Glancing at his gear wish list, it looks like he's more into action than astro. An A7R is 2500 less in the budget (camera + EF adapter). Personally I would love one for portrait and landscape work, but I can not justify the expense. I suspect I'd get more use from that tamron 150-600 and a new tripod.
So, while I'd like an A7r for my landscape photography, it is actually one of the worst possible choices for astrophotography. I do landscapes sometimes, wildlife and birds most of the time, and astrophotography every time there is a clear night.
I was looking at the A7R with adapter for landscapes, but then I read on Thom Hogan's site that Sony uses lossy compression on their RAWs (unless I misread him), and you can't switch it off!
Why would they do that?
On that basis, it may have amazing DR but then it surely will just smudge out some of the detail for err, actually I'm not sure for what benefit...
Hmm, I hadn't heard of that. If they do, it's foolish, and you really no longer have a RAW image. I am a bit skeptical of that...it doesn't seem logical, but who knows.
Had a look at that astro link - it's a whole new language there If I understood correctly, then it's a 2000mm lens? And optically is it better than your 600mm lens with a 1.4x and 2.x attached? Just curious as to the benefits. Thanks.
Reflecting light tends to produce superior spots at the sensor plane in comparison to refracting light. Reflecting light can warp star diffraction spots due to coma and astigmatism, but that's about it. Refracting light, on the other hand, suffers from all forms of optical aberrations...which also includes chromatic aberrations, spherical aberration, etc. The RC, or Ritchey-Chretien, telescope design is one of the more superior designs. It's the same design used in all the major earth-bound telescopes...the huge ones, up to 10 meters in size. It tends to produce superior results, although it does suffer from some coma and astigmatism in the corners.
There is a better telescope design than even the RC, called a CDK or Corrected Dall-Kirkham. The CDK uses a mirror and built in corrector to get one of the best spot shapes, center to corner, of any telescope design I've ever seen. PlaneWave makes CDK scopes, but they are pretty pricey. From what I've read and seen, a CDK is about the best telescope design in the world today.
As good as my lens is, and it is very good with a very flat field corner to corner, it is no RC and certainly no CDK. If I throw on teleconverters, that gets me more focal length (which is not necessarily the best thing...a LOT of nebula are even larger than I can fit in my field with the 600mm, let alone a 2000mm scope), but it also increases the optical aberrations. For galaxies, clusters, and getting close up on parts of nebula, a longer, better scope like the Astro-Tech 10" RC is better. The larger aperture, ten inches vs. six inches, also means I can resolve smaller magnitude stars, galaxies, and other details. Most scopes work with focal reducers, so while it is 2000mm natively, I can use a 0.63x reducer to make it an f/5 1260mm telescope. That is relatively fast with a moderately wide field. For planetary work, I can also throw on a 2x or 3x barlow lens, and get a 400mm f/16 or 6000mm f/24 scope, which is much better for planetary imaging (f-ratio doesn't usually matter for planetary, as you image planets by taking videos with thousands of frames for anywhere from a couple minutes to as long as a half hour...then filter, register, and stack the best frames of the video, which is basically performing a superresolution integration...that eliminates blurring from seeing, and effectively allows you to image well beyond the diffraction limit.)