This instrument was built primarily for spectroscopy. EFOSC2 is one of the most efficient instruments available when low resolution (dispersion < 1000) is the need of the hour. The different science spectroscopic modes modes are described below. Note that there is a separate page for the Multi-Object Spectroscopy mode. The observer is also referred to the pages on Acquisition images and Miscellaneous information.
The median seeing at La Silla is 0.8", so the unbinned pixel size of 0.12" spreads the light over a large number of pixels in the spatial direction. The binned modes adequately sample the resolution afforded by all slits. Thus unless spectral resolution is of first importance and one has bright targets using the CCD in the binned mode is strongly recommended. At the NTT, binning modes of 1x1 and 2x2 are currently ofered, while we plan to offer 1x1, 1x2, 1x3, 1x4, 2x1, 2x2, 2x3 and 2x4 modes in the near future.
In this mode one can obtain one or more spectra of 1-2 objects through 1 or more grisms. However, it is essential that all templates in an OB have the same slits. There is no provision for moving the object along the slit from one exposure to another.
The following acquisition templates may be used in this mode
The above example shows an OB comprising a sequence of:
The FITS file is called EFOSC_Spectrum.#.fits
Parameter Values One does not have to (in fact, should not) change the value of any parameter other than:
This is very similar to Long Slit Spectroscopy except that the telescope is moved across the extended target while integrating in order to sample the spectrum across a larger part of the target. There are 2 ways in which this can be achieved:
1. Unguided scanning across the target
One can apply a velocity to the telescope in the RA and/or DEC directions
after the shutter has opened and with the AutoGuider turned off. This is
to be used when the telescope scans many tens or hundreds of acrseconds
during an exposure or if the required velocity is large (say, less than
0.1 arcsec/sec). Since the telescope is not being guided the motion is
not precise and one may have a 1" wander around the desired track.The problem
with large translations is that the guide star may fall off the edge of
the guide probe field. In the case of large velocities the star may move
out of the target square between one guide correction and the next (done
every 5 seconds). Follow the recipe given below :
You will see the guide star starting to move away from the guide target
area once the guiding is switched off. If one wants to repeat the exposure,
with the same grism or another one, one can ask the telescope operator
to offset the telescope so as to bring the guide star back into the guide
target area - this will ensure that the telescope is again pointing to
the starting position. This should be much faster than requiring the image
with an acquistion template. An alternative is to repeat the telescope
movement backwards for the next exposure.
Some other Issues
Try and reduce the scan speed so as to be able to use the differential tracking method; aligning the slit along the major axis of the extended source is one way of soing so.
Always confirm the direction of motion of the telescope - there are several sources of confusion : whether the velocity refers to the movement of the telescope or the movement of the source across the field. One should ask the support astronomer to confirm the sense by trailing a star on the image with a velocity of known sign.
These observations require constant communication between the observer and the telescope operator and manual intervention by the latter over and above the regular actions - there is scope for a lot of confusion. Try to keep the intervention as simple as possible by, for example, confining the scan to just RA or DEC and providing clear instructions to the telescope operator.
The SpectroPolarimetric mode is very similar to the Long Slit Spectroscopy mode except for the following differences:
Any one of the 2 available SpectroPolarimetric Masks/Slits may be mounted on the aperture wheel but the name in jP2PP will be the same. The Wollaston Prism in the grism wheel must be aligned in a manner appropriate for a parallel mask.
Important When the half wave plate (HWP) is set to continuous rotation in a template the counter ends up with 360xN degrees. When a signal is sent to set the HWP to some small fixed value (say, 45 degrees) it tries to undo all the N rotations and in doing so triggers a time-out error. So we have included a command at the end of every Polarimetry template to initialise the HWP if it was set into continuous rotation mode - this just takes a few seconds. However when such a template is aborted, for whatever reason, this initialisation does not take place. The only solution is to cold-start the EFOSC2 instrument which takes about 1-2 minutes. So avoid aborting a Polarimetry template - use the STOP option instead.
A sample jP2PP observing block
The above example shows an OB comprising a sequence of
Parameter Values One does not have to (in fact, should not) change the value of any parameter other than:
The use of filters in combination with a grism can be useful to isolate
the spectral region of interest and to reduce crowding and sky background
intensity. Note that the spectral coverage depends on the position of the
objects. Measures are needed to define the wavelength ranges as a function
of the object position and the overall efficiency in this mode.
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