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WORKING WITH ACAMACAM is a combined optical imager and low resolution spectrograph. In imaging mode is has a full field of view of 8.5 arcminutes in diameter and a pixel scale of 0.25 arcsecond/pixel using the AUXCAM detector. At sky PA = 0, North is right and East is up. It has two filter wheels which hold up to six filters each (thus 7 positions per wheel to allow for a clear aperture in each). Wheel 2 (the one closest to the detector) also holds a VPH (Volume Phase Holographic) grating. This is a transmissive grating for spectroscopic mode. The grating (called V400) offers a spectral resolution of 450 with a 1 arcsend slit at 6000Å, a second higher resolution grating will be offered in the future. A slit slider at the back of the instrument (closest to the telescope, before the collimating optics) offers 6 different slit widths (0.5, 0.75, 1.0, 1.5, 2.0 and 10 arcseconds) and can also hold up to two filters for narrow band imaging or calibration masks. The slits are provided by fixed width masks rather than by a movable slit jaw as is the case with ISIS. In standard operation at least one filter / mask position in the slider will be left empty. If both are in use then there is no clear aperture left for imaging with filters in the main filter wheels, though spectroscopy could still be carried out.
DocumentationAcquisition Tool manual
ACAM Software Manual
CAGB Software manual
Filling the CryostatThere is an automated LN2 filling system for ACAM (and for other instruments). On arrival at the telescope at the start of the night, check the display of the filling system in case of problems with previous fills. Initiate an immediate transfer of LN2. This should be done early since it can take a long time to fill all the Cass cryostats if other instruments are also mounted, and the system also needs at least 10 mins to warm up before removing it from Cass to prevent the tubes cracking. At the end of the night, simply reconnect the system and it should fill ACAM automatically after a 60-second countdown, without any input.
OR: goto the observers control GUI and click the ACAM button in the 'Mirror Control' section of the ACAM Observer tab or the A&G tab.
The telescope focus is measured by the observer. At the start of each night (maybe unless the seeing is realy bad) they measure the best telescope focus taking exposures at different focus positions of a standard star, preferably in the Bessell R filter as all other focus offsets were measured relative to this. They usually use focusrun to do this. For spectroscopy, there is an automatic focus offset that is applied when the grism is deployed, but this isn't necessarily reliable, so they might want to check the focus in spectroscopic mode too if there's time.
Rotator centre and Calibrate Procedure:
Use the standard autoguider for CASS with camera AG6. If not already running, start the autoguider software from UDASDEV2 (whtdas18), with 'obssys
1', 'startobssys' and 'startag AG6'. No windowing is required. A DS9
display tool will be pop up as well as the TV Guider Control GUI. This
GUI allows you to change exposure times, take fields and
set the TV and Guider going. Images are piped directly to the display tool.
At the start of the night check that the autoguider filter AG-CLEAR is in position, by looking at the mimic. This filter is suitable for optical observations, such as with ACAM. If the filter needs to be changed, type at the TO prompt:
TO@taurus> autofilt ag-clear
For a telescope focus position of 98.05,
best autoguider focus is about 2400 (2011). This can be set from the TO prompt:
TO@taurus> autofocus 2400
NB:The autoguider focus and filter mechanisms can also be moved from the observer's GUI.
GSS2 is used for finding guide stars. To start the GUI, double click on the WHTGSS icon on OSADISPLAY2 and enter the password, the GSS2. Alternatively, open a new terminal window and type:
[whtobs@todisplay1 ~]$ ssh -Xl gss whtgss (same password)
Documentation for gss2 can be found here.
TO@taurus> prag radial theta
Note that the theta area between 70,000 and 102,000 is vignetted by
the structure holding the ACAM fold flat. Also the radial value
provided by gss2 will be too high by ~ 4000 units, but this will be corrected in the new version.
The field of view of the autoguider is 60 x 60 arcseconds. Changing
radial / theta results in X / Y movements of the star on the autoguider
Observing: Imaging AcquisitionSimply gocat to an object in the TCS catalogue. At sky PA = 0 (the standard orientation), North is right and East is up on the ACAM detector. Apply any offset the observer requests (in X/Y or RA/DEC) and the target is acquired, so you can start guiding.
Note the observer's GUI will allow them to put multiple optical components in the beam (e.g. a filter in wheel 1, a filter in wheel 2 and a slit or filter in the slider!). Obviously we only want one filter in place, so it's a good idea to check on the mimic that everything is as it should be.
A focus offset, whose magnitude varies with the filter in use, is sent to the TCS when the filter wheel position is changed, just as with Prime focus imaging, for example.
Sometimes dithering is used between ACAM exposures. This can be done efficiently during the long readout. Simply turn off guiding (F8 while in TCS USER), use the handset to make the required offset, then turn on guiding again with F9.
Observing: Spectroscopic Acquisition
Unlike with ISIS, there is no slit-reflection and no acquisition TV, however there is a new and fantastic Acquisition Tool which can be used by the TO (usually), SA (if they want to) or observer (if you show them how to use it), which speeds up acquisition considerably compared with the old method. The above document is extremely detailed but the basic acquisition sequence is described in Section 10.1.
To start the Acquisition Tool:
TO@taurus> acqtool &
Now take an acquisition image. Measure the position of the target to be observed (use the 'r' option in imexam if possible) and calculate the pixel offset between this and the slit centre. Divide by four to get the offset in arcseconds (0.25 arcsecond pixels!) and apply this using the TCS handset in XY mode.
Note you MUST have Instrument ACAM set in order for the handset XY directions to be correct!
The above procedure may have to repeated to get close enough to the desired pixel. By then the target should be visible in a through-slit image (with no grism). For any further small adjustments to position, the guiding loop should be closed to prevent loss of acquisition during the process. The object in the slit should be visible with a longer exposure, e.g. 15s for a 20 mag object was found sufficient, however the right scaling in DS9 must be used. This procedure is considerably quicker than it sounds, about 2-3 mins if the target is not too faint. Objects as faint as 22.5 mag have been acquired.
Observing: Spectroscopic Calibration
The observer will usually do all the calibrations required, so this is just for information. To direct light from the calibration unit to ACAM the old small feed
mirror is used. Either click on the ACAM Cal button (see ACAM Observer
tab on the GUI) or type:
You can turn the calibration lamps on and off via the GUI or with the commands:
Unless you have previously obtained training in changing filters please do not attempt to change them! General instructions on filter handling and care can be found here. After any filter change the mechanisms for both filter wheels and the slit unit will be in an undefined state. You will need to initialise them via the ACAM Eng tab on the GUI or via the command line with the following command:
Alternatively you can initialise each mechanism individually:
It is also essential that the filter database be updated.
Data HandlingYou can write DVDs on WHTDRPC2 (in the control room) or WHTPC1 (in the terminal area). When you put a blank DVD in either machine it will be automatically recognised and you will be given the option to 'Create data DVD with K3b'. Select this, the software is very easy to use and needs no explanation. Spare DVDs can be found in the astronomy office.
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