How does the collecting area of JWST compare to HST?
HST has a 2.4 m diameter round primary mirror. For ACS and STIS, the central obscuration by the secondary is 0.33r, where r is the 1.2 m radius. WFPC2 had a larger internal obscuration, which was oversized to ensure alignment, ranging between 0.39r and 0.43r. Using the 0.33r obscuration, the area of HST’s mirror is π (1.22) (1-0.332) = 4.0 m2. Therefore, the ratio between the 25.0 m2 JWST mirror and the HST mirror is 6.25.
