SkyView Blog

One of astronomy’s primary links to the past are the constellations. We’ve just added a set of overlay files that you can use with SkyView’s standalone package to draw constellation boundaries over an image. You can download individual constellation files, or download a tar file containing all of the constellation files. A constellation overlay can be drawn over all an image using settings like

java -jar skyview.jar position=0.,0. drawfile=all.bound survey=408mhz size=100 pixels=800

The image this produces is

The constellation boundaries used are derived from the official IAU constellation boundaries. However there are a couple of caveats. The constellation boundaries are defined in terms of corners in 1875 coordinates, i.e., only the 1875 declination or 1875 right ascension varies between two consecutive corners. SkyView transforms these corner positions to the user’s requested projection and then draws straight lines between the points. Unless the user has requested B1875 coordinates in a Cartesian projection, it’s unlikely that SkyView will match the precise boundaries. However if you are looking at an image at a scale large enough to see constellations, then discrepancies are likely smaller than the width of the borders.

Another problem can arise in projections that can tile (e.g., the Cartesian projection). E.g., if we are making an image centered at 0,0 then constellations spanning RA=180 degrees will have lines that are supposed to leave the left edge of the image and return on the right. Howver SkyView’s overlay drawer simply joins the two corners with a line the runs through the middle. Using non-tiling projections (like Tan on Sin) or only including constellations within 90 degrees of the center of the image will usually take care of this.

The Clip sampler is useful when users want to exactly preserve flux or to significantly undersample the data. Basically you can think of it as overlaying the new coordinate pixel grid on top of the old pixel grid and integrating over the new pixel boundaries assuming flux is uniformly distributed within each of the old pixels.

An area where the Clip sampler can have problems is in very low resolution surveys when a single input image covers the entire sky. E.g., suppose we have a survey (say, HEAO1A) whose input data is a single Cartesian image centered at 0,0. A user wants to generate an output image centered at 180,0. If the output image has 1 degree pixels, then the corners of its central pixel would be at the coordinates

179.5,-0.5 180.5,-0.5 180.5,0.5 and 179.5,0.5

While these are close together on the sphere, they will probably get projected to opposite sides of the input image — two on the far right and two to the far left.

The old version of the clipping sampler doesn’t handle this case properly. It sees a pixel that contains almost all of the equator.

A similar problem occurs when the input image is not convex. E.g., in the COBE maps the input image covers a sideways T in the projection plane. When we sample this image, the output pixel corners may project to a quadrilateral that contains area that’s outside of the valid region of the projection.

An updated version of the Clip sampler included with version 2.6 of SkyView begins to address these. Each input pixel that is included in the output will be checked to ensure that it is a valid pixel. Pixels that wrap around should also be handled for all of low resolution surveys that we have. Basically we assume that the user doesn’t want pixels > 240 degrees on a side and treat pixels that seem to be so large specially.

There are issues with the topology of the new TOAST and TEA projections that could cause problems if input surveys were stored in these projections, but since we have no such surveys, this is a moot point for the moment.

The new version of the sampler will also ignore any pixel in which one of the corners of the output image has coordinates that include a NaN. For example, if the output image is a large scale Sine projection, then one might see NaN data values at the edge of the circular valid region of the projection. This will now be 0.

We’ll probably include both the old and new Clip samplers in the initial releases of V2.6, but only the new sampler is likely to be available through the web page.

Occasionally we go back and take a look at where people have asked for images from SkyView. The following image gives the distribution of SkyView requests as a simple Cartesian (RA,Dec) map. The Galactic plane is very prominent, and even the Ecliptic shows up as an enhancement (even though SkyView has no solar system objects). The Magellenic clouds and M31 and many other prominent targets are clearly delineated. There remains a lot of structure whose origin is quite unclear.

This represents about 2.5 million requests since the June 2007. Requests for images in fixed projections (Aitoff, Cartesian, and TOAST) are not included since what we record are the CRVALn entries in the FITS files, and it is precisely these values that are fixed in those projections.

Due to a file system configuration problem some 2MASS images have been unavailable since December 17, 2008.  We are working to fix the situation and should have the 2MASS data back on line soon.  We apologize for the inconvenience.