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Overheads

Overheads

For Phase I observing proposals, the information presented in this section can be used to estimate the overheads associated with a new science target. This information could be also useful for the Phase II planning of your observing programs. All of the values listed on this page are included in OT calculations of the execution time of an observation. However, only the actual overheads incurred at the time of the observation will be charged to the program.The detailed overhead information is presented below.


1. Telescope and GeMS/GSAOI setup time

Acquisition overheads associated with setting up on each new science target include the time for slewing the telescope, launching the GeMS lasers, acquiring the GeMS laser and natural guide stars, closing all loops, acquiring the GSAOI On-Detector Guide Window guide star, positioning the target within the GSAOI field-of-view, and configuring the instrument. The table below gives the estimates for acquisition overheads. For more detailed information about the acquisition overheads budget see the GeMS Acquisition and Overheads web page.


GeMS/GSAOI Acquisition Overheads
Telescope Setup, Laser alignment, GSAOI configuration, NGS alignment and closing all loops 30 minutes
Re-acquisition when the offset to the sky is larger than 5 arcmin from the target position 10 minutes (1)

 

(1) See Overheads associated to a sky sequence for details.

Applicants for GSAOI time should allow for one acquisition for approximately every 2 hours observed (including overheads).

The acquisition overheads listed above are based on average statistics from the last three GeMS/GSAOI commissioning runs and are included in OT calculations of the execution time of an observation.

2. Readout overheads

The table below lists the overheads associated with taking GSAOI images in the various read modes without telescope offsetting. These values include the time to read out the array Ncoadds times and to write the final FITS image.


GSAOI overheads for each image
Read mode Overheads
Bright Objects 21s + 5.6s * Ncoadds + 6.5s * (Ncoadds - 1)
Faint Objects/Broad-band imaging 21s + 22.4s * Ncoadds + 6.5s * (Ncoadds - 1)
Very Faint Objects/Narrow-band images 21s + 44.8s * Ncoadds + 6.5s * (Ncoadds - 1)

Filter change overhead

The overhead due to filter change without telescope offsetting is in average 15 sec (depend on the position of the filter in the wheel).

3. Dithering overheads

A normal GSAOI science observation sequence can include telescope offsets for image dithering and telescope offsets for sky background images. In both cases, some specific GeMS loops (Laser, NGS, etc) must either be paused or opened, and then resumed after the offsets. Note that the gaps between the four GSAOI arrays cause gaps in the imaging field. If continuous coverage of the field is needed, then multiple exposures have to be obtained at different dithering positions. A minimum dither step of 4 arcsec in the both X-direction and Y-direction (p and q-direction on the OT) is recommended. The overhead per dither step is 30 sec. This overhead is applicable when GeMS is working with 3 Canopus NGS stars and 1 ODGW star.

4. Overheads associated to a sky sequence

The overheads associated to a sky sequence can be different depending on how large is the offset to the selected sky region.

  • If the observation requires offsets to the sky shorter than 5 arcminutes from the base position, additional 60 seconds have to be accounted for the offset to the sky position. In this case, all NGS loops and dependencies are paused, and the probes are frozen (do not follow telescope offsets), but the LGS stabilization loop remain closed. Note that when the telescope is offsetting to the desired sky position, the same overhead per dither step on the sky (30 sec) is applicable.

  • If the observation requires offsets to the sky larger than 5 arcminutes from the base position, the laser propagation must be stopped due to Laser Clearing House restrictions. Moreover, the laser has to be re-acquired and the NGS acquisition re-checked when the telescope moves back to the base position. In this case, additional 10 minutes have to be accounted for re-acquisition of the field. The sky sequence is observed un-guided and the overhead associated to telescope offsetting is 10 sec. Note that for this case, a separate GSAOI observation sequence will be required.