Jim: Sky fog reading will vary from day to day and even hour to hour. What's happening is the ground lighting is reflected of of particles in the air. Last night's cirrus clouds out at SPOC are a good example. I rated the night as poor to fair because of the weather conditions. To get an idea about what conditions will be like before I go out, I look at the weather sounding map. http://weather.uwyo.edu/upperair/sounding.html First I look at the "Precipitable water [mm] for entire sounding" and when it's less than a half an inch the night will be good. If it's a half to a full inch, I may as well stay home. This will effect even the best of dark sites as well as suburban viewing. Next I look at the sounding data looking for layers of the atmosphere where the relative humidity is above 50%. These layers will map the cirrus clouding like those that occured last night. You would think that 50% would be alright but I think what is happening is that dust and pollen are hygroscopic and are starting the formation of water droplets even at just 50% humidity. April is bad because there is a lot of water on the ground that needs to be dried out and carried away by the atmosphere before we get good transparency. September is the best time of year for the same reason. Clear Skies DT --- On Sun, 4/19/09, Jim Gibson <jimgibson00@yahoo.com> wrote:
From: Jim Gibson <jimgibson00@yahoo.com> Subject: [Utah-astronomy] Sky Fog Test with Canon Rebel XT To: "Utah Astronomy" <utah-astronomy@mailman.xmission.com> Date: Sunday, April 19, 2009, 8:18 AM
I found this article on measuring sky fog with a camera quite interesting: http://www.pbase.com/samirkharusi/image/37608572. The idea is to tell photographers a couple of things. One is to give photographers an idea how their backyard skies compare to a more preferred location. Of course they would have to conduct the same test at the preferred location, and the other thing it would tell them is how long they can take an exposure before running into sky fog problems. As Samir says, as long as the histogram is not attached to the left side and it is also half way or less then you are sky fog limited. I decided to give it a try for just those reasons. I was not really sure I could judge the visual magnitude of the sky in my back yard in Orem very well. I see many of you can figure the visual magnitude without much trouble, so I thought I better do something to learn how. In the past I would look for Polaris and see how many stars I could see in the little dipper. Usually, all I could see was Polaris, but those skies are in or near the Salt Lake glow for me. Anyway, I would figure my skies to be between vMag 2.0 and 3.0 with average clear sky conditions. In following Samir Kharusi directions, only using a tripod, I pointed my Canon Rebel XT with a Sigma 2.8 24mm-70mm lens (set at 24mm) straight up and it didn’t take long to get the disappointing news. I was reeeally hoping to get at least 180 seconds or better of exposure time, but no dice. http://www.slas.us/gallery2/main.php?g2_itemId=1790 . In this image you will see that my histogram is slightly past half way after 90 seconds. According to Samir’s chart: (This is probably not going to come out lined up.) Sec----------------1680---1062---660---420---264--168---108---66----42----27-----16.8 Mins-----------------28---17.7----11----7.0----4.4---2.8----1.8---1.1----0.7---0.45---0.28
Mag/sq arc-sec-------22---21.5----21----20.5---20----19.5---19----18.5---18----17.5---17
VLMag---------------8+?---7+?----7+?----6.6----6.1---5.6----5.1---4.6----4.1---3.6----3.1 that would put my visual magnitude between 4.6 and 5.1. Uh…I don’t think so. I went back and re-read Samir’s article and found this: > VISUAL LIMITING MAGNITUDES: For convenience we may talk of Visual Limiting Magnitudes. Note, for convenience only. The VLimMag translation scale shown above is not absolute. It is observer dependent and very subjective< But none the less, The process is a non-subjective record that I can repeat over time and in various locations and hopefully I can learn something from it. Maybe by empirical data I can match it to a visual scale for comparison use only. (Any comments or insights would be helpful) In case you were wondering how I got the histogram image I used Canon’s Digital Photo Professional. Samir says to use the back-of-camera histogram; Digital Photo Professional’s histogram looks just the same. However, it does look a little different in PhotoShop. You may have noticed that the f/stop is set at 5.6 instead of f/4. I had set the internal f/stop to 4 as suggested by Samir, but when I switched to bulb then I got the f/5.6. I am going to try and get out to the pit-n-pole in the next few days and see what I get out there running exactly the same test. I think I will also try this with the camera attached to them 100mm ED and see what the difference is. Once I decide on a procedure then I will have to stick with it for comparison reasons. Results may vary, so each photographer will have to figure their own sky fog limit. I have left out a lot of good stuff, like the minimum exposure time and comments about signal to noise ratio so be sure to read the article. My minimum exposure was about 10 seconds when the sky fog histogram mountain separated from the left side of the histogram frame :>(. That is way too short for any kind of good detail. Therefore, I feel that my sky fog limit is a very narrow band from my backyard location. Any comments or suggestions are welcome. Jim
_______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://www.slas.us/gallery2/main.php Visit the Wiki: http://www.utahastronomy.com