The Interface Region Imaging Spectrograph (IRIS) is a complementary follow-on to Solar Dynamics Observatory
Atmospheric Imaging Assembly (SDO-AIA) and funded as a member of the NASA SMEX program. This paper
presents the thermal design of the IRIS telescope front end, with a focus on the IRIS door and entrance aperture
assembly. The challenge of the IRIS entrance aperture, including the door design, was to manage the solar flux, both
before and after the door was opened. This is especially a problem with instruments that are permanently pointed directly
at the sun. Though there is an array of effective flux-rejecting coatings, they are expensive, hard to apply, harder to
measure, delicate, prone to unpredictable performance decay with exposure, and very often a source of contamination.
This paper presents a thermal control and protection method based on robust, inexpensive coatings and materials,
combined to produce high thermal and structural isolation. The end result is a first line of thermal protection whose
performance is easy to predict and well isolated from the instrument it is protecting.