[ SUSI2 ]
European Southern Observatory
La Silla Observatory


Superb Seeing Imager - 2
Direct CCD Imaging Camera at the NTT
User's manual
M. Billeres
2002 August. 
 O. Hainaut
2002 August 

2002 August

(see table at the end for minor releases) 


Table of content :

0- Introduction
1-Instrument overview
2-Instrument Properties:
    2.1 Optical Parameters
    2.2 CCDs
    2.3 Photometry and Throughputs
3-Observing with SUSI2
    3.1 Generalities
    3.2 Hints

Annexes :
Filter Transmission curves

    0. Introduction

    This manual describes the SuSI2 instrument on the ESO New Technology Telescope at La Silla Observatory. It is meant for the observers preparing observations on that instrument, or processing data produced with SuSI2. The manual exists only in HTML. The on-line version has many in-line links to other pages. Nevertheless, a printed version from this document will contain all the relevant information: links that point to critical information are given in clear in the text, the others are for general or background information.

    1. Instrument Overview

    The NTT Nasmyth focus A hosts since 1998 two new instruments: the IR imager-spectrometer SOFI and the direct imaging CCD camera SUSI2. The latter is an upgrade version of the SUperb Seeing Imager identical in concept (imaging at the f/11 focus of the telescope with one additional reflection) but with a 4 times larger field (5.5 x 5.5 arcmin). SUSI2 incorporates the first version of new ESO controller FIERA with a mosaic of two 2k x 4k, 15 µm pixel, thinned, anti-reflection coated EEV CCDs. Other novel features of the instrument are an 8cm x 8cm sliding curtain shutter which permits uniform exposures down to 0.3 seconds and a special cryostat designed to operate on a rotating Nasmyth adaptor. SUSI2 and SOFI share the same mechanical structure attached to the Nasmyth adaptor flange and the same cable derotator. The direct beam from the telescope feeds SOFI. A 45° mirror is inserted in the light path when SUSI2 is in operation.

    This manual is extensively based on the SusI-2 commissioning report and on the preliminary manual by S. d'Odorico and G.Martin.

    2. Instrument properties

    2.1 Optical parameters

    Optical Components in the Light Path:
    The optical path of SUSI2 includes the three mirrors of the telescope, the 45° reflection prism with a multi-layer coating (reflection efficiency > 90% in the range 340-500 nm; > 95% in the range 500-1100nm; this prims is known as "Mirror 4" in the NTT jargon) and the coated quartz window of the cryostat (transmission > 97% over the range 340-1100 nm).

    The filter wheel and the shutter are located in the f/11 between the prism and the cryostat.  Before Nov.1999, there was a small vignetted region at the bottom the the CCDs (~500pix). While the vignetting was flat-fielded out, this region should not be used for accurate photometry. The vignetting was caused by the mirror 4, whose position has been adjusted.

    The filter wheel has 6 positions. The filters have a diameter of 100 mm. The filters currently available are listed in the table below.

    Table 1: SuSI2 filters.
    Filter ID Central 
    FWHM PWL Transm. 
    Red leak and remarks
    Bessel U #810 357.08 52.07 360.5 58.6 -
    Bessel B #811 421.20 98.94 417.4 69.1 0.01% @ 1100nm 
    0.02% @ 1200nm 
    Bessel B #817 421.17 99.17 419.0 69.1 spare
    Bessel V #812 544.17 115.17 526.0 89.4 0.01% @ 1100nm 
    0.02% @ 1200nm
    Bessel R #813 641.58 158.89 597.0 86.9 0.06% @ 1100nm 
    0.04% @ 1200nm
    Bessel I #814 794.96 147.82 797.5 88.9 0.13% @ 1100nm 
    1.63% @ 1200nm
    Bessel Z #815 840.90 high pass 1126.0 98.2 Atmospheric + CCD cut off at large wavel.
    r #822

    not measured. Use at your own risk
    IB 609 #830 609.38 27.16 610.0 88.9 Not available anymore
    IB 662 #831 662.29 35.4 665.0 91.7 Not available anymore
    WB 490 #824 502.72 107.32 510.0 87.5 -
    WB 665 #825 665.67 123.52 679.0 97.4 -
    U' #823 368.97 low pass. 339.0 81.0 Atmospheric + CCD cut off at small wavel. 
    Maximizes transmission at l<370nm.
    He II #880 469.531 7.644 469.0 72.2 -
    H Beta #881 486.438 6.668 485.5 83.2 -
    O III #882 500.984 7.244 500.0 86.1 -
    O III/Cont #883 511.061 7.019 510.5 84.2 = O III 6000km/s
    H alpha #884 655.528 6.976 656.5 89.5 -
    H alpha/Cont #885 668.655 6.899 669.0 90.3 = H alpha 6000km/s

    Notes on the filters:

    2.2 CCDs

    The two EEV CCD 44-80 were the first in the large format with 2 x 4 k, 15µ pixels in operation for astronomy. Their properties when operated with the FIERA controller are summarized below:

    Data Format, Read-out Time and Read-out Noise:
    The two EEV chips are identified as ESO CCD # 45  and 46. The two frames are combined in a single FITS file (chip #45 has lower x values, i.e. on the left on standard display); the space between the two chips has been "filled" with some overscan columns so that the respective geometry of the two chips is approximatively preserved. The format of the file is 4288 x 4096. Along the x axis there are 50 prescan, 2048 active, 46 overscan pixels for chip # 45, followed by 50 prescan, 2048 active and 46 overscan for chip # 46.

    There is one single read-out option in which the two chips are read in parallel each through a single port at 2 x 105 pixels/sec. There are three binning options (unbinned=1x1, 2 x 2 and 3 x 3) and the possibility of defining a single read-out window (which can overlap with both chips).

    The read-out times (intervals from the closure of the shutter to the display of the image in the Real Time Display monitor) are 56, 16 and 9 seconds for the three binning options respectively.

    On the RTD, the default options show North at the top and West (chip # 45) to the left.

    The measured read-out noise are: 4.7 e- for (# 45) and 4.6 e- for (# 46).  For the latest measurements of the CCD parameters, pls check the NTT detector page at  /sci/facilities/lasilla/sciops/CCDs/  on the WWW (updated ~weekly). This page also links to an history of these parameters' evolution.

    CCD QE:
    Measurements of the Qes for the two chips in the ESO detector laboratory show that # 45 is up to 5% relatively more efficient than # 46. The table below gives the average values:

    Table 2: SuSI2's CCDs quantum efficiency
    % QE

    Saturation and Linearity: 
    Pixel saturation occurs at ~ 150000 e-. With the adopted gain of ~2.25 e- /ADU this corresponds approximately to the full rang e of the ADC converter in the unbinned mode. The CCDs have been found linear within + 0.15% over the full range 0-60000 ADUs.

    Charge Transfer Efficiency:
    Both serial and horizontal CTE are better than 0.999999.

    CCD dark current:
    <0.5 e-/pix/hour, i.e. negligible for all practical purposes.

    Bias Level: 
    To be derived from the average of prescan and overscan regions of each frame because of a slight dependence on the mean level of the charge in each row of the CCD.  0sec bias exposure average at ~300adu.
    (again, check the NTT detector web pagefor up to date values)

                        SUSI2 bias

Figure 1: Example of SUSI bias: the difference of sensibility bewteen the two CCDs is responsible for the contrast.

    Chip #45 (left) present ~15 bad columns, while #46 has some blemishes that look quite bad but actually flat-field out very well.

    Image scale, Field Size, Image Quality
    The scale at the Nasmyth focus of the NTT is 5.4 arcsec/mm. The measured pixel scale is 0.085 arcsec/pixel (0.161 arc/sec in the 2 x 2 binned mode). Each chip of the 2x1 mosaic covers a field of 5.5 x 2.7 arcmin. The gap between the chips corresponds to ~ 100 real CCD pixels, or ~ 8 arcsec. In the default position of the rotator the gap runs in the N-S direction. The optical axis -which is also the reference for pointing- is at about 60 pix left of the center of the mosaic, i.e. in chip #45 (left), very close to the inter-CCD gap.

    The angular misalignment between the two chips is smaller than 6 arcmin. Very accurate astrometry requires an independent calibration of the two subfields. The table below lists the mean astrometric parameters over the two chips.

    Variable X coefficient Y coefficient
    X +0.02239 -0.00033
    Y +0.00013 +0.02231
    XY +0.00005 +0.00004
    X2 -0.00004 +0.00003
    Y2 -0.00004 -0.00001


    No aberrations or change of focus have been detected over the full field covered by the mosaic down to an image quality of 0.5 arcsec FWHM.

    The  chip geometry is summarized in Fig.1, with the effect of an additional Rotator Offset.

    SuSI orientation
    Figure 2: Orientation of the SuSI2 detectors

    2.3 Photometry and Throughputs

    The photometric parameters have been determined during the commissioning of the instrument, and are measured regularly as part of the Maintenance Plan of the NTT. Typical values (for 2001) obtained using several Landolt standard fields are presented hereafter for the Bessel filters. It should be noted that the zero points show a slow decrease over the past two years, which is being investigated. Obviously, these values should be used only as reference: careful observers will re-measure them during their run. This is specially true for the Zero Points, whose values are strongly affected by the normalisation region used when preparing the flatfields.
    Up to date values can be found at  :

    Table 3: SuSI2 Photometric Calibration 
    (typical 2001 values,
    zero points in ADUs)
    U - u =  0.10(U-B) + 23.52
    B - b = -0.22(B-V) + 25.46
    V - v = -0.02(V-R) + 25.70
    R - r = -0.05(R-I) + 25.60
    I - i = -0.10(R-I) + 24.60

    For the purpose of observation preparation, an exposure time calculator for SUSI2 is available at this web site. It computes the counts of the instrument based on the optics and CCD data as they are known now. The results are in good agreement with the number of photo-electrons measured at the telescope. The table below gives the average measurements normalised to a star of 15 magnitude, in one second exposure and at 1 airmass, for two epochs. The stability of countrates is excellent, as illustrated by the evolution curves displayed at the above web page.

Table 4: Count rates (in adu) on SUSI2, for a mag=15 star, arimass=1

    CCD#45 4780 31200 46050 45100 18200
    CCD#46 4690 30840 45770 44920 17950

    3. Observing with SuSI2

    3.1 Generalities

    SuSI2 is entirely operated within the VLT observation scheme: "Observation Blocks" are prepared the days before the observations using P2PP, and executed at the telescope. It is also possible to create, copy and modify the Observations Blocks at the telescope; actually, the most efficient way to observe (in visitor mode) is to prepare in advance all your Target Packages and Observation Descriptions, while preparing only models of Observing Blocks, which  you will duplicate and modify while observing. If the last sentence does not make any sense to you, don't dispair: your support scientist will introduce you to P2PP. You may also want to read the P2PP User's Manual, a complete, detailed, reference document.

    The  instrument templates (describing the different possible observations, calibration and target acquisition) are operationally identical to those which have been used with the previous SUSI imager, and are described in details in the SuSI2/EMMI Template Signature File Parameters Reference Guide. A short summary is given in the table below. Curious readers can consult the EMMI user manual for a longer description of the VLT observation scheme at the NTT.

    Note that as of Period 68 (Oct.2001), a new set of SuSI2 observations templates has been introduced (known as SuSI2001). As of Period 69 (April 2002), the old templates are not supported anymore and new OBs must be prepared using SuSI2001.

    Template Correspondance with old template Description
    Table 5: SuSI2001 Templates

    Acquisition Templates    
    SUSI_img_acq_Preset SAT01 Open loop (blind) pointing
    SUSI_img_acq_MoveToPixel SAT02 Target field at location (brings the object to a given pixel after taking a short acquisition image)
    Observation Templates    
    SUSI_img_obs_Exposure SOT01 Single image
    SUSI_img_obs_Jitter SOT02 Multiple images with dithering (offsets)
    SUSI_img_obs_DoubleW - Reads two non overlapping subwindows (for fast photometry)
    Calibration Templates    
    SUSI_img_cal_Dark SCT01 Biases and Darks
    SUSI_img_cal_DomeFF SCT02 Dome flat-fields (request a series of flat at a given level)
    SUSI_img_cal_SkyFF SCT03 Twilight flat-fields (request a series of flat at a given leve, computed taking into account a model of the sky brightness de/increase)
    SUSI_img_cal_TelFocus SCT04 Through-focus sequence

    3.2 Hints

    This section lists a collection of hints and tips for observing with SuSI2. At this point it is not ordered in a very coherent way.

Figure 3: Left Panel : Dirty image in I. Right panel : Clean image in I.

Figure 4: The fringing pattern

Additionally, another template (SUSI_img_obs_DoubleW) has been introduced to perform fast photometry of two objects (e.g. a variable star and a reference star) using two non-overlapping windows. Proceed as above to define the two sub-windows.To decrease the read-out-time between two exposures, you can use the 2x2 mode or even better the 3x3: if you look for short variations in photometry, you are not really interested by the resolution, but by the minimisation of the dead time. Example of light curve obtain with this method:
    Light Curve EC05217

    Figure 5: Example of photometric light curve of variable star with SUSI2 in mmag.


  -- oOo --

    Annexes:  Filter Transmission Curves

    (click for ASCII file)
    (click on thumbnail)
    (click for ASCII file)
    (click on thumbnail)
    Broad band
     Bessel U #810  r #822
     Bessel B #811  IB 609 #830 
    Bessel B #817 IB 662 #831
     Bessel V #812  WB 490 #824 
     Bessel R #813  WB 665 #825 
     Bessel I #814  U' #823
     Bessel Z #815 

    Narrow Band

    He II #880 O III/6000km #883
    H beta #881 H alpha #884
    O III #882 H alpha/6000km #885

    Release History
    Release Date Changes and Comments
    0.1 20 Mars 1998 First version, by Sandro D'Odorico and Gabriel Martin
    0.2 20 Nov. 1998 Added filters, photometry, etc. O.Hainaut
    0.2.1 1 Dec. 1998 Hints and transmission curves. O.Hainaut
    0.2.2 18 Dec. 98 Added note on vignetting
    1.0 19 Apr. 99 several additions (B#817, orientation, corono...)
    1.1 19 Jun. 99 Added 2d throughput epoch
    1.2 20 Aug 1999 Added narrow band filters
    1.3 30 Nov 1999 fast photometry, and few typos fixed
    1.4 31 Aug 2000 New version of the zero point and count rates
    1.5 7 Feb 2001 updates (oh)
    1.6 25 Aug 2001 updates (oh)
    1.7 30 Dec 2001 updates (oh)
     1.8  26 Aug 2002

     update (mb)

     1.9  13 Aug 2004

     Most links broken. Repaired (FSE)



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