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.
This manual is extensively based on the SusI-2 commissioning report
and on the preliminary manual by S. d'Odorico and G.Martin.
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.
|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:
Filters with a dimeter < 100mm must be mounted in an adaptor.
We have a set of such adaptors for some standard diameters, and
we can manufacture adaptors for other diameters. It is critical
to organize this in advance. Be warned that it is very likely that
special filters will have a different optical thickness than the
standard filters, causing a focus offset that will have to be measured
and cause some additional overhead during the observations.
Data Format, Read-out Time and Read-out
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.
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:
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.
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)
Chip #45 (left) present ~15 bad columns, while #46 has some blemishes that look quite bad but actually flat-field out very well.
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|
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.
|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
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|
|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)|
|SUSI_img_obs_Jitter||SOT02||Multiple images with dithering (offsets)|
|SUSI_img_obs_DoubleW||-||Reads two non overlapping subwindows (for fast photometry)|
|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)|
- acquisition template: SUSI_img_acq_Preset (blind acquisition)
- a test exposure with full frame and bin 1x1, SUSI_img_obs_Jitter, with an offset to move the object of interest out of the gap.
- Once this image is displayed, measure the position of the object of interest (x,y in pix on the RTD), and define the read out window in a SUSI_img_obs_Jitter template:
- window TRUE
- start: x-50, y-50
- size: 100, 400
- number of exposures: 1
- excute that template, and check that the object is well centered. Adjust the start values of the window and retest if needed.
- modify the number of exposures to the number requested (eg 500), and start.
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:
Figure 5: Example of photometric light curve of variable star with SUSI2 in mmag.
(click for ASCII file)
(click on thumbnail)
(click for ASCII file)
(click on thumbnail)
|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|
|He II #880||O III/6000km #883|
|H beta #881||H alpha #884|
|O III #882||H alpha/6000km #885|
|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||
|1.9||13 Aug 2004||
Most links broken. Repaired (FSE)