|Ryukyu Astronomy Club Newsletter
|Volume 3, Issue 6||November/December 2004|
October and November Meetings
The November 13th meeting was held at the Naval Hospital. There were five at the meeting, including two first-time attendees, and two others came to the observing session. Mike Swanson gave a presentation on imaging planets with web cams and processing the images. We discussed the meeting schedule for next year and agreed that the second Saturday of each month would continue through 2005. The new meeting schedule is posted at out web site. After the meeting, we proceeded to Alivila for our first group observing session in several months. Mike Swanson brought his NexStar 11 GPS and John O'Briant brought out his LX-90. We bagged several objects but wrapped up around 11PM due to busy schedules the next day.
Things to See in the Night Sky
Other objects well placed for viewing:
For more objects of interest and the locations of those listed above, download the latest chart from www.skymaps.com or visit www.SkyandTelescope.com and customize the online star chart for Okinawa's general location of 128 degrees East longitude and 26 degrees North latitude.
Equipment Review - Baader Fringe-Killer Filter
Early this year Baader Planetarium announced a new filter - the Fringe-Killer. Basically it is a minus-violet filter of the highest quality with a few unique features. It is available in 1.25" and 2" format in North America from Alpine Astro (www.alpineastro.com) or in Europe directly from Baader (www.baader-planetarium.de).
Minus-violet filters have been in high demand recently due to the resurgent popularity of achromatic refractors - particularly those with fast focal ratios like the short-tube 80mm refractors from China as well as their larger 6" aperture siblings. Achromatic refractors, especially fast focal ratio models, suffer from significant amounts of "chromatic aberration" - false color. This is generally experienced as a violet halo around bright objects, particularly planets, bright stars and the Moon. The halo is caused by the fact that not all the wavelengths of light come to focus at the same point. In essence, the lens in a refractor is acting like a prism. Although it is possible to ignore the halo, the real problem is that the unfocused violet and blue light is spread all across the object you are viewing, robbing it of detail. Thus, fast refractors seldom live up to their theoretical resolution limit.
The most common "cure" for this is to simply block the violet and blue light waves. The problem is, most minus-violet filters introduce their own resolution-robbing aberrations and they typically block significant "good" light, further reducing detail. Enter the Baader Fringe-Killer.
The light spectrum above shows most of the important characteristics of the Fringe-Killer filter. Almost all violet light (light below wavelengths of 450nm) is completely blocked, while blue light (450nm - 480nm) is attenuated by 50%. Just as importantly, all other light is transmitted at higher than 95% efficiency - nearly as good as coated white optical glass. Baader also incorporated full infrared blocking (650nm and up) to make this a perfect filter for web cam and digital camera imaging. In short, the light transmission characteristics of this filter are precisely what you would want for the task at hand.
To insure the filter does not introduce any optical aberrations, Baader's Fringe-Killer is a precision ground, fully coated, optical filter - as are all the Baader filters. As noted in a recent review in Sky and Telescope magazine, this allows Baader filters to be stacked (using multiple filters at the same time) and even placed far in front of the focal plane without degrading the image - unlike competing models. In addition to starting with precision optical glass, Baader uses their unique coating techniques to insure the glass remains stress-free and perfectly flat. This involves nearly 50 dielectric coats on each side of the glass. Additionally, these dielectric coatings are very tough, allowing for cleaning without damaging the coatings. I have a full set of similarly coated Baader color filters and can attest to their durability through the 4 years I have been using them.
One last note on the uniqueness of this filter. Baader suggests that the Fringe-Killer combined with their Red filter is perhaps the best Hydrogen-Alpha filter commercially available. This combination has a 95% transmission rate at the H-Alpha wavelength and yet costs very little. H-Alpha filters are great for imaging emission nebula in great detail, though a small pass-band would produce better results.
The Fringe-Killer in Use
In short, this filter transformed the views through this f/5 achromatic refractor into what I would expect to see through an f/12 achromatic refractor. A fast achromatic refractor will never be the best choice for planetary views, but Baader's Fringe-Killer filter can certainly make it a good choice. If you've been looking for something to improve your achromatic refractor - this is it!
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