I’m getting frustrated with my lack of reach for taking pictures of the moon and other things in the night sky. Moving from the D200 to the D7000 gives me a larger image of the moon, simply because it has a higher resolution sensor, but it’s not really that big a difference. How does a budding astrophotographer get more reach without buying a telescope or breaking the bank?

Easy: get a telescope that’s masquerading as an SLR lens.

They are often called mirror lenses, and they are surprisingly cheap. They are essentially small Maksutov–Cassegrain telescopes with an SLR mount instead of an eye piece. The cheapest telescope with a mount that I can find costs about $350, while these lenses start at $50 for a nice used one. Because all of their imaging work is done with mirrors folding the light path up and down the length of the lens, they suffer from almost no chromatic aberration, which usually shows up as color fringing in cheaper SLR zoom and telephoto lenses. However, because of their design, they are also fixed aperture lenses (usually f/8 or f/11) and are almost always manual focus.

The one that I’m looking at is a Sigma 600mm f/8. Of course, it’s not autofocus and lacks things I’d really like, like hard focus stops at infinity. However, they get as cheap as $200 on the used market – mostly because people try to buy them to use as a replacement for a professional 600mm fixed lens, and find out that the professional lenses cost $4000+ for a reason. They are slow to focus by hand, they weigh a lot, they have very peculiar bokeh, they are hard to clean when cleaning is needed, and the contrast and color clarity is not what they want for wildlife photography. But for astrophotography? Perfect. When focused to infinity, there are no out-of-focus backgrounds to throw off an image, and contrast is as much determined by the lens as it is by relative humidity or cloud cover. So, it’s a cheap telescope that comes with a camera mount.

The other thing I’m looking at is getting a Sigma 2.0x APO teleconverter. It’s listed to work with my 70-200mm f/2.8 zoom, and would make it a 140-400mm f/5.6 lens. That isn’t as much of an issue with me, as the ISO performance of the D7000 is still rocking my world. Further, it would give me ²⁄₃ of the reach of the mirror lens while still being useful walking around, as autofocus would still work, and it would take much better day-to-day photographs. I think this is the smarter way to spend $200 on camera gear for photography, but the mirror lens would be smarter in terms of astrophotography.

I could also do both. Get the teleconverter used for $150 and one of the cheapest mirror lenses for $50. I’ll have to then do all metering manually, but as I’m not looking at the mirror lens for anything other than astrophotography, the minor annoyances can be dealt with. (I’ll be spending enough time in setup to justify chimping a few shots to get the metering right). I would then have both options – a 400mm lens with autofocus and a 600mm lens for strict night-time work. As I don’t have the cash for either laying around right now, it’s still just a thought experiment, but someday soon, it will materialize.