Andrew_B

From what you've described the ST80 is seriously underperforming even a decent 60mm scope which sounds odd to me. If your William Optics diagonal is the 45 degree one then I can say from experience that while it works well at low powers, going above 50x magnification will give you views that get very soft and there's a dramatic difference when using a decent non-erecting mirror or prism diagonal.

From what I've read it's much easier to apply hard coatings to the tiny lenses used in eyepieces than it is to the much larger elements in telescope objectives so your eyepieces will stand up to normal cleaning very well. When I bought my Fujiyama ortho recently I asked the seller (Barry Gooley of Magellan Science in Japan - lovely guy) about the proper way to clean them and he advised the use of Baader Optical Wonder Cloth which he gently presses into the edges of the lens using a wooden toothpick to get the last traces of grime all the way across the lens. Most of the time he uses the cloth by itself and only wets it with Optical Wonder cleaning fluid when the grime is really bad. The main thing he cautioned about was not using so much fluid that it gets into the lens mountings. Hope this helps.

Thirty years ago a 4" refractor was anything but small and even a basic achromat was a lot of money. From what I remember the bottom of the range 80mm Vixen achro with a basic alt-az mount cost almost a grand in modern money. The only thing that doesn't seem expensive is when you see that in 1991 you could buy an Astro-Physics 6" f9 Starfire for £2300. Even taking into account inflation that isn't a bad price and there weren't 20 year waiting lists either back then!

One question that came to mind is whether you'd just want to use the scope for astronomical observing, or if it would also be useful if it could double as a spotting scope. I use my little refractor for birdwatching as well as taking it out under the night sky and views it gives of birds and wildlife are fantastic. It also means I get more use out of it than I would given the limitations of the British weather and seemingly endless cloud we sometimes have to endure! If you did want something that was multi-use then a refractor or smaller Maksutov would do the job well but if you just want something for astro then it's hard to beat a Dobsonian for performance and value.

Following in from my previous post, some refractors are specifically designed for use with prisms and are better corrected than if they're used with a mirror diagonal. Zeiss did it with the APQ and I've read that it's also true of the little Tak FS-60 but I suspect that there are relatively few scopes designed this way and probably none outside of a handful of high end models. Ideally you'd test the different types side by side to see if there's a difference, but as the safer bet I'd go for a mirror diagonal with an f5.5 achromat.

I've got the William Optics 45 degree 1.25" erecting prism and it's great at low powers but starts to get soft as the magnification increases and doesn't let me get close to pushing the limits of my little 60mm scope. Above about 50x I can see a very noticeable difference in sharpness and micro contrast when using my StellaMira dielectric diagonal. Prisms tend to have shorter optical path lengths which can enable them to be used with a wider range of scopes and should have less scatter, but as Zermelo points out they might not be quite as good in faster scopes. I've also got the Baader 32mm T-2 prism which is superb. It was a toss-up between that and the Takahashi prism and I went with the Baader because of the flexibility of the T-2 system but would have been just as happy with the Tak model which I gather is optically superb. You can get astro quality erecting prisms; Baader do a couple, but they come with an astronomical price tag (see what I did there 😅).

I wonder if that chart means the current FC models when it says that the best match is the Extender TOA 1.6x, or whether they just mean the pre-FS series scopes from the 80s. On the system charts it always shows the Extender-Q 1.6x [TKA00595] as the best match (or the C2x), except for the FC-100DZ which has the Extender-Q 1.6x (50.8) [TKA36595] - also listed as being for the FSQ106ED. Those system charts might be a bit confusing at first but once you're used to them, you realise how useful they are. No messing around trying to calculate backfocus distances to work out which adapters to use in different configurations; it's all right there on the chart.

Are the Vixen HR models no longer in production? I can't afford any at the moment, but I've often seen people singing their praises.

From what I remember Jeremy, extenders and flatteners are less fussy about which scope you pair them with, but reducers are best if they're matched for the particular scope they're going to be paired wit, while other combinations can perform noticeably worse. I haven't been able to test it personally but I've seen it mentioned and I notice that on Astrograph they advise that the reducers they sell are designed to operate with specific telescopes and are not interchangeable unless another scope falls within the tolerances of that model. Takahashi have a multi-flattener that works with all current FS and FC models (and many older ones apparently), but the FS-60 has its own reducer and there's a separate one for the FC-76 and FC-100 models so presumably the focal ratio of the 60mm (f5.9) compared to the larger scopes (f7.4 and slower) is what makes the difference.

A 102mm refractor at f4.9 would be very fast for a triplet fluorite apo and even then it might struggle to match the colour correction of a really good F8 doublet. With the linked scope being a doublet achromat there's going to be a lot of false colour compared to the apo models that have been discussed in this thread. I've never looked through that scope but even as someone who isn't that bothered by a bit of false colour I think it would have more than I'd be able to tolerate.

True, although I'd be surprised if an objective was optimised for deep red as standard given that it's practically invisible to the eye in low light. The re-spacing thing is interesting - I've heard of people doing it to try and improve performance of cheap optics but didn't know it worked for an application like this.

I've got an OVL Nirvana 4mm which gives nice views of the Moon and planets. It's sharp and contrasty and has a nice 82 degree FOV.

Spherochromatism means that a scope which is well corrected for SA in green or blue could have terrible spherical aberration in deep red light.

Tak's lenses have always been made by others (Canon-Optron since the 70s I believe) but they design their objectives and manufacture the lens cells in-house and their collaboration with Canon has been a very close one and not simply that of a buyer of components. They also grind and polish their own mirrors and for a company with fewer than 40 employees they do a surprising amount themselves. Eyepiece manufacture has I think always been outsourced entirely although it would be interesting to know if Tak had any input into their design (I suspect not) or their specification (more likely).

The dedicated extenders also designed for use with specific telescopes which could allow for greater optimisation of the design. In the case of the 1.7x extenders for the FS-60CB/FC-76DCU and FOA-60 they have the advantage of having a fixed position relative to the objective so they turn the scope into a true quadruplet or sextuplet design and should be capable of even better performance.

Either that or overcome his fear of becoming a werewolf. 😛

I've also found that reading those websites seems to lead to a sudden influx of Taks and a reduction in one's bank balance! Those are some beautiful scopes and thanks for writing such a good summary of this remarkably capable and flexible system. My setup is pretty similar to your and I think I might have the same or nearly same Manfrotto fluid head as in your first pic. I started off with an FC-76DCU plus extender but it was built from parts and due to use a Feather Touch focuser. That's still on back order and I couldn't cope with having most of a telescope and not being able to use it so when I saw an ex-demo FS-60CB on FLO I couldn't resist its allure. I keep it set up permanently either on a tripod like yours or it sits in its clamshell and dovetail on a shelf in a little bookcase that's now become my shrine to Takahashi. I love every configuration and they all have their strengths such as the peerless high magnification performance of the FC-76 in Q-mode or the cuteness of the FS-60CB which never ceases to put a smile on my face with the views it gives. The FS-60Q makes the perfect spotting scope with the Baader Hyperion Mk IV zoom with its very useful 25-75x magnification range and the regular DCU is a brilliant and compact grab and go for the Moon and planets. When the focuser arrives I'll be able to keep the FC-76DCU set up permanently but I'll still be left with an empty shelf on the bookcase. That FC-100DC is looking awfully tempting! 🤣

Totally agree, and I should have made it clear that I was talking about designs preferred for very large optics that are far beyond the budget of any amateur that are used for space telescopes and the latest observatory-class instruments. The great thing about a Newtonian is that your budget only needs to pay for a single large, curved mirror so you should be able to get more aperture and better quality for the money than with any other design. One of the more interesting concepts I've seen is unobstructed reflectors based on three-mirror designs which could have incredible performance but the need for asymmetric optics would presumably make them expensive and keep them out of the amateur market for a while yet.

I should have qualified that by high end I mean the kind of optics used in spy satellites and state-of-the telescopes like the ELT. I don't know if there's any three-mirror designs (three curved mirrors, not just a Nasmyth-Cassegrain) on sale aimed at the amateur market. If you're an amateur and especially one who's interested in visual observing then you'd be hard pressed to beat a good Newtonian.

Newtonians have their advantages but there's a reason that two, and increasingly three-mirror designs are the standard at the high end. More mirrors allow you to have greater correction across the entire field but they obviously cost more to make and collimation is more of an issue.

My guess would be that it's residue from using the wrong cleaner, a lens cloth that wasn't clean or that it hasn't been cleaned enough to completely remove whatever was on it. The hard coating for those fluorite lenses is apparently much harder than you would expect so it's not that easy to damage.

What's the padding like in those Geoptik cases? The smaller version seems like it could be a reasonable choice for my FC-76DCU but I've never seen any pictures of the inside showing the thickness of the padding.

That is a really nicely made dovetail but it is a little bit narrower than some others (41mm) which can be an issue with some clamps and saddles - ADM's Vixen-type dovetail adaptor for the AZ GTi comes to mind. Worth checking the minimum width in advance.

Don't forget that you also need to keep it in an atmosphere that's been completely purged of oxygen and water vapour to prevent mould and bacterial growth inside the lens cell. It's a wonder any of these pieces of junk work at all. 😆

Thanks Jeremy. I've now had the chance to give it a quick go in my little FS-60CB to look at some terrestrial targets and yet again the ortho has been impressive. It's super sharp and seems to get that little bit extra resolution and contrast out of the scope that I don't remember seeing before (although I should do a proper side by side comparison). That sharpness extends right to the edge of the FOV and distortion is non-existent, and in combination with the shallow DOF of the baby Tak it gives that perfect snap into focus that's a sign of good optics.