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There is no slew to locate the star, or telescope motion to center it up, whence comes the term "inplace focus". The focus setting value used as the center of the sweep is called focbase; the script first sets the focus to that value. The routine then "acquires" the star by defining a standard sized (200x200 now 300x300 at 2:2 on LMI) subframe around the given position and taking a single image. Analysis is performed on this image- if the star is sufficiently bright, in terms of instrumental mag. and maximum pixel value, but not saturated, and the fwhm is reasonable, the subframe is redrawn to place the psf in the center of the image. If the image appears reasonable, but either too bright or faint, the exposure time will be adjusted within certain limits and the exposure retried. If this is still out of specification, the focus script ends with a failure status.
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x | x coordinate in pxls of star on a full frame image at the selected binning; there is no default. | |
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y | y coordinate in pxls of star on a full frame image at the selected binning; there is no default. | |
title | this is used for a heading in the focus log; may be multiple tokens- default is empty string | |
bin | integer for binning factor- LMI default is 2. 1:1 has also been tested. Note x,y coordinates must be in the same scale. | |
exptime | exposure time in seconds for acquisition and focus sweep images. May be adjusted. | |
filter | name (like "R") of the filter setting to do the focus sweep. "Nominal" means use the filter as previously set. Leave this blank for LMI. | |
focstep | delta for focus setting per sweep position. (defaulted to 40 microns on LMI) | |
focbase | setting of focus for acquisition and sweep center. This has a reasonable default (800 microns on LMI). | |
flags | strings, all optional, defined in any order, as follows: | |
| save ( put images taken in focus sweeps in a subdirectory in the main image directory.) |
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| conditioning ( focus run is a dry-run; the final focus setting is always focbase.) |
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| noexptimeadjust ( suppress autoadjust of exposure times for both acquisition and focus measures.) |
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| lowlight ( use lower thresholds for light level in acquisition & focus ) | ||
low ( same as lowlight) | ||
| longexposure (allow up to 60 seconds exptime) | ||
| long (same as longexposure) |
Exptime is a floating point value for exposure time in seconds, defaulted as 1.0. If specified as two tokens, the second value is the number of singles to be taken, defaulted as 1. If specified as three tokens, the second value is the number of basic occultations to be taken, and the third value is the number of frames (NAXIS3) for each occultation. Occultations not currently available for LMI.
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- The LMI version of focus can't adjust the filter (because even though LMI LOIS can't, now).
- The 'fom' ratio is peak/flux- it would be better to use peak/(flux*etime). This could only affect the focus outcome if the etime varied during the sweep, which is possible with the exposure time adjustment. In practice this rarely happens, because the adjustment is already done in acquisition.
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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.
LMI Definitions Applicable To The Focus Script
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biaslevel | 1000.0 | average bias level for A amplifier |
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occul_avail | 'no' | 'yes' or 'no', whether or not occultation exposures can be specified for focus |
foc_coloravail | 'none' | available filters for focus- 'none' means current (nominal) only |
foclog | 'testlmifocus.log' | log file name is obs acct home directory |
focfilter | 'nominal' | filter default when not specified to inplacefocus or stdfocus |
focstep | 40 | default size of focus steps in applicable units (microns for LMI) when not specified to inplacefocus or stdfocus |
focstep_max | 200 | maximum focus step size that can be specified to inplacefocus or stdfocus |
focstep_min | 5 | minimum focus step size that can be specified to inplacefocus or stdfocus |
focbase | 800.0 | default focbase in applicable units (microns for LMI) when not specified to inplacefocus or stdfocus |
focbase_max | 1500.0 | maximum focbase that can be specified to inplacefocus or stdfocus |
focbase_min | 10.0 | minimum focbase that can be specified to inplacefocus or stdfocus |
focexptime | 3.0 | default exptime in seconds when not specified to inplacefocus or stdfocus |
focexptime_min | .1 | minimum exptime that can be specified to inplacefocus or stdfocus |
focexptime_max | 10.0 | maximum exptime that can be specified to inplacefocus or stdfocus |
| focexptime_supermax | 60.0 | maximum exptime that can be specified to inplacefocus or stdfocus (with longexposure flag) |
focfaint | 14.0 | minimum instrumental magnitude allowed for a focus image source. |
focminfwhm | 3.0 | minimum full width half maximum allowed for a focus image source. |
focminpeak | 2000 | minimum max pixel in ADU's allowed for a focus image source. |
| focminpeaklow | 1400 | minimum max pixel in ADU's allowed for a focus image source, if the lowlight flag set. |
focsaturatedpeak | 50000 | maximum max pixel in ADU's allowed for a focus image source- set to 0 for no saturation test. |
focadjustlims | { .6 1.6 } | minimum and maximum ratio of adjusted exptime to original for focus step retries |
focadjustetimelims | { .05 15.0 } | minimum and maximum ratio of adjusted exptimes in seconds for focus step retries |
foctry | 2 | total tries allowed for a focus step. |
focbin | 2 | default camera binning when not specified to inplacefocus or stdfocus |
focphotannuli | { 20 28 35 } | size in unbinned pixels of object, inner sky and outer sky annuli for focus sweep photometry |
focdelay | 10000 | delay in msecs after a focus move during focus sweep- large for LMI because move is open loop |
focretrydelay | 3000 | delay in msecs before a focus retry image is taken |
acqminpeak | 10000 | minimum max pixel in ADU's allowed for a acquisition image source. |
| acqminpeaklow | 2800 | minimum max pixel in ADU's allowed for a acquisition image source, if the lowlight flag set. |
acqsaturatedpeak | 30000 | maximum max pixel in ADU's allowed for a acquisition image source. |
acqadjustlims | { .2 4.0 } | minimum and maximum ratio of adjusted exptime to original for acquisition retries |
focadjustetimelims | { .05 15.0 } | minimum and maximum ratio of adjusted exptimes in seconds for acquisition retries |
acqtry | 1 | total tries allowed for an acquisition- really 2. |