Table of contents:
WHAT ANY CFH12K USER SHOULD KNOW
Any CFH12K observer should check his pointings using this tool to ensure that no major bright stars related problems (see items 1, 2 & 3 below) will badly obliterate the data. Of course, there are times when the object of interest can not be moved significantly enough on the CFH12K field of view to avoid contamination, but there are ways to at least minimize the impact of these bright stars.
Note that the CFHT prime focus bonnette can not be rotated, hence dodging bright stars can only be accomplished through translations along the alpha and delta axis.
TFM respects the size and gaps between the CCDs as well as the off-centered position of the telescope pointing (the point of the sky corresponding to the telescope pointing coordinates lies on the top corner of CCD02, not exactly at the center of the mosaic. See this page for further information.)
However be aware that the gaps between the CCDs are not perfectly uniform, nor are the CCDs perfectly aligned in respect to each other (typical alignment angle is 0.3 degree), and can play a role at the scale of 10 arcseconds (see the "Mosaic Geometry" section on this page for further information.). So don't count on TFM to position an object precisely in a gap for example. Worse, the rotation of the camera on the sky is defined by the alignment of the telescope prime focus top-end at installation at the beginning of the observing run. Due to innacuracies in the process, the alignment of the instrument changes by up to 0.5 degree from an observing run to another. TFM maps the sky on the CFH12K field for a rotation angle of 0 degree, hence a small rotation induced by the mounting process will shift all the positions by as much as 20 arcseconds in the outer parts of the field. Also, the telescope pointing accuracy is roughly 10 arcseconds unless a SAO star is used to reset the pointing just before. A SAO star pre-pointing allows a pointing accuracy on a nearby field of 1 arcsecond but the process of calibrating on the SAO stars eats up 2 to 3 minutes. So, all in all, TFM can not (so could not any other tool) be used to prepare CFH12K pointings more accurate than 10 arcseconds.
A sinus projection is used to map the Guide Star Catalog objects on a two dimensional plane. Moreover, the CFHT prime focus wide-field corrector optical distortion is included in the star positions mapping. A common effect for such optics, this radial distortion increases the actual distance of objects to the center of the field (see Cuillandre et al. PASP, 1996, 108, 1120).
THE MAIN REGIMES OF BRIGHT STARS CONTAMINATION
This slide illustrates how bright stars falling within or near the CFH12K field of view can badly degrade the quality of the data. Here are described the four typical cases of contamination caused by bright stars: reflection halos, blooming and scattered light from the edge of the focal plane. The letters pointing the various phenomena on the slide are described:
Baffling and surfaces blackening have been greatly improved at CFHT's prime focus prior CFH12K first light in 1999. While UH8K used to suffer a lot from stars within the field of view of the wide-field corrector in particular (0.5 degree radius), the effect on the CFH12K has been strongly reduced (factor of 10). The tests shown on this slide illustrate the effect caused by the star Sirius, the brightest star in the sky (the star was positioned at various locations around the field of view of CFH12K as illustrated by the small images distribution). For all the other stars in the sky, the effect will be lower of course (scaled by their relative brightness), often dominated by the sky photon noise, but still present.
3: Beyond 1 degree radius stars
Stars beyond that limit are not a concern anymore. This is illustrated by this slide.
TIPS TO MINIMIZE CONTAMINATION FROM BRIGHT STARS
2: Look at the TFM output for the wished pointing. If there is a bright star of magnitude less than 7 inside the CFH12K field of view (the mosaic), try to compromise the pointing to put the star outside the focal plane. However, you have to expect the following item 3. Iterate using the "Display GIF Image" option.
3: Look at the TFM output for the wished pointing. If there is a bright star of magnitude less than 6 outside and very near (less than 1 arcminute) the CFH12K field of view, try to compromise the pointing to put the star further away (like 2 arcminutes). However, you have to expect the following item 4. Iterate using the "Display GIF Image" option.
4: Look at the TFM output for the wished pointing. If there is a bright star of magnitude less than 8 outside and near (more than 1 arcminute) the CFH12K field of view, try to compromise the pointing to put the star further away (3 arcminutes). Iterate using the "Display GIF Image" option.
HOW TO EFFICIENTLY USE TFM
A radius of 1.0 degree is used for checking for bright stars contamination. As shown above, even extremely bright stars are not a problem beyond the 1 degree radius.
TFM can also be used to produce CFH12K finding charts (use a of radius of 0.4 degree). Access this fonction with this link.
TFM also runs in ``
Known problem: TFM fails if a pole (+90.0 or -90.0) is included in the field (projection problem).
EXAMPLE OF TFM OUTPUT
Comments on the CFH12K pages to Jean-Charles Cuillandre: email@example.com
``Telescope Field Mapping'' was developed by Frédéric Magnard (CFHT) and Jean-Charles Cuillandre (CFHT).
CFH12K is the CFHT CCD wide-field imaging mosaic.