Question
NGC 4546 [R.A. = 12h35m29.5s, Dec. = -03d47m35s] has a very compact (point source like from the ground) companion NGC 4546-UCD1 [R.A. = 12h35m28.7s, Dec. = -03d47m21.1s] which is now known to be the remnant centre of a once much more massive galaxy, which was tidally destroyed by the gravity of NGC 4546. Calculate the zenith angle and airman of NGC 4546 when observed at an LST 0 hrs (i.e. at upper culmination, when it is at highest in the sky) from the Large Binocular Telescope [latitude: 32.7 degree N]. Astronomers prefer to observe astronomical objects at airmass as close to 1 as possible. The most accurate observations are often limited to airmass = 1.41 (zenith angle = 45 degree). For how long is NGC 4546 above airmass 1.41? Other than atmospheric refraction, name and describe two other effects which are worsened by observing at large zenith angles. How far apart do NGC 4546 and NGC 4546-UCD1 appear on the sky? Give your answer in arc seconds. Atmospheric dispersion causes light to be spread as a function of wavelength. If at zenith the red and blue light from NGC 4546-UCD1 is coincident, how far apart will the blue and red images of NGC 4546-UCD1 be at a zenith angle of 60 degree? Assume the refractive index of air is 1.000280 for blue light, and 1.000276 for red light.
Explanation / Answer
d) Far apart = 456 * 21/65
= 147.32 arcseconds
e) far apart blue and red images = (1.000280 - 1.000276) * 147.32 * cos60
= 2.9464 * 10-4 arcseconds
