PDS_VERSION_ID = PDS3 OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = ESO INSTRUMENT_ID = SUSI OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "Superb Seeing Imager" INSTRUMENT_TYPE = "IMAGER" INSTRUMENT_DESC = " Instrument Overview =================== EMMI is a flexible instrument which allows a wide range of observing modes, from wide-field imaging to high-dispersion echelle spectroscopy, including long-slit and multi-object spectroscopy. This manual also describes SUSI (SUperb Seeing Imager) which is mounted in the other Nasmyth focus of the NTT and complements the imaging capabilities of EMMI. A brief description of the active optics system of the NTT and its basic operational principles are also provided in this manual. The driving concepts in the instrument definition were the high image quality foreseen for the NTT, the need to complement or improve on the instrumentation on the 3.6m telescope, and the need to minimize instrument change-overs. The concept which was adopted is that of a dual-beam instrument, fully dioptric, and based on the white pupil principle. CCD detectors were foreseen for the two arms with the possibility to adapt to the geometric characteristics of future detectors by changing the cameras only. The main advantages of this type of design are the high efficiency in both channels and the easy conversion from wide field imaging to grism and grating spectroscopy. After the first observations with the NTT, it became clear that the telescope and the atmosphere at La Silla could provide stellar images with diameters as good as 0.3 arc sec. Images of this quality could not be sampled adequately with EMMI, which scale had also to be adapted to the spectroscopic modes of observation. Thus, SUSI was designed and built for the other Nasmyth arm of the NTT. The design is such that switching between EMMI and SUSI is done in a matter of minutes, so that they can be considered as different parts of the same instrument. The Superb Seeing Imager (SUSI) is physically distinct from EMMI but complements its observing capabilities. A supporting plate is mounted on the adapter of the Nasmyth A focus of the NTT. On the plate, a mirror with 3 positions is mounted. The first position sends the light to SUSI, the second feeds an IR camera (not available), and the third position is free for the operation of IRSPEC. Between the diagonal mirror and the CCD is a filter wheel with 8 positions. In the control software, SUSI is also referred to as DIFA: the Direct Imaging FAcility. SUSI has two advantages over EMMI. First, the small pixel scale which gives a much better sampling of the point-spread function. Second, the lack of optics which gives better throughput. It also has two disadvantages: the CCD is much less efficient in the blue than the one in th blue arm of EMMI, and the field is small. SUSI is therefore well suited for either periods of good to very good seeing. Switching from EMMI to SUSI only takes a few minutes. However, it is necessary to move the telescope to point at the zenith: only in that position can M3 be turned, to move the beam from one Nasmyth platform to the other. The EMMI control software should be terminated, and SUSI started up as above. You will have to redetermine the focus, and if you are switching because of significant improvements in the seeing, a new image analysis should also be done to take advantage of the new situation. System Performance ================== Telescope focus is critical to obtain good images. The slow angle of the NTT beam at the Nasmyth focus (F/11) facilitates focusing the telescope with SUSI, which must be done using through-focus sequences. The parameters for these sequences are entered in the SUSI Exposures form. Tests done on reasonably good seeing (0.75-1.0 arc sec) showed that the optimal focus step is 30 microns. Step the telescope by 10 arc sec in the most convenient direction depending on the field. 7-9 focus exposures give the best results. The following table gives approximate focus values for SUSI filters. U -3.48mm B -3.47 V -3.50 R -3.47 I -3.43 Z -3.43 Calibrations ============ As with all CCDs, the afternoon should be used to check that the CCD is in good working condition. Because SUSI is not used as often as EMMI, faults can go unnoticed for some time and the night is not the time to find out about those! Take a few biases and check the read-out noise and check for the presence of patterns. The CCD is normally used in the slow mode, especially when used with binning: the time gained in reading out in fast mode is 25 sec in slow mode and 5 sec with 2 by 2 binning CCD is linear up to 150000 electrons/pixel. Dome flats have been found to flatten images to about 3 %. Sky flats are generally more accurate: SUSI shows some vignetting which is not well removed with dome flats. For twilight flats Tyson sequences have been used with good succes: the program to calculate these is available on the off-line computersystem (lw0--lw10) under the directory { /astro/progr1/twilight}. Twilight flats should preferably be taken in the evening. If the background in your science images is bright enough, you can also create flats out of these. A photometric system for SUSI has not been established. The normal calibrations using the Landholt standards or other CCD standards can be used, but due to the small field size it is difficult to get more than one star on the CCD. If you have determined colour terms for SUSI, please let us know! Cameras and Detectors ===================== Image scale for EMMI and SUSI EMMI BIMG #31 TEK 1024 F/4 24 micron/0.37 arc sec CCD characteristics for EMMI and SUSI # 31 slow mode 1.7e/ADU conversion 2.5ADU RON 280.2ADU BIAS 8e DARK fast 3.4 1.6 275.2 8 Saturation is in most cases defined by the ADU converter, at 65 kADU. The actual well depth is around 2 x 10^6 e so that the linearity is good up to digital saturation. The exceptions are EMMI red and SUSI when read out in fast mode. For EMMI red in the fast mode, exposure levels should be kept below 40 kADU, and for SUSI below 24kADU. Otherwise, the linearity of the CCD is better than 0.5%. The measured linearity curves can be found in the CCD test reports. The fast readout mode has as main advantage a reduced readout time. This becomes important on EMMI red where fast readout saves two minutes. It is much less important in SUSI. The disadvantage is increased readout noise and digitization noise, and sometimes increased electric interference. For broadband imaging and many spectroscopic programmes, the readout noise is not important compared to the photon background, and the fast readout mode would be recommended. Electronic interference could be larger in fast readout: when in doubt, it is worthwile to take a few biases in the afternoon to check on the readout noise and on the presence of pattern noise. Remember that, in general, calibration frames such as bias, flat fields, and darks taken with slow readout cannot be used for correcting fast exposures, and vice versa. CCDs have an electronic bias that can be evaluated by averaging several 0s dark exposures and subtracted from the science images to take it out. By using these exposures the observer can check the CCD readout noise and possible pick-up patterns in the electronic background. At least one, but if possible more, long (at least 1 hour) dark exposures are important to monitor the dark current of the CCD (if possible, take a dark longer than the longest science exposure). Filters ======= The SUSI filterwheel has 8 positions of which 7 are available for filters. In contrast to EMMI, SUSI uses 60-mm filters which are the same size as the ones used for EFOSC. EFOSC filters can therefore also be mounted in SUSI (if they are not required by the EFOSC observer!). There are also a large number of 60-mm filters which are not allocated to a particular instrument and which can be requested. The ESO Image Quality Filters Catalogue (Gilliotte, 1992) contains a list of available filters and transmission curves. More recently (1995), a number of new filters have been acquired and all transmission curves re-measured. Processing ========== The data from the EMMI and SUSI detectors are simultaneously transmitted to IHAP and MIDAS databases. MIDAS runs on a Unix workstation equipped with a DAT tape unit. IHAP uses standard 1/2~inch 2400~foot tapes at 6250 BPI. The FITS headers of CCD files contain all the information necessary for the scientific use of the data, that is all the telescope, instrument, and detector parameters. Most of these parameters are stored in so-called hierarchical keywords. MIDAS can read these keywords, but some other packages may not since these are an extension of the FITS standard. If you are not using MIDAS, it is worth to check the actual FITS header for further information which may be useful. Electronics =========== The NTT is controlled by two HP1000/A900 computers, one for the telescope (called NTT) and one for the instruments (called NTI). The control software of EMMI is organized in such a way that EMMI is presented as five sub-instruments called RILD, REMD, BIMG, BLMD, and DIMD. Depending on the type of observations, the user selects one of these modes and the control software automatically moves the functions to be set for this mode. This leaves only the parameters of the particular type of observation to be defined. For instance, when setting up an exposure in RILD, the required mirror unit and the upper red folding mirror are automatically set. The observer must only specify the camera focus, the choice of slit, filter and/or grism, and exposure parameters (see section Getting started). The user interface (UIF) consists of a RAMTEK monitor where mouse driven menus and forms are displayed, and a CRT (LU:53) monitor where messages from the system about the instrument are given and commands may be entered. Parameters are entered by filling in forms on the RAMTEK screen. Once all optional optical elements are installed by the operation group, according to the observer's request, a setup form is produced. A printout of this form is left in the control room so that the observer can verify the setup and can use it as reference during the night. The positions in the wheels of filters, grisms,and slits, and the gratings in the housing will be displayed, on the RAMTEK UIF in sofar as it is used in the chosen mode, whenever a setup in that mode is defined. Ancillary Data ============== Bias and darks It is not safe to assume the bias to be always a scalar and therefore it is recommended to take many bias exposures. It has proven to be extremely difficult to isolate the CCD electronics from electrical interference from components in the NTT adapters/rotators. Therefore to some extent, the EMMI CCDs show pick-up patterns in the electronic background (the bias). This noise is minimized in SLOW readout mode, but may be rather strong in FAST readout frames. The patterns are not stable, but change from one exposure to the next, so it is difficult to remove them completely by substracting bias frames. However, some reduction can be achieved and, therefore, it is recommended to take a good number of bias frames throughout the observing run. Should strong patterns (i.e. more than a few ADUs) appear on SLOW readout bias frames, call the NTT coordinator (93-50). Note that spurious patterns are introduced if images are displayed with demagnification factors. At least one, but preferably more, long (1 hour) dark exposures should be taken to monitor the dark current and any exposure dependent features. Flat fields The linearity of CCD No 31 is very good up to 160,000 electrons/pixel. The most accurate results for flat fields in broad band imaging are obtained using sky flats. This may be achieved by median filtering of science images, if they are not too densely populated with stars and do not contain very extended objects, or by doing multiple exposures of sparsely populated fields, using spatial offsets. A list of such fields is available in the control room. Another approach is to use morning and evening twilight. Shutter timing A time delay of 0.80 seconds has been measured for the shutter in the blue F/4 camera. Because of the location of the shutter in the optical path, the exposure time over the field is constant and equal to the chosen time plus the average shutter delay. If critical for your application, it is recommended that you check the shutter timing by either taking exposures of increasing duration on a star, or using one of the internal lamps and a pinhole in the aperture wheel. Typical count rates In blue imaging the efficiency of EMMI is the product of the transmission of the atmosphere, three reflections in the telescope, the transmission of the blue coated optics of mode BIMG, filter, and quantum efficiency of the CCD. The efficiency in B and U was checked for the F/4 camera and TEK CCD No 31 in 1993. The count rates in e/sec deduced for a 15th magnitude A star with approximately zero colour are U: 2200, B: 16,900 at unit airmass. Wallander A.: 1993, Remote Control of the 3.5m NTT User Guide, ESO Operating Manual No 17. Dekker. H., Delabre, B.: 1987, Applied Optics, 26, 8, 1375 Dekker, H., Delabre, B., D'Odorico, S.: 1986, SPIE, 627, 339 Gilliotte, A.: 1992, Image Quality Filters Catalogue, Internal ESO publication Melnick, J., Dekker, H., D'Odorico, S.: 1989, ESO Operating Manual #4 " END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "N/A" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END