Hi, I was listening to the discussion of wcsremap and hotpants, and I would like to give a couple of comments:
- Only deprojecting the template, but not the image (i.e. leaving the image 'pristine'): * If the image is not deprojected, it still contains higher-order distortions. One of the side effects of this is that the platescale is varying across the CCD. If your flattened image is physically flat (i.e. the sky background has the same number of counts everywhere), then this flattened image is photometrically not flat (since the pixel area projected on the sky is variable). So when you project your template onto the image, you have to make sure that the projected template has the same photometrically 'un'-flatness, just to ensure that the kernel sum is constant over the complete CCD. The difference image has then to be either deprojected, or a correction to the photometry has to be applied on the dB level. I don't know how much the platescale varies for LSST, but on the 4m it varies enough that the effect on the photometry is in the % range. * We did exactly that at the beginning of the Supermacho project: we 'remapped' the template to match the image. We had a failure rate of several %, and we never could really get it down. By using the WCS we completely eliminated this failure mode. I think one of the reasons is that one has to find the transformation of one disorted image to another distorted image. This is intrisically more difficult than having two single transformation to a tangential plane. Especially for LSST there will hopefully be a non-linear WCS solution that is based on the real physical distortions (e.g. it has radial terms etc), and not just a random polynomial representation. In that case it will add another layer of complexity to try to find the transformation from one distorted system to another (instead of from the distorted to a simple tan. plane) * In the Supermacho and ESSENCE project, we project ALL images of a given field to the same tangential plane. This has the big advantage that a given star is always at the same x,y position, and all images are physically and photometrically flat. Clearly, LSST cannot do it this way since the exposures are dithered, but I thought I still point it out. I think my bottom line is: I don't think the advantages gained by not deprojecting the images outweigh the trouble it causes. - oversampling the template One big reason we don't oversample is simply disk space: If both template and image are oversampled, the required disk space can easily double or quadruple. For the reasons stated above we opted to deproject both, the image and template. - in hotpants: why not use the complete stamp to determine the kernel, why substamps? besides the time issue: * There are two extreme cases: sparse and crowded fields: In sparse fields, you don't want to use too much sky, so it's better to use only subareas with significant flux. For crowded fields, the saturated stars are the problem: if your stamp is too big, then there is a very high chance that there is a saturated star in the stamp. If you use the complete stamp to determine the kernel, all stamps with saturated pixels have to be kicked out. In crowded fields, that can be the majority. If you have several substamps for each stamp, there is a very good chance that at least one substamp does not have saturated pixels in it. - hotpants failure modes: As Andy pointed out, on very rare occasions a satellite can make it fail. There are some cases where an extremely bright star is in the image. The halo of that star cannot be subtracted out correctly, and hotpants then tries to compensate that with the background and kernel. One solution to that is to have a list of very bright stars, and pass a mask that indicates to hotpants to not use any pixels within a big aperture around this bright star. The most common reason for not catastrophic, but low-quality difference images: * bad seeing. At CTIO in september (end of winter), it is not uncommon that there are nights with extremely bad seeing (>2.5"). These difference images are just ugly. Don't know if it is worth to look into that since these images are anyway of low scientific use. * lots of saturated stars, or a star cluster. Cheers, -Armin _______________________________________________ LSST-data mailing list [email protected] http://www.lsstmail.org/mailman/listinfo/lsst-data
