Announcements • This weeks lab: SDSS DR7 Advanced Projects Hubble Diagram • Next week: Monday is Veterans Day …no class • Week after (Nov 19): last two Observing Nights. Monday 11/19 and Tuesday 11/20. Set-up starts at 6:15pm • Homework: Chapter 7 # 1, 2, 3, 4 & 5 • Turn in your logbook for grading! For those in the Wednesday night class you can turn it in Wednesday evening.
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.
Once the extinction coefficient is known, the magnitude can be corrected ml is the measured magnitude and ml0 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
Refraction is also wavelength dependent Atmospheric dispersion causes rainbows for bright objects at high zenith distances
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. Watch Jupiter and Saturn videos from Astrophotography
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
AO is even available for amateurs The SBIG AO8 is a tip-tilt mirror system that attaches to most CBIG CCD cameras
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