I'm looking at an ad for a used Canon 1200mm f/5.6L EF USM Autofocus Lens where they are asking $180,000. Yes, that's not a typo.
I don't get it.
I'm an amateur astronomer and I know I can buy a killer 1200m refractor for well less than $180k. What makes this so expensive? I know that a good refractor only has a triplet (or a doublet) as the objective and a small, but complex, eyepiece. The Canon lens has 13 elements, 2 of which are flourite.
This isn't about this particular lens, and this question may well be better for an astronomy forum
But why are camera lens so much more expensive than refractor telescopes?
Rarity. There were only approximately 20 of these now out of production lenses ever made. When they were in production they sold for about $90,000 (US). Due to the time needed to grow the large fluorite crystal used in the 3rd element of the lens, once ordered they took about 18 months to produce.
Autofocus Capability. These lenses include auto focus capability. Moving focus elements as large and heavy as this requires mechanisms that are both robust and extremely precise. Moving them fast enough to be used to photograph sporting events means they must also be very powerful.
Maximum Aperture The EF 1200mm f/5.6 L requires an entrance pupil of 214mm (8.4 inches). A 1200mm f/8 telescope needs only a 150mm wide objective. When you compare the areas of a 214mm circle to that of a 150mm circle, you see that it takes at least twice as much material to create a lens 214mm wide compared to 150mm wide. And that is before you consider that the larger lens element must also be thicker in the center to maintain the same amount of curvature on the surfaces.
Optical Image Quality While things like coma and chromatic aberration are expected at the edges of the field of view of a large refracting telescope, they are not as acceptable in a camera lens. And the larger the diameter of a lens is, the more correction must be applied to obtain the same image quality. This requires more elements in the lens, and these additional elements are almost always made of exotic materials with a higher refractive index and lower mass density than normal optical glass. They must also be precisely shaped to almost insane tolerances to perform as designed.
Image Circle Diameter A telescope need only to cast an image circle the size of a human eye's pupil: approximately 8mm in diameter. A camera lens intended for use with a Full frame camera must cast an image circle approximately 44mm in diameter.
Minimum Focus Distance Many telescopes are designed to focus only at longer distances, some even only at infinity. Camera lenses such as the EF 1200mm f/5.6 L can focus at infinity but are expected to focus at shorter distances as well.
Different tools for different jobs. A telescope is for looking at objects with the naked eye, a camera lens is for recording high resolution images.
It is acceptable for a telescope to have large distortions around the edges and nobody much cares about colour correctness. This makes it simple (cheap) to make the lenses which are good enough to do the job. Now add the sheer size of the optics required to get an f-stop of 5.6 from a 1200mm focus length.
---- edit to correct the edits ---
A 1200mm telescope is for looking at Stars, a 1200mm f/5.6 lens is for looking at Celebrities. If you expect to sell a picture of Venus then you really don't want it to look like a picture of Jupiter. If you expect to sell it for over $30,000 then it had better look like Venus.
Michael Clark missed one important point in his otherwise correct answer:
focal length. This normally means that the distance from the center of the lenses to the place where the image forms is 1200mm. (Yes 1.2 meters!) On your telescope I guess that you pretty much see these 1200 millimeters as the telescope is around 1.2 meters in length. On your Canon you're probably not carrying a lens that is 1.2 meters long. The lens in question is 83.6 centimeters. This significant shortening of the physical dimensions can be achieved by optical elements, most probably the ones with fluorite.
Astronomers use reflectors mostly but are indeed interested in accuracy.
The lens in question is much faster than a telescope.
Part of the reason is doubtless that professional photographers earn more money than professional astronomers, and so can be milked a bit more for their cash.
Score: +5 Funny