MODS GS Selection
The MODS Patrol Field is a 5 x 5 arcmin FOV. MODS has its own integrated AGw unit. For both MODS1 & MODS2, part of the patrol field (the footprint of the pickoff mirror) is totally vignetted. This area is ~150 arcsec wide x 85 arcsec high surrounding a selected guide star. Field of view overlays in the Position Editor of the OT have been designed to help the user visualize this area. Guide stars should fall in the 11.5 to 16.5 magnitude range.
The OT has tools such as the Field of View overlays and Catalog Query Parameters in the Manual Guide Star Selection that will help the user select a suitable guide star. The following will discuss how to select and verify suitable guide stars.To start, bring up the field in the Position Editor and overlay the NOMAD or UCAC4 Guide Star Catalogs. This can be done 2 ways:
- In the Target Element, select the ManualGS button from the right side.
- Open the Position Editor by clicking the Image Button in the OT Main Toolbar. In the Position Editor, select the ManualGS from the Position Editor Main Toolbar.
This will bring up the Position Editor, along with the Guide Star Selection pop-up.
From this pop-up the user can select which catalog they desire to have overlaid, UCAC4 or NOMAD.
By default, all Viewing Options in the Position Editor Left Toolbar are selected. The position editor typically will show an overlay of the science field, guide patrol field, mask (if one is in place), any user added targets, etc. If these Field of Views are not overlaid if may be good to overlay them.
Once a catalog is selected from the Guide Star Selection pop-up, those will be overlaid with either green circles (UCAC4) or turquoise squares (NOMAD). Make sure that appropriate Catalog Query is selected in the Viewing Options in Position Editor Left Toolbar or else it may not be visible.
Note that an image of the field is not automatically brought up. An image of the field can be downloaded from an image server one of two ways. Hit the Images button in the Position Editor Main Toolbar to query the the default image server using the base coordinates of the observation selected in the Target Element. Or one can use the Catalog button to open the Catalog Navigator. Keep in mind that the catalog overlay may not align perfectly with the image. There is some distortion in the DSS images.
To add a Guide Star, activate the +MODSGS button. Click within a catalog overlay box. An overlay of the probe vignette area will appear over the selected Guide Star if it falls within the Patrol Field and the +MODSGS button is activated.
If the user clicks within the catalog overlay box, the catalog information will then be sent to the Target Element and recorded as a numbered MODSGS element. If the user clicked outside the catalog overlay, the coordinates of the clicked point will be recorded to the Target Element as the numbered MODSGS element.
There are 2 ways to do this:
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- In the Target Element enter all possible PA’s for the observing period. The PA will be reflected in the orientation of the Field of View overlays.
If no suitable Guide Star falls in the patrol field at the new PA, as the user cycles through the previously selected guide stars a red message will appear at the bottom of the Target Element that reads “This Star is not reachable at all positions”. Select Guide Stars that fall within the Guide Probe Patrol Field for each PA following the procedure described above.
Note that the others in the list will not be usable for this particular PA, and will show the error message, but the goal is to have only one usable Guide star at every desired PA for an observable night.
- In the Target Element enter all possible PA’s for the observing period. The PA will be reflected in the orientation of the Field of View overlays.
- Using the Drag Mode in the Position Editor, use the Position Angle Handle (turquoise triangle at top of the Science field) to rotate the field. Rotate the Field until the Guide Star(s) previously selected fall outside of the Guide Probe Patrol Field. Add new guide stars that pall within the Patrol Field using the method described above.
If this is a previously observed target or well known field the user may have a guide star in mind and can also manually enter it. With Target Element: Select New in the Target Element. In the drop down menu, select MODSGS to create a new guide star element in the Target Environment. You will need to manually enter a valid SIMBAD designation or the RA DEC for the Guide star.
