Hi all

From about the beginning of 2017, the team at the SGF noticed some increased instability in our terrestrial range calibration values at the level of 2-3 mm.  It was particularly clear on the first calibration of the day being a low value and so we learnt to take at least two calibrations before starting an observing duty.

2 weeks ago we also saw an increase in the RMS of the calibrations and some periodic behaviour in the calibration range values.  We consequently took the kHz laser offline and began using our older 10Hz laser, which had very stable calibration values and RMS.

After some investigation and discussion with High-Q, the team found that the flow rate of the cooling water was much lower than it should have been.  The water used is a reservoir of distilled water, but after 10 years of use something had built up to block the waterways.

Temperatures in the laser bed were not being kept at a stabilised constant, causing energy fluctuations and range errors from the start diode. The temperature would change when the laser began firing after being off for a period (20+ minutes). It would stabilise after approximately 10 minutes, during which time calibration range differences were seen.

Running the flow backwards and using a descaler (Durgol was recommended) brought the flowrate up to specification levels.  Setting the best operating temperature removed the signals in the range values and now the calibration RMS and repeatability are now within acceptable levels.

Needless to say, we will be checking the flowrate more often from now on.

Matt

What experience has anyone had with laser beam spectral line width?

Is it correct that there is some broadening of the laser wavelength spectrum with shorter beam pulse lengths?

Has anyone measured this effect or know what their laser spectral pulse is?

At Herstmonceux we use a narrow band filter for daylight ranging, but we are concerned that we cannot select the narrowest filter possible because we must consider the spectral width of our kHz laser.

Hi

It is now essential that ILRS stations are able to estimate, or measure, the angular divergence of laser pulses emitted during satellite laser ranging. With this divergence value, along with the laser pulse energy, it is possible to calculate the energy density at satellite heights.  This is of high interest to satellite mission operators, particularly those with on-board equipment that could be sensitive to incident laser light.

A procedure to measure beam divergence was drafted by the NESC and is available to download http://ilrs.gsfc.nasa.gov/docs/2016/BeamDiv_procedure_201606.pdf

Stations are using this procedure and other methods to determine beam divergence with some success, particularly after some practice.

It would be helpful for forum members share their experience, advice and issues here so that all stations can get a reliable divergence measurement.

Matt

Hi

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Enjoy!

Matt