higher magnification which results in an image with higher contrast.

While we’re on the subject of higher contrast I should point out that the optimum combination of TFOV and high contrast in 1.25" eyepieces is found in wide-field eyepieces (about 68 degrees AFOV) with focal lengths of 24mm. This gives an almost identical TFOV as a 32mm Plossl but you will get both higher magnification and contrast.

If you’re fortunate enough to have a telescope with a 2" focuser you will be able to use eyepieces that will produce very wide TFOV’s. A 2" eyepiece is 1.6 times larger than a 1.25" eyepiece and has 3x the area. A low powered 2" eyepiece is excellent for viewing the Milky Way and cruising through the sky in general. Combined with a short-tube refractor you can see over 7 degrees worth of sky, the main trade-off being a rather low magnification of 7x. But don’t automatically think you should try to buy all of your eyepieces with 2" barrels.

Below a certain focal length, the area of sky seen with a 1.25" eyepiece and a 2" eyepiece with identical focal lengths will be the same, and the same is true for the brightness of the image. The cut-off point will be different for each eyepiece/telescope combination but in general low-powered eyepieces that generate exit pupils in the 4–7mm range are the only ones that benefit from the extra size.

Eyepiece DesignsThere are many different types of eyepieces for

astronomers to choose from, but to keep things simple I’ll limit the discussion to those that are most likely to be used by the first-time buyer.

The eyepiece that came with your telescope was most likely a Plossl. This design is over 100 years old and is far and away the most popular eyepiece used in amateur astronomy. That’s because it has a decent AFOV of about 50 degrees, has good color correction, is free of ghost images, and is relatively inexpensive. Modern variations of the original design also correct very well for astigmatism, coma, and field curvature.

The main drawback with Plossl eyepieces is that the amount of eye relief decreases proportionally as the focal length gets shorter. Earlier I mentioned that eye relief starts to become tight in focal lengths of 10–12mm, but if you need to wear eyeglasses when observing you might be limited to focal lengths over 16mm. The novice should be aware, however, that eyeglasses are only

Plossl Eyepieces

�BPAA newsletter summer �007necessary for observing if you have astigmatism. If you are near- or far-sighted you can just tweak the focuser to suit your eyes. I should also add that many people with astigmatism don’t require glasses when using high powered eyepieces because the small exit pupil falls on a much smaller area of the cornea and the effects of astigmatism will be less noticeable.

A lesser drawback to a Plossl eyepiece is its AFOV. Up until the early 1980’s this was not a drawback at all since the only eyepiece with a larger AFOV was the Erfle, developed in 1921 for military use. The Erfle has a 60–70 degree AFOV but is pretty much unusable at high powers because of astigmatism and ghost images, although they are very good at low powers showing just a bit of distortion at the edge of the field. The competition changed dramatically, however, in the early 1980’s when Al Nagler of TeleVue Optics introduced the Nagler line of eyepieces with 82 degree AFOV’s. Since then many companies have jumped in, offering a broad selection of wide-field and super-wide-field eyepieces.

Wide-Field and Super-Wide-Field EyepiecesThe 50 degree AFOV of Plossl eyepieces is about

the most an observer can take in at one time with the acute area of the eye. Anything larger than this falls into the realm of peripheral vision and is more sensed than seen, so eyepieces in the “wide field” realm can be considered more luxuries than necessities. But there’s no denying the fact they give luxurious views.

If you watch a person observing with one of the super-wide-field eyepieces with AFOV’s of 80 degrees or more, you will see them moving their head around as they position their eye left, right, up, down to take in the whole field of view. You would almost think that instead of observing the sky, they were observing the inside walls of the eyepiece, and the experience has often been likened to looking through the porthole of a spaceship.

A number of manufacturers now produce eyepieces with AFOV’s over 65 degrees using designs generated by computer, with lens curves so extreme that they were impossible to produce until the advent of modern grinding machines. But as sophisticated as these modern designs are, none of them can reverse optical laws and all have to give up something in order to achieve their expansive views.

One of these trade-offs, and probably the most common, is called the “pincushion” effect, where straight lines appear to have progressively more

curve as they move from the center of the view towards the edge. It is also called the fishbowl effect and is not very noticeable until you pan your telescope across the sky. This effect can even nauseate some observers. But for more stationary viewing, pincushion is not a serious problem and is tolerated by eyepiece designers because it’s the natural outcome of correcting for astigmatism, a problem where stars no longer look like points of light and begin to grow wings the closer they get to the edge of the field.

Pincushion is a characteristic of the eyepiece itself and will present itself regardless of the telescope used. A similar problem is called field curvature. Field curvature is a characteristic of both the telescope and the eyepiece. Most telescopes have a final focal plane that seems to curve away from you toward the edge of the field (a convex or positive curve) and many eyepieces have a negative, or concave, curve. If the two characteristics happen to match perfectly you will end up with a perfectly flat field, but this is rarely the case in eyepieces that give wide fields of view or, for that matter, in telescopes designed to give a wide field of view. This is one of the major reasons why different eyepieces perform differently in different telescopes and a good reason to try them out before buying.

Since many of these designs require lenses with extreme curves and extra lenses to correct for various aberrations, they all contain a fair amount of glass which makes them heavy compared to more conventional eyepieces, weighing as much as 2 pounds or more. If you have a relatively small telescope or one with a very narrow window for balance, these heavier eyepieces will throw off your balance. Most telescopes have ways to easily rebalance but this can become a nuisance if you’re frequently changing eyepieces. To find out how heavy an eyepiece your telescope will take without rebalancing, experiment by filling a sock with some sand, adding or removing sand until you find your telescope’s limit.

Along with the extra weight, these eyepieces carry extra cost, some with price tags that rival the amount you could spend on a moderately priced telescope. Prices will vary widely but this is definitely a case where you get what you pay for. Most inexpensive wide field eyepieces will exhibit some astigmatism toward the edge of the field and only you know how much you can tolerate when compared to cost.

Heading the list of premium eyepieces in this class are the Panoptics (68 degree AFOV) and the Naglers (82 degree AFOV) made by TeleVue Optics and the comparable Super- and Ultra-Wides made by Meade. A very good bargain in the low-power category is the 30mm

10BPAA newsletter summer �007

Widescan Type II marketed by Apogee. This eyepiece has a reported AFOV of 85–90 degrees, and costs roughly half the price of the equivalent premium eyepiece. The tradeoff is that the image starts getting soft about 70% of the way from center, but is very sharp on-axis. Another brand of eyepiece that’s an excellent bargain is the Speers-Waler, made in Canada. They also give AFOV’s of 84 degrees or more at very reasonable price. The tradeoff on these is that they are very long and may require an extra amount of in-travel to come to focus.

Eye ReliefEarlier I mentioned eye relief, the distance you

need to hold your eye away from the eyepiece in order to see the whole image. With modern eyepiece designs and the use of exotic glass it became possible to make eyepieces specifically designed for very generous eye relief in any focal length, not just the long ones. This is good news for eyeglass wearers who have astigmatism since 20mm, the standard amount of eye relief, allows them to observe without having to remove their glasses. The most noted eyepieces in this class are the Radians, made by TeleVue, with AFOV’s of 60 degrees, the XL

line made by Pentax with 65 degree AFOV’s, and the Vixen Lanthanums with AFOV’s which vary from 50–65 degrees depending on the focal length of the eyepiece. There are several other brands which offer eyepieces with generous eye relief and if this is important to you be sure to check out an eyepiece’s specifications before you buy.

If you are buying your first set of eyepieces for your first telescope and you stick with the hobby for any length of time you will probably undergo what is known as “aperture fever,” the desire to upgrade your telescope to something with a wider aperture and greater light-gathering ability. In fact, if you are still in the hobby several years from now the odds are better than even that you will have upgraded to a different telescope. But this is not true of your eyepiece set if you buy quality from the start. Premium eyepieces will perform well in almost any telescope and can remain with you long after that first telescope has been passed down to your grandchildren.

If you have questions about eyepieces, feel free to email me at dtanaka@seanet.com

Anna Edmonds Astronomy 0.001

While the Big Bang is thought to have happened only once, refried beans keep reappearing on our

menus. (See BPAA Newsletter, Spring 2007, p. 11) But is there any reason to continue to compare them?

Like beans, or dry seeds, the theory is that about 0.05 seconds after the Big Bang there were “seeds,” or protons—the future parts of atomic nuclei. It was at this instant that both time and space began. In about three minutes the protons combined to form deuterium, or heavy hydrogen. A minute or so later some of the hydrogen fused—melted together—to form helium. The temperature at the moment of the Big Bang was extremely hot, about 3 x 109 K, and the point of the Bang was extremely dense—all matter and energy was contained at one point. This nuclear fusion of hydrogen and helium continued, with the two elements accumulating and space expanding. Within the short

time of about a hundred years, beryllium and lithium had formed. By around 500 years after the Big Bang the temperature had cooled to only 108 K, and it was too cold for this primordial synthesis to continue. (Too cold for elemental creation; much too hot for refried beans.)

Physicists trying to understand the theory of the Big Bang think that some of its evidence can be found in the earliest proportions of isotopes of hydrogen, helium, beryllium, and lithium—a great cosmic mishmash. They see these isotopes in the spectroscopic analysis of the light coming from galaxies as far away and therefore as far back in time as they have probed so far. They calculate that this light is about 14± billion years old.

Time is thought to have begun about 15 billion years ago, calculated according to our human—and therefore earth-bound—definition. (This may be a conservative estimate; another suggests tens of trillions of years for the beginning.) The place of primordial space is a bit less definable. Because both time and space in these meanings are intangibles, they are dependent on human reasoning and understanding. While we base our mathematics and sciences on our time-space logical systems, we cannot exclude the possibilities that other systems (realities) are equally existent.

At 108 K helium fuses to create beryllium for only a fraction of a second. When the beryllium fuses with helium, the result is the creation of a different, stable

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11BPAA newsletter summer �007

element, carbon. (All of these events of fusion are violently hot and explosive.) Carbon fusing with helium produces another stable element, oxygen. Carbon and oxygen are the most common elements found in all forms of what we call life. In relation to our human perspective, it would seem that a great lot of carbon and oxygen have been created, but when we think in terms of time and space, the immensities of both dwarf even our solar system almost to nothingness.

When the accumulation of protons, electrons, and neutrons combine into elements and clump together making a critical mass, thanks to electromagnetism and gravity, the whole begins to “burn” hydrogen and radiate the light that we can see. (This is part of what is happening in our Sun.) Thus we can see a new galaxy and new stars forming. The hydrogen in any one of the billions of stars will be burned up in time—millions

or billions of years—resulting, if the mass is sufficient, in what appears to us as a supernova with its elemental beauty and its gut-terrifying violence. In a supernova elements are split apart and flung off in billions of fragments. It is out of the repeated heat and violence of supernova explosions that the heavier elements such as iron and silver and gold and uranium are created—the ancient alchemists’ dream!

What does all this have to do with something as earth-bound as refried beans? The beans soak, combining with the different elements of water (H2O). They are cooked until their skins split open, and mashed into fragments. Then they are subjected to the high heat of frying, sometimes with the addition of other elements, such as garlic and salt and cayenne pepper, before they become part of our bodies as we eat them.

Further similarities are left to the reader’s imagination.

observe the sun and solar flares with a Coronado Solar filter system. In order to check out these telescopes you must be a present member, and have been a member for at least two months. You must be trained on the use of the telescope by a qualified member. Telescopes and accessories (such as eyepieces and filters) can be checked out for 30 days. Contact Russell M. Heglund to arrange training and check out of telescopes. (206) 842–8758 rmheglund@yahoo.com).

Telescopes for Loan Russell M. Heglund photos by Ragna C. Blanco

The BPAA has nine portable telescopes

available to loan to members. Three of them are 8” Schmidt/Cassegrain Telescopes (SCT), a very popular and sophisticated design. Two of these SCT’s have motor-drives. We also have a Meade ETX 90mm Maksutov/Cassegrain with motor-drive. This ETX 90 Telescope has excellent optics and is very portable. We have 4”, 6”, 10” and 16” Newtonian

reflectors on Dobsonian mounts. These Dobs are very easy to use and are good telescopes for beginners or if you just want an easy-to-use telescope. Also available is a motor-driven 90mm Orion Refractor that is set up to

8” Schmidt/Cassegrain

Meade ETX 90mm

Dobsonian

Motor-driven 90mm Orion Refractor

The Fourth of July is coming, and BPAA will be there. So far plans include our traditional booth, and parading through Winslow with truck, telescope, and giant Jupiter model. All members, loved ones, friends, and acquaintances are invited to participate. Call Russ Heglund (206) 842–8758 rmheglund@yahoo.com about parade matters, and Harry Colvin 206 842-6617, hcolvin1@comcast.net about the booth.

1�BPAA newsletter summer �007BATTLE POINT ASTRONOMICAL ASSOCIATION

P.O. Box 10914, Bainbridge Island, WA 98110 http://www.bpastro.org/

Ritchie Observatory, Battle Point Park (206)842-9152

Officers Harry Colvin, President

206 842-6617, hcolvin1@comcast.netGeorge McCullough, Vice President360-697-3525, geomac@sprintmail.com

Russell M. Heglund, Secretary(206)842-8758, rmheglund@yahoo.com

Eric Cederwall, Treasurer(206)842-8587, ecederwall@bainbridge.net

Nels Johansen, Facilities Officer(206)842-7968

Malcolm Saunders, Chief Astronomer(206)780-1905, saunders@drizzle.com

Edward M. (Mac) Gardiner, President Emeritus/Founder

(206)842-3717, macg@bainbridge.net

Ed Ritchie, Chief Astronomer/Founder 1993-1997 John H. Rudolph, Facility Director/Founder 1993-2003

In thIs Issue

Battle Point Astronomical AssociationP.O. Box 10914Bainbridge Island, WA 98110

Newsletter Editor Vicki Saunders. BPAA Newsletter is a quarterly publication. Submissions due on the 10th of the month before the quarter begins. Email to: newsletter@bpastro.org. Do not embed photos or other graphics in articles.

CALENDAR: June-July-August 1

CALENDAR NOTES 2

IN BRIEFPresident’s Message 3 Twenty-Inch Telescope Sees First Light 3 Big Time Optics 4

ARTICLES Selecting Eyepieces 5Anna’s Big Bang 10 Telescopes for Loan 11The Fourth is Coming 11