Having the direct experience with carbon tube newtonians (zero focus shift during the night), precision of ASA focuser, quality of image (spots and almost real apochromaticity) produced by a superior coma correctors I have decided to have an own, small, ultra portable „refractor“ (best refractor is, for me, a custom built reflector) to be build. I have also considered to get the nice Vixen VSD 100 F/3.8, but for the price… knowing it won’t hold focus and won’t give small spots in the very corners of a large, smallish pixels camera, I managed to persuade myself to not go that route. The first thing I’d do with that would be a focuser upgrade anyway (for this scope I’d see a FLI’s Atlas as the right choice) which asks for almost 8K of money and still a lot of homework and custom built adapters.
The goal was to have zero vignetting – that could only be achieved by sacrifying the speed of the objective (F-stop) and the only option was a Paracorr VIP 3″ from TeleVue to get the focal plane out of the tube. I was assuming to get an effective F/5.6 objective (after including loss due to the large central obstruction from secondary mirror) that would have almost zero vignetting (goal met) and give reasonably great (good) star shapes in the very corners of a 36x24mm camera (goal not met).
Unfortunately, it turned out that the Paracorr is NOT suited for short focal length telescopes (below 900mm). I have had a call with Al from TeleVue on this matter for over a quarter of an hour and got a few days later e-mail response that there’s no remedy to this. Therefore my project for almost 4K of money failed.
Klaus Helmerichs Carbon Tube – 300 EUR
Orion UK 6″ Research Grade Primary mirror – 220 GBP
Orion UK 82mm Secondary – 72 GBP
ASA 3″ OK3 Focuser – 999 EUR
TeleVue VIP-3010 3″ Paracorr – 1290 EUR
Lacerta El-Panel + DewShield ~ 300 EUR
Manpower Costs and Other Material ~ 1000 EUR
UPDATE (spring 2018):
Since the 6″ Newtonian seems not to work with any available corrector, for a 36x24mm camera, I will give it a try to convert it to a small FOV, high-speed astrograph. I will try to use it with the well known 2″ 0.73x reducer and corrector from ASA. This would give a rather smallish image circle (the Panasonic MN34230 chip (4/3″) won’t be fully covered in the corners!) and therefore a small latest-technology CMOS chip might do well here. Considering to try it for narrow band using something like IMX183mono or IMX428mono when available. Paired with 5nm Astrodon narrow band filters, imaging at F/2.8 might be a „way to go“ let’s see…