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Dear Taichi-- > It is nice to hear the policy of AIP for Windows to compute heliocentric > corrections. But the existence of "JD heliocentric correction" in the > AIP for Windows output headers brings some concern. Is this value > an adjustable parameter by the user? Or the value is invariably zero as > you have suggested? > > > JD calculated from image time plus half of exposure duration. > > JD time zone correction added = 0 hours. > > JD user correction (True - Log) applied = 0 seconds. > > JD heliocentric correction added = 0 seconds. I am very sorry, but the values are not invariably zero. These values may be optionally entered by the observer: 1) time zone correction 2) computer clock error correction 3) heliocentric correction. Julian Day creates a special difficulty because it contains many elements. We cannot ignore heliocentric correction because many observers want it. However, we force them to compute it themselves (which is probably good) and we place an "audit trail" in the file header. As I noted, this was a major concession. If the observer computes a heliocentric correction, the program will display the value that is specified, and apply it to the time given in the image FITS header. We display this correction in the file header so that it is a permanent part of the reduction record. However, please note that all of the original information from the image is reproduced in the data record. The date and time are preserved, and are not altered in the calculation of the Julian Day. This means that you can independently compute the Julian Day when you use the data. Here is the complete header: -------------------------------------------------------------------------------- ------ AIP4Win Multi-Image Photometry Tool Analysis of 38 images from directory: C:\My Documents\2003FEB09-10 Radius of star diaphragm: 7.5 Sky annulus inner radius: 9 Sky annulus outer Radius: 15 Search Radius: 20 Initial Comparison Star coords: X=375.28, Y=284.79 Initial Variable Star coords: X=192.55, Y=194.93 Initial Check Star coords: X=350.79, Y=239.39 Image time = time in FITS file header or log file. JD calculated from image time plus half of exposure duration. JD time zone correction added = 6 hours. JD user correction (True - Log) applied = 0 seconds. JD heliocentric correction added = 0 seconds. Image Date Time Exp ADU Com ADU Sky ADU Var V-C mag ADU Chk K-C mag Julian Day 00000060.USNO_Star.FIT 2003-02-10 00:26:21 60 665643.4 732.9 153985.6 1.589 342729.1 0.721 2452680.76864583 00000061.USNO_Star.FIT 2003-02-10 00:31:20 60 672387.8 736.9 152109.4 1.614 346072.0 0.721 2452680.77210648 00000062.USNO_Star.FIT 2003-02-10 00:33:11 60 670922.8 733.4 150533.8 etc. -------------------------------------------------------------------------------- ------ Of course there is no way to check the accuracy of the observer's computer clock. --Richard
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