Announcements No lab this week due to observing night last night There will be a lab after class next week. If the skies are clear expect to stay out until at least 10pm The Telescopic Observations of the Moon lab is due next Thursday. You can check out a Dobs for the weekend tonight or tomorrow. Homework: Chapter 7 # 1, 2, 3, 4 & 5

Atmospheric Effects

Earth’s atmospheric windows We will be concerned mostly with just the visible window

The amount of atmospheric effect depends on the “zenith distance” z One airmass is defined as the amount of atmosphere directly overhead. If we include Earth’s curvature, the airmass, X, is given by

There are several things that cause atmospheric extinction Rayleigh scattering affects the shorter wavelengths more than longer wavelengths. Ozone absorption is almost entirely in the UV. Dust absorbs all wavelengths uniformly

The Extinction Coefficient, k, must be found by observation The extinction coefficient, k, is the slope of the magnitude versus airmass plot. It is wavelength dependent and varies from night to night. Because of this, absolute photometry is extremely tedious. Many astronomers use differential photometry.

The extinction coefficient varies greatly with wavelength

Once the extinction coefficient is known, the magnitude can be corrected m is the measured magnitude and m 0 is the magnitude that would be measured above the Earth’s atmosphere

For high precision measurements, a second order term is needed Where C is the color index of the star under observation

Atmospheric refraction is also important

A consequence of atmospheric refraction is flattening near the horizon

Refraction is also wavelength dependent Atmospheric dispersion causes rainbows for bright objects at high zenith distances

“Seeing” is a combination of several effects

High altitude turbulence can contribute to bad seeing

Turbulence can cause extended objects to defocus

Scintillation is the changing in brightness of an object Changes in the density of the air above lead to an effect similar to the light and dark patterns at the bottom of a pool of water

Correcting for atmospheric effects: Adaptive Optics The simplest AO systems use a tip-tilt mirror. More sophisticated systems use a deformable mirror.

Most AO systems require a bright star to adapt to Since there isn’t always a bright star near the target, large observatories use a laser generated artificial star

The use of an AO system can make a tremendous difference

Most major observatories now use AO systems

AO is even available for amateurs The SBIG AO8 is a tip-tilt mirror system that attaches to most CBIG CCD cameras

The Ring nebula with and without AO

A double star with and without AO While not as dramatic an improvement as the professional systems, amateur AO systems can definitely improve the image quality