wtable2 MISTRAL spectrograph camera
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MISTRAL spectrograph camera

These pages provide a basic tutorial for the use of the MISTRAL (Multi-purpose InSTRument for Astronomy at Low-resolution) spectro-imager. We recommend however to read the Cook Book before planning any observation with MISTRAL. More specific informations are also available in the Appendix.

MISTRAL basics

MISTRAL is a low resolution spectro-imager adapted to the folded-Cassegrain focus of the 1.93m telescope via a focal reducer, in parallel with the SOPHIE adapter housing. A 45 deg mirror send the beam in one of the output sides to feed the MISTRAL instrument. This allows a very simple switch-over between the two instruments, without any mechanical operation.

The MISTRAL optical path is populated with an ANDOR deep depletion CCD 2K×2K camera (iKon-L DZ936N BEX2DD CCD-22031). The cooling is made by a 5-layer Peltier device. The operating temperature can reach -95℃ to -100℃. The dark current proved to be lower than 3 electrons/hour/pixel at -95℃. Full details can be found here.

MISTRAL hosts two dispersors plus two empty slots on a mobile plate. They cover the full spectral range with a resolution of the order of 700. The instrument includes four Thorlabs motorized stages used to move/remove elements from the optical path: the slit, the grisms, the filters and the calibrating mirror. The FLI filter wheel has 12 positions for 50 mm filters (available : SDSS g', r', i', z' + Y, galactic H, OIIIa&b, Hα, SII). The main filter characteristics are summarized here.

Calibration lights (Hg Ar Xe spectral calibration lamps and Tungsten spectral flatfield lamp) are inserted within the optical path by four optical fibers via the calibration mirror which needs to be moved in. In order to facilitate the operability and stability of the instrument, all the calibration lamps, power supplies and electronic modules have been integrated directly in the mechanical structure of the instrument.

MISTRAL can offer two operating modes: regular observing runs in visitor mode and Target of Opportunity (ToO) in service observing mode for fast transients. This ToO mode (not available for now), will be activated under triggers.


Science Basics

With the advent of new sky surveys, both from the ground and from space, the exploration of the variable sky is entering a new era. The high cadence of those surveys, and the large area covered allow a much larger coverage of the physical parameter space than ever before. As a result, a wealth of new phenomena and classes of objects are discovered, enlarging the physical diversity, and the statistics of previously known, but rare phenomena is greatly improved.

On the high-energy side, Gamma-Rays bursts (GRBs) are now observed in large numbers, and classified into two categories, the short- and long-duration GRBs. On the Supernovae (SNe) side, it appears that stellar explosions are not just core-collapse, or thermonuclear explosions of CO white dwarfs, but new categories are discovered, from ultrabright SNe to faint and fast decaying type I SNe, and passing through He detonations, Ia objects or luminous red novae. The range of underlying physical mechanisms must therefore be much more diverse than previously thought, but is still not understood. On a somewhat quieter side, Luminous Blue Variables, or numerous peculiar binaries await a better understanding too.

What is most necessary to progress is enough ground-based observing time to follow the variations of a series of representative examples of all those categories, both in photometry, and, even more so, in spectroscopy and in near infrared (Y band) spectroscopy: only with long time series of spectroscopic variations, accompanying the light-curves, it is possible understand the underlying physical mechanisms. Small to medium sized telescopes are best suited for that, being now more available than before (with 8m telescopes) provided they are equipped with efficient versatile spectro-imagers. This is the purpose of the MISTRAL instrument, mounted at the 1.93m telescope of OHP. With a possibility of rapid changeover from the other available instrument (SOPHIE), it will allow fast response to transient objects when this mode will be offered.

MISTRAL can also follow non transient targets in the framework of e.g. spatial missions covering fields as galactic HII regions and their exciting and triggered stars (e.g. Herschel) or nearby contributions to extragalactic surveys as for example XXL or XCLASS (XMM-Newton).


Observing Modes

MISTRAL allows several observing modes, accessible from a dedicated GUI, depending on the position of the different elements along the optical path. These elements are the filter wheel (12 positions), the spectral dispersors (blue and red VPH, associated with a blue and red intrance lens), and the slit (1.9 arc-sec wide). These elements are summarized in Table 1 and organised following the different operating modes. The Cook Book also gathers other useful informations about the CCD reading modes, the fringing occuring at the spectral red-end domain and the CCD optical distorsion in imaging mode.


Planning the Night

1) Working environment

The working environment for MISTRAL inside the T193 control room offers a personal visiting astronomer's place, where you can install your personal laptop and connect it to an additional screen (through a VGA connection). To the right is a four-screen wall. Two are for the control of the spectrograph, and the two others are for preparation of the observations (exposure time calculators, night planner, etc.), data reduction (spectrum and images quick look tools...), web pages, etc. At the extreme right, you have a (cable-)phone (04 92 70 64 48).

The two upper screens are controlled by the MISTRALtube PC. This PC is physically mounted on the T193 telescope and it directly pilots the instrument itself. It offers a GUI to launch predefined MISTRAL observing sequences. There is the Maxim DL windows for image visualisation after acquisition, along with some pre-processing tools able to perform basic operations on the images (extraction of the flux along a line, a box, etc. + basic statistics on the regions).

The two lower screens are dedicated to observation preparation (exposure time calculators, night planner, etc.), data reduction (spectrum quick look tool...), web pages, and all personal observer's tasks. They also offers a coordinate server: This is the window showing the telescope coordinates, in principle located in the lower right screen. If not present, you have to launch the "telnet_coord.exe" icon (in the lower left screen).

The generated fits files are first stored within PC MISTRALtube. They also are duplicated in the other PC MISTRALburo. They are located by default in the "DATA/username/date". Username is the one requested in the Command Control MISTRAL window (upper right screen). Date is the current date.

2) Object observability

A planner is e.g. available from the IRIS telescope, it allows to compute the visibility of any object from the OHP site. It also allows to predict the distance to the moon and the moon illumination.

3) Estimated exposure times

— The Cook Book gives informations about the brightest observable objects still allowing a linear CCD/shutter response.

— A spectral exposure time calculator (ETC1) is available to give to the observer a typical exposure time for his/her targets in spectroscopic mode. It offers the choice of the chosen wavelength range (blue/red), of the expected seeing, of the target V band magnitude, of the required S/N for the expected most intense spectral line, of the nature of this line (absorption or emission), and of the physical shape of the target (point source or extended source modelled by a Gaussian). In order to give a quick flavour of the faintest reachable objects you can hope to measure with MISTRAL, Table 2 summarizes the V band magnitudes corresponding to a total exposure time of 1 hour, with a minimal S/N of 3, for point sources, and under a seeing of 2.5 arc-sec.

— Two other exposure time calculators (ETC2, ETC3) are available to give to the observer a typical exposure time for his/her targets in imaging mode. ETC2 gives you the exposure time needed to detect objects at a given magnitude with the requested S/N. ETC3 gives the exposure time needed to detect objects at a given magnitude with a probability larger than the requested one. As for the spectroscopic ETC1, Table 3 gives a quick flavour of the relation between exposure time and reachable magnitudes, for grizY bands and different seeing conditions.

Other useful information (e.g. the OHP sky light pollution) is available in the Cook Book.

4) Overheads and typical operating times

The maximal durations of different observing steps as recorded during MISTRAL qualification runs is given here. These maximal durations correspond to objects very difficult to locate as e.g. transients embedded in large galaxies. Most of the time, steps are therefore achieved faster than the listed durations. Some steps also depend on the astronomical object characteristics and Table 4 gives these durations as a function of several object V-band magnitude.

5) Guiding

For now, MISTRAL is using the native T193 guiding in a mode allowing to find suitable stars in 95 % of the sky regions. The main limitation of this mode is that, depending on the telescope position, some mechanical flexions can appear. This induces, in the worse cases, slew of the target position by typically 1 arc-sec within 15 minutes (without loosing the guiding star). This amplitude is the typical size of the slit. It is therefore recommended to not take MISTRAL individual spectral exposures of more than 15 minutes, and to check between two individual exposures if the target is still within the slit. This is a very short procedure (see Overheads and typical operating times) of typically less than 2 minutes.
An internal MISTRAL guiding, without such mechanical flexion, is under construction and should be ready before the end of 2021. It will offer enough sky coverage and magnitude depth to be able to guide everywhere on the sky.
More detailed analyses of the guiding capacities are given in the Cook Book.

6) Focus

In addition to times in Table 4, observer has also to schedule in his observing plan at least one telescope focus per night. This is done in general at the beginning of the night and the required duration for this operation is generally less than 30 minutes. Other (much shorter) focusses may have to be done during the night if external conditions are strongly varying.

7) Field rotation

Despite the fact that the MISTRAL instrument or slit can not be rotated in itself, the T193 telescope also allows to rotate the field. This operation can be useful when several objects are visible in the field in order to fit more than one target within the MISTRAL slit. This task is not automatized for this telescope but can be manually done by the night assistant. The typical duration of such a task is of the order of 5 minutes if you already know the slit P.A. you want to apply..


A Typical Night

0) Before the night

It is strongly recommended to estimate imaging and spectral exposure time prior to the beginning of the night. These can always be adapted during the night depending on the observing conditions.

— It is crucial to know the magnitude reachable in imaging mode versus the exposure time. This allows to predict exposure times needed to e.g. detect transients and to place them through the slit for spectral purposes. The basic way is to use the previous tables, but it is recommended to use the imaging ETCs (ETC2, ETC3) to have an exposure time more adapted to several parameters as airmass, seeing, sky transparency, ..etc... ETC2 predicts an exposure time for a given magnitude and detection percentage, disregarding the signal to noise. This case is adapted to be sure to detect a target to place it within the slit, without beeing intrinsically interested in the object image itself. ETC3 gives you an exposure time to detect an object at a given magnitude and at a given signal to noise. This is more adapted to studies requiring to use the images for themselves, for example if you want to make some basic morphological studies as star/galaxy separation.

— It is even more crucial to know for how long you need to expose your target in spectroscopic mode to reach your scientific goals. This exposure time can be estimated using ETC1. It offers the choice of the chosen wavelength range (blue/red), of the expected seeing, of the target V band magnitude, of the required S/N for the expected most intense spectral line, of the nature of this line (absorption or emission), and of the physical shape of the target (point source or extended source modelled by a Gaussian).

— Let's assume these steps have been satisfied and that you are in the telescope control room, in front of your screen wall. In principle, you should find the whole system online when you arrive at telescope. If this is not the case, the process to follow is described in Section I 9) of the Cook Book.

1) first step : offsets

These are 0 sec CCD exposures with closed shutter before or after the night. This can even be done during day time. This is the less penalizing step because offsets are very stable with MISTRAL (see the test report) and could be approximated by subtracting a constant. We note that observing Darks is not mandatory (the Dark Current is less than 3 e-/hour/pixel). They do not increase significantly the reduction quality with MISTRAL.

2) second step : imaging flatfields

This is crucial in order to correct for the CCD response inhomogeneities across the field of view. This simply consists in observing a uniform light source and then deducing the CCD response. This uniform light source can traditionally be a white screen enlighted by some continuous lamp (domeflats), or the sky itself before the dark night, when stars are still below the sky level (skyflats). The T193 telescope has no flatfield screen on the dome, so the best way is to use the sky technique.

  • Do not forget to observe skyflats in all the filters you plan to use, as imaging flatfields can be VERY different from a filter to another one (see the test report).
  • Do not observe a single flatfield image per filter as you will have no way to get rid of statistical variations. Usually, observing five flats per filter is a good compromise between statistics and time needed to achieve the task.
  • Exposure times are very variable and depend on the sky level. However, we do not recommend to expose for more than 1 minunte because it may cause the first stars to be detected (or the flat to be saturated)

More details are available in the Cook Book.

3) Getting a usable spectrum

The process of getting usable spectra of a given astronomical object consists in the following steps. Automated procedures are designed to move the MISTRAL elements for each of these steps.

— (1) point telescope at the right place: this is done in imaging mode with the most favourable filter according to the object characteristic. Starting from theoretical coordinates, the telescope is approximately pointed and a first image is acquired (mode 3 of Observing Modes: "science image", then "start exposure") and compared to a finding chart. The usual pointing accuracy of the telescope (but depending on hour angle and declination) is presently small enough to have your target within the MISTRAL field of view. The process is iterated until a satisfactory telescope position is reached. The mode (3) save the images you got. Finally a guiding star has to be found. This is the task of the night assistant. Given the actual T193/MISTRAL capacities (sensitivity, FOV), he should be able to find such a suitable star in 95% of the sky regions.

— (2) Whatever the target, you then have to determine the slit position. For this, you have to use the "Search Slit Position" mode (mode 5 in the GUI) within the Command Control MISTRAL window and press "start exposure". This moves the slit on the optical path. An image (rapid reading mode) is then automatically taken and shows the sky through the slit. You have to note manually the slit "x" coordinate (if possible in the center of slit to avoid minor potentially curvature effects).

— (3) Place target at the slit position. Two situations are possible: target is a relatively bright object and you can see it in imaging mode with exposure times typically shorter than ~10 seconds, or, target is too faint to be detected with exposure times of ~10seconds.

— (4) You can now launch a "Science Spectrum" + "start exp" (mode 7 of the GUI)

— (5) get spectral calibrations (modes 8 and 9 of the GUI): these two last steps (wavelength calibrations and spectral flat fields) involve the injection in the instrument of the light from Hg Ar Xe spectral calibration lamps and then from Tungsten spectral flatfield lamp. They can be done after step (4) and are detailed here.

— (6) Panic mode: not that it will (systematically) happen during a "typical night", but you may experience troubles with the system. So we summarize different steps to exit this panic mode.

4) Data Archival

Raw MISTRAL data will be automatically archived (see CookBook, section V4) within a database hosted by the CeSAM. Raw data are visible but not accessible during a proprietary period of 12 months to people other than PI. All calibration data are immediately public. A possibility is also offered to the observers to store/make available their final reduced data and added values through the ASPIC national service.


Night Spectral Data Quick Look

A local reduction tool is provided. It is able to give you a real-time basic spectral reduction (= a 1D spectrum) to judge if your data are good enough for your science goals. This code is based on the Automated SpectroPhotometric Image REDuction package (author: Marco Lam) and has been tuned to the MISTRAL needs. This is a Python code, available at the T193 observing room (and not requiring any Python knowledge). Iconized in the lower right screen of the mistralburo PC, it will first ask you the files you want to involve in the data reduction through a graphical interface, and the places in the raw 2D spectrum where you want to extract the object and the sky. Then, it will automatically produce a 1D wavelength calibrated spectrum.

More precisely, it starts from a raw 2D science spectrum, and uses spectral flat fields, wavelength calibration 2D spectra, and offsets that you provide. These files are listed in a ascii file created by the graphical interface (which you do not have to edit). An example of such an ascii file content is here.

With this input list, the code will automatically:

  • provide an automatic detection of the objects in the science spectral image, extract and draw a non wavelength calibrated 1D spectrum
  • make an automatic wavelength calibration along the object path : calibration lamp automatic line detection and identification
  • provide a (non flux-calibrated) final 1D spectrum taking into account the observatory extinction curve and give a visualisation of this spectrum available in linear or log scale.

The whole process is taking a few dozen of seconds and only requests to launch the "quicklook.py" icon on the reduction PC, selecting the files you want to examine, and providing the code with approximate initial and final wavelength (Lambdamin, Lambdamax), the Y line where the object to extract resides (row), minimal level detection of lines in the wavelength calibration image (level), and widths of objects and sky extraction (a, b, and c). The style of command the icon is launching is: python3.6 quicklook.py Lambdamin Lambdamax level row a b c

We give in the Cook Book the contents of "quicklook.py".


Dernière mise à jour : 05 Apr 2021  --  Cette page a été visitée   198 fois depuis le 6 mars 2021