Introduction

The telescope's pointing origin is the physical position within the focal plane corresponding to the currently selected right ascension and declination. Having different pointing origins allows you to specify that a certain object should be placed in the center of the DEIMOS detector field-of-view rather than within the field-of-view of the offset guider, for example. The name of the current pointing origin is always displayed in the lower right corner of the Facilities Summary (FACSUM) window. The pointing origin can be changed by the OA or by using the poname command on polo.

Notes:

• XIM and YIM denote the location of the pointing origin in the telescope focal plane coordinate system.
• XCCD and YCCD denote the approximate location of the pointing origin in the DEIMOS science mosaic coordinate system.

Pointing origins and mosaic extensions

This section illustrates the location of some of the most relevant DEIMOS pointing origins as well as the naming of the different mosaic extensions.

The next figure shows a DEIMOS direct image of the open cluster M67 taken with de DUAL:A+B amplifier mode. The approximate location of the LVM slit pointing origins for the 0.7", 0.8", 1.0", 1.2" and 1.5" slits is represented by cyan croses. The yellow cross shows the location of the longslit pointing origin. The green crosses represent the direct imagin pointing origin and the MOS slitmask center pointing origin.

Additionally, the image shows, from top to bottom:

• Green numbers represent the consecutive number of the fits extension. Since the image was taken using both amplifiers, A and B, for each CCD, and there are four CCDs, then we have a total of 8 extensions. Each CCD is separated from the adjacent ones by the thick solid red vertical line. The amplifiers of each CCD are separated by vertical red dashed lines.
• Yellow numbers represent the value of the keyword AMPLOC. The name of each extension in the CCD mosaic is video_input_1, video_input_2, etc. Note that the video board numbering is flipped for each of the CCDs.
• The name of each CCD is shown in red.
• Cyan markers at the bottom of the CCD represent the name of each amplifier, e.g. 2B is the amplifier B of CCD2.

Important note (Jan, 2018): Normally, the amplifier mode DUAL:A+B is used in direct imaging mode, because the readout is faster. This is particuarly important for mask alignmnent in MOS mode. However, we are experiencing high bias noise in amplifiers 2A and 3A. Therefore, we strongly recommend to use the amplifier mode SINGLE:B for imaging observing programs. The commands oneamp and twoamp can be used in any of the DEIMOS server terminals to switch between one amplifier modes.

Image of the open cluster M67 where the different pointing origins are shown as well as some of the key names used in the FITS extensions to define the mosaic.

Location of science spectrum in longslit spectroscopy

This section illustrates the location of the science spectrum when a target is acquired in the longslit spectroscopy pointing origin.

The left image below shows a flatfield taken with the 1.0" long slit. In the image shows the CCD gaps and the bridges between the slitlets forming the full long slit. The right image shows a standard star slitless spectrum. The approximate location of the PO with a yellow circle in both figures.

The nearest CCD gaps to the PO are located 1024 pixels to the left and right of the PO. Since the DEIMOS plate scale is 0.1185 "/pix, the nearest CCD gaps are located 2 arcmin away from the long slit spectroscopy PO. The gaps are 8 arcsec wide. Additionally, the longslit is made of 12 slitlets, 1.37 arcmin long each, with small bridges (about a couple of arcsec wide) between them.

Internal flat taken with the 1.0" longslit. The approximate location of the long slit pointing origin is represented by a yellow circle.	Slitless standard star spectrum taken with the 1.0" longslit. The position of the standard star spectrum is indicated with a yellow circle.