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You are more likely to see the shadow when viewing bright objects and the pupil is contracted.

And of course the secondary shadow is enlarged, which I have found less of a problem in practice, than some others apparently do.

No reason why any telescope should not be used with any eyepiece which has an accessible focal plane, like a plossl or ortho. I have used a matchstick in a 20mm plossl to occult Sirius though I have yet to see to pup...

Only with a coma corrector...

I have mixed feelings about the Clicklock and similar twist fit eyepiece clamps. They are good for security, but both my APM and Baader 1.25" adaptors have too much friction to ensure keeping the view when changing to a high magnification eyepiece on a low friction balanced set up, like a Dobsonian. The Takahashi diagonal clamp does not have this problem but it does take a lot of turning. Although set screws, with or without a brass ring, are mechanically flawed in principle, I currently prefer them for accessories that I change when viewing. However with more practice this opinion may change...

Congratulations! Of course Olle will be not be happy that you have taken no notice of his advice. And, getting two good refractors so at ones is greedy, so there will be no TEC 140/ 7 for you! Of course the strehl of 0.985 is surely only in green. If only this was a polystrehl, then Vlad would declare it "sensibly perfect"... For those unfamiliar with the work of Thomas M. Back, an amateur astronomer who died suddenly age of 49, there is a good obituary here.

I should have read more closely! I would definitely want a finished one. However his project is now on hold, while I work on my 350mm dobsonian. Many thanks

Yes. The issues of delivery are not negligible. Reading this made me think again. Instead now, I aim to pimp my current 350mm F/4.6, I think. It is light enough for me for now. I will see how the current mirror performs at both 350mm and 250mm (so at F/6.44) and either replace it or keep it. I will replace the current straight spider with a curved one and the 63mm diagonal with a 50mm, and make some other minor changes. Thank you all for the suggestions for the source of any replacement mirror. The intention would to have a fantastic 250mm with the mirror masked down. I would of course retain the current "light bucket" capability with it unmasked (albeit with some vignetting when viewing a wide field).

I am about to buy a high quality 250mm F/6 (or longer) dob. I will invite Orion UK to tender, but who else would you go to?

We have friends who live about 1km away in of sight and they will, I am sure, be willing to host a flat mirror. I got prices for a small flat from: Orion Optics UK (elliptical): £42+VAT+carriage; Antares (elliptical): $49.99+carriage; Surplus shed (round): $20+carriage. Of course, I will go for the last of these. I shall also order optical bits to make an bath interferometer, sometime or perhaps never...

Testing

Just a few more points in a new post since I have already edited my earlier post too often: * Dobsonians take up surprisingly little storage space; * I would probably specify a spider of this form: Royce_Curved_spider or perhaps a straight 3-arm spider and then retro-fit the curved one. * I would reverse the fan direction and make a baffle for the back so air flows down the tube, past the mirror rather than coming in at the back; * I would probably mask the outer 2mm or 3mm of the mirror; * I would fit a stainless screw pipe clamp to allow the tube to be rotated; * I would fix a removable flexible dew shield with velco. That is all, I think.

Orion UK scopes are light, so handling is easy even on the larger scopes. Though above your budget, the VX10-10L on a dobsonian mount is the one I would go for. At F/6.3, I would not want a coma corrector for visual use. It is the longest instrument that is usable at zenith without a step, unless you are less than about 1700mm tall (5ft 7in), so it might cure aperture fever, or perhaps not. I have a good Southern horizon so an equitorial mount is of no interest. Push-to would be nice, though. I would also want a curved spider of some kind, but might leave that as a retro-fit. If I were to be buying a made to order instrument, then I would expect any delivery date to slip.

That would not be entirely unexpected.

One small refractor, say 70mm to 90mm for travel and for when you are feeling very lazy. One large refractor, say 120mm to 140mm for quick use at home on less than perfect night. One large dob, say 200mm to as large as you can handle for when you have time and the sky is dark and transparent. Somehow I have gone wrong because I have two of each of these...