First, watch this nice intro to the topic:
Then read Chapter 2 of Gray's Stellar Photospheres. Read enough to understand Figure 2.4 (this was incorrectly labeled as Figure 2.8 in a previous version of this post. Fortunately the hyperlink went to the correct figure.), which you should memorize for life.
We'll think about telescope optics and astronomical instrumentation mostly using 1-d visualizations. As additional reading you can check out this introduction to FT for 2-dimensional functions (images).