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Club Address:
65-1158 Mamalahoa Hwy.
Kamuela, HI 96743
An electronic publication of The West Hawaii Astronomy Club -
Club Officers:
Co-Presidents:
Craig Combes,
Craig Nance,
Doug Summers
Secretary:
Laura Kinoshita
With Support From:
Canada-France Hawaii Telescope and
W. M. Keck Observatory
This website is currently maintained by
Doug Summers
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February 2006 Club Meeting Report
Agenda:
| | Welcome / Greetings |
Craig Combes/All |
10 min |
| Show and Tell? |
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| The Sky This Month |
Craig Nance |
15 min |
Featured talk,
Diffraction Effects in Telescopes |
Mark Baril from CFHT |
40 min |
| New Club Telescope discussion |
Craig Combes |
10 min |
| Club Business |
TBD |
TBD |
| Group Discussions |
All |
Finish by 9:00pm |
The club meeting was held on February 14, 2006 at 7pm in the Hualalai conference room at Keck with Craig Combes facilitating.
We had a small but dedicated Valentines Day crowd. New faces were welcomed! We had 17 members in attendance. During introductions Joann Durst shared a "Mauna Kea Astronomy Society" newsletter from February 1993. We understand that this is not the only "extinct" club that served our area. Scott Hartman said that he's heard rumors there was a third club as well. The newsletter was scanned and is on the WHAC web site for historical reference. Makes for very interesting reading!
For the Gear Show and Tell, Pierre Martin shared the "Observers Handbook 2006", which he called 'the best thing out of Canada, other than maple syrup.' It has everything an observer needs in one concise book. Learn all about it here.
Craig Nance presented The Sky This Month, and the Featured Object of the Month. Craig deferred a planned presentation on the Orion Nebula in favor of the Supernova 2006X which occurred last week in M-100. This is a bright (telescopic) supernova in the Virgo cluster galaxy M100. The supernova is expected to be near peak brightness of ~12M by the next observing session in late February. There was agreement and enthusiasm that this will be a prime object for observing that weekend.
Marc Baril, Instrumentation Engineer for Canada-France-Hawaii Telescope, was the featured speaker of the night.
He spoke about diffraction effects in telescopes, and how to improve the performance of our telescopes. Marc started with reviewing the basic physics of diffraction, and how light is best understood as a wave, not as a "ray of light".
He showed that due to the wave nature of light, a "perfect" telescope will give you a star that consists of a bright core, called the Airy Disk, surrounded by ringlets of light. The best theoretical performance is 84% of the light being in the Airy Disk.
Because of the wave nature of light, and the existence of the Airy Disk, the resolving power of your telescope is determined by its aperture. Focal length is not a factor. Marc showed that optics experts such as him use Fourier mathematics to predict the effects of complicated optics, such as secondary supports and central obstructions.
He concluded the background math by introducing the concept of Strehl. Although it can be quite involved, for most situations Strehl is a ratio of the energy in the Airy Disk of your telescope divided by a "perfect" telescope.
A Strehl of 1.0 indicates that your telescope is "perfect". Most telescopes are not perfect for reasons to follow.
Marc then showed how increasing the size of the secondary obstruction spreads light out of the Airy disk, resulting in lower (worse) Strehl. Plots of Strehl versus obstruction were presented. The bottom line is that the smaller the obstruction, the better. Pierre Martin mentioned an optical rule-of-thumb: if you subtract the diameter of your central obstruction from the diameter of your primary, the result is the aperture of a "perfect" telescope that performs like yours. For example, if you have a 10" telescope with a 2" central obstruction, your telescope performs like a "perfect" 8" telescope.
Marc showed that strength diffraction spikes from spider vanes are proportional to the area of the vanes. He showed that a 3-vane spider is only slightly better. For most reasonably well made telescopes, there is no advantage to three vanes over four vanes.
Marc showed that curved vanes are not that great. They have three advantages: 1. greatly reduced diffraction spikes, 2. looks cool and impressive, 3. Increased springiness for off-road handling of your telescope. The disadvantages are that curves supports are hard to make stiff, and optically have significant light scattered into the halo, making it difficult to observe objects near bright stars.
Marc discussed apodizing masks. The word "apodizing" means "to remove the feet". The idea is to remove the sudden drop in intensity from the edge of the mirror to outside the mirror. An apodizing mask has very impressive performance. Because the mask reduces the aperture slightly, the Airy Disk will become a bit larger. Marc explained that seeing (the observed turbulence of an image...twinkling of the star...) follow Kolmogorov power law spectrum.
This can be run through computer models and allow calculations of "real" telescope performance in "real" seeing. In moderate to poor seeing, the apodizing mask is exceptional, especially for planetary observing. Marc wonders why these are not made and sold on the amateur market, as their performance is so good.
An apodizing mask has several caveats. If the central obscuration is >20%, don't bother as the mask will not help. Remember that the Airy disk will grow due to the effective decrease in aperture, and this also reduces the light gathering power of the telescope. With those caveats aside...
A classic apodizing mask is an optical window that varies in transmission as a function of radius. This is not practical for most, if any, amateurs to make. There is a easy, cheap, and highly effective apodizing mask that anyone can make, called a "Fly Screen Apodizing Mask", or "Window Screen Apodizing Mask"
Details can be found here: http://www.csastro.org/gallery/article4.htm and here: http://www.kitgear.com/apodiz/index.htm.
You can make one of these for a couple of bucks, and improve observing of planets on nights with lousy seeing.
In summary, large central obstructions are bad news for telescopes, curved vanes are not a great solution to diffraction spikes as they introduce a host of other problems, and fly screen apodizing masks are a cheap, effective way to improve telescope seeing on marginal nights.
Carlton Lane shared a couple of web sites about the upcoming solar eclipse, visible in Turkey and other locales. Carlton is heading to Turkey, and will certainly report on the eclipse upon his return.
Craig Combes presented our first-ever club telescope. It is a classic 1970's Celestron C-8. Full details about the telescope are on the WHAC web site: Club Telescope. Do not be fooled by the Meade-esque paint job. Everyone is very appreciative of the hard, dedicated work Craig C. did bringing this telescope to life. The good part - if you want to use this telescope, contact Craig Combes.
The business part of the meeting was fast and furious:
- Doug Summers reported the January deep sky observing session was rained out. Doug will send out a reminder as the February observing session approaches. Check the WHAC Yahoo group message board for upcoming announcements here.
- Herb Conrad announced that he is inviting everyone over to his place in Kohala Ranch on Saturday May 27. That afternoon WHAC telescope wizards will help with assembly of his new telescope, then celebrate first light for the telescope that evening. More details to follow.
- Astroday is coming up - Saturday May 6, 2006. WHAC has been asked to set up telescopes at Hale Pohaku that evening. More details to follow.
- Hoku project - Doug mentioned that there is an invitation for WHAC to set up telescopes as part of a dance performance celebrating the stars. More details to follow.
- The Filippenko lecture series is nearly complete. Starting February 28, the "What's New in Astronomy" 16-lecture series will begin. See the WHAC Yahoo groups message board for details.
Meeting adjourned at 9pm!
Reminder that the next WHAC meeting will occur on Tuesday March 14, 2006 at 7pm at the meeting room at CFHT. At 6pm that evening many club members gather for the customary pre-meeting dinner at the Parker Ranch Food Court.
Previous meeting minutes:
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