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Two of these parameters deserve some explanation. The bin value is not editable and is pre-filled based on the setting chosen in the previous full frame image. In the case of the LMI, the Focus Base is adjusted from the value in the config file by the differential focus offset value for the current filter. So if the value in the config file is 750 µm 1540 µm and the current filter is V, the default value would be 920 µm1710 µm.
The first time the "Focus Run Parameters" dialog box comes up, the values will be the default values which are read from the config file. Most of the times, the default values are a very good place to start. But if for some reason, the user decides to change some of these values, they will be saved and used the next time the dialog box comes up. If a pre-filled value in the text box is not the same as the default, the color will change to red and the suggested default value will display in front of the text box. In the example above, the focus base is set to 850 1540 which would be the saved value from the previous run and we see that the suggested default is 20.
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The focus value is ordinarily defined to be the position of a focusing element. On LMI, the definition is somewhat abstract: it is the offset in microns from the nominal focus selection of the AOS for M2 as mentioned above. This is typically between 750 - 900 around 1540 microns with no filter in place, the open position. Note that there is no internal focusing element in the LMI instrument.
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You should preset the focus offset to 800 1540 microns (or any known happy focus value from experience) using the stdfocus button or asking the telescope operator. You will also use this for focbase.
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- Changing the focus takes too long, because it is being run open loop; defining when a focus offset move is complete turns out to be tricky.
- There is a bug in the CCD firmware that will cause anomalous saturation artifacts to appear on the left hand part of a subframe (given an A amplifier readout) either in a bright background situation (twilight or near the Moon) or if a star that is considerably brighter than the star being used for focus is located to the left of the focus/acquisition subframe. If the pixels of the artifact intrude into the focus/acquisition analysis box, these will be picked up instead of the star and usually cause a focus failure. The size of the focus/acquisition subframe has recently been increased, without expanding the analysis box size, which has substantially mitigated the problem. It's now possible to focus within a few degrees of the Full Moon.
LMI Definitions Applicable To The Focus Script (THESE VALUES AREN'T CURRENT)
These are all in the LMI TCL definitions file. It is not recommended for observers to modify these.
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