Announcements No lab this week since we had an observing night Tuesday. Next week: 1st Quarter Nights Tuesday and Thursday. Set-up will start at 6:30pm so no class or lab next week. Week after next: Thanksgiving so won’t meet again until November 29 and that is our last meeting Homework: Chapter 10 # 2, 3, 5, 8 & 10

Lab due November 29 Variable Star Photometry NAAP lab found at http://astro.unl.edu/naap/vsp/vsp.html Be sure to go through all the background pages

Astronomical Photometry

The old way: photoelectric photometry Discussed in last chapter. Not used much anymore.

Problems with photoelectric photometers Can only observe one star at a time To measure sky background requires moving the telescope To measure a comparison star requires moving the telescope Susceptible to variations in sky so requires photometric conditions. Because of required movement of telescope, cannot do short period varaibles

Typical observation sequence KSCVCVCSK K = check start …non-variable to check constancy of sky S = sky…no stars C = comparison star, close to same temp as unknown V = variable star. Observations need to be close to each other so movement of telescope is minimized

CCD Photometry Advantage: can check sky and comparison stars in the same field of view, no need to move telescope

In CCD Photometry its all about signal to noise ratio Higher SNR is the goal of any experiment

The CCD Equation NR is obtained from the bias image…average count per pixel Nsky is the average count per pixel in the sky annulus around the star. It should not include any of the star in it.

Getting the magnitude from photometry data C is an offset constant that depends on the specific telescope/camera set-up

Aperture Photometry is the most commonly used technique

How big should the star aperture be? You should include most of the star but not necessarily all of it The Point Spread Function (PSF) is a graphical display of the stars light

The sky annulus should avoid any of the star

Standard Stars Star fields that have been repeatedly observed under the best sky conditions and “calibrated” so that their magnitudes are known

Observing Procedure Plan your target list ahead of time. If a target is a transiting object, pre-calculate transit times Observe targets as close to meridian as possible Determine exposure times for targets ahead of time Observe standard star fields at low, medium and high altitude several times during the night Frequently check the skies visually