The astronomical object which is under study.
This is the object used for the adaptive optics correction. It must be located less than 30 arcsec away from the science object. Even better, it may be the science object itself, providing its intrinsic angular size is smaller than 3 arcsec. The visual magnitude of the reference star can reach 12.5.
Because residual errors remain on the adaptive optics wavefront
sensor, a deconvolution process should be applied. This is done using
the point spread function (PSF) determination of a so called PSF
star. In order to get the same correction quality with the
If the science object is not used as the reference object, then the best deconvolution results are obtained by using a binary for which the binary separation is the same as the separation between the science and reference objects. One binary component will be used as the reference and the other as the PSF. Both components should obey the flux and positional constraints listed above. In practice, you have to be lucky to find a suitable nearby binary.
Closing the loop
When the loop is closed, adaptive optic corrections are taking place. That is, the wavefront sensor is monitoring the difference between the actual atmospheric abberations and the most recent position of the deformable mirror.
There are a number of ways to close the adaptive optics loop. The most
frequently used methods are as follows:
Close loop with zonal optimization - This is much faster (around 1 minute) than closing with modal optimization. However the final correction is not as good. In zonal control, each zone or segment of the mirror is controlled independently by wavefront signals that are measured for the subaperture corresponding to that zone. If the object is a photometric standard or simply a test than the loop may be closed with zonal optimization. Note: For standards, most observers simply close the loop without optimization (see below).
Close loop without optimization - This method closes the loop without acquiring a new optimization. It requires less than 3 sec and is used, for example, when the telescope returns to a science object after a series of sky frames or when the object is a PSF or standard.
Close loop on the PSF - The correction for the PSF must be kept as near as possible to that made with the science object. To do this the telescope operator must adjust the filters and high tension on the wavefront sensors in order to achieve the same flux and noise as the previously observed science object. This takes about 2 mins. Then the loop is closed without optimization (see above).
The observer need only decide what method he/she would prefer. The telescope operator will execute the commands.
Correction of residual abberations
It is very likely that at some point during the observing night a further correction will have to be made for static abberations. Such abberations can appear when observing a scientific target and can change with airmass. To correct for them a series of adjustments are made (e.g. focus, astigmatism, coma, triangular and spherical) by the telescope operator. This process can take up to 15 minutes. The observer need only decide what method he/she would prefer. The telescope operator will execute the commands.
Selecting the centre of your desired Adonis image can be confusing if you don't know the difference between the following centres:
Infrared (IR) centre - This is the centre of the camera field of view and is defined by the observer using the ON/OFF mirror with a specified offset in arcsec. Usually this position corresponds to the centre of the desired science image.
Moving the telescope
In this case the on/off (M6) mirror remains fixed. The telescope operator opens the adaptive optics loop and applies an offset to the telescope pointing. Once the sky frame is taken the telescope returns to the original pointing position and the loop is closed without optimization.
They are several reasons to use this mode:
Moving the M6 on/off mirror
In this case the on/off mirror chops so that the camera field of view can be centred on different positions within the available field of view. Double or triple chopping is possible. The observer defines the frequency and amplitude of the chopping via the ADOCAM software.
They are several reasons to use this mode:
Example 1: The scientific target is the reference star for the Wave Front Sensor (WFS).
Example 2: The scientific target is distinct from the reference star.
In order to plan correctly the time needed for an observation, the astronomer should be aware of the following overheads:
Data are written to one of two disks (/s3 or /h1) on the ADOCAM computer.
Data do not entirely follow the FITS standard. The dimensions of each image are 256x257 pixels. The extra 257th column is used to write the UT time of each individual frame. To reformat the ADONIS FITS to a standard FITS, you can use the ECLIPSE refits command.
The output images are cubes. Each cube contains data from 1 observing cycle, with each plane corresponding to an individual frame.
Here is an example of an Adonis FITS header.
At the beginning of the night the observer must enter a name and a nickname which is 3 letters long (e.g. xxx). A directory named xxx_yymmdd_hhmmss.DIR will be created (where y, m, d, h... stand for year, month, day, hour...). This directory will contain a logfile (xxx_yymmdd_hhmmss.LOG) and the raw data files (xxx_yymmdd_hhmmss.FITS).
For each cube, all the settings of the detector are logged automatically in a file. The settings include integration time, lens scale, filter, observing mode, sequences and ON/OFF position. The user has the option to include the target name and coordinates.
The detector has no communication with the telescope and so does not know what the airmass, seeing or coordinates are. Therefore we suggest that the observer fills out an Adonis log sheet (provided).
The telescope operator will keep a log of the adaptive optic corrections made for each observed source. This log includes the flux and noise measured on the wavefront sensor, the estimated strehl ratio and seeing, type of optimization used and the parameters r0 and lcor. If you would like a copy of this log then please inform the operator.
Each day the operations staff write the previous night's data to dat tape using WDAT. Two copies are made. One copy is for the observer and the other is for ESO.
At the end of each night the observer must list (on a form provided) what directories are to be backed up. If this is not done then the operations staff will not know what directories to backup.
Once the observer receives their dat tape it is essential that they
verify its contents as soon as possible. In most cases their data will
be deleted from disk before the start of the next night. The dat tapes
must be read with WDAT. This is part of the
reduction package and is available on all machines in the Observer's
With a DAT driver called /dev/nrst0
To write in the file "listing.lis" the content of the DAT tape:
wdat -d /dev/nrst0 -l= listing.lis
Note: Under no circumstances can data be transferred from the ADOCAM computer to another work station during data acquisition. This will cause ADOCAM to crash and halt observing.
The observer should have finding charts for each science object as well as for any PSF or standard stars. The telescope has a centre field camera with a field of view of 2.5 x 2.5 arcmin. The display is orientated North (down) and East (left).
It is possible for the telescope software to use a source catalogue file. The file must have the extension.cat and the following format:
source 033603.2 162802.6 2000.0 2000.0 0.0 0.0 m
where "source" is the object name, "033603.2" is the RA, "162802.6" is the Dec and "2000.0" is the epoch. The line must end with "0.0 0.0 m" which sets the proper motion offsets to zero. The separations must be spaces and not tabs.
This file can easily by ftped to the telescope computer at the beginning of the night.