The Official Publication of the Bucks-Mont Astronomical Association, Inc ________________________________________________________________________________________________________________________________________
VOL XV, NO 8 AUGUST Scott Petersen, Editor
Ó2000 BMAA, Inc
What You Might Have Missed... Last Year!
- by Alan Pasicznyk
Two clear starry nights with steady skies in the crisp early October season, the infamous flea market with incredible buys on Saturday morning featuring Pocono Mountain Optics, Hand Optics, and Paul Rini with his legendary yet affordable hand-made eyepieces, a Saturday night banquet with an evening talk given by a NASA speaker on "Asteroid Impacts", doorprizes including a 2" Meade eyepiece, and last but not least the pleasant fall-like weather, all made this particular event worth not missing.
That is, unless you missed it!
Just in case you haven't guessed by now, it was called Stella Della Valley XIII, our annual astronomical campout/convention extravaganza which takes place at Camp Onas near Ottsville Pennsylvania.
But you don't have to just take my word for it. Deep sky aficionado and BMAA member Bernie Kosher described the Saturday night sky in an article he wrote last year: "As an example of how clear parts of the night were, a planetary in Aquila (NGC 6781) at magnitude 11.8 was hunted down with a 4.5" refractor by a crazed planetary nebula freak." I wonder who that could've been? There
was also the spectacle of seeing two dueling telescopes do battle, a new Televue 4" and a Brandon 4". Who won? If you were there, you probably know.
This year the location for SDV will be the same, we will also have one night as well as weekend registration, but in an effort to even further improve this year's event we will be moving the evening speakers to an afternoon slot. It seems that far too many amateur astronomers want to start observing as soon as possible after having dinner and, of course, getting their doorprizes!
So fill out your registration form now, and take advantage of the early-bird discount. Time is a wastin'..Tick, tick, tick...
WE'LL BE BACK AFTER THESE MESSAGES...
As always, running an event like this requires the involvement of as many BMAA members as possible. If I've said it once, I've said it a BILLION times: the more people that help out, the less that each person has to do! From the show of hands at a meeting after last year's event, it is overwhelmingly obvious that members want us to continue with this event this year. So lets do it! Just a little bit of your time and effort will insure that we have as great a time this year as we did last year.
I will begin filling slots for the various positions at the September meeting, and also calling the rest by phone, but if you're really smart you'll pick your favorite position from the list below as soon as possible rather than being asked to fill one later. Besides, SDV attendees usually rather enjoy pitching in when they see everyone else helping out, so come and join the team! Hope to see you there.
Think Clear Skies!
Stella Della Valley XIV
FRIDAY: - Assist in Parking Cars (a 1.5 hour shift of this, and you're FREE from volunteer work the rest of the weekend!)
- Post and later remove signs to SDV.
SATURDAY: 8:00 - Setup for Breakfast
- Setup for Flea Market
10:00 - Texas Bldg. Chair Setup
- BMAA Table (2 Hr. Shift)
4:00 - Setup for Dinner
SUNDAY: 8:00 - Setup for Breakfast
- Pre-Departure Cleanup
(Patrol the observing field for litter, tidy up dining hall, generally restore the site to original condition).
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AND NOW.... From our home office in Danboro, Pennsylvania...
ALAN'S TOP TEN LIST FOR ATTENDING SDV XIV
10 - The crisp, beautiful Fall weather
9 - You get to see Bob Post go nuts buying flea market stuff
8 - Clear skies, Clear skies, Clear skies !!!
7 - The really great "all you can eat" pizza banquet
6 - The Tastykakes after the banquet
5 - Meeting with old friends you saw last year
4 - Meeting with young friends you saw last year
3 - Prizes, prizes, and even more prizes!
2 - The astronomical compu-dudes-afternoon-so-you-can-observe-earlier-lecture
1 - You get to see the dueling four inch refractors go totally ballistic before the end of the REAL Millennium.
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- BMAA member Alan Pasicznyk has been volunteer coordinator for SDV longer than anyone can remember. [ -ed]
Late - Breaking News!!!
- by Jim Shearer
Friday night at SDVXIV, September 29, Questar will be joining our StarParty as part of their 50th anniversary! An area of the observing field will be devoted to Questar users' group. This may be one of the largest gatherings of Questar instruments ever! Don't miss this opportunity to catch views through both the legendary Questar 3.5 and its larger sibling, the Questar 7.
On Saturday, Questar will display their wares at the Pro-Am Flea Market and present a talk on the history of Questar. Make it a point to come see what Questar has to offer!
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- BMAA member Jim Shearer is co-chair of Stella-Della-Valley XIV. Contact him at firstname.lastname@example.org. [ -ed]
The next BMAA General Meeting is scheduled for Wednesday, August 2 at 8:00p
The CONSTELLATION is the official publication of the Bucks-Mont Astronomical Association, Inc, a 501(c)3 non-profit organization incorporated in the Commonwealth of Pennsylvania and exists for the exchange of ideas, news, information and publicity among the BMAA membership, as well as the amateur astronomy community at large. The views expressed are not necessarily those of BMAA, but of the contributors and are edited to fit within the format and confines of the publication. Unsolicited articles relevant to astronomy are welcomed and may be submitted to the Editor.
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BMAA MESSAGELINE - 215/579-9973
BMAA WEB SITE: bmaa.freeyellow.com/
Bucks-Mont Astronomical Association
2000 Calendar Of Events
StarWatch Chairman: Ed Radomski - 215/822-8312, email@example.com
Information Line - 215/579-9973
Tips for August
Eyepieces - part 3: types and applications
- by Bernie Kosher
In installment 2 we discussed some aberrations and their effect. Each eyepiece is troubled by some sort of problem or limitation. Let's see which are the most practical for our uses.
A note.... these discussions pertain more to the viewing perfectionist who is troubled by the slightest lack of contrast and image purity. Most of us, especially for casual viewing, are quite happy with a generic ocular which provides a reasonable field and minimal aberration at reasonable cost. I add myself to the list of casually happy users.
However, on those perfect nights when the turbulence and seeing settle down, when the scope is working at it's best and the target is high in the sky one appreciates the difference between the higher cost Plossls and their lower cost knock-offs. That last little bit of contrast on the planets separates seeing the white spots and festoons on Jupiter from a generally 'good' view. Admittedly, experience and the scope itself play very important roles. But the weakest link in the chain is the limiter. Don't let a few dollars subtract from the overall quality.
Back to the subject. Here's a quick description of various types:
RAMSDEN - simple, and very old, two element design. A pair of equal plano-convex lenses with convex sides facing separated by about the focal length of either. The overall focal length is about equal to the F of either lens. About a 25 degree usable field. Poor below about f/8 and over f/12 or so. Tends to have short eye relief, not good for spectacle wearers. Although not too bad on the long f/ratio side. Can be fine for planetary in the center of the field if well made. Poor for wide field. Inexpensive. Awful if poorly made. If made to original 1-1-1 specs the field lens and any dust are in the focus of the eyelens. If not coated can be haunted by ghosts. In the 'achromatic' type the eyelens is a cemented doublet, provides better color and a wider field.
HUYGENIAN - two plano-convex lenses with convex sides facing the incoming light. Ancient, and discounting single lenses, the original eyepiece. Poor below f/12 or so, but fine for long f refractors. Lots of aberrations off axis. Limited to about 20 degree usable field. Original formula had an eye lens of 1/3 the focal length of the field lens and separated by half the sum of the individual focal lengths, thus is referred to as 1-2-3. Lots of different formulae over the years. Often provided on cheap dime store scopes, as above.
KELLNER - very similar to RAMSDEN. Uses a double convex field lens and sometimes a third element to improve field of view. Originally designed to give sharper edges of field in shorter f/ ratio Newtonians. Not bad up to about 40 degree fields, although some will work fine at wider fields with longer f/ratio scopes. A bit more costly than above two. Seen in many moderate priced refractors as standard supplied eyepiece, especially in the small diameter barrel found on many imports.
ORTHOSCOPIC - (literally, without distortion) a two element type with a flat field up to about 45 degree. The original had a triplet cemented field lens and simple eyelens. Zillions of variations. Outstanding for narrow field if well made and fully coated. Perhaps the best planetary eyepiece on the market, and priced well below lots of other types. Some of the newer types with computer optimized curves and spacing, along with modern glasses, are supposedly fine at low f/ratios and have improved eye relief and field of view. I have not had the opportunity to try them out. I have an old and badly scratched Edmund 6 mm ortho and it works just fine for planetary, although the eye relief and field of view leave much to be desired.
PLOSSL - somehow, I fail to see the difference in these and SYMMETRICAL eyepieces. Although they tell me the symmetrical has equal curves and faces differently, the modern PLOSSL uses as many different formula as there are manufacturers. The original of each had two cemented doublets almost in contact. Some of my Polish's use three elements. These oculars are all new generation, in that they were designed by ray trace and many are computer maximized. Excellent for about any application, as they have a wide field of over 50 degrees, usable at short f/ratios on Newts, and are sharp across most of the field. I recommend these to amateurs due to the wide usability and reasonable price. My current favorites are an 18mm Celestron Ultima with 2x Barlow, and a 10 mm Televue PLOSSL used on my 4.5 inch refractor for lunar and planetary, each yielding about 150X. They provide wide dark fields and sharp contrast. I prefer the Ultima with Barlow for planetary.
ERFLE - a wide field ocular, is usable to about f/4 and can provide fields up to about 70 degrees of which the central 50 or so degrees can be called sharp. The softening around the edges is not that annoying. Much less expensive than the following types and a good general low power eyepiece. Many types and optical layouts. Avoid some of the old military types, as they may be corrected for a specific objective and yield poor results on your scope. However, I recall the old Erfle's from Edmund as being pretty decent. Haven't tried one on a scope in a while, and would be interested to see how they stack up next to the following types.
KONIG - wide field ocular, although the edges are blurry even in my f/14 refractor, and the eye relief is almost non-existent in the 6.5 mm one I have. I must put my eye almost in contact with the eyelens to use it's whole filed. Even then, with my refractor, I get noticeably clearer views with a 10mm and 2X Barlow, or the 18mm and 3X Barlow. Unfortunately, I must rank this one 'not my favorite'. In defense of this ocular, many people love them.
NAGLER and other SUPER WIDE FIELD types are very expensive, but are superior for wide fields clear to the edge. On short f/ratio scopes they are the cat's meow. On a truly fine scope with one of these, almost all of which have 2 inch barrels, the field is evenly illuminated to the edge, reasonably sharp and will make you a believer. These eyepieces are chock full of zillions of elements and weigh tons. They cost more than the GNP of some countries. Due to their weight, many Dobs ( actually, most other types too) must be rebalanced if the target object is low in the sky. I have yet to see one perform on the planets as well as a good PLOSSL or Ortho, due, I suppose, to scattered light and absorption. Even with all my complaints, I highly recommend these. One has only to look at M42 on a good night with a good large Dob. My personal recommendation is buying one which will yield about 6-8 power to the inch of aperture in your scope. Thus 60-80x for a 10 inch or 72 -96X for a 12 inch. While these powers are not the lowest usable, they give greater contrast and the enormous fields make up for the lost field due to the higher power.
There are many other types and names, most of which are just variants of these.
Be advised, the ocular can only enlarge what is there: if the objective is poor the image is poor, and no ocular can fix it. However, some oculars will improve the performance of some scopes if the aberrations are such that they annul the aberrations of the objective.
Be very picky. Try many eyepieces before sinking good money into one or several. Those of us who are older than about 50 probably can only accept a 5mm or so bundle of rays. This limits our low power to more like 5 to the inch. Believe it or not. a lower power may look good, but is actually not using the full diameter of the objective.
Also beware that the secondary mirror in many scopes is small: wider fields vignette as the ray bundle from the objective misses it.
Well, it's easy to talk on and on, but in defense of the oculars, a wide field of view can be much dimmer at the edges and not be noticeably inferior to a fully illuminated one due to increased sensitivity of the eye off axis. Only the critical and experienced planetary observer will be able to tell the difference between good and excellent. If your eyepieces are not performing as well as you feel they should, try them on another similar scope. Try them on short and long f/ratio scopes. If they still do not perform as well as other ones, use them as finder eyepieces or sell/trade them. Eventually you will become so displeased they will affect your entire observing session, and that's what it's all about.
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- BMAA member Bernie Kosher provides 'Tips' monthly. He can be reached at firstname.lastname@example.org. [ -ed]
- by John Deitz
RESOLUTION: The ability to see (or photograph) fine detail. All other things being equal, the larger the telescope the greater the detail or resolution. Why should this be? Let's explore this issue of resolution.
The key to understanding the concept is this: the minimum size of the image of a star is determined by the F/value, and only the F/Value! It does NOT matter what kind (design) of telescope, or the aperture. If each scope is made with the same very high precision, and the F/value is the same, each will yield the same diameter of star image. This minimal size is set by the nature of light, and is called the Airy disc. Moreover, the faster the system the SMALLER the disc.
Diameter of Airy Disc (inches):
Every image you see through the telescope is composed of these tiny balls of light - the Airy disc. This is true even for the moon and planets. So, to return to the original question, why does the larger aperture yield better resolution?
Consider two telescopes, each of f/10, with apertures of 4in and 8in. Each shows the same Airy disc - determined by the f/value. Keep in mind the f/value is a ratio of focal length to aperture. What is the focal lengths of each of the telescopes considered here?
The 4in has a focal length of 40 inches.
The 8in has a focal length of 80 inches.
If the four inch scope shows two stars almost separated (think of their edges overlapping), the 8 inch scope will show them twice as far apart at the focal length and well separated!
So bigger is better, right? We will explore this further in later issues.
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- BMAA member John Deitz builds telescopes, mirrors and lenses. He can be reached at email@example.com. [ -ed]