I thought that a "large" but portable catadioptric or similar would suit me better, as if I take the (steep) learning curve, I will have the telescope I really needed anyway, and I won't have to upgrade and spend even more money.
OK. Now I understand what you are asking and I think we are on the same page. By all means, start with an aperture in the size range you are exploring (7" - 9.25").
There is a reason that an 8" SCT is so popular. It has enough aperture to be a very satisfying 'scope: not only for beginners, but for folks who have enjoyed amateur astronomy for decades. I have had a few 8" SCTs over the years. I have also had a Celestron 9.25 Edge. The 8" is lighter weight, can be used on a lighter (and less expensive) mount, and is easier to transport. The 9.25" allows you to see a bit more and a bit brighter views at comparable magnifications - but the difference minimal, and probably not nearly as much as you might think. As a general rule, the benefits of portability and ease of use tend to trump those of aperture - especially in the long run. Ease of transportation, ease of setting up and taking down (in the dark), translates into ease of use which, in turn, means you will use it more. But personally speaking, I never had any trouble with my C9.25 Edge.
I am going to recommend an 8" Celestron on an Evolution mount - or, because you do not want coma, something unconventional: buy a new Evolution mount with a 6" SCT, sell the 6" and then buy and mount an 8" Edge on the Evolution mount. (Celestron does not currently sell the Evolution mount with Edge series telescopes).
Another possibility is an 8" Edge on an AVX mount, but that set up will be at least 2 - 3 Kg above your weight limit; and transporting a mount with counterweights is a bit of a PITA. The Evolution mount will be easier to transport and much easier to set up and use: especially for visual astronomy.
So let's examine your list:
- It has to have a focal length larger than 800mm. The 8" SCT has a FL of 2032, but you can buy a .7 x focal reducer for the 8" Edge about $300. It will result in an f/7 scope with a FL of 1422mm
- It HAS to be portable. That is, it CAN'T be longer than 85cm (preferably shorter than that), and CAN'T be heavier than 18 kilos. The 8" Edge is only 17" long. The Evolution mount, plus tripod, weighs about 26 lbs. Total weight of the 8" Edge on an Evolution mount, plus the tripod that comes with it, weighs just a bit over 40 lbs (... a bit over 18 Kg).
- it has to be 180-250mm in aperture, preferably 7-9,5 inches. The 8" Edge fits right in the middle.
- It HAS HAS HAS to be cheaper than 4000$, and preferably cheaper than 3500$. The 8" Edge on an evolution mount will cost about $1,800. - I know the Evolution mount with an 8" SCT costs only $1,600; but if you follow my suggestion and buy an Evolution mount with a 6" SCT, sell that 'scope and the buy an 8" Edge to put on the Evolution mount, you will incur an additional cost of about $200 - $300 as you will have to take a loss on the lale of the 6". Still, the cost is way below your limits.
- It has to have very little or no chromatic aberration/spherochromatism, so that it can be later (probably) used for spectroscopy. I've done spectroscopy with an 8" SCT (non-edge) using an SBIG Spectrophotometer. I do not know if the Schott glass corrector lenses in the Edge scopes effect spectroscopy... but I tend to think not.
- MUCH preferably faster than f/10 The 8" SCT is a native f/10, but for $300 you can buy a .7x focal reducer for the 8" Edge. You'll then be able to use it at f/7 with a FL of 1422 mm.
- It has to be very good for DSO astrophotography with 35mm chips, that is, it has to have a flat field. Very good for DSO astrophotography? Yes, and no. The Edge series have a flat field. The Evolution Mount can be used successfully for short exposure AP, so you can certainly use it on the Moon and on Planets. For DSOs, you be best off buying an Hyperstar adapter. It will convert the scope to an f/2. That will allow you to take very nice APs of DSOs with short exposure times. However, the FL will be reduced to about 400 mm. - Tools are often best at performing specific jobs. In Astronomy, there is no "one size fits all." - BTW, Since you want to explore AP, I do not recommend the 9.25" SCT on the Evolution mount. Add a camera and the weight will be too much to allow the mount to track well enough for AP. You'd be pushing it with the 8".
- Diffraction spikes don't annoy me, I actually find them very nice. If you like diffraction spikes, you can always add s simple cross made of heavy wire, or thin strips of balsa wood (painted flat black) to the front of the SCT... although I don't know why anyone would want to do that (other than to use it temporarily in order to use the diffraction spikes as an aide for focusing).
- Good contrast is welcome. The contrast is good with an SCT. Not as good as with a refractor, but everything is relative. And, btw, contrast is something we are concerned about for visual use. For AP, one enhances the contrast and the sharpness via image post-processing. So the contrast of SCTs is not an issue when it comes to AP.
- Point like stars are very welcome. Stars will look very good - again, not quite as pinpoint as with a refractor, but very good indeed.
- Coma is NOT welcome. No coma with the Edge 'scopes. It is this requirement of yours that made me suggest you swap out the SCT that comes with the Evolution mounts and get an Edge scope.
Note: while I think Meade also makes very good SCTs, I did not recommend them for two reasons: They are heavier than Celestron scopes, so they will exceed your stated weight limit requirement. Second, while they have no coma, they do not have a flat field (which is another of your requirements.
There are, of course, other solutions to fill your needs (wants). But since an 8" SCT is such a good, all around scope - for beginners and advanced hobbyists alike (even for Consumers), it's a pretty safe bet that the above recommendation will make you very happy. And if not, I'll be happy to buy it from from you - at a slight discount, of course.
Hope that helps....
Edited by elwaine, 17 August 2014 - 12:42 PM.