How many photons are contributed by the sky background and by the star to the seeing disk over this exposure?

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What fraction of the eclipse will be observable?


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1. What will the separation in degrees between the galaxy M95 and the moon be, on 10:00 UT, 13 March, 2014? Specify your observing location. Show all your work (but feel free to use a software package to check your result). Lunar ephemeris data can be found online at All the basic data on M95 can be found in SIMBAD at t


2. A primary eclipse of the detached binary star YY Sagittarii is predicted to begin on Julian Date 2456794 at 16:42 UT. A friend asks you to use the UTAS H127 telescope at Bisdee Tier to photo metrically monitor the star during the eclipse. The position of the star on the date of the eclipse is RA 18h44m35.9s, dec –19º23’22.7”. Assume that the star cannot be observed within 1 hour of sunrise or sunset, or below 20º altitude. Since this is just a “preliminary planning” type of calculation, times only need to be accurate to 5—10 min, and we can neglect detailed timing issues, the uneven motion of the Sun, the effect of refraction on the apparent position of the star, etc.

a. Determine whether or not the star will in fact be visible from Bisdee Tier (long. 9h49m09s E, lat. 42º25’53” S) at the beginning of the eclipse.


b. The eclipse is predicted to last 8 hours in total. What fraction of the eclipse will be observable?



3. Detection limits for telescopes in broad and narrow bandpasses. Consider a d = 1.0m telescope used for direct imaging, with a detector having average quantum efficiency q = 0.3 over a bandpass Δλ = 100 nm from 425-525 nm. This telescope is at a suburban site with a sky brightness of Σ = 18.0 mag arcsec–2 and a seeing disk diameter d* = 3.0 arcsec. You can presume that a zero-magnitude star in this bandpass has a photon flux F(0) = 104 photons s–¹ cm–² nm–¹.

a. Write down a formula for the signal/noise ratio obtained for a star of magnitude g within bandpass Δλ and exposure time Δt, assuming that the mean sky background brightness Σ is known perfectly and only contributes noise via the photon counting statistics of the light it contributes within the seeing disk of the star.


b. How bright must the star be to obtain S/N = 3 (a practical working minimum for useful data) for Δλ = 100 nm and Δt = 3600 sec (1 hour)? How many photons are contributed by the sky background and by the star to the seeing disk over this exposure?


c. Repeat part a, for a narrow band filter centered on the Ca II H and K lines (λ = 395 nm, Δλ = 10 nm). Assume the seeing disk in the violet is 4.0 arcsec, the quantum efficiency is q = 0.1, and the sky brightness is the same as in part a. The photon flux of a zero magnitude star in this band pass is ~6×103 photons s–¹ cm–² nm–¹


d. How long would you have to integrate to obtain S/N = 10 using the setup in part c if the target star has magnitude 10?



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