feat: add getFocalRation() utility to optics module in @observerly/as…

…trometry

feat: add getFocalRation() utility to optics module in @observerly/astrometry

src/optics.ts

@@ -6,6 +6,29 @@
 
 /*****************************************************************************************************************/
 
-export {}
+type FocalRatio = `f/${number}`
+
+/*****************************************************************************************************************/
+
+/**
+ *
+ * getFocalRatio()
+ *
+ * @param apertureWidth - the aperture of the optics
+ * @param focalLength - the focal length of the optics
+ * @returns the focal ratio as a string formatted in the standard focal ratio, e.g., f/x.
+ */
+export function getFocalRatio(apertureWidth: number, focalLength: number): FocalRatio {
+  // Check that the aperterure is a sensible number, e.g., > 0:
+  if (apertureWidth < 0) {
+    throw new Error(`Invalid focal ratio as aperture is negative`)
+  }
+
+  if (focalLength < 0) {
+    throw new Error(`Invalid focal ratio as focal length is negative`)
+  }
+
+  return `f/${focalLength / apertureWidth}`
+}
 
 /*****************************************************************************************************************/