Finding Charts

Finder Chart Requirements

A Finder Chart should have the following:

• The primary target should be clearly indicated.  If the primary target is not visible (too faint or part of a diffuse object), then please consider using a blind offset and if possible setting the slit position angle so that there will be a brighter target also on the slit. On the finding chart, indicate the reference star and the target.
• The target name or OB name should be clearly indicated on the finder chart.
• North and East should be clearly indicated.
• The image should be rotated when possible such that the requested PA is “up”.
• The scale needs to be indicated by drawing a bar and writing the bar length in arcseconds or arcminutes.
• MODS acquisition images are usually taken using the red channel and the r_sdss filter, but the script can be set to obtain acquisition images in i_sdss or z_sdss, or for blue targets, using the blue channel and g_sdss or u_sdss. The image in the finding chart should use the same or a similar bandpass as the acquisition image. It’s not helpful to have a finding chart taken through H-alpha.
• The image should reach about the same depth as expected in an acquisition image at the LBT. In 30-sec, MODS reaches r_sdss ~21.
• The entire 6′ x 6′ acquisition FOV should be shown, though a zoom about the target is helpful for faint targets.

How to Generate a Finder Chart

For brighter targets, a finder chart can be generated using modsView with the –finder option.  By default modsView uses an image from the digitized sky survey, but if you have a FITS format image with a valid (and correct!) world coordinate system applied, you can use that image to create the finder.  Using this method creates a 12′ x 12′ FOV finder, which is very large compared to the 6′ x 6′ longslit acquisition field.  It is best to supplement this!  It is best to use an SDSS image when possible.  A 30 second MODS image is approximately the same depth as a SDSS image.

For faint targets, it can be useful to have charts that show the target in relation to nearby objects.  Ensure that whatever source you use to obtain your image, that your finder chart meets all requirements listed above.  Some images may clearly show the target, but if taken in an unsuitable wavelength or if no scale/orientation is shown, the clearest image will not be a suitable finder chart.