Version 1.7 - Jul. '00
by P. Bizenberger
with help from
T. Herbst, R. Lenzen, D. Thompson
In this manual, we are assuming that the reader is
already familiar with infrared observing in general, as well as the reduction
of data into a final, usable form. If not, the reader is directed to the
MAGIC
manual, where much of this information can be found. We attempt here
to provide enough information so the user can prepare for and execute a
successful observing program with Omega Cass.
Integrating for much longer than the time needed
to reach the background limit does not provide any advantage to the observer,
since a series of independent exposures will also increase the S/N ratio
as square root of time. In fact, the variability of the sky level and other
factors make it advantageous to end an exposure when the background limit
is reached. For broad band images with Omega Cass, this usually requires
only a few seconds.
Minimum integration time is 0.84 seconds but the total time to get an image is 1.68 seconds because of the two required reads.
This mode is the standard readout mode and it is used to verify the correct performance of the electronics and cabling. Usually the electronics are set-up correct by the staff but the observer is welcome to double check. The procedure to test the performance is described in the following:
1.
Set up the camera in the GUI Control Window with:
Filter K, Optics Wide Field, Lyot blank, itime 1 sec, Repeat 3
2. Read a stack of images
3. Save the 3rd image as an individual file
4. Select this file as 'sky' in the GUI Display and subtract it from the
displayed image
(all pixels should be zero now)
5. Read a second stack of images
6. Set the radius (of the zoom) in the GUI Display to a number > 20
7. Check the standard deviation (dev) of frame 3 in the GUI Display
The displayed data (fResetReadRead minus ResetReadRead)
should be a flat image (no gradient) with a mean of ~ 0 counts and
a standard deviation (dev) of < 7 counts. If the numbers are
different or the image is not flat (some obvious stripes) please contact
the night assistant for help.
The noise is the same as for the Double Correlated Read. Minimum integration time is 1.68 seconds.
The common double correlated read out modes are 'frame orientated' i.e. you reset the hole array, read the hole array, read again the hole array and subtract the two frames. Assuming a very fast reset, this takes twice the time to read the array, to achieve an image with minimum integration time. In this case, a single pixel integrates light only as long as it takes to read the array once. The resulting efficiency is for minimum integration time 50%, changing to better values for longer integration times.
The new Full MPIA mode is 'line orientated'. You read one line, reset the same line and read it again. Do this for the hole array. Next cycle is the same. You read one line, reset the same line and read it again. To archive a double correlated image, you subtract the second read of the first cycle from the first read of the second cycle. Repeat this for the hole array. The efficiency is in this case almost 100%, it is not exact 100% since the reset is not infinite short and the first read of the first cycle (as well as the second read of the last cycle) is lost and counts as an overhead. This overhead becomes negligible when taking many frames in a row. The graph is for a stack of 15 images (Repeat 15). For a single image, this mode shows no advantage to the MPIA mode, it rather has the disadvantage of the longer minimum itegration time.
The minimum integration time increases from 0.84 to 1.68 seconds because still two reads are necessary for a double correlated read. But the integration of photons is done during the hole read process.
The readout speed depends on the size of the subframe (see table below).
/ pixel |
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for all frames / sec |
per frame / msec |
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The minimum integration time for a subarray depends of the location on the array. The pixels of a quadrant are clocked line by line. To read a subarray, all previous lines to the subarray must be clocked as well as all previous pixel in a line where the subarray is located. To avoid an overhead by clocking unused lines and pixels, the best location for a subarry is in the corner of an quadrant where the clocking of a quadrant starts. In the following image, the quadrants are labeled #1 to #4 and the direction of clocking is indicated. Taking in account that the optics have less aberrations in the center region, it is best to locate the subarray in the lower left corner of quadrant #4. At this position, the best performance for subarrays is archived. See the square, orange box.
The best position for subarrays using ALFA is the lower right corner of quadrant #1. This is due to an additional mirror of the image by the ALFA optics.
All filters, grisms and polarisiers are close to the pupil position in the parallel beam. The optics are designed for the whole spectral range from 1.0 to 2.5 µm. There is no refocusing necessary due to a filter change or optic change.
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For all temperature sensors is only one monitor available, to display all sensors switching cables is necessary.
Usually, the camera will be mounted and aligned by the Calar Alto staff but the observer must double check to guarantee a perfect set-up. Aligning will take place in the beginning of the first night of observing since you need a star to do this. To align the camera with the telescope optical axis is important in terms of background reduction. The entrance pupil of the telescope must be aligned with the Lyot stop of the camera. If you are not familiar with this procedure please see the MAGIC manual for basic instructions.
To align the entrance pupil of the telescope with the Lyot stop of Omega Cass, you have to apply two different methods for the two directions (north-south and east-west). The mounting flange can be tilted in one direction which allows the alignment in the east-west direction. The north-south direction can only be aligned by rotation of the Lyot stop wheel.
Aligning a single wheel can be done with the following
command wheel # rel xxxx in
the interpreter window of the software.
Where # is the number of the wheel (0 = optic wheel, 1 = grism wheel,
2 = lyot wheel, 3 = filter2 wheel, 4 = filter1 wheel, 5 = mask wheel) and
xxxx is the number of steps. All wheels have a different gear ratio i.e.
a given number of steps moves the wheels a different amount.
If you have aligned a wheel, you must edit the corresponding info file with the new position. Otherwise the software will remember the initial position and will use it again for further movements. After updating the info file, you also must tell the software that the info files have changed. Apply the command wheel rdb and this will update the software automatically.
Note! The info files are located in a privileged area. A password
is required to allow access, ask the night assistant for help.
Omega Cass offers you the following pixelscales for
the following telescope configurations:
3.5 m - f/10 | ~ 0.3 "/pixel |
~ 0.2 "/pixel | |
~ 0.1 "/pixel | |
3.5 m - with ALFA
f/25 |
~ 0.12 "/pixel |
~ 0.08 "/pixel | |
~ 0.04 "/pixel | |
3.5 m - f/45 | ~ 0.067 "/pixel |
~ 0.044 "/pixel | |
~ 0.022 "/pixel | |
2.2 m - f/8 | ~ 0.6 "/pixel |
~ 0.4 "/pixel | |
~ 0.2 "/pixel |
Omega Cass offers a set of broad band and narrow band filters for direct imaging. See the Technical Characteristics for a full list of filters.
At present, two grisms are available for Omega Cass:
Two additional grisms are in preparation. They are installed and available to the observer at a shared risk base.
Objects can be centered into the slit by first taking
a direct image for the selected wavelength band. The image of the object
can now be moved to the required position on the detector. Now the slit
is switched in, a direct image
is taken through the slit. Recentering is provided if necessary. For
a last step, the required grism is positioned.
Wavelength calibration can be done by using sky spectrum or by switching in a calibration lamp (Argon). The argon spectrum is given in the following wavelength ranges: 1.0 - 1.25 µm, 1.25 - 1.50 µm, 1.50 - 2.0 µm, 2.0 - 2.5 µm and 2.5 - 3.5 µm. The last range is not used for Omega Cass of course but it is for completeness.
In principle, the spectroscopic mode of Omega-Cass
can be combined with the adaptive optics system ALFA, however, using this
combination no slit rotation at the sky will be possible, the fixed position
angle is -14 deg. A rotator, that will provide slit rotation even in combination
with ALFA is in preparation.
The second mode uses wire-grid polarizers which provide
single linear polarized images. Four single images have to be taken through
the four offered analyzers which are mounted at position angles of 0, 45,
90 and 135 deg.
A plot of the relationship fot the detector in Omega Cass will be placed here. (soon)
Note that the linearity plot uses the median counts for all pixels. Some pixels are more nonlinear and others less. Observers who need accurate photometry will want to correct all their exposures for nonlinearity before proceeding with the standard data reduction. We recommend using a second or third order polynomial fit to each detector's response.
A linearization matrix is not yet available.
Telescope and Camera Configuration | J Filter | H Filter | K' Filter | K Filter |
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3.5 m f/10 - 0.2 arcsec/pixel | ||||
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3.5 m f/25 - 0.2 arcsec/pixel | ||||
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