Astronomical Seeing B Waddington 61510 Just To Be

Astronomical Seeing B. Waddington 6/15/10

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

Cornell University lecture notes

Why Is The Wavefront Distorted? n Air has a refractive index (n), which affects n n 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 n Just like hundreds of lenses moving in and out of your line of sight n This is not a good thing. . . n

Physics of Seeing Plane light wave moving through nonuniform medium undergoes phase and amplitude fluctuations n When focused, the wave front creates an image that varies in intensity, sharpness, and position n

Physics of Seeing n These “seeing” effects are called n n 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 Cornell University lecture notes Long exposure, blurring

Cell Sizes Typical size of these thermal “cells” matters n Referred to as R 0 in seeing and optics models (dependent on wavelength) n Roughly equivalent to maximum cell size that produces only “tilt” distortion n Diameter of telescope relative to R 0 (D/ R 0) determines seeing characteristics n

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

Cornell University lecture notes

Here’s the Kicker R 0 is usually in the range of only 10 – 20 cm in the visual range n May be as high as 40 cm in very best locations n Gets larger with longer wavelengths – (e. g. 10 cm to 20 cm, violet to red) n

Measuring Seeing n Common amateur measure is FWHM of a non-saturated, well-formed star near zenith n n 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 n n Guiding, focus, collimation issues Mechanical flaws, vibration, wind deflection

Measuring Seeing n “Seeing monitors” typically take a different approach by measuring image motion n 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… n But where do they come from? n Can we use meteorology or atmospheric models to forecast seeing? n What can we do to mitigate the effects? n

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

Upper Layer (6 -12 km) Typically a function of the jet streams n Worst where mixing occurs at the margins of a jet stream n It’s thermal mixing that kills you more than the actual wind speed n This layer typically contributes the least to the problem (maybe less than 0. 5”) n

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

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

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

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

Improving The Situation n Equilibrate outside and inside temperatures n n n 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 n Eliminate or shield heat sources near the telescope n Keep your body out of the optical path, especially in cold weather n

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

Good seeing conditions, worldclass planetary imager

Poor seeing conditions, same imager

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