Use case: observe with a CCD photometer

Purpose: to do fast, time-resolved photoemtry.

Actors: observer.

Outline: the instrument is a dedicated CCD photometer with a frame-transfer CCD. The user sets up small (e.g. 20x20 pixels) windows over a few (<10) regions of interest. For each run, the user specifies an integration time, a number of integrations, and the time-spacing between the start of integrations. The system do the integrations as a shutterless, frame-transfer operation and delivers each integration into one plane of a data cube.

Related use-cases:

Standard sequences:

Typical course of events:

User action System response

1. The use case begins when the user has started up the TCS, DAS, CIA and ICS successfully. The entries below can be combined in any order and with any number of repetitions.

2. User defines the number of readout cycles in each observation. System remembers this number as the number of planes in a data cube needed to store each observation.

3. User asks for a glance run, stating the exposure time. When the data arrive, the user uses them to place the windows and choose a better exposure time. System sets up a file for a single, planar image. System closes the shutter, clears the CCD, preflashes, exposes and reads out. In parallel, system executes the sequence make FITS header and read and store pixel data, completing the FITS file.

4. User sets the details of each readout (binning etc.) by running the use case set readout format. Typically, three small windows are used, one covering the science object, one covering a comparison object and on covering part of the bias strip. System notes that these new detauils apply to all subsequent readouts.

5. User asks for a bias frame. System sets up a file for a single, planar image. System closes the shutter, clears the CCDs, preflashes, exposes reads out. In parallel, system executes the sequence make FITS header and read and store pixel data, completing the FITS file. DAS executes the sequence save to archive.

7. User asks for an observation stating the exposure time and title. System sets up to receive a data cube of the previously given depth. System closes the shutter and clears the detector once. System opens the shutter and repeats this sequence for each plane of the cube:

Waits for the given exposure time.
Moves the charge from the exposed pixels to a masked area of the chip which is not exposed to light.
Reads out the image stored in the masked area.
Executes the sequence read and store data, producing a FITS image.
Records accurately the start time and length of this exposure.
Operations 1, 3 and 4 happen in parallel. Given a suitable arrangement of windows, the readout/data-save time is less than 200ms (possibly as low as 10ms with suitable CCDs) and the camera integrates all the time for exposures longer than this. After the last readout of the series, the system closes the shutter. System executes the sequence make FITS header. Additionally, system writes the timing data to a FITS table following the FITS image in the observation file. System executes the sequence save to archive.

8. User repeats the steps above in any order.

	User action	System response
1.	The use case begins when the user has started up the TCS, DAS, CIA and ICS successfully. The entries below can be combined in any order and with any number of repetitions.
2.	User defines the number of readout cycles in each observation.	System remembers this number as the number of planes in a data cube needed to store each observation.
3.	User asks for a glance run, stating the exposure time. When the data arrive, the user uses them to place the windows and choose a better exposure time.	System sets up a file for a single, planar image. System closes the shutter, clears the CCD, preflashes, exposes and reads out. In parallel, system executes the sequence make FITS header and read and store pixel data, completing the FITS file.
4.	User sets the details of each readout (binning etc.) by running the use case set readout format. Typically, three small windows are used, one covering the science object, one covering a comparison object and on covering part of the bias strip.	System notes that these new detauils apply to all subsequent readouts.
5.	User asks for a bias frame.	System sets up a file for a single, planar image. System closes the shutter, clears the CCDs, preflashes, exposes reads out. In parallel, system executes the sequence make FITS header and read and store pixel data, completing the FITS file. DAS executes the sequence save to archive.
7.	User asks for an observation stating the exposure time and title.	System sets up to receive a data cube of the previously given depth. System closes the shutter and clears the detector once. System opens the shutter and repeats this sequence for each plane of the cube: Waits for the given exposure time. Moves the charge from the exposed pixels to a masked area of the chip which is not exposed to light. Reads out the image stored in the masked area. Executes the sequence read and store data, producing a FITS image. Records accurately the start time and length of this exposure. Operations 1, 3 and 4 happen in parallel. Given a suitable arrangement of windows, the readout/data-save time is less than 200ms (possibly as low as 10ms with suitable CCDs) and the camera integrates all the time for exposures longer than this. After the last readout of the series, the system closes the shutter. System executes the sequence make FITS header. Additionally, system writes the timing data to a FITS table following the FITS image in the observation file. System executes the sequence save to archive.
8.	User repeats the steps above in any order.

Variations:

A slower photometer may be made without using a frame-transfer CCD; a general-purpose CCD could be used. System operates this camera as above, but has to close the shutter during each readout. In this case, the cycle time between exposures is not guaranteed to be any better than 5 seconds.