|Home > Astronomy > Site Quality > Atmospheric Seeing Measurements at ING|
Atmospheric Seeing Measurements at INGING provides its observers with online seeing measurements, updated throughout the night. These measurements are taken by its Robotic Differential Image Motion Monitor (known as RoboDIMM).
RoboDIMM is intended foremost to provide ING's observers with a real-time guide to seeing conditions. For this purpose it is useful, however we would like to stress that calibration tests suggest it is NOT a reliable instrument for long-term statistics or site testing, as discussed in the next section. There are some caveats attached to using RoboDIMM as a real-time seeing monitor, and they are described here. The main requirement for a site testing monitor is to provide reliable statistics and the sources of error on measurements must be fully characterised. This is not the case for RoboDIMM and we have not published a figure for measurement error. RoboDIMM probably lacks enough sensitivity at low seeing values to be considered a "site testing" instrument. In any case, extensive site testing data has been taken at ORM, as evidenced by campaigns listed here.
In 2007 ING installed a MASS-DIMM turbulence profiler next to the RoboDIMM. As a result, data of turbulence strength at different altitudes, isoplanatic angle and the characteristic time scale are now being taken on a regular basis at ORM. We hope to make these data, or derived statistics, available through these pages before long. Scientific reference for the MASS-DIMM instrument
Accuracy of RoboDIMM dataWe stress that RoboDIMM was not designed as a site characterization tool. If data are used for the purpose of site characterization, we urge the user of the data to pay attention to calibration issues.
We have made it known since 2004 that RoboDIMM measures good seeing less frequently than expected from the statistics of previous campaigns (see DIMM 1995-98)) , but we do not ascribe this to worsening seeing at ORM. Rather it appears that RoboDIMM is progressively less sensitive as the image motion, hence seeing, decreases.
For this reason, I am reluctant to publish statistical seeing values from RoboDIMM, because it might give the impression that median seeing had increased since our 1995-98 campaign for Dome Seeing measurements .
Shortly after it started operating in August 2002, several samples of a few hour's length were used to show that RoboDIMM seeing agreed with the the IAC's DIMM seeing monitor to within better than 10%. However, this result is valid in the range 0.63 to 1.1 arcsec and for one night only. RoboDIMM also shows a similar level of agreement with SloDAR and JKT-SCIDAR monitors in summer 2004 and January 2005. More detail of this comparison is available. The algorithm used by RoboDIMM to calculate seeing is made publicly available here .
RoboDIMM descriptionThe Monitor: a Meade 12" telescope with an SBIG ST-5C CCD detector at Cassegrain focus, controlled via serial link by a PC running custom software. Instrumental focus is also under PC control. The same camera head has been used since installation in August 2002! The controller was changed for an identical model after a lightning strike in 2007. Image scale is 0.72 arcseconds per pixel.
The telescope forms four separate images of the same star on the CCD thanks to a pupil mask with 4 subapertures covered by small-deviation glass prisms. This allows four independent differential motions to be measured, from which the monitor program derives four simultaneous estimates of seeing.
Tower: The telescope is mounted 6m above ground, thus escaping much of the ground level turbulence, on a tower that also provides a high degree of vibrational stability, thanks to its serrurier truss-type structure. The original Arcetri observatory tower has been modified to improve access and lightning protection.
Dome: The monitor and PC are housed inside a fibreglass clamshell dome (from Astrohaven, Canada) which is mounted on an access platform at approximately 5m above ground.
|Top | Back|