rl

Perhaps it's dual function. Can you turn it around and use it as the objective with one of their HR series to make a complete mini-scope?

If you're going to drop 500 quid then a few more details might help. What do you expect to improve by changing the scope? What other accessories are you using and can they be carried over to your new scope? For example, are you using a coma corrector and off-axis guider? These would go with a bigger Newt, but will the mount then be adequate? Personally I don't see a massive gain in going to a 8" f/5 Newt, or possibly even an 8"f/4. Would you be better off improving the camera or the guiding? An ED refractor in this price range will have a shorter focal length and larger field (and no diffraction spikes) so is better suited to a different class of objects. But you will also need the matching flattener and may need to change your guiding arrangements. Would a better mount allow longer exposures with your existing scope? Specific recommendations if changing the scope really is the way forward; I've owned a ZS61 which is just about in budget secondhand with the flattener and is fairly undemanding on your mount, and gives good results, and makes a useful travel scope . If you are lucky you might get a secondhand TS65Q which has the flattener built in and makes a really good solid well-built dedicated imaging scope. I paid #380 for mine on ABS. Some of the early ones had pinched optics in cold weather but the issue is easily fixed, and it works visually only with a 1.25" diagonal. As to Newts. I'm using a CT8 f/4.5 which is a beautiful scope but secondhand ones are rare as rocking horse manure and tend to come in at #600 plus. But a "wanted" ad on ABS costs nothing...

Both mine are several years old. Both have the OO Crayford. They are manageable but could be better. One works fine providing you don't rack in too far (wherupon it falls apart). ..The other is either stiff and lumpy or too loose to be useful ..there is no really happy mean. I've tried a couple of the new ones and both have been beautiful..smooth and tight at the same time.

I currently have both in my possession, both bought secondhand at prices far below the new rate. I'm currently pondering which one to keep. Nice problem to have.. The VX14 gives you the choice of mirror accuracy. I have the 1/10 wave and it makes a superb dob. It's the largest thing I can reasonably lift. Having said that, the tradeoff is that the tubes on the VX series are a little thin and prone to dents if you aren't careful. The CT12 comes as 1/10 wave as standard. I have the f/4 option. It is more rigid than the VX and less prone to dents. It holds collimation better if you're going to be carrying it around., not that the VX is bad in this respect but, as previously stated, the tube is more flexible. The CT12 also seems quite a lot lighter due to the smaller aperture and shorter length..I don't recognise the 22kg number but I might be wrong on this. Note..the lovely carbon fibre finish is easily scratched and very prone to dewing up on the outside, (but not on the inside where it matters). The improved rigidity and smaller coefficient of thermal expansion will be plusses for astrophotography. One annoyance of the CT12 is that you can't stand it on end without sitting it on the collimation bolts risking the adjustment. Not a problem one it's on the mount. I fitted some some spacers to get around this niggle. Both mirrors are superb, but to be honest on 98% of nights the atmosphere is the limiting factor unless you're on top of the Pyranees. Both are capable of really good results on planets even with the central obstruction. The mirror cells on both are well made and easily adjusted, especially if you disassemble them and grease the bolts. The CT12 seems better made but the VX is easier to adjust having thumbscrews rather than Allen bolts. The VX14 cell has 2 mounting options about 30mm apart giving you the choice of setting the focal plane in the most convenient place. I'm not sure if this was an intentional feature but it is useful. The 3 metal blocks on the edge of the cell have 2 sets of holes. A lot of the CT12 metalwork is machined from aluminium plate. The end rings on the VX14 are pressed out of thin sheet and feel a little flimsy if you take the whole scope to pieces. I generally have no conscience about drilling holes in Newts for accessories but I make an exceptIon for both these scopes, especially the CT12. On balance I think the CT12 is the better all-rounder. It's almost the same aperture, more portable, better adapted to photography. But in my case, the VX14 mirror is exceptionally good according to the zygo report, having a 99.1 Strehl and virtually zero astigmatism. The newer focusers (Baader Steeltrack?) are a big improvement on the older OO home grown offering on both my scopes. They are the only Crayford types I have any confidence in. A lot of the choice may come down to the mount. The CT12 is just about useable visually on an AZ-EQ6 with extra counterweights and an extension bar. The VX14 really gets you into EQ8 territory. I had an EQ8 once and bitterly regret selling it.... One costs a small fortune new, the other costs a large one, but this is the inevitable result of UK manufacturing costs and lack of economy of scale. Both have the potential to be a scope for life. Both are much better value secondhand but make sure you get the Zygo certificate. The OO customer experience can be "interesting" . Personally I've not had too many issues but you will find alternative viewpoints on this site. Hope this helps. RL

As others have commented, the ES92 17 is a truly stonking lump of glass, IMHO better corrected than any of the ES100 range I have owned or looked through. I've detected Cassini's division with telescopes of smaller aperture than the eye lens on the ES17. Depending on your observing environment, the excellent eye relief can let in quite a lot of stray light if you happen to be afflicted by local street lighting or houses. I used to think that the Ethos was a bit tight on eye relief and wished for a bit more, but having owned and used both in a light polluted environment I think the Ethos really has it just about right providing you don't need glasses. Raising the eye cup on the ES helps, but it's not a complete solution. And if you are observing with glasses, you may find that the 92 degree field gets cropped top and bottom so you don't get the full benefit of that magnificent field.

Aperture is king...except when you have to carry it around! I think you need to be very realistic here. Exactly how far are you going to be carrying the kit? Are you taking it to the park in a car and just lifting it out of the boot, or is it a bus job followed by a 200 yard hike? The bigger scope costs more, is more clumsy to handle in the dark, and will depreciate more after you've put a few dents in it at 2am. Got the T-shirt..I had (actually still have) a 5" f/10 refractor I took everywhere for years when I was in my 30s but it certainly gained a few battle scars on the road. A decent quality carrying bag or case is a good investment. I have the 102mm Evostar bought secondhand for peanuts because old-fashioned achros aren't cool any more. It's a good scope and the residual CA is not at all prominent. It's there if you get picky, but not to the gross detriment of planetary detail. And it's light. It's not a bad route to go down if you've ruled out a 6" Newt.

Basically it's all down to using a local atomic reference, which is not subject to drifts due to ageing or temperature..the atoms just work with absolute consistency for much the same reasons that the best clocks are atomic in principle. The basic principle of operation is pretty much immune to small slow temperature variations. A standard grating is subject to all sorts of environmental effects and such spectrometers are usually housed in air-conditioned rooms with exquisite temperature control, fixed down to vibration-damped benches. Yes, I believe you can get somewhere near the same sort of accuracy if you are careful enough....I believe the first exoplanets were found this way..but it is harder. The atomic based spectrometer is not perfect..it still has issues with the analogue electronics drifting with temperature and optics shifting, but the level of effort required to fix the secondary drift problems is less. Sure, there are ways of messing it up if you build one yourself; there is a learning curve. And you only get data on one line..it's a one trick pony. Horses for courses... Our spectrometers were mounted on a standard equatorial mount open to the weather and still gave very stable results in spite of getting tilted as the mount moved. They worked even better given the same environment as a precision echelle. You need to have a good look at the photon statistics..most of the starlight is thrown away using an atomic based spectrometer since youre only using the 2 wings of one line. Not a problem on the sun and 1st magnitude stars with a big mirror, but if you're looking at 12th magnitude stars with a 6" mirror you might need a very long integration time just to get a decent signal-to-noise ratio. A grating makes more efficient use of the light available. And you have the option of calibrating with an atomic standard at regular intervals. The point I'm making is that no one technique is the best in all observing situations. Sorry if that's obvious..! If you've got a genuine university project have a look at all the measurement technology out there today...my information is 25 years out of date. There is a lot to consider in order to get the most precise measurement given the constraints of observing time, development costs and operational convenience/ reliability.

Some 25 years ago I had the privelige of working for Birmingham University Physics and Astronomy dept under the late Professor George Isaak, who, for the record was one of nature's natural gentlemen. The BISON group did a lot of the early work looking at oscillations of sound waves on the sun, from which models of the solar interior could be derived. We made spectrometers that focussed on a single atomic absorption line ( usually Potassium at 770 nm) using potassium vapour as a local atomic reference. The idea is to isolate 2 wings (called red and blue) as areas in the absorption line either side of the minimum. the spectral line in a star is broadened due to thermal motion so the stellar absorption line has a very distinct width and shape. Your local reference sits at typically 100C and has a much smaller width, so can be used a a probe to look at the details of the wider stellar line. If the relative velocity was zero, the red and blue wings would be of equal intensity being halfway up the line profile. If you define an intensity ratio of (R-B)/(R+B) this gives a function which is independent of light intensity and scales with the relative velocity. When the stellar line shifts due to the doppler effect, one wing will approach the line minimum while the other will will move up towards the continuum, thus the ratio changes. How do you look at the 2 wings? Some elements have spectral lines that split into two components in the presence of a magnetic field....you put your potassium cell between the poles of a strong magnet and it is possible to look simultaneously at the red and blue wings using polarised light. It's called the Zeeman Effect. If you Fast Fourier the data and average the results over weeks and months we could see relative motions of millimetres per second in the sun since the photon statistics are really good, and about a metre per second in 1st magnitude stellar velocities. To do this we cast our own mirrors about 1-2 metres in diameter from araldite spun in a dish using the centrifugal force to produce the parabola shape. Not good enough to image with, but plenty good enough to pass starlight down an optical fibre to a spectrometer. Basically the more data the better...the precision you achieve generally goes with the square root of the number of observations. Don't confuse absolute accuracy with the ability so see small changes...you are right in that getting an accurate answer to the true radial velocity means knowing a lot of other numbers to the same precision...the moon's effect, gravitational redshift in the case of the sun, the earth's orbital velocity and barycentric motion. At the time the group had the world's most accurate radial velocity for Arcturus and one or two other stars. The whole project was a triumph of cost-effective science. We built a world-wide network of remote observatories for less than a million quid.... Have a look.... http://bison.ph.bham.ac.uk/ I can recommend the following book, with which I have no financial interest (although author Prof Chaplin is an excellent chap..it was his araldite mirrors that did the stellar work!) https://www.amazon.co.uk/Music-Sun-Helioseismology-William-Chaplin/dp/1851684514/ref=sr_1_2?dchild=1&keywords=chaplin+sun&qid=1611484535&s=books&sr=1-2 I have no idea what the current state of the art is.

Referring to the OP's comment about the possibility of using a 12" Newt on an AZ-EQ6...I've run an OO CT12 on mine successfully for visual but it's on the ragged edge. The OTA plus rings weighs in at about 16 kg. Add the Paracorr, and an Ethos 21 and you are are at the limit. And you need steps to reach the eyepiece because the OTA centre of gravity is biased heavily towards the mirror. It's not just the weight..the 12" OTA is so wide the centre of gravity is offset a long way from the mount requiring the use of the counterweight extension bar and 4 by 5kg weights. It does work, but it's not an easy setup. Skywatcher OTAs are usually a bit heavier... YMMV...

Hmmmmm ..Interesting set of replies. Lots of support for the 21E, 13E,14 Delos, big Naglers, Pentax, Vixen. To pick on one eyepiece is probably a silly question (but I'd volte for the 21E), but maybe the best range is not. Funny thing is, in the 13 years since the Ethos line was first released, every serious manufacturer out there has cloned them and most reviews rate the clones nearly or as good as the real deal. And yet I see no votes yet for the clones...is this just a statistical selection effect or did Al Nagler get something very seriously right that's hard to match? I sold my Ethoses when times were hard and tried several cheaper equivalent options. But as the years have gone by I've ended up replacing the clones as fortune allows....I'm still looking for a cheap 13!

A poor thing but mine own..an OO right angle finder. Can we have a thread for the silliest OTA- eyepiece combo? Can anyone match a 30mm ES 100 to a 60mm OG?

Good job... I regularly used to split zeta bootis as a figure-of-eight with a 5" f/10 John Owen achromat when the separation was about 0.9"given decent conditions. It's worth making a drawing with companion stars in the field...in the future you can actually see the orbital rotation over a couple of years if you have a reference point.

You will probably need to keep the Nagler just for a counterweight....

I don't know the particular model you are considering but I've owned several WO scopes over the years...ZS61, ZenithStar 80 mk2, GT81 MK1, Megrez 72...all bought secondhand. All have had excellent optics. The astro-bling comment is valid, as is the criticism about the rate of change of models. There is only so many ways you can repackage an 80mm lens and make it look exciting and original by changing the paint job but they keep trying. But, Pandora bling apart, they are basically very good small scopes optically. Not all the focusers have been so good....as you're looking to use it for AP I'd steer clear of the Crayford-equiped models from experience. The rack-and-pinion scopes seem to take a camera weight much better. The GT81 is one scope I'd probably take to the grave with me (along with my CT8 newt...and a 6" refractor..)

EOS1000D is good. I've got a pair of 1100Ds which have been very reliable. Canon seem to be the best supported by a long way from the astronomy point of view. You can get away without LiveView but it's really painful; I think all APSC format above 450D have this feature but I might be wrong on this one. Other than that they're all pretty much the same. Controlling it through a laptop is a good thing to do..screen is much easier to see. Spending a few quid on a 12v battery adapter is also worthwhile. Check the shutter count before you buy.

Nothing worse than being out on that perfect moonless night and having the battery fail on you....especially when the weather forecast for the next week is all cloud and rain. And when the rain stops, the moon is back up...... 20 quid is the cost of a spare battery, but frankly I 'd take the camera batteries out of the equation altogether and run everything off one massive battery charged up before you go out. Mains is a better option, safety precautions observed. Changing a camera battery risks disturbing the focus and the pointing. Batteries often have less capacity in cold weather, which might well catch you out. Money spent ensuring you make the most of the limited AP opportunities in this country is usually a good investment. Just my 2p worth from painful experience.

My big newts have always been stored outside in either a shed or garage. Helps enormously with cooldown and I've never had much in the way of dew issues. A mothball hidden in the tube somewhere keeps the creepie-crawlies out..I've had strange diffraction patterns in the past with spiders webs. The modern mothballs are not the old napthalene..there is no smell but they seem to work.

Advertising is free..just try it on here or ABS!

I'm not familiar with this particular scope but it's probably reasonable to assume that the cell fixings will contract faster than the mirror as the scope cools down. You need to allow enough slack so that the glass wont get pinched even in the coldest nights. I always leave enough clearance to get a couple of pieces of paper in between the metal and the glass. If you're that worried about movement a couple of blobs of silicone goo will hold the mirror without putting excessive force on it.

It's not just CA you have to worry about...the large cheap achros also suffer from varying levels of spherical aberration as well. CA is at least designed in given the glass type and focal ratio. SA is as much a feature of how spherical the figuring really is. I've owned 2 examples of the ST120 one of which was far better in this regard than the other. Other people have similar stories about the 120mm f/8 variants. All the optical aberrations seem to get better with longer focal ratios; the Lyra 102 f/11 is probably a good choice in this respect.

From the UK corner....6" triplet made by John Owen circa 1991. None of your fancy coatings or ED glass...just your plain vanilla crown-flint-crown and it works a treat. Which reminds me.. I was going to get the blackboard paint out and tart up the cell a bit!

They are certainly very impressive, driven largely by the automotive market. But the specs need to be treated with a bit of a pinch of salt.....150 amps at 0.002 ohms is still 45 watts of heat which needs decent cooling. The specs often apply with the case artificially force-cooled to 25C. The on resistance usually doubles as the die heats up towards its maximum temperature which makes the heat sink even more important. And clean switching on the gate is important, or the instantaneous power will exceed the pulse rating. But for more civilised currents they are indeed fantastic components.