Graz kHz SLR Station

Measuring Atmospheric Seeing with kHz SLR

Georg Kirchner, Daniel Kucharski, Franz Koidl, Jörg Weingrill

Canberra, Oct. 2006 Beam Pointing: Not always stable ...

- Laser Beam, night time - Backscatter, seen by ISIT - 25 Frames per second - Pointing is NOT stable - Pointing Jitter: up to 50 µrad - Frequency of this wobbling: Few Hz up to few 10 Hz

Canberra, Oct. 2006 Possible Reason: Laser Beam Pointing ?

We installed a Laser Beam Monitor at the laser box beam exit: A mirror reflects a small portion (<< 1%) of the laser on a CCD chip; CCD Image: Monitored by a PC, with up to 30 fps; Center Coordinates of Beam (X / Y) calculated, and stored.

PC CCD

Y

X <<1%

To Coudé Path kHz Laser

Canberra, Oct. 2006 Possible Reason: Laser Beam Pointing ?

Only few µrad (<<1“) wobble (mainly measurement accuracy); EvenConclusion: THAT is DEcreased Wobble by is following NOT caused beam expander; by BiggerLaserDRIFTS Beamonly Pointing at switch-on Instability / warm-up ... (not shown here)

Beam Monitor: Laser Beam Pointing Stability 125 X/Y-Coordinates of Beam Center at CCD (5 fps) 0 123 -2

121 -4 X-Coord Y-Coord 119 -6 Y [µrad] X [µrad] 117 -8

115 -10 73200 73300 73400 73500 73600 73700 73800 Seconds of Day; 600 seconds plotted Canberra, Oct. 2006 Other possible reason: Atmosphere ...

Beam Wobble caused by atmospheric micro-turbulences => Atmospheric „Seeing“: Expected Amplitudes: Some Arcseconds Laser Beam Wobble: Up to 50 µrad (= 10“) Expected Frequencies: From few Hz up to few 10 Hz Can be more than 100 Hz, but at decreasing amplitudes ... Laser Beam Wobble: Up to 10 or 15 Hz visible at ISIT images

Canberra, Oct. 2006 Seeing Effects for SLR ???

Graz Beam Divergence (< 10“) and Seeing (≈ 2“ – 7“): Both with Similar Magnitudes ... Seeing for SLR can be WORSE than Astronomical Seeing: => Fast moving telescope, faster changing atmospheric conditions Degrades laser pointing accuracy May reduce return rates; No problem for LEOs, but may have effects for GPS etc.

kHz Laser DIV What worsens „Seeing“ ?

Seeing Values are influenced by:

Actual atmosphere: Wind; layers, gradients etc. Elevation of Satellite: Seeing is worse at lower elevation Temperature differences: Seeing is worse in winter time: Graz Observatory rooms are heated; but isolation is almost ZERO This causes lot of turbulences around station in WINTER time …

Motion of telescope: Seeing is worse at higher speeds !!!

Canberra, Oct. 2006 Measuring Seeing: Standard Method

Hartmann – Shack: 2 Holes at the entrance pupil; Observing Polar Star: Gives 2 spots on the CCD sensor; Variation of spot distances gives Seeing Value

Canberra, Oct. 2006 Seeing Values with Hartmann-Shack

Our „Best“ Seeing ever measured: Below 3“ during 3 hours Deriving Seeing Values from kHz

Laser Beam Backscatter is monitored by ISIT; frames into PC Real Time Image Processing: Determine Peak of Laser Beam; Coordinates of Peak determine Seeing Area; FWHM of this area => Astr. Seeing (arc secs);

Compare with Hartmann – Shack Results Canberra, Oct. 2006 Seeing Values derived from kHz Laser

Seeing / Fixed Pointing 14

12

10

8

6

Seeing [arc secs] 4

2

0 64500 64600 64700 64800 64900 65000 65100 Seconds of Day Canberra, Oct. 2006 Seeing Values Derived from kHz Laser

Day 2006/037: Seeing vs. Elevation

16 90 14 Seeing 80 Elevation 12 70 60 10 50 8 40 Seeing 6 30 Elevation [°] 4 20 2 10 0 0 76100 76300 76500 76700 76900 77100 77300 Seconds of Day

Ajisai / Day 037/2006: Seeing changes with elevation .... Canberra, Oct. 2006 Seeing Values Derived from kHz Laser

Lageos1 322 03: 43° EL

60 14 elevation 50 seeing 12 speed 40 10

30 8

Elevation [°] 20 6 Seeing [arc secs] [0.01°/s]Speed

10 4

0 2 14500 15500 16500 17500 sec of day Canberra, Oct. 2006 Seeing Values Derived from kHz Laser

ENV03319: Seeing

12 30 - Envisat: Day 033 10 25

- < 30° Elevation 8 20

6 15

4 10 Elevation [°] Seeing [arcsecs] Seeing ENV03319: Seeing2 vs. Azimuth 5 Elevation 0 0 69100 69200 69300 69400 69500 69600 69700 69800 69900

12 Seeing Seconds120 of Day 033/2006 Azimuth 10 100 8 80 - At 90° Azimuth: 6 60 Azimuth [°] 4 40 => Obs. Roof !!! Seeing [arcsecs] 2 20 - Heating Influence 0 0 69100 69200 69300 69400 69500 69600 69700 69800 69900 Seconds of Day 033/2006

Canberra, Oct. 2006 Conclusions:

- It is possible to derive Seeing Values from a kHz Laser Beam; - Seeing Influence is BIG enough to spoil Laser Beam Pointing; - This might reduce Return Rate from High Orbiting Sats; - Biggest Influence in Graz: Heating of Observatory - We have to live with it – or freeze to death ☺

- Plans to reduce effect with a Fast Steering Mirror, controlled by Seeing Offsets derived from kHz Beam (10“ max, 40 Hz max);

Thank You ☺

Canberra, Oct. 2006