The process of "building sequence files" has many different facets, the combination of which results in a proper instrument set-up and ultimately a successful HUT observation. It is somewhat unfortunate that the nomenclature developed over the years causes some intrinsic confusion as to what one means when one refers to a "sequence file." Hence, we attempt to clarify some terminology first and we will use this terminology consistently below.

When a science plan (or SCIPLAN) of potential observations is generated in the mission planning process, one has a basic set of information about each potential observation -- the length of the planned observing period, the day/night breakdown expected, a unique numerical identifier (or "sequence number"), etc. For each planned observation, HUT scientists must decide how to set-up the instrument and generate a sequence object load (also sometimes called an MMU load because they are loaded onto the Shuttle's Mass Memory Unit). This is an ascii file containing information in the proper formats and units to drive the HUT instrument into the proper configuration for an observation (when loaded into the HUT Dedicated Experiment Processor, or DEP, on orbit). The full ensemble of HUT sequence object loads forms a single MMU file called the HUT user file. Because the units of these parameters (as used by the DEP) are often not very user-friendly, we choose instead to have individual scientists work with sequence database files. These files contain a broad range of information on each target and/or each observation in a more user-friendly format.

Sequence database files, or SDFs, contain several different kinds of information from several sources. These include 1) information about the planned observation (coordinates, requested and scheduled times, etc.), 2) assorted reference information (magnitudes, fluxes, etc.), 3) information about potential guide stars for the HUT CCTV, and 4) information on the expected count rate and specific instrument configuration to be used. Items in category 1 come mainly from the SCIPLAN file, are entered automatically, and should not be changed by the user (i.e. they are placed in the file mainly for reference purposes). Items in category 2 are obtained from the literature or other sources and are entered into the file by the user. Guide star information is retrieved from the HST Guide Star Catalog (whenever possible) and entered automatically into the file, to provide a starting point for guide star selection. However, it is up to the user to actually decide which guide stars to use and edit the entries appropriately (see section 3.2). Finally, items in category 4 arise from an analysis of the expected count rate (usually obtained from a reasonably high fidelity simulated spectrum) plus an input of scientific judgement about what is required from the observation. To assist with this process, a manila folder for each object is also kept on file; these folders, labelled with the 4-digit target ID number and the target's name, are used to store the guide star plots and overlays, reference information on each target, hard copies of your simulated spectra (if appropriate), and any other useful information or notes on the target.

Hence, as a user, you will see and be working with SDFs. But it is important to realize that these files are a "means to an end", which is the actual sequence load that drives each observation. Because of the importance of getting these files right (note: important for instrument safety as well as getting your observation!), we generally have a massive group review of each SDF followed by detailed checks of the generated sequence object loads before generating the actual HUT user file.

The process of filling in the SDFs is an iterative one because all of the information is not available simultaneously. As soon as a SCIPLAN is finalized, "template" SDF files are created for each planned observation; these are placed in a sub-directory of the "rp" account on our "hut4" computer. These files are stuffed with the appropriate entries from the SCIPLAN for your reference. Also, sequence numbers are assigned to each observation (increasing sequentially through the planned timeline) and the HUT team member responsible for updating each file is assigned. This latter assignment is important because the permissions are set so that only the "responsible" party can edit a given SDF (except for times when a superuser stuffs additional information into the files -- see below). As soon as these files are available, users can begin filling in any supporting information and checking coordinates, etc. In parallel, users can generate simulated spectra of their objects using our "HUTSIM2" program, both to determine the proper instrument set up to use (which slit, door state, etc.) and to assess the quality of the data to expect and the expected count rate. When the information on potential HUT guide stars is available, this will be placed into the SDFs by a superuser. Users then assess the guide star situation, select guide stars, and choose the "locate mode" for the observation. One can then fill in all SDF entries that drive the instrument set-up. Finally, when actual IPS roll angles are returned from MSFC, these are entered into the SDFs by a superuser. One can then do final checks on the availability of guide stars, whether roll angle conditions (if any) were met, and make any other adjustments. When all SDFs are finalized, a separate program is run to generate the HUT user file for the MMU.

It is important to have a SDF for each potential observation where the HUT instrument is expected to be active. This includes all WUPPE and UIT observations, as well as planning ahead for contingencies (such as dwarf novae or other variable targets that may need to have alternate SDFs available). Also, separate files are needed for multiple observations of the same target since each pointing could (potentially) be set-up in a different manner. Observation planning is a lot of work in the wonderful world of Astro, and this is one of the main activities where substantial help from Guest Investigators is required.

In the sections below, we describe observing configurations in detail, and provide the necessary information for filling in SDFs.