As materials are heated or cooled, they will expand and contract. However, the rate at which this happens will vary from material to material. Changes in temperature can also affect the refractive index of optical materials. The cumulative effect of these small changes will result in your lens going out of focus as the temperature changes. In order to ensure that a lens remains in focus throughout a wide temperature range, all of these variables must be carefully considered to create an athermalized lens.
Read technical note: https://www.resolveoptics.com/developing-lenses-that-operate-over-a-wide-temperature-range/