How good can a Apochromatic telescope be?

[Macavity]

Not when it comes to resolution, and simple light grasp.

Not my point. I did add the proviso of "personal circumstance" - If I can't LIFT the sucker (conveniently) it won't get used. <G> Deep-sky penetration is attractive, even without resolution, light grasp, or production of the consummate imaging presented here...

"Aperture fever" is ever going to be easier for the Physically Fit. Idem, purchase of large APOs - The domain of the *financially* fit? Prerequisites sometimes (usually) ignored by advocacies. Ersatz they may be, but small (GoTo) scopes, with integrated (instant) imaging etc. seems to be a coming thing...

<Part Serious> It is a "law of physics" (quirk of nature) that the typical focal length of Newtonians (~1250mm) is "Neither Fish nor Fowl" re. (visual) size match to many celestial objects. Budget (small chip) imaging can be equally frustrating too. As you say, most people NEED several scopes. 'Fracs (and MAKs) seem to offer more interesting prospect in this respect.

[angusb1]

most people NEED several scopes. 'Fracs (and MAKs) seem to offer more interesting prospect in this respect.

That's why my next purchase after the APO will be a 150 Mak for planetary observing.

[damnut]

Another great SGL Thread.

We dont 'need' several scopes, we 'want several scopes'. I have a 10"sct, a 127mm f9.5 frac & 120mm f5 short tube frac. We each have our own journey through this great compulsion (hobby!). My kit has been acquired over the years, selling some buying some. I wonder what Gallileo / Newton /Huygens would have made of todays refractors with CA, I think they would have thought they were amazing.

So each to their own I guess, and whan I cannot lift the 70lb lump of sct onto the tripod I'll have to rethink again!. But I am with Skybrowser's view of the APO's cost and alleged portability and some ep costs :-).

[aa6ww]

There's no one scope that does everything well, but to me, when you just really want the absolutely best performance out of a telescope, nothing can touch an APO on crisp beautiful lines and sharpness. Its something you will have to experience over time. At some point when your observing, your going to wish focus was just a little more crisp and the stars were just a little sharper, and the back ground skies were just jet black, etc.

If you just think about cost, you're probably going to just always end up with a dob.

APO refractors are just something you have to experience. You have to live with them for a few seasons to appreciate how beautiful they are and how they present the objects in the sky to you.

This discussion can last forever, but in the end, you have to just own one and experience what it gives you, that no other scope can.

Every telescope tries to achieve the clarity, sharpness and beauty than an APO refactor gives you. Its the optical standard every instrument tries to achieve.

I have a TSA-102 and a TOA-130. I have about 10 scopes total including a C14, C11, C8, MK-67 and a 7" TMB Achro etc, in the end, the TOA and TSA are the ones Id hold onto, and maybe the C11.

[glowjet]

How good can an Apochromatic telescope be ? was the original question, the answer to that is very good indeed, in terms of scopes like the Astro-physics 155 Starfire, or the TEC 140, used in the USA were the seeing is often very good, the Starfire has been used with magnifications approaching 700X, for more info have a read of the reviews on the Cloudy Nights forum. The semi Apo in the range 80mm to 125mm give very rewarding results in the UK, as most owners will tell you.

John.

[ollypenrice]

The apos are dreams to use for wider field astro-imaging and here I would defend them against all comers. They are quick and fuss free to use, cool quickly, snap into focus and hold it. I can see why an ultra fast corrected Newtonian astrograph at F2.8 would be great but I don't want thousands of diffraction spikes and small square stars in my widefield images. Long FL images are a different matter.

In visual use, to understand why some people (like me!) love refractors you have to borrow a line from the song; It ain't what you see, it's the way that you see it.' Because I'm 'in the trade,' in a manner of speaking, I can justify having a range of scopes including small to large apos, a 10 inch SCT and a 20 inch Dob. The big Dob gets to things invisible in anything else and gives a huge Wow factor but the TEC140 shows what it shows perfectly. The stars are tiny, the sky black, the focus unequivocal, the edge of field exactly like the centre. I get a buzz out of the sheer beauty of what's there. I don't feel myself worrying about what isn't there. I can always image it!

On the planets I feel it usually beats the 10 inch SCT, absolutely brains the 20 inch F4 and the other night beat a very fine 20 inch F5 that was visiting as well. The big one was disabled by imperfect seeing. Small aperture can see through bad seeing better than large.

Fine apos are certainly the worst value for money in astronomy, but you could say something similar about Ferraris, perhaps...

Olly

[Macavity]

I sense the question is a bit like the previous: "How good are Teleview

eyepieces", really? And the answer not too dissimilar?

[rwilkey]

I think both the frac and the flec offer different takes on the night sky, as you can see from my signature, I have both, albeit, a doublet, but I love them both. Health problems means I use the doublet more these days, it is so much easier to set up and use, views are fantastic and incredibly clear, especially with well chosen ep's.

[Stu]

I can only agree with Olly that there is something wonderfully pure and clean about the view through a good refractor. I was viewing the double cluster the other night with a 31mm nagler giving over 3.5 degree fov and it was wonderful. I could have looked at it for hours, so many tiny pin points of star light beautifully resolved just at the limit of visibility and both clusters framed perfectly. The double double is also perfectly split with it.

It does seem to always perform, rather than being at the mercy of poorer seeing like the mak.

One day I shall have a look through Olly's TEC 140 :-)

As has been said though horses for courses and there's nothing like a big dob for spectacular deep sky nebula/galaxy viewing

Stu

[Uranium235]

I think Olly more or less covered it all. APOs are so much easier to use, and yield sharper, more contrasty results (IMO). However, quality, speed and the type of glass used (and the amount of it) varies, hence the huge price differences.

I have both a newt and an APO, and the ED80 knocks the socks off the 6" newt despite being half the apeture, a shorter FL and slower. As a bonus you get more focuser travel for additional gear and you wont be waiting 40+ min for it to cool down before you can start your session. This is from an imaging point of view though, since I rarely do any visual work.

[michael.h.f.wilkinson]

At a given exit pupil, the higher contrast of the apochromat will make it easier to see such detail as there is to be seen, no doubt about that. However, under the best seeing conditions the 16" RC (Officina Stellare, if I am not mistaken) of our university will beat any 6" apochromat on planets. This is just a matter of physics. In well corrected optics, the resolution limit is simply determined by the ratio of wavelength to aperture. In this respect, if a 6" Newtonian is beaten by an 80mm ED, on planetary detail, the optics of the Newtonian should be checked for collimation (in particular if it is an F/8, which is a brilliant planetary scope).

I have to say getting 700x out of a 155mm scope without blurring out the image is only possible if your visual acuity is not that great, not to say downright bad. I have a visual acuity of 1.6 (60% sharper than average), and I have looked through some great scopes (including Olly's TEC 140 (NEAT!!)). However, whenever I start approaching the magnification limit of twice the aperture in mm, i.e. 160x on my 80mm APM triplet (no slouch at all) things get visibly blurry to me, simply because my eyes start resolving the point-spread function (PSF) of the optics. On my C8 I can handle 160x and above quite routinely, simply because the central peak of the PSF of the optics is tighter in the C8, that in my APM 80mm.

At 700x you are at 4.6x the aperture in mm of a 6" scope. Only if your visual acuity is below 0.5 (50% worse than normal) will you not be able to resolve the central peak of the PSF of the optics with ease. If you can resolve the central peak of PSF, the image will become mushy. That's just physics. I have sometimes pushed my C8 to beyond 400x magnification, on both the moon and Mars, just for larks on a day of near perfect seeing. The image is still usable on such object, but I cannot resolve more detail than at 300-320x with my eyesight. People with less visual acuity may benefit from pushing the limit, but for many it will not help.

Note that a very simple way to find out which scopes give the highest resolution images is checking out the planetary imaging section. Almost all the best shots are with 11" and above SCTs and Newtonians. Very few indeed with APOs. This is not to say that the APM 530mm could not beat them, but that costs 800,000 euros or thereabouts (no mount, just OTA). In imaging you can alleviate the slightly lower contrast by postprocessing, of course, but you cannot increase resolution.

Again, I do like the contrasty views of my apo, and it is the better wide field instrument by far (and imaging scope), but I do use the C8 more, both on planets and on DSOs. For a given aperture, the apochromatic design can give near perfect images, in ways few others can, but you pay WAY more for a given aperture, especially as they go beyond 6".

[newman]

I've been to a local star party recently, and looked through a lot of scopes; from those large oil drums their owners also call C11 SCTs, large dobs, to both achro and apo refractors. Sure some of the larger instruments see more than your typical refractor, and I really enjoyed the sheer deep sky power a large newt offers, that beats a refractor every time.

But what the refractors did see was sharp and contrasty. There was no image "mottling" due to the tube not being cooled down yet. No collimation issues and no coma which is a complete immersion breaker for me. Even my achro beat a lot of the scopes present when it came down to sheer quality of picture, especially on brighter DSOs. To be fair, though, I suspect a lot of Newt owners present didn't have their instruments properly collimated - especially since they just transported them over, some for hundreds of km.

It all depends on what you want and what issues are you prepared to deal with. I'm perfectly willing to give up a bit of sky penetrating power (since all scopes are limited somewhere, anyway) when I get an instrument that has quality optics, great views, and looks great while it's at it

Not to mention portability, which is big issue for me - I have a lot of stairs and no elevator leading to my flat; that pretty much means a dob or anything too bulky is out of the question.

[MartinB]

One problem with all the scope types along with EPs is that it creates a real temptation to study a scopes optics rather than the astronomical target and to seek out the "wow!" views. Observing wise anyway. It seems a bit like hifi buffs listening for bass definition rather than the music.

For observing get yourself a sturdy, easy to set up scope along a good range of trusty EPs and then vow never to read another "what scope type" thread on SGL or any other forum again, ever. Allow yourself plenty of time to dark adapt and then spend time on a single target. A good observer will see more through a 4" achromatic than an inexperienced one will through a 10" newt. It's a little like imaging, viewing time on target isn't totally disimilar to exposure time.

The fact that I've posted in this thread shows I don't practice what I preech

[glowjet]

I appreciate what you say Micheal, there was a slight mislead, the mag was 695X as in this quote from the reviews section of the CN website ------

"Lunar? AP wins. 'Sharpness' and contrast are a tossup. Both give stunning views and soak up stupid amounts of magnification. (Both would hold a 5mm Takahashi and a 2.5 Powermate on Plato. If I did my numbers right that is 695X for the Tak (1390 divided by 5 times 2.5) and 542 for the AP (1085 divided by 5 times 2.5 And that was condition limited, not optically limited.) Both scopes took magnification well. Contrast on both scopes was outstanding". This was a review of the Tak 152 and the 155 Starfire

[Rossco72]

For my 2p worth, a refractor for imaging, a reflector for visual.

However I use my refractors for visual from time to time and my dob for imaging so what do I know....

[Capricorn]

Each scope type has their own characteristics and you have to look through each to determine what you want.

How good can an apo be, well very good. Part of the problem is what reflector are you comparing one to.

The Skywatcher/Meade mirrors are probably made by GSO and not to put them down they are not high accuracy. Look at the cost of an Orion which is about 1/10 wavelength not 1/4. There will be ones that are more accurate then Orions somewhere, also more expensive.

The point that you pay for the accuracy and quality of a scope immaterial of type. And some of the apo are very good.

The only answer is look through both types and note the difference, then determine what it is that you prefer. Then buy the scope that gives what you want.

[michael.h.f.wilkinson]

I appreciate what you say Micheal, there was a slight mislead, the mag was 695X as in this quote from the reviews section of the CN website ------

"Lunar? AP wins. 'Sharpness' and contrast are a tossup. Both give stunning views and soak up stupid amounts of magnification. (Both would hold a 5mm Takahashi and a 2.5 Powermate on Plato. If I did my numbers right that is 695X for the Tak (1390 divided by 5 times 2.5) and 542 for the AP (1085 divided by 5 times 2.5 And that was condition limited, not optically limited.) Both scopes took magnification well. Contrast on both scopes was outstanding". This was a review of the Tak 152 and the 155 Starfire

If you compare two scopes of similar aperture at silly magnifications, you might get some sense of differences in PSF between them. But optics, not conditions limit 6" scopes to magnifications well below 695x, or even 542x. Claims like these make me take reviews on CNN with a fair dose, rather than just a pinch of salt.

This does not mean I would not love to use, or better still, own either of the two scopes.

[Moonshane]

maybe my eyes are not great but I have honestly used 400-500x on the moon with my 6" f11 dob on an eqp and Nagler zoom on one night of ridiculously good seeing. I also routinely use this sort of mag on doubles at the zenith. detail on the moon was still sharp and contrasty.

[John]

The variations in seeing conditions in the UK are huge, from the dismal nights when you can't use above 100x even with an excellent scope to the, unfortunately rather rare, nights like Shane describes when you can (almost) throw as much magnification as you want at something like Saturn and still get lovely views. The seeing can also go almost from one extreme to another in a matter of a couple of hours - and back again

I guess we stay in the hobby and "live" for those moments / sessions when things do fall into place and it's great to have optics, of whatever design, that can respond accordingly to the opportunity

[michael.h.f.wilkinson]

maybe my eyes are not great but I have honestly used 400-500x on the moon with my 6" f11 dob on an eqp and Nagler zoom on one night of ridiculously good seeing. I also routinely use this sort of mag on doubles at the zenith. detail on the moon was still sharp and contrasty.

Mileage varies a lot. If 500x works for you, use it. My old 6" F/8 was pretty good (1/10th-1/12th lambda optics they claimed). I did try the thing at 300x with a circle T ortho 4 mm. On a high luminance object like the moon or bright double, this sometimes worked, but I could already see the stars become more than just points of light. I tended to use it at a more sedate 140 to 200x.

A 6" scope has a resolution limit of 0.77" (seconds of arc). At 700x (give or take 5x) you get a centre peak of the PSF of 9' (minutes of arc), or 0.15 degrees. That is equivalent to the size of Mare Imbrium with the naked eye. I can readily resolve Mare Imbrium with the naked eye. At 200x I reach 2.5' for the PSF central peak diameter.

[MjrTom]

I can echo some of the comments, I have used 333x magnification on epsilon lyrae with the 102mm F/9.8 on a good seeing day a few weeks back.

This is far in excess of the 50x aperture (80x) which is generally accepted as the optical limit of most scopes.

[Stu]

Would agree with that. I think I've used x400 on saturn on two occasions with the mak and it was stunning. Not managed that for a few years though, but I know I've missed quite a few nights when it might have been possible :-(

Stu