Astronomical Seeing B. Waddington 6/15/10

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

Cornell University lecture notes

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

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...

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

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

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

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

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

Cornell University lecture notes

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)

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

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

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?

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

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”)

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

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

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

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

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

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

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

Good seeing conditions, world-class planetary imager

Poor seeing conditions, same imager

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