From owner-fitsbits at marmoset.cv.nrao.edu Sun Jan 5 13:50:34 1997 Received: (from majdom at localhost) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) id NAA02931; Sun, 5 Jan 1997 13:49:59 -0500 Received: from fits.cv.nrao.edu (fits.cv.nrao.edu [192.33.115.8]) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) with ESMTP id LAA01971; Sat, 4 Jan 1997 11:12:53 -0500 Received: from solitaire.cv.nrao.edu (news at solitaire.cv.nrao.edu [192.33.115.79]) by fits.cv.nrao.edu (8.8.3/8.8.0/CV-2.2) with SMTP id LAA20399 for ; Sat, 4 Jan 1997 11:12:52 -0500 (EST) Received: by solitaire.cv.nrao.edu (4.1/DDN-CV/1.10) id AA29746; Sat, 4 Jan 97 11:12:49 EST To: fitsbits at fits.cv.nrao.edu Date: 3 Jan 1997 20:03:48 GMT From: thompson at orpheus.nascom.nasa.gov (William Thompson) Message-Id: <5ajon4$ecg at post.gsfc.nasa.gov> Organization: NASA Goddard Space Flight Center -- Greenbelt, Maryland USA Path: solitaire.cv.nrao.edu!hearst.acc.Virginia.EDU!aruba.odu.edu!news.larc.nasa.gov!news.msfc.nasa.gov!centauri.hq.nasa.gov!newsfeed.gsfc.nasa.gov!orpheus.nascom.nasa.gov!thompson Subject: Solar coordinates Newsgroups: sci.astro.fits Sender: owner-fitsbits at marmoset.cv.nrao.edu Precedence: bulk Before I left for Christmas vacation, there was some talk about how to store solar data in FITS files using the WCS formalism. Unfortunately, those messages are gone from the usenet server, and there appears to be something wrong with the traffic archive on fits.cv.nrao.edu, so I'll have to work from memory on this. As I see it, there are three coordinate systems relevant to solar data. One of the things that was originally not evident to me was that they all share the same PC matrix. This is because, although the coordinate systems are rotated with respect to each other, the rotation is out of the plane of observation. Much of my earlier confusion was because I was thinking of these coordinate systems as requiring different PC matrices for each. Also, it wasn't obvious to me that the same PC matrix would be applied to the secondary axis descriptions as well as the primary ones. That should perhaps be made clearer in the documentation. The three main coordinate systems are: I. Heliographic coordinates. This is a spherical coordinate system, well described by Mark Calabretta's earlier message. The one point of ambiguity is that the sun actually has several spherical "surfaces" of interest. What is most commonly understood as the surface of the sun is the photospheric surface, which is well defined at most visible wavelengths. However, for some wavelengths, a more appropriate surface to use in describing the data is the base of the chromosphere, which is perceptibly above the photospheric limb. Other heights may be appropriate for other data. This ambiguity could be removed by including an additional keyword to define exactly what surface is meant. I suggest that the keyword SOLAR_R be defined as an optional keyword whose meaning would be the apparent solar radius in kilometers of the surface used in determining the solar latitude and longitude. If omitted, the default value would be the photospheric value. II. Heliocentric coordinates. This is a three-dimensional X,Y,Z coordinate system which is defined to have the following characteristics: 1. The Z axis points from sun center towards the observer. 2. The Y axis is perpendicular to the Z axis, and lies in the plane containing the Z axis and the solar north rotational axis. 3. The X axis is perpendicular to both the Y and Z axes, and completes the right-handed coordinate system. It points towards the west (right) solar limb. It is not a spherical coordinate system. The Z coordinate is, of course, not observed. In my opinion, this is the primary coordinate system for coaligning solar data taken at different observatories. In a strict formal sense, no solar observations are ever made directly in this coordinate system, but it is generally treated as if it did. The important assumption is made that the distance from the observer to the object is large compared to any of the dimensions of the object. This is similar to the assumption that -mu >> 1 in Mark Calabretta's message. Traditionally, data in this coordinate system is given in one of three different units: Arcsec (or arcmin): Actually, what is meant here is not the angle itself, but the tangent of the angle. Expressing the data in arcsec or arcmin is really an expression of the fact that one is operating in the regime of the small angle approximation. Since this isn't truly a spherical coordinate system, the use of these pseudo-angular dimensions is not really in the spirit of the WCS. Also, the advent of spacecraft observing the sun from beyond low earth orbit, such as SOHO, means that the apparent angular size of the sun can be different for different sets of data. For example, SOHO sees a 1% larger sun than ground-based observatories. Solar radii: It's common to speak of data as being so many solar radii out from disk center. This is used for both on-disk data (0-1) and for off-limb data (>1). In Mark Calabretta's earlier message, it was suggested that the PC matrix be defined in such a way that it converted the data into units relative to the solar radius. However, I don't see where this is necessary. In order to properly express the position of the center of the sun, one needs proper CRPIXn and CRVALn values (or CmPIXn and CmVALn values). Those keywords imply that one also has a corresponding CDELTn (or CmELTn) keyword, so I don't see any utility in using the PC matrix to force CmELTn to be 1. Also, the coordinate system calls for the full complement of axis description keywords, including CTYPEn and CUNITn (CmYPEn/CmUNIn). It shouldn't be a "poor sister" of the heliographic coordinate system. Another objection to using the PC matrix to convert the data into solar radii is that it appears to go against the sense of the PC matrix. According to the documentation, the PC matrix is intended to serve as a rotation/skew transformation--the conversion to physical units is supposed to be done with the CDELTn/CmELTn keywords. Note also that expressing the data in solar radii suffers from the same potential ambiguity as the heliographic coordinate system as to exactly what radius is meaningful for the data. My suggestion is that the part about using the PC matrix to normalize the coordinates be removed from the PLON/PLAT proposal. Kilometers: Expressing the data as a physical distance removes all ambiguity due to either observing distance or to solar radius definition. III. "Helioprojective" coordinates For some data, the small angle approximation can no longer be maintained, and the fact that one is seeing the sun projected against the celestial sphere cannot be ignored. Such data require a modification to the heliocentric coordinate system, to convert it into a spherical system. Such a coordinate system would have the following properties: 1. The solar disk center would be on the equator at latitude and longitude both equal to zero. 2. The projection of the solar north polar axis would be on the great circle representing zero longitude, on the positive side. Since this is a spherical coordinate system, the units would be in degrees. I'm not sure what to call this system, which is why I put the name above in quotation marks. A very important point is that all three of the above coordinate systems use the same PC matrix. This makes it possible to have a combination of these coordinates occur in the same FITS file. Mark Calabretta's suggestion was to have the primary coordinate system be heliographic, and then one could have the secondary coordinate system be heliocentric. Is it also possible to do it the other way around? To my mind the heliocentric system is a more direct representation of the data, and the heliographic coordinates are a more derived quantity. However, I wasn't sure if the keywords LONGPOLE, etc. applied to the secondary coordinates as well. The sentence in the WCS documentation that The alternate coordinate descriptions are computed in the same fashion as the primary coordinates. would appear to imply that they did, in the same sense as the PC matrix. William Thompson From owner-fitsbits at marmoset.cv.nrao.edu Mon Jan 6 08:58:56 1997 Received: (from majdom at localhost) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) id IAA04285; Mon, 6 Jan 1997 08:58:52 -0500 Received: from fits.cv.nrao.edu (fits.cv.nrao.edu [192.33.115.8]) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) with ESMTP id TAA03084; Sun, 5 Jan 1997 19:49:11 -0500 Received: from solitaire.cv.nrao.edu (news at solitaire.cv.nrao.edu [192.33.115.79]) by fits.cv.nrao.edu (8.8.3/8.8.0/CV-2.2) with SMTP id TAA04305 for ; Sun, 5 Jan 1997 19:49:09 -0500 (EST) Received: by solitaire.cv.nrao.edu (4.1/DDN-CV/1.10) id AA05337; Sun, 5 Jan 97 19:49:06 EST To: fitsbits at fits.cv.nrao.edu Date: Sun, 05 Jan 1997 17:30:02 +0000 From: Andrew Gawthrope Message-Id: <32CFE51A.258D at paddocks.demon.co.uk> Path: solitaire.cv.nrao.edu!hearst.acc.Virginia.EDU!uunet!in1.uu.net!134.222.90.2!EU.net!uknet!usenet1.news.uk.psi.net!uknet!dispatch.news.demon.net!demon!paddocks.demon.co.uk!not-for-mail Reply-To: a.gawthrope at paddocks.demon.co.uk Subject: Viewer (FITS) required for OS/2 Newsgroups: sci.astro.fits Sender: owner-fitsbits at marmoset.cv.nrao.edu Precedence: bulk I am looking for a viewer for OS/2. Does anyone know of the existance of such an application. The PC I am using does not run DOS or Windows just OS/2 can anyone please help ? RGDS A.Gawthrope MIAP a.gawthrope at paddocks.demon.co.uk From owner-fitsbits at marmoset.cv.nrao.edu Mon Jan 6 09:04:23 1997 Received: (from majdom at localhost) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) id JAA04299; Mon, 6 Jan 1997 09:04:21 -0500 Received: from fits.cv.nrao.edu (fits.cv.nrao.edu [192.33.115.8]) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) with ESMTP id IAA04270; Mon, 6 Jan 1997 08:49:22 -0500 Received: from cv3.cv.nrao.edu (cv3.cv.nrao.edu [192.33.115.2]) by fits.cv.nrao.edu (8.8.3/8.8.0/CV-2.2) with ESMTP id IAA09896 for ; Mon, 6 Jan 1997 08:49:21 -0500 (EST) Received: from xebec.gsfc.nasa.gov (xebec.gsfc.nasa.gov [128.183.126.205]) by cv3.cv.nrao.edu (8.8.3/8.8.0/CV-2.3) with SMTP id IAA04959 for ; Mon, 6 Jan 1997 08:49:18 -0500 (EST) Received: (from arots at localhost) by xebec.gsfc.nasa.gov (LHEA9504/950407.s1) id IAA00777; Mon, 6 Jan 1997 08:48:03 -0500 From: Arnold Rots Message-Id: <199701061348.IAA00777 at xebec.gsfc.nasa.gov> Subject: Re: Solar coordinates To: thompson at orpheus.nascom.nasa.gov (William Thompson) Date: Mon, 6 Jan 1997 08:48:02 -0500 (EST) Cc: fitsbits at fits.cv.nrao.edu In-Reply-To: <5ajon4$ecg at post.gsfc.nasa.gov> from "William Thompson" at Jan 3, 97 08:03:48 pm X-Mailer: ELM [version 2.4 PL25] MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-fitsbits at marmoset.cv.nrao.edu Precedence: bulk At the risk of sounding pedantic, William Thompson writes: > ... > This ambiguity could be removed by including an additional keyword to define > exactly what surface is meant. I suggest that the keyword SOLAR_R be defined > as an optional keyword whose meaning would be the apparent solar radius in > kilometers of the surface used in determining the solar latitude and longitude. > If omitted, the default value would be the photospheric value. > ... > Kilometers: Expressing the data as a physical distance removes all ambiguity > due to either observing distance or to solar radius definition. > ... > William Thompson > > Let's not forget that the FITS standard prescribes (or, at least, strongly recommends) SI units. Meters would be more appropriate. - Arnold Rots From owner-fitsbits at marmoset.cv.nrao.edu Thu Jan 16 01:11:29 1997 Received: (from majdom at localhost) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) id BAA19273; Thu, 16 Jan 1997 01:09:48 -0500 Received: from fits.cv.nrao.edu (fits.cv.nrao.edu [192.33.115.8]) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) with ESMTP id WAA19163; Wed, 15 Jan 1997 22:45:09 -0500 Received: from solitaire.cv.nrao.edu (news at solitaire.cv.nrao.edu [192.33.115.79]) by fits.cv.nrao.edu (8.8.3/8.8.0/CV-2.2) with SMTP id WAA22717 for ; Wed, 15 Jan 1997 22:45:08 -0500 (EST) Received: by solitaire.cv.nrao.edu (4.1/DDN-CV/1.10) id AA09539; Wed, 15 Jan 97 22:45:06 EST To: fitsbits at fits.cv.nrao.edu Date: 14 Jan 1997 18:36:09 GMT From: seaman at noao.edu (Rob Seaman) Message-Id: <5bgjmp$3fe at noao.edu> Organization: National Optical Astronomy Observatories, Tucson, AZ, USA Path: solitaire.cv.nrao.edu!hearst.acc.Virginia.EDU!aruba.odu.edu!news.larc.nasa.gov!news.msfc.nasa.gov!news.sgi.com!news.corp.sgi.com!enews.sgi.com!news.mathworks.com!howland.erols.net!cs.utexas.edu!ennfs.eas.asu.edu!noao!seaman References: <32DB12FF.2865 at axres.com> Subject: Re: image orientation on the screen Newsgroups: sci.astro.fits Sender: owner-fitsbits at marmoset.cv.nrao.edu Precedence: bulk Mike Newberry writes: > Since there is no hardwired spec and no keywords for this, I wonder if > there is a commonly adopted "standard": > > Where should pixel (0,0) of a FITS file be displayed on the screen-- in > the upper left or lower left corner? Well, IRAF/Ximtool/SAOimage (and others) put the origin at the lower left. A FITS file is usually considered to be in fortran storage order, so the lines fill left to right before they fill bottom to top. Data cubes and higher dimensional images provide further complications. The computer graphics "standard" is to have the origin at the upper left. Many systems both in and out of astronomy allow the origin to be placed in any corner (either overtly, or via syntax that flips axes). Ximtool, for instance, provides buttons that flip the display along either axis. Transposing about diagonals requires outside operations. > And for which ever is correct, should the screen image match the > orientation (i.e., no mirror reversal) of what the CCD saw? Depends. It isn't entirely clear how to interpret the orientation of what a detector array (not limited to CCDs, or physical detectors in general for that matter) "sees". An orientation may only be defined at the point that the image is actually displayed to the user. To be less pedantic, note that a scientist and an engineer may want to view different orientations of the same field. An engineer (or a paranoid scientist, for that matter) will want to view an array in "readout order" - the first pixel/line read out should be the first pixel/line displayed. This allows unambiguous interpretation of any imaging defects - for just one example: do defects precede or follow known bad pixels? A CCD, of course, may be mounted any which way on the telescope, and may be read out through any one of the corners. Multiple readout CCDs complicate this further (half the pixels are shifted each way - or perhaps four different ways). Mosaic detectors may have 8 or 16 readouts... On the other hand, an astronomer will often prefer to see the image "correct on the sky". Note that "correct on the sky" is itself a rather fuzzy concept - only in one hemisphere might north be at the top and east to the left. To get more pedantic again, note that the computer monitor is rarely placed in any correct orientation relative to the sky. Note that FITS WCS provides a (potentially) more rigorous mechanism to address these issues. (FITS might also conceivably benefit from the experiences of the remote sensing/GIS communities.) Rob Seaman -- seaman at noao.edu, http://iraf.noao.edu/~seaman NOAO, 950 N Cherry Ave, Tucson AZ 85719, 520-318-8248 PGP: 98 8D 8B 49 74 9A 41 88 3A 43 87 54 51 BF 30 4B From owner-fitsbits at marmoset.cv.nrao.edu Thu Jan 16 01:11:53 1997 Received: (from majdom at localhost) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) id BAA19284; Thu, 16 Jan 1997 01:11:37 -0500 Received: from fits.cv.nrao.edu (fits.cv.nrao.edu [192.33.115.8]) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) with ESMTP id AAA19253; Thu, 16 Jan 1997 00:44:38 -0500 Received: from solitaire.cv.nrao.edu (news at solitaire.cv.nrao.edu [192.33.115.79]) by fits.cv.nrao.edu (8.8.3/8.8.0/CV-2.2) with SMTP id AAA23628 for ; Thu, 16 Jan 1997 00:44:37 -0500 (EST) Received: by solitaire.cv.nrao.edu (4.1/DDN-CV/1.10) id AA13800; Thu, 16 Jan 97 00:44:35 EST To: fitsbits at fits.cv.nrao.edu Date: 13 Jan 1997 21:01:07 -0700 From: Mike Newberry Message-Id: <32DB12FF.2865 at axres.com> Organization: Axiom Research, Inc. Path: solitaire.cv.nrao.edu!hearst.acc.Virginia.EDU!news-server.ncren.net!newsgate.duke.edu!agate!spool.mu.edu!uwm.edu!www.nntp.primenet.com!nntp.primenet.com!news.primenet.com!news.primenet.com!not-for-mail Reply-To: mnewberry at axres.com Subject: image orientation on the screen Newsgroups: sci.astro.fits Sender: owner-fitsbits at marmoset.cv.nrao.edu Precedence: bulk Since there is no hardwired spec and no keywords for this, I wonder if there is a commonly adopted "standard": Where should pixel (0,0) of a FITS file be displayed on the screen-- in the upper left or lower left corner? And for which ever is correct, should the screen image match the orientation (i.e., no mirror reversal) of what the CCD saw? Mike Newberry From owner-fitsbits at marmoset.cv.nrao.edu Thu Jan 16 11:50:55 1997 Received: (from majdom at localhost) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) id LAA00347; Thu, 16 Jan 1997 11:50:43 -0500 Received: from fits.cv.nrao.edu (fits.cv.nrao.edu [192.33.115.8]) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) with ESMTP id LAA00332; Thu, 16 Jan 1997 11:34:50 -0500 Received: from solitaire.cv.nrao.edu (news at solitaire.cv.nrao.edu [192.33.115.79]) by fits.cv.nrao.edu (8.8.3/8.8.0/CV-2.2) with SMTP id LAA00534 for ; Thu, 16 Jan 1997 11:34:48 -0500 (EST) Received: by solitaire.cv.nrao.edu (4.1/DDN-CV/1.10) id AA02839; Thu, 16 Jan 97 11:34:45 EST To: fitsbits at fits.cv.nrao.edu Date: 15 Jan 1997 12:13 EDT From: bschlesinger at nssdca.gsfc.nasa.gov (Barry M. Schlesinger) Message-Id: <15JAN199712130457 at nssdca.gsfc.nasa.gov> Organization: NASA Goddard Space Flight Center -- Greenbelt, Maryland USA Path: solitaire.cv.nrao.edu!hearst.acc.Virginia.EDU!uunet!in1.uu.net!205.252.116.190!feed1.news.erols.com!howland.erols.net!news.mathworks.com!enews.sgi.com!ames!newsfeed.gsfc.nasa.gov!nssdca.gsfc.nasa.gov!bschlesinger References: <32DB12FF.2865 at axres.com> <5bgjmp$3fe at noao.edu> Subject: Re: image orientation on the screen Newsgroups: sci.astro.fits Sender: owner-fitsbits at marmoset.cv.nrao.edu Precedence: bulk In article <5bgjmp$3fe at noao.edu>, seaman at noao.edu (Rob Seaman) writes... >Mike Newberry writes: > >> ... I wonder if >> there is a commonly adopted "standard": >> >> Where should pixel (0,0) of a FITS file be displayed on the screen-- in >> the upper left or lower left corner? > >Well, IRAF/Ximtool/SAOimage (and others) put the origin at the lower left. >A FITS file is usually considered to be in fortran storage order, so the >lines fill left to right before they fill bottom to top.... > >The computer graphics "standard" is to have the origin at the upper left. >Many systems both in and out of astronomy allow the origin to be placed >in any corner (either overtly, or via syntax that flips axes).... > >> And for which ever is correct, should the screen image match the >> orientation (i.e., no mirror reversal) of what the CCD saw? > >... It isn't entirely clear how to interpret the orientation of what a >detector array (not limited to CCDs, or physical detectors in general >for that matter) "sees". An orientation may only be defined at the >point that the image is actually displayed to the user. > > [detailed discussion omitted] > >Note that FITS WCS provides a (potentially) more rigorous mechanism to >address these issues. (FITS might also conceivably benefit from the >experiences of the remote sensing/GIS communities.) > The current draft of the World Coordinate Systems (WCS) proposal has some discussion on these points and can be found via http://fits.cv.nrao.edu/documents/wcs/wcs/html Barry Schlesinger FITS Support Office GSFC/ADF From owner-fitsbits at marmoset.cv.nrao.edu Mon Jan 20 11:04:27 1997 Received: (from majdom at localhost) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) id LAA07853; Mon, 20 Jan 1997 11:03:03 -0500 Received: from fits.cv.nrao.edu (fits.cv.nrao.edu [192.33.115.8]) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) with ESMTP id XAA06681; Sun, 19 Jan 1997 23:39:32 -0500 Received: from solitaire.cv.nrao.edu (news at solitaire.cv.nrao.edu [192.33.115.79]) by fits.cv.nrao.edu (8.8.3/8.8.0/CV-2.2) with SMTP id XAA07862 for ; Sun, 19 Jan 1997 23:39:31 -0500 (EST) Received: by solitaire.cv.nrao.edu (4.1/DDN-CV/1.10) id AA27192; Sun, 19 Jan 97 23:39:29 EST To: fitsbits at fits.cv.nrao.edu Date: 17 Jan 1997 13:21 EDT From: bschlesinger at nssdca.gsfc.nasa.gov (Barry M. Schlesinger) Message-Id: <17JAN199713211819 at nssdca.gsfc.nasa.gov> Organization: NASA Goddard Space Flight Center -- Greenbelt, Maryland USA Path: solitaire.cv.nrao.edu!hearst.acc.Virginia.EDU!aruba.odu.edu!news.larc.nasa.gov!news.msfc.nasa.gov!news.sgi.com!news.sprintlink.net!news-peer.sprintlink.net!arclight.uoregon.edu!enews.sgi.com!ames!newsfeed.gsfc.nasa.gov!nssdca.gsfc.nasa.gov!bschlesinger Subject: ANNOUNCE: Physical Move of FITS Support Office Newsgroups: sci.astro.fits Sender: owner-fitsbits at marmoset.cv.nrao.edu Precedence: bulk The FITS Support Office will be changing its physical location on Tuesday, January 21. The electronic mail addresses and the locations of the World Wide Web pages and ftp sites will remain the same. There will be a new telephone number. During the transition, it will not be possible to reach the FITS Support Office by telephone, until the new telephone number is announced. As the host computers are not being moved, electronic mail will still be received, but as the work station *is* being moved, there may be some delay in response. Note that January 20 is a holiday in the U. S. (Dr. Martin Luther King's birthday.) We hope to be back to normal by the end of the week (January 24). Barry Schlesinger FITS Support Office GSFC/ADF From owner-fitsbits at marmoset.cv.nrao.edu Mon Jan 20 11:04:40 1997 Received: (from majdom at localhost) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) id LAA07865; Mon, 20 Jan 1997 11:04:39 -0500 Received: from fits.cv.nrao.edu (fits.cv.nrao.edu [192.33.115.8]) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) with ESMTP id AAA06695; Mon, 20 Jan 1997 00:14:51 -0500 Received: from solitaire.cv.nrao.edu (news at solitaire.cv.nrao.edu [192.33.115.79]) by fits.cv.nrao.edu (8.8.3/8.8.0/CV-2.2) with SMTP id AAA07892 for ; Mon, 20 Jan 1997 00:14:49 -0500 (EST) Received: by solitaire.cv.nrao.edu (4.1/DDN-CV/1.10) id AA28320; Mon, 20 Jan 97 00:14:47 EST To: fitsbits at fits.cv.nrao.edu Date: 16 Jan 1997 02:19:01 -0700 From: mnewberry at axres.com (Michael Newberry) Message-Id: <32ddc603.17665081 at news.primenet.com> Organization: Primenet Services for the Internet Path: solitaire.cv.nrao.edu!hearst.acc.Virginia.EDU!news-server.ncren.net!newsgate.duke.edu!news.mathworks.com!www.nntp.primenet.com!nntp.primenet.com!news.primenet.com!news.primenet.com!not-for-mail References: <32DB12FF.2865 at axres.com> <5bgjmp$3fe at noao.edu> Subject: Re: image orientation on the screen Newsgroups: sci.astro.fits Sender: owner-fitsbits at marmoset.cv.nrao.edu Precedence: bulk On 14 Jan 1997 18:36:09 GMT, seaman at noao.edu (Rob Seaman) wrote: >Mike Newberry writes: > >> Since there is no hardwired spec and no keywords for this, I wonder if >> there is a commonly adopted "standard": >> >> Where should pixel (0,0) of a FITS file be displayed on the screen-- in >> the upper left or lower left corner? > >Well, IRAF/Ximtool/SAOimage (and others) put the origin at the lower left. >A FITS file is usually considered to be in fortran storage order, so the >lines fill left to right before they fill bottom to top. Data cubes and >higher dimensional images provide further complications. > >The computer graphics "standard" is to have the origin at the upper left. >Many systems both in and out of astronomy allow the origin to be placed >in any corner (either overtly, or via syntax that flips axes). Ximtool, >for instance, provides buttons that flip the display along either axis. >Transposing about diagonals requires outside operations. > >> And for which ever is correct, should the screen image match the >> orientation (i.e., no mirror reversal) of what the CCD saw? > >Depends. > >It isn't entirely clear how to interpret the orientation of what a >detector array (not limited to CCDs, or physical detectors in general >for that matter) "sees". An orientation may only be defined at the >point that the image is actually displayed to the user. > >To be less pedantic, note that a scientist and an engineer may want >to view different orientations of the same field. An engineer (or >a paranoid scientist, for that matter) will want to view an array >in "readout order" - the first pixel/line read out should be the >first pixel/line displayed. This allows unambiguous interpretation >of any imaging defects - for just one example: do defects precede >or follow known bad pixels? > >A CCD, of course, may be mounted any which way on the telescope, and >may be read out through any one of the corners. Multiple readout CCDs >complicate this further (half the pixels are shifted each way - or >perhaps four different ways). Mosaic detectors may have 8 or 16 readouts... > >On the other hand, an astronomer will often prefer to see the image >"correct on the sky". Note that "correct on the sky" is itself a rather >fuzzy concept - only in one hemisphere might north be at the top and >east to the left. To get more pedantic again, note that the computer >monitor is rarely placed in any correct orientation relative to the sky. > >Note that FITS WCS provides a (potentially) more rigorous mechanism to >address these issues. (FITS might also conceivably benefit from the >experiences of the remote sensing/GIS communities.) > >Rob Seaman Rob, This confirms what I thought was the state of things. I have been pretty close to FITS for some years in astronomy, but I was starting to wonder if ther had been some improvement or convention that I hadn't yet heard of. First we would need to agree that pixel (0,0) or (1,1) goes in the lower left corner. Then we need to find a way to avoid mirror reversals of the image with respect to the sky, or the literal view outside of astronomical imaging. In TIFF and other pictorial formats,there is header information that allows the reading software to show the correct orientation even if there is no world coordinate system attached to the image. It is unfortunate that there is no adopted protocol in the FITS world without going all the way to WCS calibration. It would be really useful if there were two keywords that would go into an image header at the point of origin that could later be used to give the correct display in the "mirror" sense. This would leave only rotation as ambiguous but, since CCD's are usually mounted on telescopes with North or South up, images could be displayed nearly with East left and North up (or left left and up up). Along the lines of the keywords XOVRSCAN and YOVRSCAN, or X-ORIGIN, or X-BIN, etc., we might use X-FLIP and Y-FLIP, X-MIRROR and Y-MIRROR, XREVERSE and YREVERSE, or X-INVERT and Y-INVERT, as logical (T/F) keywords. The case of no reversals would be keywords not present or XREVERSE = F and YREVERSE = F. We don't need to emend the FITS standard to do this; some of us in the camera control software and image processing software world just need to agree on the implementation of the additional keywords. Personally, I like XREVERSE and YREVERSE. Mike Newberry From owner-fitsbits at marmoset.cv.nrao.edu Mon Jan 20 11:28:57 1997 Received: (from majdom at localhost) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) id LAA08897; Mon, 20 Jan 1997 11:28:48 -0500 Received: from fits.cv.nrao.edu (fits.cv.nrao.edu [192.33.115.8]) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) with ESMTP id LAA08762; Mon, 20 Jan 1997 11:26:05 -0500 Received: from solitaire.cv.nrao.edu (news at solitaire.cv.nrao.edu [192.33.115.79]) by fits.cv.nrao.edu (8.8.3/8.8.0/CV-2.2) with SMTP id LAA00615 for ; Mon, 20 Jan 1997 11:26:03 -0500 (EST) Received: by solitaire.cv.nrao.edu (4.1/DDN-CV/1.10) id AA18633; Mon, 20 Jan 97 11:26:01 EST To: fitsbits at fits.cv.nrao.edu Date: Fri, 17 Jan 1997 15:45:46 -0500 From: Doug Mink Message-Id: <32DFE4FA.42D2 at cfa.harvard.edu> Organization: Harvard University University Information Systems Path: solitaire.cv.nrao.edu!hearst.acc.Virginia.EDU!aruba.odu.edu!news.larc.nasa.gov!news.msfc.nasa.gov!www.nntp.primenet.com!nntp.primenet.com!howland.erols.net!news.bbnplanet.com!cam-news-hub1.bbnplanet.com!cam-news-feed3.bbnplanet.com!das-news2.harvard.edu!oitnews.harvard.edu!news Subject: Re: image orientation on the screen Newsgroups: sci.astro.fits Sender: owner-fitsbits at marmoset.cv.nrao.edu Precedence: bulk Mike Newberry writes: > It would be really useful if there were two keywords that would go > into an image header at the point of origin that could later be used > to give the correct display in the "mirror" sense. In my world coordinate setting software, I have had to deal with image rotation and mirroring on an image-by-image basis. I, too, would like to have some keywords to specify mirroring (XREVERSE and YREVERSE sound like the right idea) and position angle of north (NORTHPA?) standardized as either before or after mirroring. For the ground-based optical telescopes with which I have been dealing, these keyword values stay constant for at least the duration of a run. -Doug Mink Telescope Data Center Harvard-Smithsonian Center for Astrophysics From owner-fitsbits at marmoset.cv.nrao.edu Wed Jan 22 18:03:16 1997 Received: (from majdom at localhost) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) id SAA06253; Wed, 22 Jan 1997 18:02:39 -0500 Received: from fits.cv.nrao.edu (fits.cv.nrao.edu [192.33.115.8]) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) with ESMTP id RAA06193; Wed, 22 Jan 1997 17:55:06 -0500 Received: from solitaire.cv.nrao.edu (news at solitaire.cv.nrao.edu [192.33.115.79]) by fits.cv.nrao.edu (8.8.3/8.8.0/CV-2.2) with SMTP id RAA17924 for ; Wed, 22 Jan 1997 17:55:04 -0500 (EST) Received: by solitaire.cv.nrao.edu (4.1/DDN-CV/1.10) id AA09737; Wed, 22 Jan 97 17:55:02 EST To: fitsbits at fits.cv.nrao.edu Date: 20 Jan 1997 18:34:44 GMT From: hodge at bowline.stsci.edu (Phil Hodge) Message-Id: <5c0ds4$hlf at tnm.stsci.edu> Organization: Space Telescope Science Institute Path: solitaire.cv.nrao.edu!hearst.acc.Virginia.EDU!aruba.odu.edu!news.larc.nasa.gov!news.msfc.nasa.gov!news.sgi.com!spool.mu.edu!uwm.edu!cs.utexas.edu!ennfs.eas.asu.edu!noao!stsci.edu!usenet References: <32DB12FF.2865 at axres.com> <5bgjmp$3fe at noao.edu> <32ddc603.17665081 at news.primenet.com> Subject: Re: image orientation on the screen Newsgroups: sci.astro.fits Sender: owner-fitsbits at marmoset.cv.nrao.edu Precedence: bulk Michael Newberry (mnewberry at axres.com) writes: > It would be really useful if there were two keywords that would go > into an image header at the point of origin that could later be used > to give the correct display in the "mirror" sense. This would leave > ... > Personally, I like XREVERSE and YREVERSE. I don't think it's a good idea to create FITS keywords which depend on the convention for displaying images. A separate keyword for each axis is also not very useful for images taken from spacecraft. The parity of an image is easily obtained from the sign of the determinant of the CD matrix (or the PC matrix multiplied by the CDELTi, in Greisen and Calabretta's notation). The interpretation of the sign depends on how you display the image, of course. The sign is negative for images that will be displayed correctly (i.e. without mirror reversal, but possibly rotated) if the first pixel is at the lower left, with the sample direction running from left to right. Phil From owner-fitsbits at marmoset.cv.nrao.edu Fri Jan 24 17:31:57 1997 Received: (from majdom at localhost) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) id RAA10953; Fri, 24 Jan 1997 17:30:34 -0500 Received: from fits.cv.nrao.edu (fits.cv.nrao.edu [192.33.115.8]) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) with ESMTP id QAA10874; Fri, 24 Jan 1997 16:25:34 -0500 Received: from solitaire.cv.nrao.edu (news at solitaire.cv.nrao.edu [192.33.115.79]) by fits.cv.nrao.edu (8.8.3/8.8.0/CV-2.2) with SMTP id QAA09012 for ; Fri, 24 Jan 1997 16:25:33 -0500 (EST) Received: by solitaire.cv.nrao.edu (4.1/DDN-CV/1.10) id AA11200; Fri, 24 Jan 97 16:25:30 EST To: fitsbits at fits.cv.nrao.edu Date: 20 Jan 1997 16:29:02 -0700 From: mnewberry at axres.com (Michael Newberry) Message-Id: <32e3fee7.4689162 at news.primenet.com> Organization: Primenet Services for the Internet Path: solitaire.cv.nrao.edu!hearst.acc.Virginia.EDU!news-server.ncren.net!newsgate.duke.edu!news.mathworks.com!news.bbnplanet.com!cam-news-hub1.bbnplanet.com!uunet!in3.uu.net!204.245.3.50!news.primenet.com!news.primenet.com!not-for-mail References: <32DB12FF.2865 at axres.com> <5bgjmp$3fe at noao.edu> <32ddc603.17665081 at news.primenet.com> <5c0ds4$hlf at tnm.stsci.edu> Subject: Re: image orientation on the screen Newsgroups: sci.astro.fits Sender: owner-fitsbits at marmoset.cv.nrao.edu Precedence: bulk On 20 Jan 1997 18:34:44 GMT, hodge at bowline.stsci.edu (Phil Hodge) wrote: >Michael Newberry (mnewberry at axres.com) writes: > >> It would be really useful if there were two keywords that would go >> into an image header at the point of origin that could later be used >> to give the correct display in the "mirror" sense. This would leave >> ... >> Personally, I like XREVERSE and YREVERSE. > >I don't think it's a good idea to create FITS keywords which depend >on the convention for displaying images. A separate keyword for each >axis is also not very useful for images taken from spacecraft. The >parity of an image is easily obtained from the sign of the determinant >of the CD matrix (or the PC matrix multiplied by the CDELTi, in Greisen >and Calabretta's notation). The interpretation of the sign depends on >how you display the image, of course. The sign is negative for images >that will be displayed correctly (i.e. without mirror reversal, but >possibly rotated) if the first pixel is at the lower left, with the >sample direction running from left to right. > > Phil Phil, You are right about using the signs of CRDELT1 and CRDELT2. But you are talking about World Coordinate keywords whereas I'm talking about keywords that do not require an world coordinate calibration. Most CCD users do not WCS calibrate their images---all I want is North almost up and East almost left for *display* purposes. Mike Newberry. From owner-fitsbits at marmoset.cv.nrao.edu Sat Jan 25 20:56:53 1997 Received: (from majdom at localhost) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) id UAA12790; Sat, 25 Jan 1997 20:56:46 -0500 Received: from cv3.cv.nrao.edu (cv3.cv.nrao.edu [192.33.115.2]) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) with ESMTP id XAA11257; Fri, 24 Jan 1997 23:29:44 -0500 Received: from cicg-communication.grenet.fr (cicg-communication.grenet.fr [130.190.6.23]) by cv3.cv.nrao.edu (8.8.5/8.8.0/CV-2.3) with SMTP id XAA20467 for ; Fri, 24 Jan 1997 23:29:43 -0500 (EST) Message-Id: <199701250429.XAA20467 at cv3.cv.nrao.edu> Received: from gag.observ-gr.fr (gagax1-fddi.observ-gr.fr) by cicg-communication.grenet.fr (4.1/Ccomm.94021501) id AA14610; Sat, 25 Jan 97 05:30:50 +0100 Date: Sat, 25 Jan 1997 05:29:41 +0100 From: forveill at gag.observ-gr.fr (Thierry Forveille) Posted-Date: Sat, 25 Jan 1997 05:29:41 +0100 To: fitsbits at nrao.edu Subject: Re: image orientation on the screen Sender: owner-fitsbits at marmoset.cv.nrao.edu Precedence: bulk mnewberry at axres.com (Michael Newberry) writes > You are right about using the signs of CRDELT1 and CRDELT2. But you > are talking about World Coordinate keywords whereas I'm talking about > keywords that do not require an world coordinate calibration. Most > CCD users do not WCS calibrate their images---all I want is North > almost up and East almost left for *display* purposes. > If that's all you want, just set the signs of CDELT1 and CDELT2 right, using either +1 or -1 as is fit for your orientation. That will leave you with an image displayed with axes labelled in pixel units, and the (aproximately) right orientation. I'd personally argue for the value of inserting an aproximate value of the angular size of a pixel (which even amateur CCD users know to better (and often much better) than 10%), and get your axes labelled in arc seconds. But if you don't want to do that, you don't have to. Let's not add keywords where they are not needed.... Thierry Forveille Observatoire de Grenoble France PS: Technically CDELTi, CRVALi and CRPIXi are not part of the WCS definitions, which they predate by many years. They are defined in the original FITS paper. ------- end ------- From owner-fitsbits at marmoset.cv.nrao.edu Mon Jan 27 00:52:46 1997 Received: (from majdom at localhost) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) id AAA13699; Mon, 27 Jan 1997 00:51:53 -0500 Received: from fits.cv.nrao.edu (fits.cv.nrao.edu [192.33.115.8]) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) with ESMTP id GAA13281; Sun, 26 Jan 1997 06:51:26 -0500 Received: from solitaire.cv.nrao.edu (news at solitaire.cv.nrao.edu [192.33.115.79]) by fits.cv.nrao.edu (8.8.3/8.8.0/CV-2.2) with SMTP id GAA12788 for ; Sun, 26 Jan 1997 06:51:25 -0500 (EST) Received: by solitaire.cv.nrao.edu (4.1/DDN-CV/1.10) id AA22408; Sun, 26 Jan 97 06:51:21 EST To: fitsbits at fits.cv.nrao.edu Date: 22 Jan 1997 12:38:41 GMT From: "\'\u\ Message-Id: <01bc0861$57e35f20$80c5bd89 at s936723> Organization: CUHK Path: solitaire.cv.nrao.edu!hearst.acc.Virginia.EDU!news-server.ncren.net!newsgate.duke.edu!agate!newsgate.cuhk.edu.hk!news.cuhk.edu.hk!usenet Subject: TIFF -> FITS Newsgroups: sci.astro.fits Sender: owner-fitsbits at marmoset.cv.nrao.edu Precedence: bulk I use Meade Pictor View(416) to get CCD images in TIFF format(16 bit). I can only save it to 8 bit FITS format. Actually, I want to save it in 16 bit FITS format. Are there any software convert it for me? Thank you. edward lee From owner-fitsbits at marmoset.cv.nrao.edu Tue Jan 28 22:39:33 1997 Received: (from majdom at localhost) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) id WAA17014; Tue, 28 Jan 1997 22:39:18 -0500 Received: from fits.cv.nrao.edu (fits.cv.nrao.edu [192.33.115.8]) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) with ESMTP id VAA16916; Tue, 28 Jan 1997 21:30:10 -0500 Received: from cv3.cv.nrao.edu (cv3.cv.nrao.edu [192.33.115.2]) by fits.cv.nrao.edu (8.8.5/8.8.0/CV-2.2) with ESMTP id VAA16666 for ; Tue, 28 Jan 1997 21:30:05 -0500 (EST) Received: from crux.rp.CSIRO.AU (crux.rp.CSIRO.AU [130.155.194.32]) by cv3.cv.nrao.edu (8.8.5/8.8.0/CV-2.3) with SMTP id VAA20284 for ; Tue, 28 Jan 1997 21:30:00 -0500 (EST) Received: from grus.atnf.CSIRO.AU (grus.atnf.CSIRO.AU [130.155.194.37]) by crux.rp.CSIRO.AU (8.6.10/8.6.10.A) with ESMTP id NAA20989; Wed, 29 Jan 1997 13:28:27 +1100 Message-Id: <199701290228.NAA20989 at crux.rp.CSIRO.AU> To: thompson at orpheus.nascom.nasa.gov (William Thompson) CC: fitsbits at fits.cv.nrao.edu Subject: Re: Solar coordinates In-reply-to: Your message of Fri 1997/01/03 20:03:48 GMT <5ajon4$ecg at post.gsfc.nasa.gov> Date: Wed, 29 Jan 1997 13:28:25 +1100 From: Mark Calabretta Sender: owner-fitsbits at marmoset.cv.nrao.edu Precedence: bulk On Fri 1997/01/03 20:03:48 GMT, William Thompson wrote in a message to: fitsbits at fits.cv.nrao.edu > As I see it, there are three coordinate systems relevant to solar data. The PLON/PLAT proposal that I forwarded before xmas provided an example of a WCS application. Basically I have no interest in it other than as a demonstration of usage (where it appears to have been largely successful). The next steps are up to you. If the solar community can decide on the exact details of PLON/PLAT usage then you should refine the proposal. You may also want to define related coordinate systems such as ZLON/ZLAT (say) with the pole at the sub-viewer point. It may or may not be possible to register this/these in the WCS draft depending on timescales. Mark Calabretta ATNF From owner-fitsbits at marmoset.cv.nrao.edu Wed Jan 29 09:13:25 1997 Received: (from majdom at localhost) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) id JAA18018; Wed, 29 Jan 1997 09:13:15 -0500 Received: from fits.cv.nrao.edu (fits.cv.nrao.edu [192.33.115.8]) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) with ESMTP id WAA17023; Tue, 28 Jan 1997 22:46:52 -0500 Received: from cv3.cv.nrao.edu (cv3.cv.nrao.edu [192.33.115.2]) by fits.cv.nrao.edu (8.8.5/8.8.0/CV-2.2) with ESMTP id WAA17245 for ; Tue, 28 Jan 1997 22:46:51 -0500 (EST) Received: from crux.rp.CSIRO.AU (crux.rp.CSIRO.AU [130.155.194.32]) by cv3.cv.nrao.edu (8.8.5/8.8.0/CV-2.3) with SMTP id WAA20985 for ; Tue, 28 Jan 1997 22:46:47 -0500 (EST) Received: from grus.atnf.CSIRO.AU (grus.atnf.CSIRO.AU [130.155.194.37]) by crux.rp.CSIRO.AU (8.6.10/8.6.10.A) with ESMTP id OAA07208; Wed, 29 Jan 1997 14:45:17 +1100 Message-Id: <199701290345.OAA07208 at crux.rp.CSIRO.AU> To: mnewberry at axres.com (Michael Newberry) CC: fitsbits at fits.cv.nrao.edu Subject: Re: image orientation on the screen In-reply-to: Your message of Mon 1997/01/20 16:29:02 PDT <32e3fee7.4689162 at news.primenet.com> Date: Wed, 29 Jan 1997 14:45:15 +1100 From: Mark Calabretta Sender: owner-fitsbits at marmoset.cv.nrao.edu Precedence: bulk On Mon 1997/01/20 16:29:02 PDT, Michael Newberry wrote in a message to: fitsbits at fits.cv.nrao.edu >You are right about using the signs of CRDELT1 and CRDELT2. But you >are talking about World Coordinate keywords whereas I'm talking about >keywords that do not require an world coordinate calibration. Most >CCD users do not WCS calibrate their images---all I want is North >almost up and East almost left for *display* purposes. The default image orientation is independent of the coordinate system; that is, the orientation is not determined by the sign of the CDELTn (e.g. you could have secondary axis descriptions with conflicting signs). While this does not preclude a particular FITS viewer from transposing an image in order to transpose the coordinate system, you couldn't expect all FITS viewers to do the same thing, and in general I think it unreasonable to expect them to intuit an orientation from curvilinear coordinate systems which need not be parallel to the pixel axes, nor even orthogonal. You were on safer ground with your X/YREVERSE proposal. The image orientation convention was added to the WCS proposal to formalize a long-standing defacto FITS convention. Strictly speaking it has nothing to do with WCS (and is unrelated to the FORTRAN array storage convention). Mark Calabretta ATNF From owner-fitsbits at marmoset.cv.nrao.edu Thu Jan 30 16:34:11 1997 Received: (from majdom at localhost) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) id QAA11554; Thu, 30 Jan 1997 16:33:30 -0500 Received: from fits.cv.nrao.edu (fits.cv.nrao.edu [192.33.115.8]) by marmoset.cv.nrao.edu (8.7.6/8.7.3: $Revision: 1.1 $) with ESMTP id QAA11541; Thu, 30 Jan 1997 16:32:26 -0500 Received: from solitaire.cv.nrao.edu (news at solitaire.cv.nrao.edu [192.33.115.79]) by fits.cv.nrao.edu (8.8.5/8.8.0/CV-2.2) with SMTP id QAA06145 for ; Thu, 30 Jan 1997 16:32:25 -0500 (EST) Received: by solitaire.cv.nrao.edu (4.1/DDN-CV/1.10) id AA22482; Thu, 30 Jan 97 16:32:22 EST To: fitsbits at fits.cv.nrao.edu Date: Wed, 29 Jan 1997 09:06:22 +0000 From: Martin Beckett Message-Id: <32EF130E.16B4 at ast.cam.ac.uk> Organization: Institute of Astronomy, University of Cambridge Path: solitaire.cv.nrao.edu!hearst.acc.Virginia.EDU!uunet!in3.uu.net!199.94.215.18!news.bbnplanet.com!cam-news-hub1.bbnplanet.com!howland.erols.net!rill.news.pipex.net!pipex!warwick!lyra.csx.cam.ac.uk!news Reply-To: mgb at ast.cam.ac.uk Subject: fitsio without file access Newsgroups: sci.astro.fits Sender: owner-fitsbits at marmoset.cv.nrao.edu Precedence: bulk I need to read and write fits files without necessarily having a random access file, eg. to shared memory or Tcl i/o streams. Idealy I would like the fits file to be created as just an area in memory. Unfortunately CFITSIO needs a real file. I could create a temporary disk file and mmap() it back into memory but that seems a bit daft. Are there any other C/C++ fits libraries, or a patch to cfitsio out there ? Basically I'm too lazy to reinvent everything Martin ------------------------------------------------------------ Martin Beckett, Institute of Astronomy, University of Cambridge Tel: 01223 337093 Fax: 01223 330804 Email: mgb at ast.cam.ac.uk