SAAO - South African Astronomical Observatory

UCT CCD Photometer

The UCT CCD was retired at the beginning of 2013.  Please see the SHOC page for details of SAAO's new high-speed cameras for photometry.

Introduction

The UCT CCD is a Wright Instruments Peltier-cooled camera, available for use on the SAAO 1.9-m, 1.0-m and 0.75-m telescopes.  It is used in frame-transfer mode for high-speed photometry of rapidly variable stars.  It is no longer possible to use it as a conventional imager in full frame mode.

Detector Specifications

The CCD is a thinned, back-illuminated EEV CCD giving good UV sensitivity. Its specification is given in Table 1.

Table 1. CCD properties

No. of pixels

576x420

Pixel size

22µm

Dark current

~0.05 e-/pix/s

Readout noise

10 e-/pix

Gain

1 = 10 e-/ADU

1 = 2.5 e-/ADU

ADC max

32768 ADU (16 bit)

CCD saturates

~250000 e-

The CCD is Peltier-cooled (making for very straightforward operation) to approx -50°C, yielding typical dark count rates of ~0.005 e-/pix/s. With readout noise of 10 e-/pix this implies total noise of 14 e-/pix for a 1000 s exposure. Dark count can be ignored for short exposures in frame transfer mode (1-30 s). There are two gain settings:-

  • Gain 1 which gives 10 e-/ADU, useful for bright objects
  • Gain 4 which gives 2.5 e-/ADU, better for faint targets

The Analog-Digital Converter (ADC) saturates at 32,768 ADU and the CCD saturates at ~250,000 e-). 

CCD Format

Table 2 gives the UCT CCD detector properties applicable to each SAAO telescope.  The CCD has 576x420 22 µm pixels. Frame-transfer operation requires half of the CCD to be masked. This, combined with the unthinned and therefore insensitive "bathtub", gives a usable data frame of 380x260 pixels (or 127x87 with 3x3 prebinning). The corresponding sky coverage can be found in Table 2.  The minimum readout times (in frame-transfer mode) are also given; the minimum exposure times in High-Speed Photometry mode, where there is no dead time between exposures, is just slightly longer than this (add 0.5 sec).

With typical good seeing of 1 arcsec, and with the various plate scales at the different telescopes, the CCD should be prebinned by the amounts specified in Table 2. In bad seeing, more prebinning is required. The maximum permitted is 6x6.

Table 2. CCD format
Field of view (arcsec) 50x34 109x74 142x97
  1.9-m 1.0-m 0.75-m
Plate scale (µm/arcsec) 166 77 59
22µm pixel size (arcsec) 0.13 0.28 0.37
1 arcsec seeing prebinning 3x3 2x2 1x1
Number of pixels 127x87 190x130 380x260
Readout time (sec) 1 2 7

Detector Sensitivity

The sensitivity is given in Table 3. For Johnson broadband filters, the count rate in photons/second for a star with V=18 and B-V=0 is given. Scaling by the aperture will yield count rates for the other telescopes. Note that count rates depend on the state of the mirrors, atmospheric extinction etc. and cannot be precisely predicted.

 

Table 3. CCD count rate in phot/sec for star with V=18, B-V=0 on 1.0-m telescope.

U

B

V

R

I

7

76

105

91

41

Operation

The device is mounted on a simple turntable, so the orientation can easily be changed.  The two larger telescopes have autoguiders that can be used with the UCT CCD. 

The camera is driven by a 486 PC which collects the data, displays images as they are read out, and permits simple image manipulation. The PC is linked to a Linux box, which is connected to the Sutherland Ethernet. The Linux machine is used as the principal data storage medium (as the instrument PC has very little disk storage available), and for on-line reduction and plotting of the data.

Manuals

Further information can be found in the UCT CCD User & Technical Guide.