Greg Roberts schreef: > I do NOT suggest you use the 28mm lens - the field is far too wide and > this reduces the positional accuracy Greg, Gerhard, A 28 mm lens in principle will do fine for satellite work. With the correct astrometric software, the resolution is sub-pixel. Using a 50 mm lens and the 12 megapixel sensor of my 450D I get 5" to 7" (arcsecond!) astrometric accuracies on stellar objects (tested it on asteroids). That is much better than needed for good satellite positions. My off-track residues on satelllite trails are seldom larger than 0.01 degree, or 0.5' (arcminute). Sometimes I use a 17 mm lens (see below) and the accuracy is then still good. I mostly use the 50 mm and sometimes a 17mm (VLEO or objects with large time uncertainty). For HEO objects I use a 100 mm but solely because that lens goes fainter in objects. The best argument to go to a slightly higher focal length is actually not resolution, but that you will catch fainter objects. It is a trade-off however with smaller FOV, which means your aiming has to be better (I use a 8 Mw green laser parallel to the camera optic axis to aim. It gives a visible beam at night, showing you where the FOV is pointing. It is a simple green laserpointer that you can buy for a handful of dollars nowadays) > With LEO sats you will have problems re the time of the image as ideally to 0.1 seconds is required - Marco achieves this with his digital camera so is the man to approach re this aspect. Indeed, it is the time resolution that sets the limits to your accuracy, more so than the astrometric accuracy of the camera + lens. You need to calibrate the timing of your camera, plus a good time source. In my case, I use a radio-controlled clock which receives and synchronizes to the DCF77 signal of Mainflingen (Frankfurt). I am not sure whether something similar exists for South Africa. Before each observing run, I forcefully have the clock re-synchronize by shutting it's power off (taking the battery out and then putting it in again). What you do, is that you attempt to press the shutter button of your camera (wire release!) on a round time, e.g. 20:12:20.00, as the second digits change on the display. I have found that making a series of "mock" presses during the last few seconds of the countdown helps you attain the rhytm of changing seconds and improves accuracy. Of course, you have to calibrate the camera timing first. There is a small lag, due to the camera processing electronics, between you pressing the button and the camera actually opening. In my case (EOS 450D), that is approximately 0.30 seconds. Besides of that, for shutter speeds > 1 s the accuracy of the indicated exposure times is not quite accurate. For example, the "10 second" shuter speed of my 450D is 10.05 seconds in reality, and the "15 seconds" is 16 seconds (!) in reality. This matters if you measure both start and end of the trail. Calibration is best done on satellites with tightly fixed orbits for which accurate tle's are available. The Iridium satellites are very suitable. Just spend a few days catching and measuring as much of those as you can, and looking at your residues in delta T (e.g. using Scott Campbell's SatFit software) and you'll see the pattern appear of your equipment-induced delta T's, which yields your time lag corrections. I second Greg's remark (and already sent you a note about that) of changing your F-settings. I use F2.8, not F5, combined with a 10 second (actually 10.05s) exposure and either 800 ISO or 1600 ISO (the latter for very faint HEO). - Marco ----- Dr Marco Langbroek - SatTrackCam Leiden, the Netherlands. e-mail: sattrackcam@wanadoo.nl Cospar 4353 (Leiden): 52.15412 N, 4.49081 E (WGS84), +0 m ASL Cospar 4354 (De Wilck): 52.11685 N, 4.56016 E (WGS84), -2 m ASL SatTrackCam: http://home.wanadoo.nl/marco.langbroek/satcam.html Station (b)log: http://sattrackcam.blogspot.com ----- ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
This archive was generated by hypermail 2b29 : Tue May 19 2009 - 11:58:34 UTC