Apos - how do they compare?

[Alfian]

This is a bit of a spin off from another thread of mine asking for advice on scopes and picks up on something I'd noticed in JamesTs quest for something better.

Other than that the more advanced optical design of Apos gives them better optical abilities giving less /no CA - how do they compare to say Maks or Reflec tors in terms of resolution. I get the impression by the way they are seemingly rated as a bit special, perhaps mistakenly, that for instance an 80ED will gather as much light and resolve as well or better than say a 102 mak or a 120 Achromat, or a 130 Reflector. I'm just plucking examples randomly out of the air - but in real terms how do they equate (if thats the right expression) to other different types of scope.

To illustrate what I mean A skywatcher Evostar 80ED is rated as having a theoretical max magnification of 240x. This is 3x aperture compared to the usual guideline of 2X aperture. Does an 80ED therefore have the "equivalent" light gathering of 120mm objective?

A puzzlement to me.

[Mr Onions]

No it doesn't.

APO's generally have better correction for CA { purple colour fringing round very bright objects,Venus,Moon etc ) a good 102mm achro will resolve more detail than your 80ED.

But for achro's to have a good image quality with as little CA as possibble they have to have long focal lengths,i.e. very long tubes.

But at the end of the day a little CA is nothing too much to worry about and some great bargains can be had with achros.

Forget magnification,quality of image is THE important factor.

[Alfian]

Thanks Mr Onions,

Linking this to my other thread "Next scope" - its image quality I'm really driving at. I'm just trying to tease it out in my mind. I think much of what of whats rattling around in my head has also been covered in Steves discussions in "maximum magnification, reflector vs refractor" which I've found interesting. Sorry for the duplication and thanks again.

[JamesF]

As regards maximum resolution, that's a function of aperture. If you want to be able to resolve smaller details, you need more aperture.

There's an interesting piece on magnification here, by the way:

http://starizona.com/acb/basics/equip_magnification.aspx

James

[John]

What really matters is the quality of the lens or mirror in terms of it's figuring and polish. A scope with high quality optics does seem to handle higher magnification better than a mediocre one, seeing conditions allowing.

There are excellent and mediocre achromats just as there are excellent and mediocre apochromats of course.

[cs1cjc]

Achromats, particularly in shorter focal lengths and at higher magnifications, do show colour fringes which does reduce contrast (though some will dispute this). The Baader fringe killer filter reduces colour fringing by cutting out deep violet and red light.

A true Apochromat will be colour free and is often available in a short focal length. Doublet scopes with an ED glass element are generally somewhere between an achromat and an apochromat and can be made reasonably short with, for visual use, little colour fringing. The ED scopes from Sky-Watcher have gained an excellent reputation based both on user experience and independent optical reports.

[rwilkey]

Quality of image is a high priority for me, and an ED doublet does this for me, the views are super clear and well defined with good quality eyepieces in my view.

[Mr Fiery Jack]

Quality of image is a high priority for me, and an ED doublet does this for me, the views are super clear and well defined with good quality eyepieces in my view.

Indeed, I am very much the same. As much as I enjoy my dob the optics just cannot compete with my refractor, and that's with research grade optics inside! There is lots of silly talk about masking down dobs to produce so called "apo killing" views, they don't produce anything close to it. Refractors and Reflectors dob different jobs, fuzzy blobs for dobs and for those razor sharp views of open cluster binary stars and solar system objects an apo/ED wins every single time.

[cs1cjc]

A properly cooled and collimated newtonian with a coma corrector will stand comparison with an ED refractor in my experience. The time to cool to a big mirror to ambient temperature even with a fan is an issue as is maintaining it at ambient as temperature falls, particularly at this time of year. YMMV.

[Stu]

Having seen Jupiter through Moonshane's 16" dob, masked to a clear aperture of 170mm, I can honestly say it was the best, most detailed and contrasty image I have ever seen. I have a 4" apo and 8" mak to compare with. Admittedly I did not look at any star clusters through it so don't know how the stars appeared. I love the pin point stars through my apo, but it is most certainly not 'silly talk' to compare the masked dob to an apo, at least on planets, in my opinion.

Stu

[Alfian]

Whooah guys,

I started this because I was curious about the mystique that I perceived Apos seemed to have compared with anything else - I did not intend to kindle/re-kindle the refractor/reflector debate! (re "silly talk") Having said that, many thanks for filling me in - its really interesting. More food for thought though.

[John]

Whooah guys,

I started this because I was curious about the mystique that I perceived Apos seemed to have compared with anything else - I did not intend to kindle/re-kindle the refractor/reflector debate! (re "silly talk") Having said that, many thanks for filling me in - its really interesting. More food for thought though.

I don't think any war of words is about to break out in this thread - we don't tend to do that on SGL

There are other forums where this topic would stoke up all sorts of heated points of view which can get quite nasty at times. But not here thank goodness

I have 2 refractors which have ED doublet objective lenses in them. I love the views through both but I also appreciate the views that my much larger aperture newtonian delivers, especially of deep sky objects

Maybe apocromat refractors did used to have a "mystique". A decade or two back they were rare and expensive pieces of kit so relatively few folks had used them. I think that time has gone now though and apos are much more widely available and have found their place in the ranks of scope designs with knowledge of their strengths and weaknesses being much more widely appreciated.

[haitch]

I've owned a couple of 6" achros, a Megrez 88 ED Doublet, an 8" SCT and a 14" Dob.

I've still got the dob and would never sell it because I love it's ability to gather so much light. The SCT is quite a nice scope although it hasn't yet really been through it's paces so time will tell on that.

I miss the Megrez far more than the Achros and I wish I hadn't had to sell it when I did. It gave beautiful pin sharp colour free views almost from the moment I set it up. Apart from the planetary performance (which does require rather short focal length EPs which often come with little eye relief) the other beauty of the apo is their fast optics which also allows a nice wide fov. For instance the Andromeda Galaxy looked better through the Megrez than the dob simply because I could fit more of it in.

[Stu]

Don't worry Alfian, I don't have the inclination, let alone the energy for any kind of heated debate, apologies if it looked that way :-)

I would echo the comments above, and say that the 106mm apo I own is my favourite and most used scope for a variety of reasons. It is relatively compact, is ready to use very quickly and gives wonderful views at a wide range of magnifications. It will give a 4 degree fov with a 41mm Panoptic, yet will still perform very well as a planetary scope up to x230, or even x276 using a Nagler zoom. Lovely views of Mars last opposition and Jupiter recently.

My favourite thing though is simply the beautifully tight star images you get, tiny pin point stars in the middle of open clusters which I could look at for hours, and lovely clean splits of doubles such as the double double. I guess that is what does it for me, and even though it doesn't show nearly as much as my mak, and is very limited on fainter dso's, I do prefer the views through it on most targets other than high magnifications on planets, and particularly globular clusters which the mak eats for breakfast, resolving M13 pretty much to the core.

Not sure if that makes sense, certainly not trying to argue they are any better or worse than other scopes, all scopes are compromises to a degree anyway, just that apo's do certain things very well which I enjoy.

Stu

[Moonshane]

I think apos are good at what they do, giving decent sharp images of planets, moon and stars at a wide field if required. you cannot get more resolution than the aperture allows. more magnification does not = more resolution.

personally though I have never seen anything come close to my 'silly' scope masked down. best thing to do with this sort of thing is try it. if you have a dob of 10" or more, make a mask and then see what you think. you'll be surprised I think. if not then nothing lost. I am lucky as I have good optics in my newts. with the paracorr, the star shapes (and there are lots with 16" aperture even in LP city) are good too.

ultimately, there's different scopes for different folks thank goodness.

[Moonshane]

Indeed, I am very much the same. As much as I enjoy my dob the optics just cannot compete with my refractor, and that's with research grade optics inside! There is lots of silly talk about masking down dobs to produce so called "apo killing" views, they don't produce anything close to it. Refractors and Reflectors dob different jobs, fuzzy blobs for dobs and for those razor sharp views of open cluster binary stars and solar system objects an apo/ED wins every single time.

if you ever fancy a friendly challenge with any scope of your choice against my masked off dob, you'd be welcome in Manchester any time

[ollypenrice]

I have no axe to grind because I have lots of scopes around the place for work. I do like the small stars and high contrast of premium refractors and binoculars but you don't get the light grasp of a bigger scope. I do find the premium apos have a magic, though. And they are so convenient, apart from at the stage of paying for them!!

Olly

[Chris]

One day not in the too distant future I'll build a largish Apo binoscope, then hopefully I'll have the best of both worlds, high contrast tight optics with reasonable light grasp, plus the comfort and sudo 3D effect of using both eyes:)

[Moonshane]

I have no axe to grind because I have lots of scopes around the place for work. I do like the small stars and high contrast of premium refractors and binoculars but you don't get the light grasp of a bigger scope. I do find the premium apos have a magic, though. And they are so convenient, apart from at the stage of paying for them!!

Olly

I have no axe to grind either Olly. Like you I just know what I want, what I like and what works for me. That's the key I think. I have no worries if others don't agree as it's me that's using and enjoying my gear and vice versa (I hope ) .

[skywatcher58gb]

During last years Stargazing Live our society gave a series of Jupiter nights, I set up my Skywatcher ED80 outside our Observatory which has a 14" SCT. Although the image size was much smaller, quite a number of the general public commented to me how much crisper the view of Jupiter was compared to the 14" and they prefered this. Its horses for courses though, and I prefer my 8" SCT for visual DSOs and the ED80 for imaging.

[Chris]

During last years Stargazing Live our society gave a series of Jupiter nights, I set up my Skywatcher ED80 outside our Observatory which has a 14" SCT. Although the image size was much smaller, quite a number of the general public commented to me how much crisper the view of Jupiter was compared to the 14" and they prefered this. Its horses for courses though, and I prefer my 8" SCT for visual DSOs and the ED80 for imaging.

I definately agree with the general public, I've only recently bought an old Celestron ED80 and I did give a wow when I trained it on Jupiter, the crsip detail was magic! Makes me want a 4" apo, or two strapped together with about half a dozen diagonals:D

[Damo636]

I also have had a lot of success with an aperture mask. As per Shanes recommendation, I managed to squeeze in a 120mm hole between the vanes of my 12" newt. It is so effective I now only ever view the moon or double stars with the mask in place. Contrast is cranked up a notch and stars are much more pinpoint in apperance. It may not quite rival a top of the line apo, but for the grand sum of £3, it'll do me!

[Dunkster]

During last years Stargazing Live our society gave a series of Jupiter nights, I set up my Skywatcher ED80 outside our Observatory which has a 14" SCT. Although the image size was much smaller, quite a number of the general public commented to me how much crisper the view of Jupiter was compared to the 14" and they prefered this. Its horses for courses though, and I prefer my 8" SCT for visual DSOs and the ED80 for imaging.

Small aperture masks bad seeing. Unfortunately, the weather doesn't take public events into account when deciding what to throw at us

[Alfian]

Hi all,

I hadn't disappeared - PC down - PSU went AWOL - now fixed. Thanks for all the interest. Thanks John & BigMak' for the re-assuring words - I was just a tad uncertain! All the comments re Reflectors, Achros and Apos -just reinforce that idea that no scope is good at eveything - and moreover that whatever we have or would like the idea of the quality of view, is the ideal to aim at. My new PSU has come out of my Astronomy Equp't fund - (curses) - so whatever scope is down the line - its now a little bit futher! Thanks again for the insights and have a good Xmas.

[ollypenrice]

One day not in the too distant future I'll build a largish Apo binoscope, then hopefully I'll have the best of both worlds, high contrast tight optics with reasonable light grasp, plus the comfort and sudo 3D effect of using both eyes:)

Whether having two scopes in a bino configuration has any perceptible effect on apparent brightness is a moot point and complicated. Doubling the optical surface area in a single instrument has a clear effect. Doubling up two scopes does not seem to have anything like that effect, but can be 'nice' for other reasons, notably the impression of three dimensionality, though it can only be an impression.

When you close one eye the view in the other one does not get dimmer.

This is all very strange!

Olly