With a sufficiently isolated vacuum flask containing the tender and with an optical opening only large enough to view the moon, you wouldn't even need to concen

With a sufficiently isolated vacuum flask containing the tender and with an optical opening only large enough to view the moon, you wouldn't even need to concentrate the scattered light from the moon. The temperature of the inside of the flask would eventually reach the approximate blackbody temperature of the reflected light from the Sun (corrected for whatever frequencies are absorbed by the Moon). The only reason the moon's surface doesn't reach such high temperatures is that the moon's surface is not thermally isolated from the rest of the night sky or from the body of the moon itself (which is also not isolated from the night sky).