Image acquisition with the MU script
Customizing the MU script
Focus parameters (continued)
- minHFD, maxHFD. These two parameters define a range
of acceptable half-flux diameters for an in-focus star image.
minHFD should be positive to prevent the acceptance of spurious focus
measurements with zero half-flux diameter; such spurious measurements have been produced
on occasion by FocusMax.
useTempFocusRelation. Set this parameter to
True if you wish to use a linear
focus-temperature relationship defined by FocusIntercept
and FocusSlope, otherwise set it to
If useTempFocusRelation is set to True
is greater than zero (so actual focus
measurements will be performed during scheduled focus breaks), the
focus-temperature relationship will be used to reject any focus measurement which deviates
from the focuser position predicted by the relationship by more than
focuser steps. In this case, if all focus measurements
fail or are rejected, the focuser position will be adjusted using the focus-temperature
relationship. If useTempFocusRelation is set to
focusSampleSize is zero
(so that no actual focus measurements will be performed),
the focuser position will be adjusted using the focus-temperature relationship. This provides
the fastest way to focus the telescope.
- FocusIntercept, FocusSlope. These two parameters are
the coefficients A and B in the linear focus-temperature relationship
Focuser position (steps) = A + B x Temperature (degrees Celsius).
Here the temperature must be provided by a working
To determine these coefficients, you should perform several observing runs on nights spanning
a wide range of temperatures (ideally, the whole range of temperatures which occur at the
observing site over a typical seasonal cycle). In these runs used to collect focus-temperature
should be set to a positive value, so that
actual focus measurements are made by FocusMax. The focuser position and temperature
for each focus measurement can be collected from the MU
log files, as illustrated by the following section of
a log file:
2004-07-03 04:45:29 Target: Focus Break (# 36 of 354)
2004-07-03 04:45:29 Starting image download...
2004-07-03 04:46:05 Download complete.
2004-07-03 04:46:10 Exposure saved to c:\users\PRH\040703_tel-1\images\PRH00034.fit
2004-07-03 04:46:10 Download/save time = 21.0 sec
2004-07-03 04:46:10 Camera temperature: -20.6 C.
2004-07-03 04:46:11 Using clear filter.
2004-07-03 04:46:11 Timing error: actual - scheduled = -36.2 sec.
2004-07-03 04:46:16 Temperature = 28.7 C
2004-07-03 04:46:16 Trying to focus telescope (2 measurements)...
2004-07-03 04:46:17 132 catalog stars found within search radius.
2004-07-03 04:46:23 Focus measurement #1
2004-07-03 04:46:28 Chosen focus star has magnitude 7.3
2004-07-03 04:46:28 Slewing to target: RA = 04.35211 h, dec = -28.6209 deg,
2004-07-03 04:46:28 az = 006.8583 deg, alt = 82.4534 deg,
2004-07-03 04:46:28 HA = -00.07703 h, airmass = 1.009,
2004-07-03 04:46:28 dHA = -01.84383 h, dDec = +66.3456 deg,
2004-07-03 04:46:28 dist = 071.1965 deg, MJD = 53189.19894
2004-07-03 04:46:30 Waiting for slew to end...
2004-07-03 04:47:35 Slew complete.
2004-07-03 04:47:35 Exposure time: .45 sec
2004-07-03 04:47:35 Starting automatic focusing...
2004-07-03 04:48:59 Automatic focusing successful. Half-flux diameter = 3.9
2004-07-03 04:48:59 Focus position = 2697.0
2004-07-03 04:49:00 Focus measurement #2
2004-07-03 04:49:04 Chosen focus star has magnitude 7.7
2004-07-03 04:49:04 Slewing to target: RA = 04.35791 h, dec = -28.5342 deg,
2004-07-03 04:49:04 az = 347.4309 deg, alt = 82.2724 deg,
2004-07-03 04:49:04 HA = +00.14383 h, airmass = 1.009,
2004-07-03 04:49:04 dHA = +00.18052 h, dDec = +.0554 deg,
2004-07-03 04:49:04 dist = 002.1062 deg, MJD = 53189.20074
2004-07-03 04:49:06 Waiting for slew to end...
2004-07-03 04:49:20 Slew complete.
2004-07-03 04:49:21 Exposure time: .52 sec
2004-07-03 04:49:21 Starting automatic focusing...
2004-07-03 04:50:45 Automatic focusing successful. Half-flux diameter = 3.5
2004-07-03 04:50:45 Focus position = 2704.0
2004-07-03 04:50:49 Set focuser position to 2700.5
All the focuser position and temperature data should be collected on a text file
containing two numeric columns (temperature and focuser position):
This data file would then be processed with program regress
(or any other linear regression program) to produce the coefficients A and B in
the above relationship.
FocusTol1. This parameter specifies the maximum
acceptable deviation of a focus measurement from the focuser position computed using the
focus-temperature relationship defined by FocusIntercept
and FocusSlope. A focus measurement which deviates from the
computed value by more than FocusTol1 focuser steps will be
rejected. Note that this rejection process only takes place if
is set to True
is greater than zero.
- defaultFocus. This parameter specifies a default
initial value for the focuser position. Some time after sunset (and before any calibration or
science images are taken), the focuser is set to this default position provided that a
(usually much more accurate) focus-temperature relationship is not enabled.
- FocusTol2. This parameter specifies the maximum
acceptable deviation of a focus measurement from the default focuser position defined by
defaultFocus. A focus measurement which deviates from
defaultFocus by more than FocusTol2
focuser steps will be rejected. Note that this rejection process will only apply if
is set to False or
is set to True but a current temperature measurement is not available.
- focusTimeOut. This parameter specifies the maximum time
(in minutes) which FocusMax will be allowed to work on a focus measurement. If a
measurement is still unfinished focusTimeOut minutes after it
started, the script will abort it and consider it unsuccessful. This feature is useful to prevent
certain situations when FocusMax will take a very long time to finish a focus measurement.
- SetDefaultFocusAtStart. Set this parameter to
True to focus the telescope before any calibration or science images
are taken, otherwise set it to False. If
is set to True,
the focuser position will be computed from the focus-temperature relationship. If
is set to False,
the focuser position will be set to defaultFocus.
focusAtCurrentPosition. This parameter affects the
sky position where the telescope will point at the time the focus is adjusted during a scheduled
It has no effect unless
focusSampleSize is set to
0. In this case, if
focusAtCurrentPosition is set to False,
the telescope will slew to the sky position defined by
before adjusting the focuser position. This is convenient
when the telescope control software automatically compensates for focus changes due to altitude
variations (as is the case with the SciTech telescope control software), so that focusing should
be performed at a standard altitude (such as at zenith). If
focusAtCurrentPosition is set to True,
the focuser position will be adjusted with the telescope at whatever position
it was pointed before the beginning of the focus break.
- giveUpFocusIfFMGoesBad. This parameter affects the
way the script will handle situations when FocusMax behaves unreasonably; such events,
which have been observed on several occasions, may involve errors when creating or
destroying focus control or focuser objects, setting exposure times, etc. When one of these
events occurs, one may experience persistent problems in subsequent calls to FocusMax,
which may be avoided by giving up all subsequent operations involving FocusMax (which
may result in poor focus later in the observing run). If
giveUpFocusIfFMGoesBad is set to True,
all operations involving focus will be cancelled if the script notices signs that
FocusMax is behaving unreasonably. If
giveUpFocusIfFMGoesBad is set to False,
the script will not cancel all focus operations even though FocusMax may give
signs of unreasonable behavior.
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© 1999-2004 Paulo Holvorcem