# Infrared Telescopes

1. Dec 7, 2015

### user57304

I'm trying to remember back to my undergrad physics days. Can anyone point me to a good text book to compute the following?

Given an infrared telescope with given specifications, such as the Infrared Astronomical Satellite and a star of defined intensity in the field of view at a specific point in time, how can I compute what the in-band irradiance on the focal plane would be?

Thanks for any references.

2. Dec 8, 2015

### Andy Resnick

First let me sketch out a basic analysis, then provide some caveats:

If you are given the source intensity, you can calculate the power (Watts) that enters the telescope. The star is imaged to a spot of defined size (given by the optical properties of the telescope), and so you can then determine W/area of the focused image.

Now the caveats: I noticed the words 'intensity' and 'irradiance' which implies a radiometric analysis, which uses a different definition of 'intensity' than you learned in intro physics. Intensity is Watt/steradian, and you characterize the telescope in terms of angular variables- what is the angular subtense of the star? This is different than calculating the incident irradiance from the star (which is Watt/area). Either way, if you can calculate the amount of power entering the telescope and the size of the image, you are basically done.

Does this help?