9.1 Introduction
The preliminary design of the DEIMOS software and data handling system is still in progress and will not be reviewed at this November 15, 1994 DEIMOS PDR. A separate software PDR will be held when the preliminary software design is complete, and that is currently scheduled for mid-May 1995. There are several reasons for this:
First, the UCO/Lick software manpower available to work on DEIMOS has been constrained by demands of other UCO/Lick and Keck instrumentation projects, which are only now starting to wind down. As a result, the DEIMOS software design effort to date has been extremely limited, although this effort will be ramping up as current projects ramp down.
Second, several pivotal software design choices cannot yet be decided. For example, CARA has proposed that the EPICS system (a distributed instrument control system in widespread use in high energy physics and planned for use by several major telescopes, including Gemini) be used for the Keck II telescope Drive and Control System (DCS). CARA are suggesting, although not requiring, that EPICS also be used for control of Keck II instruments such as DEIMOS. To promote the use of EPICS for Keck II instruments, the Project has invited the instrument software development teams to a CARA-sponsored EPICS training session the week of December 12. The decision on whether to use EPICS for DEIMOS awaits that session.
Third, based on the experience of software development for both HIRES and LRIS, it is important to avoid ramping up the software development effort too quickly, before the rest of the instrument design has stabilized to the point where the software functional requirements can be defined, and to conserve software manpower for the later phases of the project, where it will be most needed. This is and has been a fundamental assumption on which the current software budget and schedule is based.
Accordingly, rather than present an incomplete skeletal design for the DEIMOS software and data handling system, this chapter will provide a progress report on the current design efforts, and will:
9.2 Data Flow and Data Rates
The default data system described in Chapter 4 is essentially a scaled-up of the HIRES system, but using second-generation UCSD Controllers. The following discussion assumes a 2 x 4 mosaic of three-side-buttable 2K x 4K CCDs. Each chip has 2 readout amplifiers located at opposite ends of a single 2 kilopixel serial register, which is split in the middle. During image readout, 1024 image pixels per row will be clocked into each readout amplifier. If prescan and overscan pixels are included, there will be about 1060 pixels per amplifier per row. Thus, for full-frame readout, each readout amplifier will see 4096 rows x 1060 pixels per row, or 4.34 megapixels.
For reference, the per-pixel readout time for the HIRES CCD system is 33.5 microseconds in single-amplifier readout mode. This is adequate to ensure our minimum performance spec of 120 sec to read all chips on DEIMOS. However, for the purpose of calculating the required bandwidth for the component data paths, we assume a per-pixel readout time of only 10 microseconds to allow for the possibility of reduced multiplexing and for improvements in the performance characteristics of the CCDs and/or the UCSD system, which seem probable. As noted in Chapter 4, 10 microseconds is our new readout goal.
There are 16 amplifiers per dewar. For a full-frame readout, each amplifier will produce a data stream of 4.34 megapixels, yielding 69.44 megapixels per image. Since each digitized pixel produces 2 bytes of data, the image from each dewar will contain 138.88 megabytes (MB). Given the 10 microsecond per-pixel read time, each amplifier has a data rate of 100 kilopixels sec-1, or 200 kilobytes sec-1. Since all 16 amplifiers in each dewar will be read simultaneously, the aggregate data rate per dewar is 3.2 MB sec-1, or 6.4 MB sec-1 from both dewars.
A block diagram of the data system was shown in Figure 4.1. As noted there, the data rates reflect a 20 microsecond readtime per pixel and hence should be multiplied by two to meet our more stringent current goal of 10 microseconds. The exact number of analog boards and other components will also differ, since Figure 4.1 refers to the first-generation UCSD Controller whereas our plan is to use the second-generation Controller. Regardless, the major system components will include:
With the UCSD CCD Controllers, each timing board transmits its data to the Control Room VME Crate via a pair of fiber-optic cables contained in the telescope cable wrap. The timing board/fiber cable throughput of the current UCSD system is about 3.3 MB sec-1, while the aggregate data rate from each dewar is 3.2 MB sec-1, so capacity in this link is just adequate. With the second-generation UCSD system, the corresponding throughput will be higher, about 5 MB sec-1.
The VME Crate connects to the Instrument Computer via an industry-standard interface over a fiber link. There will have to be two such links rather than the one shown in Figure 4.1 to accommodate the goal of 10 microsecond readtime per pixel.
9.3 Data Storage, I/O, and Display
The video display of a mosaic of 8 large CCDs is challenging. Fortunately 2Kx2K monitors are just now becoming available. A full-up scheme might utilize a mosaic of 4 of these, with 2x2 pixel binning, plus a separate monitor that would show a blowup of a subregion at full scale. Or we might use 2 side-by-side monitors to show just the upper or lower portions of each detector, or possibly just one monitor with rapid zoom and pan. The video display problem is discussed further in the next section.
9.4 Major Areas Requiring Further Research and Evaluation
As we do not yet have any direct experience with this class of disk drive, we propose to:
9.4.2 High-Bandwidth Network Technologies
9.4.4 Instrument Control Environments: KTL/Music and KTL/EPICS
We plan to investigate EPICS further later this year, and to attend the training session provided by CARA. We anticipate spending several weeks to a month on this initial evaluation prior to the software PDR. If a clear and convincing case can be made that the benefits of switching to EPICS outweigh the costs (which are non-negligible), then we will likely switch from using KTL/Music to KTL/EPICS. Otherwise, we will proceed as originally planned in following the HIRES software model, including the use of KTL/Music.
9.5 Functional Requirement Documents for the Software PDR
Software and Data Handling System Functional Requirements Outline:
A. Data readout from CCD to disk
B. Image displayJ. DEIMOS instrument simulator - TBD
K. DEIMOS interface to Keck II telescope drive and control system (DCS) - TBD
L. Longterm data storage and archiving - TBD
M. Required software services for general observers
9.6 Object Acquisition and Mask Alignment
In many cases the observer's guide star coordinates will not be accurate enough to place the mask alignment stars in their holes on the first try. In that case it will be necessary to take the first direct image with the slitmasks open. That should add another 2 minutes or so to the cycle, for a total setup time of 6 minutes. That is the goal under typical conditions. It also obtains when only one guidestar is visible in the TVs.
9.7 Software Effort Between Now and the Software PDR
In addition to those activities, DEIMOS software staff (primarily Tucker), will be engaged in developing interim engineering test software that will be used for the early rotation tests of the DEIMOS model. The only other software we will be developing during this phase are small programs for generating the simulated 8K x 8K DEIMOS images, and simple benchmark and diagnostic software for testing disk drives and network hardware that we are able to obtain for evaluation purposes. DEIMOS software staff will also assist in the selection and evaluation of the vendor-supplied software packages associated with the various slitmask cutter systems currently under evaluation by the electronics and mechanical staff.
9.8 Preliminary Schedule After the Software PDR
Software Preliminary Design Review May 15, 1995
CDR for mechanical, optics, and instrument electronics Nov 15, 1995
Critical Design Review for detectors and software June 15, 1996