Astronomical Seeing

1. Astronomical Seeing B. Waddington 6/15/10

2. Just To Be “Clear”…. • Seeing is NOT transparency, sky darkness, or a general metric of “goodness” • It relates to “turbulence”, “twinkle”, or “jitter” (none of which are correct terms) • It can be visually estimated according to the “Pickering Scale” http://www.damianpeach.com/pickering.htm

3. Cornell University lecture notes

4. Why Is The Wavefront Distorted? • Air has a refractive index (n), which affects • The speed of light (phase) • How the light is refracted (angle) • Small scale pressure/temperature changes cause changes in the index of refraction • Small cells having different temperatures can persist

5. Why Is The Wavefront Distorted? • So, neighboring air “cells” can have different optical properties • Just like hundreds of lenses moving in and out of your line of sight • This is not a good thing...

6. Physics of Seeing • Plane light wave moving through non-uniform medium undergoes phase and amplitude fluctuations • When focused, the wave front creates an image that varies in intensity, sharpness, and position

7. Physics of Seeing • These “seeing” effects are called • Scintillation (brightness and break-up) • Image motion (angle) • Blurring (combination of both) • Integration of these effects during a long exposure results in fat stars and loss of detail – blurring predominates

8. Short exposure, scintillation Long exposure, blurring Cornell University lecture notes

9. Cell Sizes • Typical size of these thermal “cells” matters • Referred to as R0 in seeing and optics models (dependent on wavelength) • Roughly equivalent to maximum cell size that produces only “tilt” distortion • Diameter of telescope relative to R0 (D/ R0) determines seeing characteristics

10. Cell Sizes • Telescope aperture < typical cell size • Scintillation, movement predominate • Resolution will scale with aperture • Typical for many guide scope set-ups • Telescope aperture > typical cell size • Resolution is capped by seeing disk size (ouch!) • Creates larger seeing disk, especially with longer exposures

12. Cornell University lecture notes

13. Here’s the Kicker • R0 is usually in the range of only 10 – 20cm in the visual range • May be as high as 40cm in very best locations • Gets larger with longer wavelengths – (e.g. 10cm to 20cm, violet to red)

14. Measuring Seeing • Common amateur measure is FWHM of a non-saturated, well-formed star near zenith • Largely independent of focal length • Accuracy requires image scale of <= 1 asp • Exposure time > cell “coherence” time (10+ seconds • Actual seeing may be better than measured • Guiding, focus, collimation issues • Mechanical flaws, vibration, wind deflection

15. Measuring Seeing • “Seeing monitors” typically take a different approach by measuring image motion • Measure position of a single star at 5 ms intervals (SBIG) • Use an aperture mask to measure differential displacements of 2-4 images of same star (DIMM) • In any case, they compute an equivalent FWHM value

16. Now About Those Cells… • So, atmospheric cells of different temperature/density are bad news for seeing… • But where do they come from? • Can we use meteorology or atmospheric models to forecast seeing? • What can we do to mitigate the effects?

17. Seeing and Atmospheric Models • There are four major regions (heights) that are involved • Upper (free atmosphere) ~ 6km – 12km • Central (boundary) ~ 100m – 2 km • Near-ground (surface boundary) ~ 0-100m • Local – telescope and immediate proximity

18. Upper Layer (6-12 km) • Typically a function of the jet streams • Worst where mixing occurs at the margins of a jet stream • It’s the thermal mixing that kills you more than the actual wind speed • This layer typically contributes the least to the problem (maybe less than 0.5”)

19. Central Layer (100m – 2 km) • Large-scale topography upwind of the site • Mountains, dense urban areas vs. flat terrain or large bodies of water • Strong convection zones vs. temperature inversions or even fog • Turbulent vs. laminar flows

20. Near Ground Layer (0 – 100m) • Convection at or near the observing site – pads, paved surfaces, neighboring buildings, trees • Turbulent air flow created by adjacent structures, landforms, vegetation • Wind is not necessarily your enemy – but “thermal mixing” is

21. Local Environment • Convection in observatory • Tube currents • Thermal layer at air/glass boundary • Heat sources inside telescope or observatory (including bodies)

22. Improving The Situation • Keep the big picture in mind • Thermal equilibrium is good • Thermal mixing is bad • Elevate the observatory (you wish) • Avoid big sources of convection – blacktop, rooftops, trees

23. Improving The Situation • Equilibrate outside and inside temperatures • Open the roof when temperatures are dropping • Keep sunlight out of the observatory • Use fans • Note that roll-off roofs have the advantage here • Consider using tube fans – boundary layer over mirror is a major source of tube currents

24. Improving The Situation • Don’t set up immediately downwind of a structure • Eliminate or shield heat sources near the telescope • Keep your body out of the optical path, especially in cold weather

25. Don’t Lose Your Perspective • You may never see “excellent” seeing • What’s ‘good’ for one person is ‘lousy’ for another, largely based on location • Anecdotal comments about seeing aren’t very reliable without a specific context • Don’t underestimate the importance of seeing in high-resolution work

26. Good seeing conditions, world-class planetary imager

27. Poor seeing conditions, same imager

28. What About Forecasts • Forecasts using only jet stream maps not very useful – a minor contributor • CSC and MeteoBlue forecasts are helpful • “Near-ground” and “local” layer forecasts are up to you – and these are the most important