Use case: make a FITS header

Purpose: construct the header for an observation file.

Actors: (observer), ICS and TCS.

Outline: the system casues a number of FITS-header packets to be collected. It concatenates these into the header of the FITS file.

Typical course of events:

Action System response

1. The use case begins when the observer has caused an observation to take place.

2. The first exposure of the observation starts. System signals ICS and TCS to start making header packets. System specifies the packets to be collected and the cameras to which they apply.

3. TCS and ICS start collecting header data. System creates the standard part of the FITS header that defines how the image is stored.

4. Last exposure of the observation finishes. System signals TCS and ICS to complete their header collection.

5. ICS and TCS output their header packets into an agreed directory of the observing system. System reads the packets from the ICS and TCS.

6. The run number for the observation is known. System creates the header packet observation, and includes the run-number.

7. The telescope and instrument packets are available to the system. System creates the header packet WCS. If the instrument is an imager, the system defines right ascension and declination. If the instrument is a spectrograph, system writes a unit transformation (pixels to pixels) as there is not enough information to define world coordinates. System writes a comment into the packet explaining that this coordinate system is an approximation.

7. All required packets are available to the system. System copies the packets into the FITS header.

	Action	System response
1.	The use case begins when the observer has caused an observation to take place.
2.	The first exposure of the observation starts.	System signals ICS and TCS to start making header packets. System specifies the packets to be collected and the cameras to which they apply.
3.	TCS and ICS start collecting header data.	System creates the standard part of the FITS header that defines how the image is stored.
4.	Last exposure of the observation finishes.	System signals TCS and ICS to complete their header collection.
5.	ICS and TCS output their header packets into an agreed directory of the observing system.	System reads the packets from the ICS and TCS.
6.	The run number for the observation is known.	System creates the header packet observation, and includes the run-number.
7.	The telescope and instrument packets are available to the system.	System creates the header packet WCS. If the instrument is an imager, the system defines right ascension and declination. If the instrument is a spectrograph, system writes a unit transformation (pixels to pixels) as there is not enough information to define world coordinates. System writes a comment into the packet explaining that this coordinate system is an approximation.
7.	All required packets are available to the system.	System copies the packets into the FITS header.

Variations:

Either the ICS or the TCS or both do not respond to the request for header packets. The system replaces the missing packets with comment descriptors noting that the packets have been left out of the header.
Because of lost packets, system cannot determine the transformation for the WCS packet. System writes a unit transformation instead.