The following table lists overheads as determined during the LBC-Blue and -Red commissioning runs in 2006-2007 and 2007-2009 respectively (*) as well as from image timestamps.
Overheads for a single-filter OB amount to about 25% of the total open-shutter time.
|Telescope preset*||1-3 minutes|
|Collimation (dofpia or dohybrid)*||15 minutes at beginning of night
3-5 minutes after slews to new targets and every 30-40 minutes for longer programs
|Pointing/Co-Pointing correction||5 minutes|
|Sending OB to telescope*||10-15 seconds|
|Mirror preset (focus)*||4-30 seconds (longer at beginning of OB, shorter between exposures)|
|Camera preset*||~20 seconds|
|Exposure preset and post-exposure operations*||10-12 seconds|
|Image readout*||LBCB 20-22 seconds, LBCR 17-21 seconds (see below)|
|Filter change||7-24 seconds|
|Time between dithered exposures||~40-50 seconds|
The overheads for image readout in the table above come from the Commissioning Reports released in 2009 and are for a full-frame readout. For whatever reason, the detector overheads measured recently are slightly higher, but still higher for the Blue than for the Red channels. Several series of biases taken over the last year 2018-2019 lead to the following results for the full frame detector overheads.
When observing binocularly with LBCB and LBCR, and especially with exposures of unequal durations (for example, taking a long U-Bessel exposure and 2 shorter i-SLOAN exposures), taking into account the blue and red detector overheads can reduce the amount of ‘dead’ time.
|nrows||t_overhead(s) LBCB|| t_overhead(s)
Sub-windowing is often used for high cadence observations, e.g. for exoplanet transits. For the LBCs, only the number of rows can be restricted; the sub-window must cover all 2098 columns. See Setting up sub-windowing below for instructions on how to set up sub-windows in your LBC scripts.
Detector overheads for several different sub-windows are listed below. These were measured from a series of ~25 biases taken on 20190203, and the values in the table are the average values +/- the standard deviations. The values below are for all 4 chips being read out. But since each chip is read out independently, the overheads when reading out all 4 chips and only chip 2 are similar.
It appears that there is little advantage of setting up subwindows that cover fewer than ~1000-1500 rows, as at that point, irreducible overheads dominate.
|nrows||Y_extent (arcmin)||YMIN||YMAX|| t_overhead(s)
|4608||17.3||1||4608||34.2 +/- 1.5||26.8 +/- 1.8|
|3000||11.25||1416||4415||26.7 +/- 0.9||19.0 +/- 0.2|
|2000||7.5||1916||3915||22.2 +/- 0.5||15.5 +/- 1.6|
|1500||5.6||2166||3665||20.3 +/- 1.3||15.4 +/- 1.9|
|1000||3.75||2416||3415||18.9 +/- 1.3||15.5 +/- 1.3|
|500||1.875||2666||3165||18.9 +/- 2.0||15.5 +/- 1.4|
Setting up sub-windowing:
How to center the sub-window on the optical axis
The sub-window should ideally be centered on the rotator center, which is also the optical axis. The LBC scripts have always required that the same subwindow be used for both Blue and Red, so only the average value of the Y coordinate of the rotator centers on Blue and Red is used. This is 2916.
|Rotator Center LBCB (x,y) [pixels on Chip 2]||Rotator Center LBCR (x,y) [pixels on Chip 2]|
To center the sub-window on the optical axis, use the YMIN and YMAX values listed in the table above, or for a different size subwindow, calculate the YMIN and YMAX based on the Y value of the rotator center, Y=2916.
The LBCs can read out all 4 chips or only the central chip, chip 2; these are the only two options. The scripting tool creates a sub-window only for chip 2. If you would like to use all 4 chips with the subwindow, then please contact your ISA.