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”

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