Andrew_B

Have you done much astrophotography just using basic camera lenses? Shopping secondhand can get you some real bargains and you might be surprised how much detail you can capture with something like a short telephoto. There are a few advantages with this approach - shorter focal length means tracking errors are less of an issue and camera lenses typically have faster focal ratios than scopes so even when stopping them down a bit to control aberrations you can normally get away with shorter exposures. You're still learning the same workflow that applies to shooting with a scope but it's easier and less likely to cause frustrations and astrophotography can be tricky enough without making things harder than they need to be when you're a beginner.

Plastic construction has a bad reputation in astronomy that's not entirely undeserved given that it's mostly used in very cheap products and is often poor quality. The right plastics used in the right way are every bit as good as metals and it's worth remembering that even things like missile re-entry vehicles are made of reinforced plastic so it's hardly weak or easily damaged stuff. Have you ever had a chance to compare the Tak prism to prisms from other manufacturers?

Fuji cameras tend to have the best red sensitivity out of the box and they're mirrorless so it gives you space to play with back focus which can be useful. Downside is that compatible lenses are less available and remote control via PC is a non-starter. The cheaper models are actually better for AP than the fancier ones because they don't have noise reduction applied to the RAW files. Another option is to look for a secondhand pre-modded camera on somewhere like eBay. I picked up mine for just £180 and it's a 24MP full spectrum mod that can record anything from UV right through to surprisingly far into the infrared.

The Tak prism does have a plastic body but from what I've read it's not some kind of soft and flexible plastic, it's more like a reinforced composite and is fairly substantial and perfectly good for the job. I've not read about any issues with its strength or durability but its optics are very good and it's decent value for money. I've got a Baader prism which is also great but I wouldn't mind picking up the Tak one to compare at some point.

I think you'd struggle to get the best out of a big scope of any kind here in the UK compared to many places in the US. I've also read that obstructed optics tend to be more affected by poor seeing, then there's factors like temperature and weather changes that can favour doublets over triplets and Newtonians over SCTs simply because they reach peak performance so much quicker even if another design might be better when both are acclimatised. I have some bad news, it's total junk. Good news is I'll take it off your hands for proper disposal and because I'm such a generous person I won't even charge you for shipping! 😂

Interesting point about the prism. I did a comparison between one and a dielectric diagonal and felt like the prism did give a bit more contrast and show fine detail better although the test wasn't ideal for showing the difference. Prisms have less scatter so I'd guess that's what we were both seeing. You see a lot of mirrors advertised with 1/10 wavelength surface figure but apparently with dielectric ones this nearly always refers to the quality of the substrate rather than the finished item. Apparently it only takes a tiny variation in the thickness of each dielectric layer to quickly add up until the end result is several wavelengths off being flat. Like you say though, it might not be an issue if it's being used with a budget achromat at moderate magnifications. Prisms are supposed to be better suited to longer focal ratio instruments but I use one with my little f/6 refractor and while it may cause a bit more false colour there isn't enough to be a problem for me and I like the contrasty views it gives me.

What kind of magnification were you using? I found that at low magnifications diagonals were all pretty similar but it was as I got to higher powers that differences became readily apparent.

If it's based on the quality of the view through the eyepiece then that's fair enough but I don't think anyone would be surprised that more aperture tends to be better, and that high quality kit scores better than lower quality at the same aperture. In other news, the Pope is suspected of being Catholic.

Not just fiction but in the fantasy section. Anything like this is little more than personal opinion and a bit pointless unless there's some explanation of the strengths and weaknesses of different scopes and why they received a particular score.

Aperture seems to be the biggest factor, which is one way of looking at it and you'd expect a mirror to win in that context. Mind you, the lack of anything over 300mm is a bit peculiar given the high end nature of much of the gear on the list, so wouldn't something like a relatively ordinary 400mm Dob utterly dominate purely on the basis of aperture? I'm a bit confused as to the purpose of the list. Am I supposed to learn which scopes are best at any given aperture or is it telling me that all things being equal aperture is king (which seems a bit obvious)?

Just looked at the Svbony website for the specs (it's in the hunting->spotting scopes section) and it's a very compact and lightweight package. Price isn't bad either. 70mm f/11.2 and 792g which I presume is with the included eyepiece.

Apparently there isn't exactly a standard 2" filter thread. Most manufacturers seem to use M48x0.75 but some use M48x0.6 and Baader have adopted a pitch of just under 0.75mm to try and maximise compatibility with the range of M48 threaded products on the market based on detailed measurements of hundreds of examples.

Thank you, that's useful advice about the eyepiece retention system. Saved me getting the wrong thing. I saw a a few models that looked like that Antares BV which came with a pair of Barlow lenses (1.85x and 3x). I suspected that their quality can't have been up to much given what the entire package was selling for. Good to know. I ended up ordering a binoviewer sold under the Sky Rover brand who also have a few nice eyepieces that look the same as more familiar offerings from OVL and APM. I chose it mainly because it comes with a proper case but it also includes a 1.6x nosepiece in addition to the standard one so I wonder if that's the same model that W.O. were selling? I'll try it out and if it's junk it hardly matters for what I paid.

That's useful to know. I'll try my Hyperion 2.25x Barlow first and I can always invest in a GPC later on.

Many thanks Mike, that's really helpful. It looks a lot like the binoviewers I've been looking at although they used an all black colour scheme and some appeared to have a kind of twist lock holder for the eyepieces with others using normal locking screws. I don't have a Barlow like yours but I do have the Baader Hyperion 2.25x Barlow which should thread either onto the nosepiece or possibly in place of the nosepiece which I would imagine would enable different amounts of magnification. I find cyclops viewing gets a bit tiring after a while, especially when planetary viewing so I'm keen to try one of these out and the cost of a basic binoviewer and a couple of cheap orthos to start me off won't break the bank.

Thanks John. That's quite a smart case they come with.

Mike, I've not seen any Revalation branded binoviewers on sale. Are they the kind of model you see sometimes branded (usually Celestron) but often unbranded on various sites selling bargain Chinese optics? The ones I've seen usually have an aperture of 22mm, BAK-4 prisms, 54-75mm inter-pupillary distance, what I presume is diopter adjustment, and usually one or two Barlow lenses, which seems remarkably good value for money. Did yours come with a Barlow or do you use a separate high quality one? Thanks

I know you can get lightweight mirrors with honeycomb structures for some scopes but I wonder if anyone makes them for use in Newtonians? I'm sure the answer is yes, if you have the budget for a custom optic but I can't say I've seen them advertised as off the shelf products.

I've got a 3" refractor and it gives beautiful views (I haven't had a chance to take it out under properly dark skies yet though) and will take 50x per inch magnification very easily and go a fair bit higher in decent seeing on brighter objects and double stars. I've also got a little 60mm scope that's perfect for grab-and-go and doubles as a spotting scope - its views of the night sky are surprisingly good and last night I used it to see Rigel's companion for the first time, despite Orion being pretty low in the sky. For a long time 60mm was pretty much the standard size for a decent entry level refractor and many astronomers will have learned their craft using just such a scope from brands like Carton and Unitron (before my time though). Optics are better but eyes are the same so the light gathering of a small scope will be a limiting factor (imaging is a different matter where small refractors come into their own), so if you can stretch to a 4" scope then it'll be worth getting. What you have to remember is that when Sir Patrick was giving that advice, modern apo refractors didn't exist (or were new on the market in the 80s and came with an eye-watering price tag), a 3" achromat was surprisingly expensive by today's standards and a 4" model was a rare thing that cost a fortune. Achromats also needed much longer focal ratios to maintain acceptable levels of colour correction so if your 4" scope had to be f/15 then you're dealing with a very long and heavy scope with quite serious mounting requirements. I'm itching to get a 4" refractor and the nice thing with today's offerings is that I can get one that is barely any heavier than some of the 70-80mm scopes out there and much shorter than the old achromats so it would be very portable and work with the same mount I use for my smaller scopes.

I've got the 6mm Fujiyama ortho and it's a superb eyepiece - razor sharp and distortion-free with loads of contrast so I'd expect the 7mm to be a great planetary eyepiece as well. Orthos are also very small and light but the small eye lens, short eye relief and narrow AFOV aren't for everyone and can take a bit of getting used to, but if you're using a tracking mount then that last issue is far less of a problem.

Comparison of optical standards isn't helped by manufacturers playing fast and loose with their specifications. Is that 1/10th wave mirror quoting the surface accuracy of the substrate or the finished product after the application of the dielectric coatings? If it's the former then the resulting mirror can be nowhere near 1/10th wave but this isn't always made clear to the buyer. Ultimately there are enough factors affecting the quality of the final image that it's hard to judge that much from spec sheets and the best comparison is at the eyepiece. Edit - Baader are one of very few manufacturers who quote surface quality of their dielectric mirrors after they've been coated.

For an inexpensive fast achromat I think a mirror diagonal is the safest bet unless someone has tested a prism with that particular scope and found that a prism works better. Nobody is going to bother trying to optimise their objectives to work best with one or other outside of a handful of very high end models like the now discontinued and fantastically expensive Zeiss APQ apo refractors, so choice of diagonal is more likely to be based on overall quality, size, weight, optical path length and other more practical considerations. My reason for buying a Baader T-2 prism diagonal was based on the need to have a short enough optical path to allow my eyepieces to work with the limited range of back focus available and prisms tend to have shorter paths than mirrors. Prisms should have less scattered light, but again the difference compared to a reasonably good mirror diagonal probably isn't worth worrying about if you're using it with an inexpensive scope.

There are manufacturers that design objectives with a view to them being used with a prism diagonal, but it's not common. Here's an explanation from CFF about choosing a diagonal to go with their different refractor models: The color correction of Visual-tuned lenses is designed to give peak Strehl ratio where human eye is the most sensitive. For this reason, these lenses deliver best possible image sharpness and contrast for visual observations. Technically speaking, they are tuned to reach peak Strehl ratio in a limited range of wavelengths (430 nm – 700 nm). Our recommendation is for these Visual tuned lenses to be used with mirror diagonals. This type of lens gives excellent photographic results when used with IR/UV filters. The color correction of Photographic-tuned lenses is designed for covering a sensibly wider range of wavelengths. These lenses will be better corrected for near IR and violet wavelengths. Depending on the type of the photographic sensor used and its sensitivity range, images can be taken without the need of an IR/UV filter. The acceptable corrected wavelength range is usually 400 nm – 1000 nm. As photographic sensors are sensitive over this range, this type of color tuning nicely matches the needs of photographers. It has to be noted that Strehl ratio becomes slightly lower in red and blue colors and experienced planetary observers might detect the difference under excellent seeing conditions. To get optimal Visual performance from these lenses, some glass needs to be added in the light path, and in our case, this can be represented by a prism diagonal (1.25″ or 2″ size). As prism diagonals typically have better optical quality, better light transmission and less light scatter than mirror diagonals, we encourage all amateur astronomers to use them. With the Zeiss APQ 100mm f/6.4 the effect of diagonal choice was quite significant and using a mirror rather than a prism meant it behaved more like a semi-app than a super-app as it was with a prism. Their scopes were apparently designed for use with a BK7 prism with an optical path length of 35-45mm. A prism should have lower scatter than a mirror and it should have better plane surfaces for a given quality of manufacturing but how much difference the average observer would see when using either with the average refractor is another matter.

Colour correction still matters for narrowband. The main problem is spherochromatism where the degree of spherical aberration varies with wavelength so you could have a scope that's well corrected in green (as they normally are) but shows severe SA in red or blue. If it was just longitudinal chromatic aberration then it wouldn't be a problem provided you refocused for each filter.

I've not had a chance to look through the 90 degree version, but I would be surprised if it didn't have similar issues and the tendency to show diffraction spikes is inherent to amici prism diagonals. Baader sell a couple of astro-spec erecting prisms but they're expensive - several times the cost of your entire scope! A standard mirror or prism diagonal should be a lot better although the mirror diagonal will be the better choice for use with your scope due to its fast focal ratio.