Upload
lawrence-rich
View
11
Download
0
Embed Size (px)
DESCRIPTION
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 e xpect to stay out until at least 10pm - PowerPoint PPT Presentation
Citation preview
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
2 1sec 1 0.0012 sec 1 sec
cosX z z where z
z
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
Xkmm 0ml 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
CXkXkmm C 0
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