Minutes of UKATC meeting on 10/03/2000

Attendees

• Vik Dhillon
• Tom Marsh
• Derek Ives (Project Manager, Detectors)
• Tully Peacocke (Optics, Opto-mechanical)
• David Atkinson (Detectors, Electronics)
• Alan Bridger (Head of Software)
• Chris Tierney (Software)

Introduction

Electronics/Software

• ULTRACAM is one of the highest data throughput instruments that the UKATC will have ever built!
• One of the major decisions which remains to be made is whether to go for a VME-based data acqusition system or a PCI-based one. The latter is currently under development by SDSU, but it is likely that it will be available by the summer when work on the ULTRACAM electronics begins in earnest. The consensus of the meeting seemed to be that the PCI route should be followed if it is has been proven to work and if it is cost neutral. A final decision will have to be taken on this in the coming months.
• Another major decision which we need to make as soon as possible is the use of the hard disks. DJI stated that he could not guarantee our required data rates if we also intend to access the disk for the data reduction pipeline. It was agreed that we would not do this. Instead, we would use two disks and write one burst of data (say, 30 minutes-worth) to one disk, and then write the next burst of data to the other. Meanwhile, the data reduction pipeline would be working on the idle disk. This means we would not quite have real-time data reduction, but it does ensure we maintain the highest possible data rates.
• It was agreed that it would be possible to define at least 5 windows on each CCD chip, but the windows would all be in the same places on all 3 chips. This then requires that the detector heads be mounted on x-y carriages, which VSD agreed Sheffield would provide.
• It was also agreed that two modes of readout would be provided - a drift-mode readout for the fastest data rates, where the windows are aligned on the border with the masked region, and a normal frame-transfer mode, where the windows are allowed to be anywhere on the frame.
• There followed lengthy discussions on the data format and headers. It was agreed that the UKATC should define the format of the data files written to disk - TRM would then be told the format and his pipeline software written accordingly. The data would be provided demultiplexed (i.e. the data from the 3 CCDs would be separated before being written to disk). The goal would be to write separate files for each exposure to disk, but time might not allow this . The headers would be written to the data files on the disk, and not stored in separate files. This includes the exposure start times, which would be provided by a triggerable GPS card to 0.1 millisecond accuracy.
• It was agreed that the UKATC would be responsible for the following deliverables: 3 CCDs, 1 SDSU controller, 1 "black box" (a data acquisition system with a VME or PCI-based architecture) and 2 hard disks.

Optics

• The optics have been designed for the f/11 Cassegrain focus of the WHT. The resulting f/11 collimator and f/1.8 cameras will provide an instrument which is approximately 600-700mm long (from telescope focus to detector) with a beam size of approximately 45mm.
• It is cheaper to design and procure three identical broad-band cameras for the three arms of ULTRACAM than it is to provide three one-off camera specifically designed for use over a narrow wavelength range. This is because one-off optics are expensive, but copies of optics are cheap. The broad-band cameras can, however, each be coated differently, giving only 0.5% losses on each surface.
• The camera optics will be made of fused silica, which is cheap, available, durable and easy to work. The collimator will be made of a combination of fused silica and magnesium flouride, the latter being much more expensive. The camera lenses will be approximately 60mm in diameter; the collimator lenses will be approximately 110mm in diameter.
• The resulting instrument will be mountable on a different telescope with a different focal ratio, giving the same pixel scale (due to the telecentric design of the cameras) but a different beam size and different aberrations.
• There will be ghost images produced by the dichroics, but we hope to be able to overcome these by use of coatings and by throwing them off the detectors.
• The opto-mechanical design was briefly discussed. VSD described how Sheffield would undertake this work, in close contact with the UKATC. It was agreed that the optics should come from the manufacturers mounted in barrels to ease the job of handling, mounting and alignment.
• PTP requested TRM and VSD to now make a final decision on the following requirements:
  1. The wavelengths of the dichroic cut-offs.
  2. The required pixel scale when mounted on the WHT Cassegrain focus.
  3. The final list of telescopes we wish to use ULTRACAM on.
• PTP expressed concern that we have not budgeted enough for the optics. Working on the assumption that the collimator will contain 4 lenses, each of the cameras 4 lenses, and each surface of fused silica costs £1000 (and MgF costs more), the total cost of the optics will be at least £24k. To this must also be added the cost of the coatings, at approximately £300 per lens, and the costs of the filters and dichroics, which will be at least £5k. PTP said he would try to obtain a figure accurate to 5-10% as soon as possible, but it looks likely that we will be at least £5-10k over budget in this area. On the positive side, PTP did say that the amount we have budgeted for staff effort is sufficient to design and procure the optics. There followed a lengthy discussion on how we might fund any short-fall in the optics area from other areas of the project, with the conclusion that there does seem to be scope to make the necessary savings within the overall budget.

Detectors

• DJI confirmed that, having looked into the various options available for detectors, it appears that the EEV 47-20 is indeed the optimum detector for ULTRACAM.
• There followed a lengthy discussion on the expected performance of this chip in terms of readout noise and speed (3 e^- at 10 µs/pixel/readout channel, 5 e^- at 3 µs/pixel/readout channel), dark current (see below) and QE (see EEV).
• DJI stated that we must decide now on our dark current requirements, which in turn sets our cooling requirements. TRM stated that the worst case scenario is when observing with ULTRACAM in dark time in the U-band on a 2-m telescope (i.e. when the sky count rate is at a minimum). In this case we would expect approximately 0.3 photons/pixel/s from the sky (assuming 0.35 arcseconds/pixel). We would want the dark current to be less than this, which sets our dark current requirement as 0.2 e^-/pixel/s and our goal as 0.1 e^-/pixel/s. The dark current figures for the EEV 47-20 imply that the detectors must be cooled to 238 K to meet the requirement and 234 K to meet the goal. DJI made it clear that it is possible to achieve this level of cooling using the Peltier effect, albeit with the aid of water/glycol cooling.
• DJI stated that using Peltier cooling instead of liquid nitrogen is approximately cost neutral, and requires the purchase of a water chiller. It also requires the provision of a dry-nitrogen supply at the telescope in order to prevent condensation and the use of 2 pipes (one supply, one return) between the chiller and the instrument.
• VSD commented that Peltier cooling is much more desirable than liquid nitrogen, assuming that the dark current requirement is met. First, the instrument can be turned on and off much more quickly. Second, the weight and size of the instrument are dramatically reduced. Third, the instrument will be much easier to use on telescopes where there is restricted access to the focal station, such as on SALT.
• There followed a lengthy discussion on whether or not Peltier cooling will be able to maintain a constant temperature at the detector. This is desirable in order to maintain a constant dark current and QE. DJI said he would look into this.

Timescales

• Detectors/electronics: David Atkinson will be working on ULTRACAM from now on, but he has only just been recruited and will need a few months to get up to speed. Hence progress in this area will probably not occur until the summer.
• Software: Chris Tierney will be responsible for the ULTRACAM software. He is currently committed to other projects and will not be able to start work on ULTRACAM until the end of the summer.
• Optics: Tully Peacocke has already done some work on ULTRACAM and will be able to continue to do so from now on. VSD emphasized that this is excellent news, as the optical design work must be completed before the mechanical work at Sheffield can proceed.
• DJI agreed to send VSD a Gantt chart detailing the milestones of the project in April. The UKATC are still confident that the instrument will be delivered by Summer 2001.