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How to use ING's online seeing data
Probably you've noticed the seeing value on the ING Weather
Station web page after dark. But do you know where this value comes
from, what it means or how to compare it with the astronomical data you are
taking?
Since 20 August 2002, RoboDIMM has replaced the INT WFC
as the source of the seeing data on the Weather page. These seeing data are
provided as a guide to observers during the night, mainly to help them
adjust their instruments in real time and so ensure they are getting the
best image quality obtainable in their astronomical data.
RoboDIMM measures the distortion of the wavefront by atmospheric
turbulence and, from the statistics of that distortion, calculates the seeing
FWHM that would be obtained in astronomical data using a commonly accepted
method. RoboDIMM currently takes about 2.5 minutes to take a sample of the
turbulent distortion (200 images). Adding to this the time taken for a caulcation
and other software tasks means that the minimum time between measurements
is currently about 3 minutes. It increases when seeing is worse than about
1.2 arcsec. We hope to speed the sampling up in the future by optimising
the software. (The IAC DIMM's intensified camera allows a seeing calculation
every 16 seconds approximately!)
The seeing value published on the Weather Station web
pages is a moving average of the last 3 (?) RoboDIMM FWHM estimates. The
moving average of course tends to smooth out the variation and the time between
non-overlapping samples is thus a minimum of 9 minutes. Since the web page
updates every 5 minutes, you therefore have to wait for 2 page updates to
view the seeing from a completely new sample. The seeing value is unique
on the Weather page in that it does not indicate current conditions, but
rather those prevailing during the preceding few minutes!
Individual RoboDIMM measurements can be viewed in the
database, accessible in ASCII form on the web. Each measurement really consists
of 4 simultaneous FWHM estimates, calculated from the image distortion in
different axes. These values should on average be equal and are intended
to be used as a check on the reliability of the RoboDIMM data. The value
displayed on the Weather pages is an average of all four, with the intention
of making it less susceptible to bias, particularly that caused by dominant
wind directions in the various turbulent layers.
The randomly varying nature of turbulence means that the
10 second exposure you just took, having perfectly tuned the telescope and
instrument, will not necessarily give the same FWHM as that displayed on
the web page. Depending on how variable the seeing is, you must average the
image width in your data over a period of at least about 5 minutes before
comparing it with estimates from a seeing monitor. Only then should you start
considering that other contributors to image size may be present. Successive
values in the dataabase can give an idea of how variable the seeing is.
Checks performed on data so far
Just two days after RoboDIMM's first night of operation on 18 August 2002,
ING made seeing measurements from this monitor available through its weather
pages. The intention was to provide as early as possible an up-to-date estimate
of seeing as a guide to users of our telescopes, even though over the
And here ...
Improvements planned to RoboDIMM
Wind vibration, mask weight, quicker and more continuous measurement
Dome remote control installed
And so on!
RoboDIMM operational for NAOMI
And so on!
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