Seben telescopes
Hoped you might answer some queries I have regarding a company called Seben who advertise their products on Ebay.

This company Seben, apart from also selling various accesories and entry level scopes, have two main products of interest to me.They have a 'Big Boss' catadioptric 1400 mm focal length, 150 mm aperture, EQ3 mount, two "Plossl" eyepieces and tripod for ~£100+ p+p. Or they have a Navigator II catadioptric 1000 mm focal length, 203mm aperture, EQ8V mount, three Plossls and a Barlow, 8x50 finder, pole finder and tripod for ~£200 + p+p.

I wondered if you (or anyone you know) had any experience of this make or if you think that at least on paper they seem reasonable pieces of equipment for the money. There is least one scope a day being sold and not too many "re-auctions". – Derek

 I hadn't heard of this company until you drew it to my attention. I haven't come across any of their telescopes, though that doesn't mean much as there must be hundreds of models out there that I haven't seen. But they aren't telescopes that you come across every day.

I would take a fairly confident guess that they are Chinese made, as there seem to be a number of Chinese optical factories. But I doubt that they are made by the best known, Synta, who make Sky-Watcher. The scopes you mention have very low prices, so all I could say is to remind you of the old adage 'If it seems too good to be true, it probably is.' I would venture to say that not even the Chinese can produce a good six-inch reflector for that price.

I also have serious doubts about any telescope, other than maybe one at the top end of the market, which provides a short tube by means of a corrector lens at the eyepiece end, as with both these models. They are not true catadioptrics with corrector plates at the top end (eg Schmidt-Cassegrains or Maksutovs), but are short-focus Newtonian reflectors with what amounts to a Barlow lens in the focuser, giving a longer focal length. Invariably these give worse performance than if they had no such corrector, but you are stuck with it.

If you do decide to go for one, I would love to know if my guesses are borne out in practice -- though you'd need to compare it alongside a known good telescope of similar aperture and type on the same occasion to draw any real conclusions.
I have a related query. Should I expect the RA and Dec scales on this kind of instrument to be accurate enough to use them for finding deep sky or other low mag. objects or is this just wishful thinking?
Very few people really do use their setting circles, although if they are properly engraved they should be adequate for getting you within a degree or so of your desired object once the telescope is perfectly polar aligned (which is not usually the case). This should bring the object within a low-power field of view. However, from the pictures online it seems to me that the circles on these mounts are so small that they couldn't be read accurately to a degree so I wouldn't rely on them. I must stress that I am only looking at info online.
On another subject, you have expressed doubts over the merits of GOTO technology because of difficulties in leveling a scope properly - does this apply to EQ mounts the same way?
Once a GO TO is set up accurately on two stars it doesn't matter whether or not it is properly levelled. The only value of the levelling is to help it find those two stars in the first place, and if you know the sky even rather poorly this should be easy enough. Problems arise if the GO TO scope doesn't have a finder (such as the old ETX70 and the new ETX80), where you have to level the thing really accurately in order to find the two reference stars at all. However, I have also found that the smaller GO TO mounts do not always work as they should, for unexplained reasons.

Whether an ordinary equatorial mount will work properly depends on what you want it to do. If you want it simply to help you follow an object through the sky with the minimum of effort, either using manual slow motions or a motor, almost any equatorial will be OK and the only requirement is that it should be large enough for the telescope without undue wobbling. However, if you want it to track an object accurately enough for long-exposure photography, you need to align it pretty carefully on the true pole, have a drive system capable of fine control, and have a second guide telescope on the same mount. These are fairly stringent requirements and you need to pay several hundred pounds to get even a small mount that will do the job.
And finally, (!) do think you might be able to give a short explanation of how exactly a 'pole finder' works?
It is a small low-power telescope that is usually fitted actually within the (hollow) polar axis of the mount itself. It has a reticle inside, which needs to be illuminated, that shows the offset of the Pole Star from the true pole. There has to be a means of rotating this reticle to match the orientation of the sky at the time of observation, either using a scale of dates and times (as in the Vixen mounts) or by orienting an arrow or line inside the reticle with a star such as Kochab in Ursa Minor. You have to look at the real sky then turn the reticle until its orientation corresponds. Easier to demonstrate than to explain!

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