well, the max t diff of a tec (at least the widely available off fleabay) is 67 degrees celcius
assuming an ambient temp of 25 degrees, we could easily drop to -30. this is a 55 degrees diff x 0.3 nm/C gives a wavelength of 660-(55*0.30) = 643.5.
maybe it is worth it after all
nice to hear from you sir simon, long time, no see. hope all is well there