The Mock Spectrometer

The Mock spectrometer

The Mock spectrometer is becoming available for use. The work is following a staged approach where the first goal is to be able to use it with ALFA. A second stage will be to use it in a fixed mode to observe a 1 GHz band with a single-pixel receiver and the final stage will be to offer more flexibility for single-pixel receiever observations.

The first observations with ALFA started in August 2008. Observing with the Mock spectrometer is still a bit experimental, but we are expecting to improve the situation as we get more experience with the machine. It can be used either alone or in parallel with the WAPPs for all observing modes available for ALFA observations. CIMA version 3.0 is the first version of CIMA that supports the Mock spectrometer, so far only for ALFA observations, but observers are recommended to use CIMA version 3.1 which offers a better support.

Use of the Mock spectrometer is fairly straight-forward; observations are done in the same way as before and the only major change in the appearance of CIMA is the addition of a new window for the configuration of the new backend:

The window is divided into three parts by two orange lines. The top part contains the two groups consisting of seven Mock boxes each, and the parameters given in this part can be set up independently for the two groups. The middle part contains some parameters that apply to both groups, while the bottom part contains various control buttons.

Several of the parameters in the first part of the configuration window are fairly self-explanatory. The bandwidth is selected by specifying a bandwidth reduction factor, and the actual bandwidth is then calculated as the clock frequency divided with this factor. Frequency offseting is only possible within the band if You are using bandwidth reduction. The blanking parameter allows You to select that FFTs should be blanked whenever there is an overflow detected in the A/D conversion. The four report parameters allow You to select how many overflows You are tolerating in various stages before the spectrometer should print out a warning. The 'File type' parameter allows You to select the format of the output files which can be either FITS-files or the native (and not recommended) PDEV format. The 'Disk data rate' is a calculation to show You the expected data rate for each beam. The design rate is 50 MB/s, but it seems that somewhat higher rates are sustainable.

The first two lines of the middle part deals with the clock frequency and LO frequencies of the the two Mock spectrometer synthesizers. Although You can play around with these values, for normal ALFA observations, You are supposed to use a clock frequency of 172.032 MHz and LO frequencies of 175 and 325 MHz. The 'Use default freqs' button will restore these settings to their recommended values, if You have been playing around with them. The 172.032 MHz clock frequency has been chosen so that You can select sampling times that are multiples of 1.0 milliseconds. You should also select a rest frequency of 1375 MHz and the 'Full width' filter option in the 'Receiver IF/LO selection menu to ensure full coverage of the ALFA band as shown in the following figure:

The two 'Winking cal' lines in the Mock configuration window allows You to specify whether You want the Mock spectrometer to generate an alternative winking cal signal to use instead of the default 25-Hz signal. The advantage with this is that You are free to design Your own winking cal signal that can have another frequency as well as different lengths of the ON and OFF phase. Note though that when using the Mock spectrometer as a winking cal generator, the winking cal signal is only available when the Mock spectrometer is taking data. If You want to run the WAPPs in parallel, the start times will not be fully synchonised and You may thus get a few seconds of data in the beginning or in the end of a scan WITHOUT winking cal in the data from the WAPPs. The 'Debug level' button allows You to specify how much feedback You want from the Mock spectrometer.

Of the 4 buttons in the bottom part of the configuration screen, three deals with power level adjustments: the 'Power control' button brings up the power control window where You can adjust the power levels of the different parts of the IF/LO system, the 'Power levels' button brings up the power level monitoring window (which also can be run outside CIMA as the 'powerwin' program), and the 'Adjust Mock power' button which adjusts the power levels on the Mock spectrometer (but nowhere else). The 'Fix parameters' button is a different story. It is normally disabled and is only enabled if there is one or more numeric values that needs 'fixing', i.e. it is not an allowed value and needs a bit of tweaking to become a normal value. Such values are shown in yellow background and they can be 'fixed' either by pressing the 'Return' button when the focus is in the entry field with the problematic value or by pressing the 'Fix paramaters' button. You can step up and down in the list of possible values by using the up and down arrow keys when the focus is in the entry field. In the example below, the sampling time of the second group of Mock boxes is bad, since You can't get a dump time of 0.1 milliseconds as the observer has typed in.

Hitting the 'Fix parameters' button in this situation will adjust the value to the closest possible value which happens to be 0.997 milliseconds. CIMA will also inform You in the log display window about the next lower and higher values which in this case are 0.0982 ms and 0.1012 ms. You can select these values instead by pressing the 'Up' or 'Down' arrow on the keyboard.

You can return to the main CIMA page by clicking here.

This page is administered by Prakash Atreya ( patreya (a) naic . edu ) and was last updated on 14 January 2009.