Calculate Photons/Angstrom at Cassegrain Telescope - V=9, zenith=60°

[goleafsgo113]

ok the question is,
Calculate the photons you expect to receive per Angstrom at the focus of a Cassegrain telescope of aperture 30cm for a star of V magnitude 9.0, observed at a zenith distance of 60 degrees. Assume extinction coefficient of 0.15 magnitudes per airmass, reflectivity at each surface = 90%, wavelength = 5500 Angstrom.

I'm overwhelmed with all the numbers given and cannot find any equations that are useful to this question. If anyone has an idea please give me a hint on where to start or how to approach this question. Thanks in advance.

 

[member 553083]

The first step is to calculate the total extinction of the star. To do this, calculate the airmass, which is equal to the secant of the zenith distance (60 degrees). This will give you an airmass of 2.00. Then, multiply the extinction coefficient (0.15) by the airmass (2.00) to get the total extinction of 0.30 magnitudes. Next, convert the V magnitude of the star (9.0) to a flux density in Janskys (Jy). To do this, use the formula: Flux Density (Jy) = 10^(-(V Magnitude - 0.03)/2.5) This gives you a flux density of 0.001 Jy. Now, calculate the telescope's collecting area. To do this, use the formula: Collecting Area (cm^2) = 3.14 * Aperture Diameter (cm)^2 This gives you a collecting area of 706.5 cm^2. Finally, calculate the number of photons you expect to receive per Angstrom at the focus of the telescope. To do this, use the formula: Number of Photons (per Angstrom) = Flux Density (Jy) * Collecting Area (cm^2) * Reflectivity (0.90) * Wavelength (5500 Angstroms) This gives you a total of 321 photons per Angstrom.