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. 

This is a very good chip that I have used in other applications. Analog Devices do a demonstration board which saves you some of the soldering problems...part number DC1555C available from Mouser and Digi-Key for about 25 quid. It's pricey (especially compared to a simple diode), but not as pricey as trashing your mount, and it copes with a lot more failure modes. . Don't bother omitting the extra FETs....they're so cheap it makes no sense. 

https://www.analog.com/media/en/technical-documentation/eval-board-schematic/dc1555c-2-sch.pdf

Excellent suggestion from symmetal...I've been working with these for months and the astro application never occurred to me!

Depends on your luck, and how far you are prepared to move the main subject off centre. So far I have never had to rotate (and refocus)...just moving a bit has been enough. But I've often had only one or two stars in the OAG field and they're frequently towards the edge. ..one day it's going to happen. On my main camera, (QHY183) I think all 3 OAG positions are available. On the DSLR, bits of camera clash for 2 out of 3 alternatives. 

The question is..what's the alternative? Guidescopes have their own issues with Newts at this focal length. A bigger  chip in the guidecam will always help....at extra cost as you have pointed out. But it is probably the only realistic way of improving the situation. 

The QHY5L2 is a very good OAG camera for the reasons you point out. even if the chip is a bit small. Working at 900 - 1200 mm focal length I've had issues occasionally finding a guide star. No issues in the galactic plane, but in some of the sparse parts of Ursa Major I've had to move the main object slightly off centre just to find a guide star. 

If I remember correctly, the connectors on the back of the QHY5L2 had a bit of a reputation for not being the best, but I've never had any issues myself. But I always allow a lot of strain relief.

30 minutes ago, John said:

There is a slight irony that it takes until the latter part of ones life to be able to afford excellent optics which you might be better able to exploit more with younger eyes and younger energy levels.

John...You have taken the exact phrase out of my brain that I was going to post! 

1 hour ago, billhinge said:

When I was building an 8.5" pyrex telescope mirror in the 70's the books e.g. Howard and Fullerscopes used to say aspirational optics should be 1/8 to 1/10th of a wave. Looking at modern optics you see 1/4 of a wave and it often seems 1/10th is premium grade.

Are things measured differently now or were the old 1/10th wave claims just bs? (I think the 1930's telescope books used to say 1/4 was OK but I think I saw Planewave quote 1/20)

Do the figure work the same with refractors? (just looking at APM

"For systems of diameter up to 152mm Wave front error RMS is no more than 0.035 of wave-length at 532 nm. Wave front error p.t.v. is no more than 0.24 of wavelength at 532 nm" ) )

 

Sadly a lot of these older measurements have to be taken with a large pinch of salt. In many cases the measurement technique would not have been capable of measuring consistently to 1/10 wave. And you have to specify the wavelength of the test...and the mirror has to be realistically supported to avoid sag of 1/10 wave...A Foucault check with the mirror propped up on a few books in a room with air currents just won't cut it.

I've got an 14" AE optics mirror by Jim Hysom made in 1977 with the documentation quoting tested to 1/10 wave. I've had it checked on a Zygo and it comes out at about 1/4 wave at 632nm. It's a very good mirror in general use because the atmosphere is the limiting factor, but 1/10 wave it ain't. 

Unqualified numbers like this without explanations are a salesman's dream. The Strehl number tells you more about the mirror quality but for some reason the wavelength error has stuck as the criterion. Beware of knaves setting traps for fools..ot at least the uninitiated. 

I doubt if an Off-Axis-Guider is the solution; you will need an extra guide camera plus a PC to control it. And there will be spacing issues to worry about. The dedicated ZS61 flattener comes with the correct spacing as standard which tends to rule out an OAG unless it goes in front; this just changes one spacing issue for possibly an in-focus travel issue.

Is the real problem poor polar alignment or gear cutting errors? Having owned this rig myself I'd strongly suspect the former, especially at the rather short focal length in use. The flattener will reduce the focal length by 25% which makes trailing a bit better cosmetically. 

Are you using the polarscope for PA alignment?

The balance needs to be slightly heavy on one side just to take out any backlash in the gears.

Are you using a PC to work the camera or just using it native? I'd look at using the camera and main scope to set polar alignment with one of the plate-solving methods...some camera control programs come with this feature as standard. It's very effective. Sharp Cap Pro? It would keep both weight and budget down.

I found I could go for about 2 mins with most of the subs useable just by moderately careful alignment on the polarscope.

That should be fine. You might well find less power is enough but I know how bad dew can be in the tropics...

The resistors will probably be ok but might be a little bit close to their maximum wattage. Half-watt parts are available and not much bigger physically.