May 2007 - Utah-Astronomy - mailman.xmission.com

Re: [Utah-astronomy] How do colored planetary filters work?
by Kurt Fisher 10 May '07

10 May '07

Chuck wrote - > I'd have to think on that definition, and test it at > the eyepiece... I don't think that they actually > lighten similarly-colored tones-filters should > darken them also; just not as much. > Contrast is still increased. Here's an easy demonstration of the similar-color-lightening effect. I did it this evening after work. It appears the similar-color-lightening and opposite-color-darkening effects are a basic photographic filter principle used by terresterial photographers. Maybe some of you camera buffs can chime in. Take your box of astronomy filters outside before sundown. Look for an neighborhood area that has lots of different colored flowers in it. Spring is good time of the year to do this. Fortunately, my neighbor has a green thumb and I had a good selection right outside my door. Look at a red flower through your dark red #25 filter. The flower will appear white while the green leaves will be several tones darker. Through a #15 Deep Yellow, a yellow flower looks white while the green leaves will appear darker. Look at a green leafy tree through your #58 Green filter - it will look lighter, not darker. Use a #80 Blue on a blue flower - it will appear light and white while the green will appear nearly black. My Purple filter makes green trees appear jet black, blue flowers disappear; yellows remain visible. When applied to planets, these terresterial effects probably are all dependent on getting enough light into the eye to trigger your color vision - hence the need for big light-bucket apeture. While at the U of U star party last night, I tried the one green #58 filter that they had on Saturn under poor seeing through their 14" Meade. A disk band just barely detectable in white light. The band was clearly enhanced with the green filter. The enhancement was subtle and not earth-shattering - but definitely was there. > Feel like a testing session this weekend, if I can clear > it with the boss? Good idea. Maybe John Lennon had something going with those rose colored glasses that he used to wear? Think I'll get me a pair. -:) Clear skies - Kurt _______________________________________________ Sent via CSolutions - http://www.csolutions.net

2 1

comets and an asteroid
by daniel turner 10 May '07

10 May '07

Last night I saw asteroid 4Vesta between eta and zeta Ophiuchus. It was clearly visable in binoculars as a fifth mag star that wasn't supposed to be there according to my charts. Without optical aid I could easily see the nearby 20 OPH which is 4.7 mag and staring at the correct spot I could make the asteroid pop out if I blocked the glare from Jupiter nearby. It was low in a sky of only fair transparency. I saw comet 2007E2 (Lovejoy) to the north near the head of Draco using a 17.5 inch dob. It was ninth mag and as large and diffuse. The core was just distinguishable from the coma but it had no tail, it was perfectly round. It rises in the east and is high at midnight. Monday night I picked up 2007E1(Garradd) in Gemini. At fourteenth mag this was tough. Just a patch of haze that drifted among an asterism of stars over the course of an hour. Tuesday night I failed to pick it up in the haze to the west. This one needs to be looked at during evening twilight. Clear Skies DT __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

3 2

How do colored planetary filters work?
by Kurt Fisher 10 May '07

10 May '07

Chuck, Okay, here's the short rule-of-thumb for colored filters that I was looking for: "What you see is that a filter lightens tones of objects colored similarly to the filter and darkens tones of objects colored dissimilarly." Kodak. 1998. Using Filters (Kodak Workshop Series) (The Kodak Workshop Series) (Paperback). So, a red filter on Mars lightens the reds and darkens the blue polar ice caps and the blue-green maria. - Kurt _______________________________________________ Sent via CSolutions - http://www.csolutions.net

2 1

How do colored planetary filters work?
by Kurt Fisher 09 May '07

09 May '07

Chuck, > In every case, the effect is to increase contrast between features > that are otherwise very close in hue and intensity. . . . . > The irony is that filters don't really show you more- they > show you less, overall. So, it looks like my understanding about how color filters work is on the right track. > Most of the filters sold to amateurs are probably not needed, . . . > The lowest transmission value of every color is essentially worthless. I can trace my own confusion over planetary filters back to the manufacturers' product inserts and marketing techniques. Like most people, I bought the "basic" planetary filter set first - figuring that these would be the most useful filters. The right information is not uniformly included in the product inserts or on the manufacturers' websites. So the "basic" set turned out to be the least useful. (I seem to recall muttering "worthless" and "stupid" on one cold night. -:) ) I have completed a table assembling descriptions of planetary filter uses from Meade, Orion and Celestron product inserts and websites. If you read the fine print closely, low-transmission level filters (40% transmission or lower) can really only be used with larger apetures - 8 inches or above. Some like a #25 Red (14% transmission) are really meant to be used with 10 inch apetures and above. The "basic" planetary color filter set offered first to amateurs by most manufacturers predominately contain low transmission levels. Typically, these "basic" filters include a 80A Medium Blue (30% transmission), #25 Red (14% transmission), #58 Green (24% transmission). Only #15 Deep Yellow in a basic set has 67% transmission. Looking at Orion's product inserts for illustrative purposes, their "basic" filter set product insert does not mention the limitation of low transmission filters to large apetures. The Orion "advanced" filter set does mention use with "larger scopes" for some low-tranmission filters. Not to single out any one manufacturer, Meade's color filter website and Celestron's product inserts are equally So, I suspect that most people, like myself, buy these filters and then slap them on a 4 or 5 inch refractor - on which they are not designed to work. I tried a range of filters and concluded that "they do not work." Then I went and bought "advanced" "light" color filters that have higher transmissions. E.g. #08 Yellow, Light (83% transmission), #11 Yellow-Green (73% transmission), #82A Blue, light (73% transmission). Then I started getting some useable enhancements in my 5 1/4" refractor. I'd have to write off my own frustration over using color filters properly to bad product insert sheets. > I wouldn't want to part with my light and medium greens and blues, > and my yellow-green. Because of the above, I also found my high-transmission filters more useful, but now that I know how to load up my filter wheel by planet (using my table), I'm going to re-try the use low-transmission filters on larger apeture scopes. > Now, what we haven't discussed is using colored filters to facilitate > observation of certain solar features, and I suspect the darker filters > may have a new lease on life in that application. I was aware that a #21 Orange filter could be used to give a white-light solar image an asthetic yellow-sun color, but hadn't thought about the other filters. > Those of you with green lasers can demonstrate this effect. Find a > piece of real "day-glow" pink cardboard or plastic, and shine your > green laser on it. The resulting dot will be either orange or white. > If not, your pink sample hasn't been doped with fluors- it's not > true "day-glow". Neat idea. Your are showing your 60's roots again, Chuck. Wow, man, like look at those colors! -:) - Kurt _______________________________________________ Sent via CSolutions - http://www.csolutions.net

1 0

RE: Utah-astronomy] How do colored planetary filters work?
by daniel turner 09 May '07

09 May '07

The short answer is "they don't work". The standard accessory filter set has been sold to amateurs for a very long time, dating back to the age of the refractor. Such telescopes have chromatic aberration that causes different color light to focus and different focal length, you can focus on one color but the rest of the spectrum is out of focus and thus is noise. You can get a sharp image by filtering out all but a single dominant wavelength and that's what the filter set allowed you to do. Of course the different filters sort for different colors so you can see different features in different filters, or you could use a telescope whose primary optics is a mirror. According to Harrold Sutter, modern refractors are corrected for color focus only and two or three points in the color spectrum. All colors beyond and between those colors are just plain out of focus. The modern Apochromat does a better job than the traditional Achromat, but still has the problem. I finally found a use for my color filter set, or at least the red and green ones. I use them for those "three D" photographs. Hold one over the right eye and the other over the left eye. On a more serious note, the Hubble Space telescope carries a large array of filters for an entirely different reason than amateur visual astronomers. By photographing a cluster of stars through a B filter and then through a V filter you can reduce the data and have the B-V value for each star in the cluster. Plot that versus the V value and you have a crude but reliable approximation of the H-R diagram for the entire cluster of stars without having to do spectroscopic analysis on each individual member of the cluster. The B-V value is listed in the Observer's Handbook as standard data for the list of bright stars. Study that number along side the spectral class and you will see how they vary together. DT __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

2 1

How do colored planetary filters work?
by Kurt Fisher 08 May '07

08 May '07

Daniel wrote: > The short answer is "they don't work". . . . > According to Harrold Sutter, modern refractors are > corrected for color focus only and two or three points > in the color spectrum. All colors beyond and > between those colors are just plain out of focus. Your point goes to refractors but what about reflectors? I've used them with reflectors and have seen subtle but demonstrable effects on Jupiter, Mars and Saturn. Having flipped through my astro library and filter product sheets, my main problem has been finding a consistent, no nonsense organized list of which filter to use on which planet to what effect and a no nonsense description of why a particular filter works on a particular planet in a particular way. This tends to slow me down using planetary filters. I can see what you are saying about sharpening by removing the other refracted light wavelengths, but what about color mixing and contrast effects? > Plot that versus the V value and you have a crude but > reliable approximation of the H-R diagram . . . > The B-V value is listed in the > Observer's Handbook as standard data for the list of > bright stars. Study that number along side the > spectral class and you will see how they vary > together. Garrison's (300) Brightest Star list in the Handbook is a list of nearby stars with highest precision known absolute magnitudes, where the absolute magnitude could be found from good Hipparcus parallaxes or the mass of a primary could be ascertained from binary star mass studies. I have most of Garrison's table imported as part of my Excel observing spreadsheet primarily to be able to call up reliable targets for use with my Rainbow Optics broadband eyepiece spectrometer. When you plot Garrison's stars in an H-R B-V format - they look like this: http://members.csolutions.net/fisherka/astronote/Clarkxref/img/HRDiagram.JPG Clear Skies - Kurt - and they do really appear to be clear tonight -:) _______________________________________________ Sent via CSolutions - http://www.csolutions.net

1 0

Obs Notes - 5/6-7 SPOC Ealing
by Kurt Fisher 08 May '07

08 May '07

I began around 10:30 pm doing review with a partial star hop tour of the Virgo Realm of the Galaxies on the SPOC Ealing. The Ealing DSC is down, so it was back to star hopping. Sky brightness 20.3 or an ZLM of about 5.6. The sky was clear from horizon to horizon but seeing was poor - 2/5, measured from a warm up and alignment 3 arcsec double - eps Boo. Temp was 37 F. Fogging on the Telrad, but not other finders was a problem. Using the eastern route into the Realm using eps Vir and rho Virgo as guideposts, a RA slew from eps Vir took me across m10.2 galaxy NGC4762 and its faint smaller companion NGC4754. Continuing the RA slew brought me to v8.8 M60. The v 13.3 companion galaxy NGC4649 was not visible. A 52mm was used as an eyepiece finder, while a 25mm and 20mm e.p.s at 203x and 254x gave the best contrast. Losing an eyepiece TFOV in declination and a further RA slew brought M59 and then M58 into view. Then I switched to doubles - running two of the Bootes double trio suggested in Gary Seronik's column in the new June issue of ST: del Boo and mu Boo. The companion to mu Boo has a great bluish color. >From below the horizon, the nearly 3/4 Moon was beginning to lighten the eastern sky. By this time, Sag and Oph and Jupiter were rising. This week's ST e-bulletin and the June issue of ST noted that dwarf planet Vesta (dia. 501) is currently naked eye and is at its brightest in the last 18 years. Vesta could be easily detected in the light-polluted southeast sky nearly midway between eta and zeta Oph. eta and zeta Oph and Vesta are the three brightest objects in that region. Then the Moon rose. I switched to hunting M40 in the northwest sky and after the Moon and Jupiter rose to around 15 degs alt, I took a few minutes to view both in poor seeing. With a light blue filter at 334x, Jupiter yielded up four or five bands, but as expected, little detail. Dorsa Smirnov in eastern Serenitatis was illuminated with low angle light and was visible for its entire 150km length. This was a nice end to wet weekend that otherwise curtailed visual astronomy. - Kurt _______________________________________________ Sent via CSolutions - http://www.csolutions.net

9 21

How do colored planetary filters work?
by Kurt Fisher 08 May '07

08 May '07

I have been doing some reading, trying to get a handle on the effects of color planetary filters by planet. I believe I understand it but wanted to get some confirmation not being up on color theory. I have not learned modern color theory but have a working understanding of Newton's outdated color chart: http://en.wikipedia.org/wiki/Image:2007_charles_blanc_etoile_des_couleurs_1… Primary and secondary opposite colors mix to produce a colorless white, black or grey. E.g. yellow and violet or red and green. Planetary color filters are transulcent. Like opaque objects, they obtain their color by absorbing colors other than their apparent color and reflecting or transmitting the apparent color. A red filter appears red because it reflects red and absorbs other colors. If a primary color is passed by a colored filter, then the opposite colors on the color wheel are suppressed. For a red filter, green and blue are reduced in brightness. This reduction in brightnesses reduces apparent color mixing to a grey hue as perceived in the eye. Paradoxically, this increases contrast of the opposite colors and the eye's ability to perceive details of features with the opposite colors that are now not completely mixed with its opposite dominent color. A red filter increases the detail seen in features that are colored blue and green. So the product inserts for let's say a Meade 25A dark Red filter states on Jupiter, that it "increases visiblity of Great Red Spot. Increases contrast of belt details." Presumably this means it passes some red but suppresses more blue and green, allowing more contrast in the blue and green details to be seen. Conversely, for a Meade 58 Green filter, the inserts describes how it on Jupiter it "increases contrast in lighter parts of the Jupiter disk by supressing red and blue toned structures. Strongly rejects blue and red tones. Use with 8" of apeture or greater." The recommendation to use larger apetures, e.g. 8 or 10 inches, depends on the percent of light that a filter transmits. High-transmission unsaturated filters can be used with smaller apetures. Low-transmission heavily-saturated filters need larger apetures to compensate for the loss of light. An analogous process occurs in modern digital astrophotography processing. A single color channel is suppressed - resulting in incomplete color mixing. So, instead of seeing featureless grey, you can see detail in the opposite partially mixed colors. Does this sound about right? How would you describe the operation of color planetary filters in terms of modern color theory with its three primary additive colors and three primary subtractive colors? See http://en.wikipedia.org/wiki/Color_theory - Thanks in advance - Kurt _______________________________________________ Sent via CSolutions - http://www.csolutions.net

1 0

Bryce Observatory
by Patrick Wiggins 08 May '07

08 May '07

Here's the latest from our own Joe Bauman: Bryce Observatory Taking advantage of southern Utah's clear, dark skies, Bryce Canyon National Park wants to build a $2 million public astronomical observatory. http://deseretnews.com/dn/view/1%2C1249%2C660218245%2C00.html pw p.s. Just for fun: If you head to Burning Man this summer and see an 80-foot slug belching plumes of fire as it inches across the desert, don't worry: it's not an environmental disaster. http://news.com.com/Burning+Man+takes+on+green+tech/ 2100-11392_3-6181680.html

4 3

Lawn astronomers vs sprinklers.
by daniel turner 08 May '07

08 May '07

Does anyone happen to know the schedule for the lawn sprinklers at SPOC? Last year we had them suddenly turn on during a scheduled event on a Friday night. After that the word was that Saturday nights are safe but the Friday nights the sprinklers would be running. I see this year that we are scheduling events at SPOC of Friday nights. The owners of "Lawn Telescopes" would like to know the answer to this. DT ____________________________________________________________________________________ Don't get soaked. Take a quick peak at the forecast with the Yahoo! Search weather shortcut. http://tools.search.yahoo.com/shortcuts/#loc_weather

3 2