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My opinion, having owned many scopes, is that for astrophotography and for visual - 130mm is kind of an ideal aperture.
120mm - 130mm (there are a number of high end 120mm scopes like the Tak TSA120).
102 is usually quite widefield but if the scope comes with a good reducer then you are close tot he field of view of the 102mm and at a faster F ratio which you want for astro to reduce exposure times.
I have been impressed with the quality of images someone was producing on this site using a Teleskop Services 130mm triplet.
I have an AP130 GT and part of what makes it superb is the availability of accessories, like a superb reducer and a superb flattener. These are important accessories for imaging.
The TS scope above produces similar looking images to my AP which surprised me.
I also find the 130mm aperture great for wider visual views and the pinpoint stars that other scope designs can't achieve are very memorable and absorbing views.
150mm aperture can be quite a jump in terms of weight and cost for not a lot of gain although 150mm can get some nice galaxy shots. Galaxy shots are really the realm of mirrored scopes. An RC8 is a formidable imaging machine that can get some good galaxy shots. never owned one but I have often been surprised at how good they are. I would get a carbon fibre tube if you went that way (if you had any money left over from a refractor purchase the RC8 is a compelling value).
I am not sure if it comes in under $5,000 given the weak Aussie dollar but it might.
It can be confusing these days as there are many rebranded scopes from Chinese suppliers. Also the quality of these Chinese scopes has made some serious leaps forward (the Skywatcher Esprit series for example).
FSQ's of course are famous imaging instruments. Visual performers to some degree as well, although I never really pursued that aspect of them, except once and I wasn't that enthralled having more appropriate visual scopes at the time.
I have read glowing reports by some about an FSQ's visual performance but not serious reviews.
For sure though, a 130mm aperture scope would be in another league.
Good luck with your choice.
So I would rank scopes by aperture like this:
102mm or thereabouts - widefield imaging and widefield visual (that is a lot of imaging targets).
120-130 A nice compromise between light gathering, focal length and weight. A bit more flexible for imaging targets with reducers and flatteners available (and even extenders if you want slow but long focal length imaging).
With these small pixelled and sensitive CMOS camera you can crop and zoom in or set the size of the image off the chip to half or quarter if you want to get a digital zoom and not lose much sensitivity, with smaller files as well. These cameras with a 130mm would produce images similar to previous CCD cameras on a 150mm aperture scope. So the need for a large aperture in refractors is still there but it is reduced with these cameras.
Also keep in mind these small pixelled CMOS cameras are showing up optical defects CCDs glossed over. Like coma, false colour and are much more sensitive to tilt than larger pixelled cameras.
That all means you need an excellent focuser. So make sure you review the focuser's performance on your intended scope as 90% of the problems I see with people's scopes are focuser issues.
Also, of course, you want a triplet not a doublet. Even if its a Tak doublet.
Greg.
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