SALT - Southern African Large Telescope

Considerations for MOS Observers

The following notes were kindly provided by Eric Hooper (University of Wisconsin).

Essential note

Slit masks must be designed with a position angle that is accessible to the telescope.  The range of accessible position angles depend on the target's declination.  It is easy to mess this up and design a mask that is unusable!  Observers must read and understand the "SALT Position Angle Visibility" document (SALT-3170AM0009) by Ken Nordsieck, especially the figure.  Be sure to get version 1.1 or later of this document.

 

Selecting Setup Stars

Three setup stars is the absolute minimum, but picking between 5 and 7 is strongly recommended for the following reasons:

  • Some setup stars can prove to be unusable for the following reasons:
    • They fall too close to, or inside, the chip gaps.
    • They are too close to other stars. 
    • They fall too close to the slitlets in the image of the slitmask. 
    • A cosmic ray may land near a setup star. The image can be repeated of course, but this takes time. 
    • The centroiding algorithm is not yet entirely stable, and sometimes a setup star is not found for unknown reasons.
  • A very large number of setup stars will likely be counterproductive because:
    • Real estate on the slit mask will be lost to science targets.
    • It will likely be harder, and hence more time consuming, for the SA to identify a large number of stars.
    • The SA will have to check whether each of the stars was properly centroided and eliminate those which weren't from consideration, which takes time.

The optimal magnitude range for setup stars is still under investigation. Preliminary indications are that stars in the magnitude range of SDSS r' ~ 14 - 18 work pretty well. Stars as faint as r' ~ 19.5 have been successfully centroided in a 20 second exposure, but only toward the center of the field; a star of this brightness near the edge of the field failed to centroid properly. Avoid stars that are bright enough to saturate RSS in whatever exposure time is needed to adequately expose the faintest setup stars. Don't use setup stars that are saturated in the image used to design the slit masks in the first place, as the RSMT positions for these stars will be less accurate. Some observers have had trouble finding a sufficient number of sufficiently bright, yet unsaturated, stars when using extremely deep images to design their slit masks.

Pick stars distributed widely about the field, but either avoid the outer edge of the field, or pick brighter stars there. Avoid the gaps between the CCDs. Avoid picking setup stars that have other objects, such as stars or galaxies, near them, especially if these other objects have comparable brightness to the setup star. The minimal acceptable distance as a function of magnitude difference between the setup star and a nearby object is not well understood at this point. As the centroiding becomes more robust and flexible, this limitation should become more relaxed. Avoid placing the setup stars near slitlets. The mask must be imaged to determine the precise location of the setup star holes, and the presence of a slitlet close to the setup star hole can confuse the centroiding algorithm.

Preparing MOS finding charts

  • Double check the position angle to make sure that it is observable given the declination of the target. 
  • It is probably easiest, and least prone to mistakes and confusion, to produce a finding chart with the conventional orientation of north up and east to the left, rather than one which tries to reproduce the appearance of the field on the CCD. The latter is complicated by the fact that the default position angle is 0 in the southern part of the sky and 180 in the northern part.
  • The position angle must be indicated on the chart. It is worth being explicit about this, either exclusively with words or with a combination of words and an arrow. For example, "MOS mask designed for PA = 39.6 degrees." Don't use a straight line as a graphical element to indicate position angle, as there is a 180 degree ambiguity in a line; be sure to attach an arrow head on the correct end of the line to break this ambiguity. 
  • Indicate setup stars using symbols (e.g., circles) that are specific to the setup stars. If there is any ambiguity as to which symbols indicate setup stars, explain with some brief text in an out-of-the-way location of the finding chart.
  • Indicating the locations of the slitlets on the primary finding chart may make it too cluttered. Rather, if an observer wants to include this information, it may be better to submit a secondary finding chart with these elements included. The RSMT program can save the MOS setup image with accurate representations of the slitlets and setup stars indicated. Note that the default RSMT setting will include lines denoting the extent of the spectra from each slitlet, which will make the image way too cluttered for a finding chart. To make a useful secondary finding chart, the observer can temporarily adjust the "width" field in the "spectral format" section under the "Validator" tab in the RSMT. Either make it zero to display only the footprint of the slitlet on the sky, or for greater visibility without too much clutter, set it to a small value of 50 or 100 pixels. Adjust the zoom level of the RMST to include enough slitlets to be useful for identifying the field.

Other Handy Tips

  • Enter the coordinates of the center of the MOS field into the PIPT, rather than the coordinates of the highest priority target or the first in a list. This will make acquisition faster. 
  • Observers should endeavor to make high-quality final masks and make the phase II proposal truly final before finalizing it in the PIPT, thereby avoiding the need to have an SA unfinalize it so it can be resubmitted with modifications. Currently there is a bug in the system whereby the barcodes of MOS masks change each time the proposal is resubmitted and refinalized. If a mask is cut with one barcode, and the science database has a different one, chaos that is not hilarious will ensue.