ASTRO 480- Finding Charts

Introduction

The absolute pointing accuracy of a given telescope can vary a great deal, from ~arcseconds to many arcminutes. However, in almost all cases, when you observe you never just rely upon the pointing precision of the telescope plus the coordinates of your object to locate it in your field of view. Instead, astronomers use finding charts, i.e. A small portion of the sky, showing the general field of stars and with the target indicated- then you just have to get good at pattern matching!! You have already seen two kinds of "finding charts" (M92 and GS1826-24), which use actual data taken at the telescope. Indeed, for many interesting targets, the discovery paper will include such a finding chart to guide future observers to the right star, hence the first step is to track down such charts in the literature. Nevertheless, if your target is not too faint, or there is no such published chart, you may resort to generating your own chart using '"Skyview" (complete sky coverage) or for Sloan sources, the SDSS imaging data itself.

Web resources

  1. Skyview- http://skyview.gsfc.nasa.gov/skyview.html

Instructions

  1. From the Skyview home page, go to the Basic link under Interfaces

  2. Type in the coordinates of your object

  3. Make sure the right Equinox is set, under Optional Parameters!!

  4. Click on "Digitized Sky Survey" and "DSS2 Red" and "DSS2 Blue" in the lower leftmost column to request a frame obtained from all the existing digitized photographic plate data (either the first and/or second generations).

  5. Select color table = B-W linear, so that it will print okay greyscale.

  6. Click on the "Submit" button

  7. Skyview will bring up a new browser window with the requested image. You can download a FITS file by clicking on the button below the image.

You may then choose to adjust the field of view to something most appropriate for the instrument you'll be using- use the "image size" option.

Note that you can also see how your source looks in X-ray and infrared wavelengths, etc. It's interesting to browse around!.

Refer to the online help for further assistance.

  1. Sloan Digital Sky Survey- http://das.sdss.org/auxcgi/dr1/fc

  1. Again enter the coordinates of your object, but this time you will need the RA and Dec in decimal degrees. There is another online utility that will convert from sexagesimal to decimal formats:

    http://heasarc.gsfc.nasa.gov/cgi-bin/Tools/convcoord/convcoord.pl

  2. Set your image size, and this time you can choose your resolution too.

  3. You may or may not wish to overlay Sloan sources on to your chart ("Objects to mark"), and similarly also extract the Sloan catalog data for this field ("Display Catalogs").

  4. Select both the FITS and colour composite ps file.

  5. Under "Image Options", the North/East arrows and scale markings are always very useful; you can choose whether to show a grid.

  6. Click on the "Submit Request" button

  7. Lastly, when the finder chart page loads, you'll probably simply want to save the postscript version (fc.ps) to your local disc to then print. Alternatively, at the interface stage you can opt to just produce and image in either FITS, ps or JPEG format.

Again, it's fairly self-explanatory and there is a basic 'help' available.

I suggest you play around with both of these interfaces (just find images of any astronomical object you;'re curious about), to familiarize yourself with how they work (it shouldn't take long).

Here's and example of a galaxy, from DSS2 Red:


From Sloan: