MAGPIS: Help for GPS Cutout Extractions

The GPS cutout page allows you to extract a section of an image from the VLA Galactic Plane Surveys at multiple frequencies. See the MAGPIS home page for a brief description of the surveys and the publications page for papers with detailed survey descriptions and analysis.

Cutout Extraction Form Parameters

The form allows you to specify the survey, the position and size of the extracted image, the type of the returned image (FITS or GIF), and, for GIF images, the intensity scaling used to display the image. There are forms for both single-band image extractions and for multi-band color image extractions. Use the link underneath the Submit button to access the color and non-color versions of the form.

Survey
For single-band (non-color) cutouts you can select one survey from which an image is extracted. For multi-band (color) cutouts you can select images from three different surveys for the red, green, and blue bands. The multi-band images are resampled to match the highest resolution image of the three bands being used.
Images are available from many different Galactic Plane Surveys:
- GPS 6cm — 5 GHz C-configuration VLA images of the Galactic plane. These are the re-reduced images from White, Becker & Helfand (2005) based on the data originally published by Becker et al. (1994). This covers the region
```
|b| < 0.4°  350° < l < 42°
```
  The images have a 1 arcsec pixel size.
- GPS 6cm Epoch 2 — 5 GHz B-configuration VLA images of the Galactic plane. These are images from archival VLA data acquired in March-May 2005 (program AH180). This covers the region
```
|b| < 1.0°  28° < l < 33.5°
```
  although the covered region is not rectangular in Galactic coordinates. The images have a 0.5 arcsec pixel size.
- GPS 6cm Epoch 3 — 5 GHz B-configuration VLA images of the Galactic plane. These are images from archival VLA data acquired in July-September 2006 (program AH884). This covers the region
```
|b| < 1.2°  20.5° < l < 49.5°
```
  although the covered region is not rectangular in Galactic coordinates. The images have a 0.5 arcsec pixel size.
- Old GPS 20cm — 1.4 GHz B-configuration VLA images of the Galactic plane. These are the re-reduced images from White, Becker & Helfand (2005) based on the data originally published by Becker et al. (1990), Zoonematkermani et al. (1990), and Helfand et al. (1992). This covers the region
```
|b| < 0.8°  340° < l < 120°
|b| < 1.7°  350° < l <  40°
|b| < 2.2°  100° < l < 105°
```
  The images have a 1 arcsec pixel size.
- New GPS 20cm — The new multi-configuration (B,C,D) 1.4 GHz VLA survey of the Galactic plane (Helfand et al. 2006, AJ, 131, 2525). This covers the region
```
|b| < 0.8°    5° < l < 48.5°
```
  The images have a 2.0 arcsec pixel size. These images are much higher quality than the old 20cm GPS images, especially in regions where there are bright extended radio sources such as H II regions, but they cover a more restricted area of the plane.
- GPS 90cm — The 0.385 GHz VLA survey of the Galactic plane. This combines VLA archive data from a new multi-configuration survey by Brogan et al. (2005, in preparation) with our own observations. This covers the region
```
|b| < 2°    3.6° < l < 33.2°
```
  The images have a pixel size of 6 arcsec over the range 3.6° < l < 20° (where they come from B+C+D-configuration archival data) and of 15 arcsec over the range 20° < l < 33° (where they come from our C-configuration observations).
- MSX IR — 21 µm infrared images of the Galactic plane from the Midcourse Space Experiment (MSX) (Price et al., 2001, AJ, 121, 2819). The MSX survey covers the entire Galactic plane for |b| < 5°, but the images on the MAGPIS server cover only our radio survey region
```
|b| < 0.8°  340° < l < 120°
|b| < 1.7°  350° < l <  40°
|b| < 2.2°  100° < l < 105°
```
  The images have been resampled to a grid matching the Old 20cm GPS and so have a 1 arcsec pixel size. Note that the MSX resolution is about 20 arcsec, considerably poorer than our radio images.
- MSX IR (New GPS grid) — These are the same 21 µm MSX infrared images but have been resampled onto a grid matching the New GPS 20cm survey. They have a 2.0 arcsec pixel size and cover the region
```
|b| < 0.8°    5° < l < 32°
```
  These may be useful in making direct comparisons between the New GPS 20cm and IR data.
- GLIMPSE 3.6, 4.5, 5.8, 8.0 µm — 3.6 to 8 µm infrared images of the Galactic plane taken with the Spitzer Space Telescope IRAC camera from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE). We have resampled the standard GLIMPSE images to match the New GPS 20cm survey. That degrades the GLIMPSE resolution slightly (increasing the pixel size from 1.2 to 2.0 arcsec) but makes comparison with the radio images more straightforward. The boundaries of the images are limited to the region of the northern Galactic plane currently released by the GLIMPSE project:
```
|b| < 1.0°  9.95° < l < 49.05°
```
- MIPSGAL 24 µm — 24 µm infrared images of the Galactic plane taken with the Spitzer Space Telescope MIPS camera from the MIPSGAL Survey. We have resampled the standard MIPSGAL images (downloaded in January 2009) to match the GLIMPSE pixel grid. The approximate area covered is:
```
|b| < 1.0°  4° < l < 50°
```
- ATLASGAL 0.87 mm — The APEX Telescope Large Area Survey of the Galaxy, a 870 µm survey of the Galactic Plane made using the LABOCA bolometer array at the APEX telescope in the Chilean Atacama Desert. This covers the region
```
|b| < 1.5°    280° < l < 60°
```
  The images have a pixel size of 6 arcsec, a resolution of 20" FWHM, and typical rms noise levels ranging from 40 to 50 mJy/beam. Images are available for download from the ATLASGAL server.
- Bolocam GPS 1.1 mm — The Bolocam Galactic Plane Survey (BGPS), a 1.1 mm continuum survey of the Galactic Plane made using the Bolocam instrument on the Caltech Submillimeter Observatory. This covers the region
```
|b| < 0.6°    349.4° < l < 90.5°
```
  (There are additional images available from the BGPS web site covering a larger latitude range for a few fields in the outer Galaxy.) The images have a pixel size of 7.2 arcsec, a resolution of 33" FWHM, and typical rms noise levels ranging from 30 to 60 mJy/beam.
RA, Dec & Equinox or Galactic Longitude & Latitude
Specify the central position of the image in Galactic, J2000 or B1950 coordinates. The positions can be in a variety of formats; for example, acceptable formats include
- hh mm ss.s dd mm ss.s
- hh.hhh +dd.ddd
- hh mm.mm -dd mm.mm
- hh:mm:ss.s dd:mm
and variations on these, where hh, mm, and ss indicate the hours, minutes and seconds of RA and dd indicates degrees of Declination. If the RA and Dec are not fully specified to the seconds in the combined RA/Dec field then either a sign must be given for the Declination (even if it is "+") or colons must be used as field separators. For fully specified RA and Dec values or separate RA/Dec fields, the sign is optional.
If Galactic coordinates are specified, both l and b should be in degrees. They can be expressed in any of the formats described above.
For expert users that are writing scripts to access the GPS search engine, the combined RA+Dec or l+b field is passed in a parameter named "RA". If the RA and Dec are available as separate strings then they can be passed in the separate CGI parameters "RA" and "Dec".
Image Size
A square image of the specified size (in arcminutes) is extracted from a GPS image. A typical 5 arcmin image will be 167x167 pixels in size. The corresponding GIF image will be about 18 kbytes; a FITS image will be about 121 kbytes. (See below for information about image formats.)
The largest cutout that can be extracted is 1024x1024 pixels (about 30x30 arcmin). Large images may extend past the edge of the survey area, in which case part of the image will be blank. The returned images are in Galactic coordinates with latitude on the vertical axis and longitude on the horizontal axis. Note that, as is typical in astronomical images, the longitude increases from right to left (opposite the usual direction for the x-axis). The latitude increases from bottom to top.
Image Type
There are 3 choices for the format of the returned image:
- GIF — The image will be displayed within your Web page. This is useful for visual examination, but the resulting image cannot be used for scientific analysis. Some useful information about the image is also included.
- FITS Image — The image is returned to the browser with a MIME type of 'image/x-fits'. You will need to configure your browser so that it can display a FITS images. Note that xv is not a very useful FITS viewer for these images because they have a high dynamic range. Saoimage or FITSview are better choices.
- FITS File — This option also produces a FITS file as output, but the file is written to your disk (in a location you select) rather than being displayed by some image display helper. This is the best way to get an archival copy of your cutout.
When GIF format is selected, the image displayed on the web page is linked to a FITS version of the image. Clicking on the GIF image will download a FITS file to your disk.
The FITS formats are not available for color multi-band images.
Maximum Intensity for Scaling
If the image is returned as a GIF image, this parameter determines the contrast enhancement. GPS images have large dynamic ranges, often a factor of 1000 or greater. A linear mapping of color to pixel value often allows only the brightest objects to be visible. This parameter sets the saturation level (above which all pixels appear white) in milliJanskys. If the objects of interest and the sky background appear too faint, reduce this value to a smaller number. If this field is blank, the image will be displayed with a scale set by the brightest pixel in the image.
The maximum intensity is in units of mJy for the 6cm and 20cm radio surveys and for the GLIMPSE 8µm survey, but is in multiples of 100 mJy for the 90cm survey and the MSX infrared survey. The 90cm scale factor is chosen so that extended non-thermal sources (F(nu) ~ nu^-0.5) look comparably bright at 20cm and 90cm, while the MSX and GLIMPSE scale factors are chosen so that typical thermal sources appear with comparable brightnesses in the 20cm and IR bands.
Note that the actual image intensity units are W m^-2sterad^-1 in the MSX FITS files and MJy sterad^-1 in the GLIMPSE FITS files; these units are converted approximately to Jy for the GIF image display.
There are no options to scale the intensity for color multi-band images. Currently only a single default automatic scaling is available.