From sutin Fri Nov 3 13:59:04 1995 Return-Path: Received: from sol.ucolick.org by helios.ucolick.org (helios.ucolick.org) (4.1/SMI-4.1) id AA29709; Fri, 3 Nov 95 13:59:03 PST From: sutin (Sarcast Van Zot) Received: (sutin@localhost) by sol.ucolick.org (8.6.10/8.6.10) id NAA29711 for sla; Fri, 3 Nov 1995 13:59:02 -0800 Date: Fri, 3 Nov 1995 13:59:02 -0800 Message-Id: <199511032159.NAA29711@sol.ucolick.org> To: sla@helios.ucolick.org Subject: Distortion Formula Status: RO Content-Length: 1296 Lines: 44 Steve, Here is the distortion formula. Brian ---------------------------------------------------------------------------- The distortion of the telescope and camera are below. They are polynomial fits. The argument is the radius from the center of the telescope or camera center, in arcmin of sky. The result is the radial movement of the resulting point on the detector, in pixels. telescope: dT = 8.056121663e-02 * R [pixels] - 9.854431080e-01 * R^2 + 4.585744900e-02 * R^3 camera: dC = - 2.981488078e-02 * R [pixels] + 3.837548331e-02 * R^2 + 5.818240867e-02 * R^3 + 3.123005326e-03 * R^4 - 2.396621712e-04 * R^5 So is the initial position of a star is (X,Y) in arcmin, and the camera is centered at (4.5,0.0) arcmin, then the offest of a star in pixels at the detector is Rt = sqrt( X ^2 + Y^2 ) [arcmin] Rc = sqrt( (X-4.5)^2 + Y^2 ) [arcmin] dXpix = X * dT(Rt) / Rt + (X - 4.5) * dC(Rc) / Rc [pixels] dYpix = Y * dT(Rt) / Rt + Y * dC(Rc) / Rc [pixels] The accuracy here is on the order of 0.01 pixel for each fit. The algorithm here of adding the distortions rather than multiplying adds an error of about 1 pixel at the worst. The scale used here came out to be 0.1186474 arcsec/pixel at 0.5500 microns. From sla@ucolick.org Tue Dec 14 00:41:46 1999 Received: from xocolatl.ucolick.org (IDENT:root@xocolatl.ucolick.org [128.114.22.203]) by helios.ucolick.org (8.9.0/8.8.6) with ESMTP id AAA28921; Tue, 14 Dec 1999 00:41:45 -0800 (PST) Received: (from sla@localhost) by xocolatl.ucolick.org (8.9.3/8.8.6) id AAA28901; Tue, 14 Dec 1999 00:41:43 -0800 Date: Tue, 14 Dec 1999 00:41:43 -0800 From: Steve Allen To: Mark Calabretta Cc: egreisen@nrao.edu, dwells@nrao.edu Subject: Re: Dual point-of-symmetry distortion model Message-ID: <19991214004143.C28672@ucolick.org> References: <199912140554.QAA14435@grus.atnf.CSIRO.AU> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Mailer: Mutt 0.95.3i In-Reply-To: <199912140554.QAA14435@grus.atnf.CSIRO.AU>; from Mark Calabretta on Tue, Dec 14, 1999 at 04:54:01PM +1100 Status: RO Content-Length: 2041 Lines: 48 On Tue 1999-12-14T16:54:01 +1100, Mark Calabretta hath writ: > At ADASS you showed us a plot of the distortions expected from your optics > and I offered to see how well the polynomial currently proposed for the TAN > projection would cope with it. Could you send me the code you used to > generate that plot please, or at least some expression of the distortion > that I can play with. The PS file showing the picture is at http://www.ucolick.org/~sla/deimos/swpdr/distort.ps with the source code being http://www.ucolick.org/~sla/deimos/swpdr/distort.fig this also needing http://www.ucolick.org/~sla/deimos/swpdr/disinc.eps For cross checks and reference, see the specs at http://www.ucolick.org/~loen/Deimos/deimos_specs.html These are 8kx4k (where 1k = 1024) CCDs with 15 micrometer pixels. The distortion vector field is plotted with the camera center subtracted out, but recall that in practice the imaging region is actually divided into 4 CCDs, and the imaging mode does not illuminate the entirety of those CCDs. So, for purposes of the WCS there will be a separate WCS for each of these 4 CCDs, and along the vertical direction only the central most 3000 (approximately) pixels are relevant. This means that the vector field distortions which are important for WCS should be zeroed at a point about 1500 pixels up each CCD. For the purposes of a test calculation the symmetry means that there are only 2 cases to examine, and the outer CCDs appear to have the worst distortions. These distortions are being re-examined with an aim for producing a more accurate form, but the form actually used in the plot is visible in mail stored at http://www.ucolick.org/~sla/deimos/swpdr/distort.mail If these expressions prove too cumbersome, I'll have a hack at reproducing them in a more palatable form. -- Steve Allen UCO/Lick Observatory Santa Cruz, CA 95064 sla@ucolick.org Voice: +1 831 459 3046 http://www.ucolick.org/~sla PGP: 1024/E46978C5 F6 78 D1 10 62 94 8F 2E 49 89 0E FE 26 B4 14 93