comparison of APO refractor and reflector

[marcusk]

Hi

I recently started a thread on the difference between apochromatic and achromatic - and what I would see, difference wise, through them... discussion obviously fell around imaging quality etc and was very useful BUT I'm interested to hear thoughts on the *visual* only side of a apo refractor vs a reflector.

I am interested in imaging but just want to get a grasp on what one *would actually see* through an eyepiece...

So.

I could buy a good 80mm apo for £400-500ish but I could buy a 200mm reflector for £300 (noticable less £) - presuming they are sat on identical stands and with medium eyepiece, what quality and size would you see through them? What I think (as a guess) I'm looking at the Orion Nebula and on the 80mm apo it's small, probably taking up 20-30% of the viewable eyepiece but super crisp and contrasty. On the 200mm reflector its a lot larger, maybe 50-60% of what I can see but its not quite as sharp or contrasty...

I am very aware there are a lot of variables (and opinions) and I could dig out a club but for now I'm just trying to get a straightforward handle on things...

Cheers

Marcus

[DirkSteele]

This should be fun. I am tempted to stand to the side and just watch....

In all seriousness though, there is no contest in the comparison you are speaking of, and I am an Apo nut who is currently considering an 8" Triplet Apo. Reflector every time in this contest. Aperture is king (especially that much of a difference), a well collimated reflector will always win.

Clear skies,

[E621Keith]

The 200p will laugh at the ED80 on DSO such as the Orion Nebula. Visual DSO observing is about light gathering, and a 200p has 6.25 times the light gathering area and the nebula will appears 6.25 times brighter at the same magnification.

[John]

Put simply the views through an 8" newtonian would outperform the 80mm apo on everything and by a noticeable margin. On deep sky objects the difference would be really significant and on the moon and planets, unless the seeing conditions were really awful, the 8" newtonian would show more detail, allow more magnification to be used and show more contrast.

I can't go into details about the differences in each and every object (obviously !) but there is a very large difference between these 2 scope types for visual astronomy.

Edit: I love apo refractors too but one has to be realistic here !"

[umadog]

>but super crisp and contrasty. On the 200mm reflector its a lot larger, maybe 50-60% of what I can see but its not quite as sharp or contrasty...

No way. On a good Newtonian contrast is no worse: it will be better. Smaller point spread function so better contrast. Yes, there'll be diffraction spikes around large stars that will give the impression that contrast is worse but contrast is, in fact, better on DSOs and (yes) planets. Some people don't like the diffraction spikes because they bleed into the sky background and make it look less dark. Personally, I don't care about the spikes.

The 200mm is pulling in 6 times more light than the 80 mm. That's a huge, huge, amount. That's the same jump as going from an 8" Newt to a 20" Newt(!). Comparing scopes that are so different isn't really meaningful, IMHO. They're instruments for different jobs. e.g. If I want pleasing wide-field views and grab and go I use the 80 mm. If I want to see DSOs in some detail I use at least the 8" or 10" Newt.

Also, it's not just that objects will be 6 times brighter at the same magnification but that you can magnify 6 times more and not loose brightness. That's the kicker. So the view at 120x in the Newt will have the same brightness as the view at 20x in the refractor. It's the ability to magnify more that allows you see more detail.

[DirkSteele]

No way. On a good Newtonian contrast is no worse: it will be better. Smaller point spread function so better contrast. Yes, there'll be diffraction spikes around large stars that will give the impression that contrast is worse but contrast is, in fact, better on DSOs and (yes) planets. Some people don't like the diffraction spikes because they bleed into the sky background and make it look less dark. Personally, I don't care about the spikes.

Probably best to clarify that so not to confuse as it sort of reads like reflectors out perform Apos. The PSF will better on a Newtonain of 200mm than an Apo of 80mm. Once again this comes down to us not comparing apples to apples. On a 200mm Apo, the contrast will be superior due to no diffraction from a secondary and spider vanes pushing light from the airy disc into the diffraction rings.

[michael.h.f.wilkinson]

Probably best to clarify that so not to confuse as it sort of reads like reflectors out perform Apos. The PSF will better on a Newtonain of 200mm than an Apo of 80mm. Once again this comes down to us not comparing apples to apples. On a 200mm Apo, the contrast will be superior due to no diffraction from a secondary and spider vanes pushing light from the airy disc into the diffraction rings.

Precisely. My 8" SCT hits my APM 80mm F/6 for 6 in all things visual except wide field. It is true that at the same exit pupil the 80mm gives a more contrasty image, but at the same magnification (which is what really counts) it cannot. If you look at planetary imagers, the majority use large reflectors, because aperture is king when it comes to resolving fine detail. I have seen some wonderful views through Olly's TEC 140 APO, but I much preferred looking through his 20" Dob. Furthermore, I used to have a 6" F/8 Newtonian with 1/10th lambda mirrors (claimed, not measured). Because of the small secondary, and the very thin spider, it had excellent contrast, and could give any 5" APO a serious run for its money. The slow 6" Newtonians are sometimes called APO-killers. With good reason.

[marcusk]

Very interesting - and thank you.

Taking out imaging for a second, why do some of you apo fans love them so if a sweet reflector can equal the same? Purely high quality wide field?

btw... SGL yields some of the best and intelligent answers from any forum acoss all my hobbies and interests. Nice one.

[nmoushon]

The only two things the 80mm APO would be better for (visually) than the reflector is grab-n-go set up and widefield viewing. Other than those the reflector with its extremely superior in aperture size and FL and price. You will be able to see so much more in a reflector than the small refractors. Unless you go get a 8" frac but for the price you could probably build/buy a 16"-20" newt or dob.

[michael.h.f.wilkinson]

Very interesting - and thank you.

Taking out imaging for a second, why do some of you apo fans love them so if a sweet reflector can equal the same? Purely high quality wide field?

btw... SGL yields some of the best and intelligent answers from any forum acoss all my hobbies and interests. Nice one.

Most people buy APOs for imaging. I bought mine for (future) imaging and (immediately) as quality wide-field and travel scope. Some people just like the aesthetics of refractors, and the sheer quality of the image (within the limits of its aperture). Quality matters, and there are differences. At the low end, reflectors do not always have perfectly formed mirrors, and such reflectors will show bigger differences in quality with APOs than high-end ones. A cheap synta or GSO reflector will generally not be as good as good as a high-end scope with 1/10th lambda mirrors or better (though GSO and Synta can make those as well). Likewise there are differences in apos as well. Your Skywatcher 80ED will be beaten (by small margin) by a similar aperture scope from APM or Televue.

Finally, buying (big) apo refractors is a bit like buying a sports car. It may only hold two people, and very limited luggage, but it is fun, aesthetically pleasing, and fast. Buying small apo refractors is to complement a big reflector is more like buying a small hatchback for shopping next to a large family car. Both cars are practical, but have different strengths.

[rwilkey]

As you can see from my signature, I have a foot in both camps, and since buying the C100ED I have been very impressed with its performance and portability, having said that I have been slightly disabled following a stroke and find the C100ED serves me better both in quality of viewing and comfort, esp. on an AZ mount. Mind you for good optics like this you also need good ep's, and I have the BST Explorers, a perfect match for the f/9 ratio. You will be dead pleased with either, performance for visual use is impecable on both.

[Capricorn]

Cloudy Nights seem to have a decent arrangement and an unspoken rule:

Reflectors and Reflectors are 2 SEPERATE sub forums and people do not go to the other and wind them up.

Seem to have worked so far as civil war hasn't broken out.

Mind you the mods there would I suspect jump on someone pretty fast.

It is comparing pleasant sweet apples (=refractors) and sour acid tart lemons (=reflectors), you prefer one or the other and you have the courtesy to leave it at that. :evil4:

Finally:

Why anyone would want a dinner plate sprayed with aluminium paint I cannot understand. :eek: :rolleyes: :grin:

And someone on CN did rewite a psalm 23 and dedicated it to refractors.

It was a "well balanced" rewrite of psalm 23, it criticised reflectors, SCT's and Mak's equally.

[Naemeth]

Small refractors (~80mm) are often used for wide-field set ups, grab and go, and taking the scope on holiday, and of course imaging. This is where they are strong, to try and compare 80mm aperture to a 200mm in terms of resolving power is just unfair. In many situations, the 200mm will beat the 80mm, but if you are trying to transport it for imaging on a plane, forget it. They each have their strengths .

[Earl]

For visual only id go reflector as big as I can afford, in that department the big dobs rule.

[earth titan]

If you mean a true APO v a reflector, and cost is no object, the APO! One reason is the position of the eyepiece. Hence the reason I like SCTs - best of both worlds.

But in the real World where cost is an option, as is usability, the newt for visual and the APO for imaging.

Typed by me, using fumms...

[E621Keith]

For the same aperture, an apo will be superior

For the same cost, a reflector will be better

For the same storage space, a CAT will be best.

The requirement between DSO and planets is slightly different. For DSO observing, you need to gather as much light as possible. Light gathering isn't that important for observing planet and big aperture scope are more susceptible to the effect of atmospheric distortion, so a smaller scope may perform better. You may gain more resolution with a bigger scope in theory, but only if the seeing is perfect.

[earth titan]

big aperture scope are more susceptible to the effect of atmospheric distortion, so a smaller scope may perform better. You may gain more resolution with a bigger scope in theory, but only if the seeing is perfect.

I've had some lovely views of Jupiter through a simple 80mm achromat, probably for these reasons.

[NGC 1502]

It's like asking which is best, a hammer or a screwdriver and the answer to that is so obvious, different tools for different jobs, neither is "better".

Regards, Ed.

[Moonshane]

the only thing I think fracs do better is slightly tighter stars which makes open clusters look a little more attractive.. other than that (and not discounting the fact that most open clusters look better with more aperture as the view goes deeper with a big dob) everything looks better with more aperture and a big dob is the cheapest way to get a big aperture.

to my eyes, for visual nothing matches a cool and collimated large aperture newt. I have a 16" and it's remarkable even in light pollution, on every single target. obviously dark skies make for even better views but I never regret buying it.

[John]

Very interesting - and thank you.

Taking out imaging for a second, why do some of you apo fans love them so if a sweet reflector can equal the same? Purely high quality wide field?

Looking back at the responses, I'm not sure we are any nearer answering this question - given the undoubted "performance per £" advantage that newtonians have and the compactness of SCT's, why would anyone in their right minds buy a refractor for visual use ???

And yet we do and they continue to be pretty popular ..............

[Stu]

It's strange isn't it John. Probably my most used scope is my 106mm apo, it's just gives such lovely, contrasty views with nice tight star images. I know it's nothing like the views through a big dob, or even my mak, but the convenience is often what I need as I have limited time. As fast as I'm concerned, it gives fabulous widefield views of open clusters, and very credible performance on planets too, plus plenty more, but you know all that already

If we get any clear sky, I'm going to try to convince Shane that aperture is not everything at PSP . I don't fancy my chances but will give it a go!

Stu

[Moonshane]

you've got a great chance mate, it's quite clear here now. oooohhhhhh, you mean about aperture....... I can totally appreciate the attraction of the grab and go nature of fracs but I was always left unsatisfied; like a Nouvelle Cuisine Meal. steak pudding, large chips, peas and gravy (or curry) now that's a meal! (this is a frac dob analogy BTW before you think I've lost it).

[ollypenrice]

I use refractors mainly for imaging. That's their main role. However, you'd have to be thick skinned not to enjoy a visual session with the TEC140 and there are some things that need a focal length of no more than 500mm. A fast apo (F5 4 inchTeleVue?) on the entire Veil complex, for instance. It's nice to see it as a whole.

When I do look through the TEC I pick targets within its light grasp and enjoy them. I don't say it will go deeper than an 8 inch Newt. It won't, but I just love the view.

Olly

[brantuk]

"Cloudy Nights seem to have a decent arrangement and an unspoken rule:

Reflectors and Reflectors are 2 SEPERATE sub forums and people do not go to the other and wind them up.

Seem to have worked so far as civil war hasn't broken out.

Mind you the mods there would I suspect jump on someone pretty fast.

It is comparing pleasant sweet apples (=refractors) and sour acid tart lemons (=reflectors), you prefer one or the other and you have the courtesy to leave it at that. :evil4:

Finally:

Why anyone would want a dinner plate sprayed with aluminium paint I cannot understand. :eek: :rolleyes: :grin:

And someone on CN did rewite a psalm 23 and dedicated it to refractors.

It was a "well balanced" rewrite of psalm 23, it criticised reflectors, SCT's and Mak's equally. "

I want some of whatever you're on Capricorn on lol

[glowjet]

This is a bit like religion and politics, but lets face it the seeing in this country hardly allows us to use our scopes, being refractor or reflector to their full potential, except on those very rare occasions when magnifications can go beyond their respectable limits. Then there are times when the scope of your desire comes on the scene such as a TMB 152 refractor offered for sale by one of our respected imagers and the wife remarks that you are not going to spend that ridiculous amount on that small 6" thing, I thought you wanted a large reflector thingy !!!

John.