Non-sidereal Observing with the LBCs

Procedure

The procedure for observing non-sidereal targets with the LBCs is similar to that for MODS and LUCI.

1. Upload the LBC script to the LBC user interface,
2. Before clicking “Play”, use the NSIGUI to “hijack” the target coordinates as described here. Use the “hijack” type 1, “Target override”.
3. Turn off guiding via the LBC user interface (see Figure 2 below) if the object is relatively fast moving. See LBC guiding when tracking non-sidereally for advice on deciding whether the rate is too fast for LBC guiding and a description of LBC guiding in non-sidereal mode.
4. In open-loop tracking (without guiding) LBC has observed non-sidereal targets moving at up to 1000 arcsec/hr, and much higher rates are possible.

Figure 1: To turn off guiding, in the LBC User Interface, on the OB Execution Page, uncheck the box next to “Guiding”.

LBC guiding when tracking non-sidereally.

The LBC guiding algorithm works by running source extractor on the first tech chip image to create a source catalog. When more than 20 stars are found, the catalog is limited to the 20 stars with the highest signal-to-noise. If no stars are found, the tech chip exposure time in increased and a new image taken; this repeats until stars are found or the tech chip exposure time is 32 seconds.

This first catalog becomes the reference catalog. On each subsequent tech chip image, source extractor searches for objects within a specified search radius (ASSOC_RADIUS, set to 10 pixels, or 2.25″) around each source in the reference catalog. After some time, t=ASSOC_RADIUS/rate, all of the guide stars will have moved beyond the search radius and either (1) no stars are found for guiding or (2) one or two stars have coincidentally moved close enough to the positions in the reference catalog that a spurious guide correction is made. This is illustrated in Figure 2, below, for a non-sidereal target moving at the rate of 65.8″/hr.

A general guideline is that:

1. For targets where the rate*exptime < 2″, guiding may be used.
2. But for targets with rate*exptime > 2″, guiding should be turned off. You can do this by unchecking the box next to “Guiding” on the Observation Execution page of the LBC User Interface (see Figure 1 above).

Figure 2: The image below illustrates how, as the stars drift away from their original positions at the non-sidereal tracking rate of 65.8″/hr, they are still detected so long as they remain within the 10-pixel search radius. But problems can arise when the stars have moved outside the search radius AND one or more stars drift into the search regions defined from the first tech chip image. The 1 guide star found in the tech chip image taken 113 sec after the first one (it fell into the search region circled in red) caused a spurious guide correction.