Guider system model

Paul Lotz

Contributions from

• Systems engineering meeting discussions• Chuck Claver document• Pat Hascall diagram• Gregory Dubois-Felsmann diagram• Additional conversations with Chuck Claver

and Brian Selvy

Chuck’s documentGuiding in LSST: A functional Concept The observatory guiding function is performed by the Telescope Control System (TCS) by making adjustments to the azimuth, elevation and Camera rotator tracking rates based on optical feedback derived from the changes in the measured position of stars at the focal plane. Because of the LSST’s optical design, namely the wide field-of-view (FOV) and fast f-number, the guide sensors reside behind the Camera shutter at the system focal surface. The term “guider” refers to the hardware/software system used to provide the TCS with the necessary optical feedback. There are a total of 8 guide sensors; 2 on each of the 4 corner rafts located at the periphery of the FOV. The guide sensor sensitivity requirements are similar to those for the science sensor requirements. The positional architecture of the guide sensors is described in the OSS. The architecture of the physical hardware for the guider resides within the Camera subsystem domain. It is proposed that the software used to select appropriate guide stars and compute the guider output signals also be implemented within the Camera subsystem domain based on the functional description that follows. Guide star selection will be based on a catalog provided by the Telescope & Site team. This catalog will be derived from an existing “bright star” catalog (e.g. Harvard SAO Catalog) that has been filtered for stars that are deemed to be acceptable. The criteria for guide star acceptability are still TBD, but will include brightness, spectral type, crowding, etc… The acceptability criteria will be filter dependent, thus there will be separate catalogs for each LSST filter. The TCS mount model ensures that the relationship between the visit RA-DEC at the field center to a fixed point on the focal plane array (FPA) is stable to the uncertainties of the pointing model (specified in OSS-REQ-0298 as 2 arcsec RMS). This allows a relatively straightforward conversion between RA-DEC and FPA pixel coordinates and back again based on the distortions in LSST’s optical prescription, the FPA layout, the position of the telescope and atmospheric refraction at the mid-visit field elevation. Project Systems Engineering will coordinate the delivery of a model for this conversion to the Camera team. The Observatory Control System (OCS) provides to the Camera the pointing/bore-sight coordinates (RA, DEC, EPOCH), Camera rotator angle and Zenith Angle for each visit. Receipt of this information initializes the guider in the Camera. This information is used by the Camera to determine which guide stars are available at each of the 8 sensors. This is done by converting guide sensor pixel coordinates to RA-DEC and selecting acceptable guide stars on each sensor from the provided catalogs. The Camera selects a single optimum guide star from the list of available guide stars. This selection is based on where the guide stars fall on the sensor segmentation, predicted Signal-to-Noise-Ratio and sampling rate. Once the guide star selection is complete the Camera will report back to the OCS/TCS the following information from each of the i=8 guide sensors:

CATALOG ID PREDICTED PIXEL COORDINATES [X0(i), Y0(i)] PREDICTED SNR SAMPLE RATE

Publishing of information to the OCS/TCS indicates the guider has been successfully configured and initialized. During each exposure, upon illumination of the guide sensors, the Camera will provide the TCS 8 streams of computed quantities – 1 from each guide sensor – at the specified sample rate (9Hz minimum. At a minimum these quantities include:

FLUX(i) in either electrons or counts

Gregory’s whiteboard sketch

Pat’s diagram

Guider functional system model

• https://confluence.lsstcorp.org/pages/viewpage.action?pageId=11993563