Hi Paul, Sorry to butt in so to speak but with respect I think you have got the wrong end of the stick as far as Thierry L.'s comments go. I believe he is being scientifically objective and that there is no intention for there to be any personal slight here. When folk apply a logical approach to some questions it can indeed sound 'petty and uncharitable' but scientists are used to it and respond in a similar somewhat abstract style of writing. Think of it as putting forward challenging arguments which intend to be helpful in the long run - a devil's advocate if you will. Thierry Legault has a great deal of experience (as does Damian Peach, who Legault quotes) in high-res imaging so it's always worth listening and taking heed of their comments. My two English penneth, Richard Miles ----- Original Message ----- From: "Paul Grace" <paulgrace@lookoutranch.com> To: <SeeSat-L@satobs.org> Sent: Tuesday, May 10, 2011 9:32 PM Subject: RE: Nanosail-D2 best color result Mr. Legault, good telescopes should have a Dawes resolving limit in arcseconds of 134/D mm. (see http://www.telescope-optics.net/telescope_resolution.htm) A good 10" scope should yield about 0.45 arcsecs. This is better than needed to resolve a 2m object 740Km distant, Not small details of course, but possibly the difference between a circular and a rectangular plan. At least it's not impossible. I agree with you, there must be shape and color artifacts in the image, and that the image is not free of these distortions. Mr. Vandebergh makes no claim that the image was free of error though (note he says "color tones actually exist in the image." he didn't say the color exists on the satellite), so I guess I don't really understand your position. The sarcasm you employ (e.g. "unimportant things") makes you sound petty and uncharitable though. I don't think any reasonable viewer would assume that the nanosail looks exactly like that image of it, in color or shape. I think without constructive comments to make, you could let Mr. Vandebergh's photos stand without taking the effort to sarcastically piss on another person's earnest efforts; there is nothing positive to gain. Paul -----Original Message----- From: seesat-l-bounces+paulgrace=lookoutranch.com@satobs.org [mailto:seesat-l-bounces+paulgrace=lookoutranch.com@satobs.org] On Behalf Of Thierry Legault Sent: Tuesday, May 10, 2011 13:39 To: SeeSat-L@satobs.org Subject: re: Nanosail-D2 best color result hi all well, let's talk one second about object size, atmospheric dispersion and other unimportant things. First, an object whose size is 3 meters, seen at 740 km (or farther: 740 km was the minimum distance but we do not know exactly what was the real distance for this precise image), corresponds to an apparent diameter less than one arcsec (about 0.8 arcsec). This is less than the size of the Airy disc, whose diameter is 1.1 arcsec for a 10" telescope. Thus, in the comparison of size with the Airy disc shown above, the size of Nanosail in the image is not mainly related to its real shape but caused by atmospheric turbulence or other spreading causes (manual tracking, defocus etc.). But the most interesting comparison is with atmospheric dispersion. We all know the atmospheric refraction, which changes the apparent altitude of an object above the horizon. Actually, this effect depends on the wavelength: blue rays are move deviated than red ones. The consequence is that colors are spread like a spectrum, as in the image of Venus below. Just like in this Nanosail image: red on one side, white (or green) in the center and blue on the other side (orientation of colors depend on the orientation of the camera and telescope). The diagram below, made by Jean-Pierre Prost, shows that at an altitude of 50°, the spreading across the visible spectrum is already 1.2 arcsec. That is to say, more than the apparent size of Nanosail itself. Thus, any color variation on the sail would be hidden by dispersion. http://astrosurf.com/prostjp/images/Atmospheric%20dispersion%20effect%20smal l.jpg http://legault.perso.sfr.fr/dispersion.jpg The dispersion is well known by top planetary imagers, who use either filters (green or red) or an ADC (Atmospheric Dispersion Corrector) for their luminance images. Of course, a spreading of colors may also come from atmospheric turbulence (we all have experienced the variations of colors of twinkling stars), and/or Bayer matrix sampling: an object which falls on a very few numbers of color pixels (here, the processed image is a huge enlargement of a raw image covering very few pixels), even if it is resolved, has different parts falling on red, green or blue pixels. Not to mention noise, unavoidable in such raw images. These effects are very clear on the example images I have put on this page: http://legault.perso.sfr.fr/bad_astrophotography.html A simple experience that many of us can do is taking a video of Io (apparent diameter 1.1 arcsec, larger than Nanosail) with a consumer camcorder behind an eyepiece on a 10" telescope tracked by hand, extract one single 8-bit compressed image from the video and try to draw details on the surface of Io. I'll be telling to Damian Peach that it's his next challenge, and wish him good luck ;-) In short, obtaining such color info on an object of this size, in these atmospheric and instrumental conditions, is physically impossible. At 12:37 06/05/2011, Ralf Vandebergh wrote: >The image set shows how the color tones actually >exist in the image. Left is original color, >right is auto-color correction, this process >increases contrast between the subtle color >dyes. http://freeimagehosting.nl/pics/562a0c0ea73a6b3811d80c3226860812.jpg Thierry Legault www.astrophoto.fr _______________________________________________ Seesat-l mailing list http://mailman.satobs.org/mailman/listinfo/seesat-l _______________________________________________ Seesat-l mailing list http://mailman.satobs.org/mailman/listinfo/seesat-l _______________________________________________ Seesat-l mailing list http://mailman.satobs.org/mailman/listinfo/seesat-l
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