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Does your reflector give "refractor like" images?

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#26 Starman1

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Posted 02 April 2013 - 05:19 PM

I am curious on this topic as I sometimes see that claim by scope makers. I am especially curious as to honest opinions regarding the <f/4 large dobs. We all know you get a brighter image and can see more, but does anyone feel that you get as crisp an image as an APO? :question:

If the scope is very large, the secondary will be fairly small, percentage-wise, and f/4 can give astounding views in, say, a 32" if the seeing is excellent, the scope is well-collimated, and the mirror is cooled.

My 12.5" always beats the 5" and 6" apos in the field, but that's not really a fair comparison. Alas, I have no 10-12" apo refractors handy to compare to it. But, for the price of my high-end 12.5" f/5 with Zambuto mirror, I could have bought a nice 4" apo on a cheap mount.
Nah, 12.5" will ALWAYS show more.

The point is, resolution is aperture-related.
But large apertures are all seeing-limited, too.
In my experience, the seeing always varies enough for the larger apertures to show off more in the way of lunar and planetary details than the smaller scopes. And, they're usable at much higher powers. 200X is really pushing a 4", but a 32" isn't really pushing the limits at 1000X.
Even in mediocre seeing, a 32" will handle 300X OK, while the smaller scope may be limited to <150X or even 100X.

Lots of people argue that they prefer the aesthetics of a view that, though it has lower absolute resolution, is much more stable and consistent. They use that to justify why they prefer their 4" and 5" apos to a big scope.

The thing is, the average resolution of their 4" and 5" apos will be what you see though a good 20" reflector when the seeing is bad, and during moments of truly steady seeing it will be as if the wax paper was ripped away and you are seeing the object in more detail than typical photographs.

I had occasion, recently, to spend some time on the Moon at first quarter with a TeleVue NP101, and the image of the Moon was as photographic as you can get--simply stunning. Contrast was superb.

Because we were having a period of really steady seeing, I took my 12.5" out to look at the Moon. Bear in mind that this scope very rarely sees anything that bright. I use it mostly for DSOs. The image of the Moon almost made me cry. At 304X, the image was dead-steady and the surface of the Moon looked littered with debris and not smooth at all.
Small craterlets were visible inside all the large craters and buried craters were everywhere. And each of the craters near the terminator had its walls casting saw-toothed shadows that looked so sharp and stark it was as if I was in orbit over the Moon. The images of the Moon I saw were simply so far beyond what the NP101 could do that the images were burned into my brain as examples of what can and should be seen when looking at the Moon.

Now, when I look through my excellent 5" Mak or the TV101, I am just disappointed at the softness and blurriness of the image, i.e. the low resolution. And both of those scopes are superb examples of their breeds.

So was it a fair comparison? No. And would an equivalent 12.5" apo refractor beat the reflector? Probably. But I could carry the 12.5" reflector in a Lamborghini Murcielago for the price the 12.5" apo and mount would cost (not counting the dome to house it in).

So, when you are talking high-resolution per aperture, the refractor wins up to about 5 or 6". But that's a pretty small reflector.
And the reflectors start revealing lots of details simply not visible in the refractors once you get to 12" or more. And by the time you get to a scope like a 24" or 28", no one you know or ever will know will see details in a refractor that can be commonly revealed in the reflector.

And that's only talking Moon and planets. Then there's the hundreds of thousands of other objects the big scopes can see the small scopes can only see through long exposures with an expensive camera.

Crispness of image is seeing related, and the small refractor will probably not be seeing-limited most of the time. The large reflector will be seeing-limited most of the time. But not ALL of the time, and therein lies the reason why a big reflector is also the best instrument for high-resolution lunar and planetary use. It also goes without saying it's best on DSOs.

#27 Scott Beith

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Posted 02 April 2013 - 07:44 PM

I don't worry about all these so-called "comparisons" and just worry about getting out to actually use all this equipment that I have acquired over the years. In the end, observing is the single more important thing to consider when it comes to this hobby.


I have found that in planetary observing, hours at the EP will do more to improve your ability to observe small details than a new scope design or EP (unless you jump from a really small scope to a really big one). DSO's I can't comment on since I don't chase those. ;)

Observe with what you have and don't sweat the "is so-and-so better than so-and-so" comparisons.

#28 GeneT

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Posted 02 April 2013 - 08:13 PM

Refractors give refractor like images, and reflectors reflector like images, and so on. Any telescope with excellent optics, well collimated, at ambient temperature, will give excellent optics. Different telescope types will yield images consistent with their lens configuration.

#29 azure1961p

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Posted 02 April 2013 - 09:02 PM

I don't worry about all these so-called "comparisons" and just worry about getting out to actually use all this equipment that I have acquired over the years. In the end, observing is the single more important thing to consider when it comes to this hobby.


I have found that in planetary observing, hours at the EP will do more to improve your ability to observe small details than a new scope design or EP (unless you jump from a really small scope to a really big one). DSO's I can't comment on since I don't chase those. ;)

Observe with what you have and don't sweat the "is so-and-so better than so-and-so" comparisons.


Thats an undersung attribute in these discussions, that the person who observes more sees more and sometimes if not often, beyond the few percentiles that get such heated debate in equipment comparisons.

Pete

#30 jgraham

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Posted 02 April 2013 - 09:03 PM

Hmmmm, sorry, my experience has been no. My "big" refractor is a 6" f/8 achro while my reflector cut a broad swath from 4.5" f/4 to 16.5" f/6.5, Newtonians, MCTs, and SCTs. The image through the unobstructed refractor is just so darned clean. You can also easily max-out the exit pupil without having to dodge around the shadow of the secondary.

I luv my reflectors, but my biggo refractor is a special piece o'kit.

#31 Mike B

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Posted 02 April 2013 - 09:23 PM

The image through the unobstructed refractor is just so darned clean. You can also easily max-out the exit pupil without having to dodge around the shadow of the secondary.

That's certainly understandable... yet you're simply describing a refractor-like trait (no CO shadow) to depict why your refractor is "special" to you. Aside from "just so darned clean", there's no reference to the views in your (many) reflectors. None of them have "clean" images?

Even the OP's question utilizes soft words that could easily be construed to answer "yes, my reflector gives views that are LIKE those of a refractor". Indistinguishable from? Hmmm, maybe not quite THAT... but very much like them. ;)

This is dangerously close to complaining about a 0.95 Strehl mirror, and discussing the concept of sending it off to get it refigured to 0.98 Strehl. :lol: :foreheadslap:

Refractors give refractor like images, and reflectors reflector like images, and so on.


This addresses the matter quite well!
:waytogo:

#32 Datapanic

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Posted 03 April 2013 - 01:48 AM

Wait a minute! Who has a 12.5" APO?

#33 David Knisely

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Posted 03 April 2013 - 02:17 AM

Wait a minute! Who has a 12.5" APO?


Not too many people do, but if you have a *lot* of cash (and some time to wait for them to be built), you can get one up to as large as 20 inches from APM Telescopes in Germany. A 12 inch (304 mm) optical tube assembly will only set you back about 198,000 euros (around $254,232 at today's exchange rate, and of course, that does *not* include the mount). My house didn't cost anywhere near that much!

#34 jpcannavo

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Posted 03 April 2013 - 04:46 AM

And comparing a large Newt to a small APO is comparing apples to oranges.



Maybe comparing apples to peas? :)

Jon


Fujis to crab apples

#35 jpcannavo

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Posted 03 April 2013 - 05:29 AM

For the record, I am a hands down Newtonian fan, and feel they are best all-around visual performers. But that having been said, a bit too much blame is placed on central obstruction and spiders. The equally if not more significant issue may be surface scatter due to roughness and contamination. Reflecting surfaces are inherently more sensitive to such issues and therefore must be made to a significantly higher standard than a transmitting surface. Interestingly, some of the relevant literature here comes from studies of solar telescopes/coronagraphs - surely the most critical "velvet black" standard!

Scatter

And, with all due respect to its typical high optical quality and spiderless view, the Mak-Newt gets no free pass here.

But assuming the best possible newtonian optics - excellent substrate surface, clean, and highest quality coating - do this:
Buy a large dob, say 16+ inches, with premium optics. Build a 6 inch off-axis mask, and knock yourself out enjoying extra-diffraction-free (Mak-Newt like? ;)) views of planets and bright stars when the mood hits you. And when you've had enough, crank up the aperture and resolution and do some serious observing!
Joe

#36 azure1961p

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Posted 03 April 2013 - 06:25 AM

But then there's thermal issues. At worst the column of air in front of the mirror is a defocusing lens the refractor camp doesn't have issue with to the same degree to say nothing of the sheer thickness of them. I'd place thermals as a close second behind large CO contrast degredation.

Its almost silly when you consider the thermal issues a reflector has to endure...

Imagine an apo where an observing buddy is breathing on the side of your dewshield every time you look in the ocular. That's no different than what happens at the reflector focuser when observing.

Imagine you have a push-to refractor and every time you need to shift it you reach up with your hand on the dew shield edge to nudge it with all the thermals pouring off that - something I think we've all seen with reflectors - particularly in out if focus stars on collimating.

Then there's the length of a dew shade on an apo compared to the length of a truss or closed tube. It's simply more room to further create the lensing effect of this warmer air.

The sheer thickness of the glass is what Gary Seronik refrs to has the effect of having a heat battery. Even a very large apo at least has the glass in thinner sections*.

Anyway, having dealt with and been (finally) successful for the most part dealing with the defocusing effects of thermals Im a true believer in it being one if the serious potential detriments to reflecting systems when left unabated.

Pete
*theres been mention apos over 7" have very long cool down issues but this doesn't seem like it has to be if there is air between the objective elements why not a cell that blows air through these spaces and be done with it. A reflector should only have such an easy option.

#37 Galicapernistein

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Posted 03 April 2013 - 06:59 AM

My 6" f5 shows pin point stars, as long as I'm using my 24mm Panoptic. At low power the distortions caused by bad seeing and a so-so mirror aren't magnified enough to see. I won't be using it to split close doubles or see details on the planets, but for wide-field views, it's a cheap substitute for a 4" APO that would cost ten times as much.

#38 Ed Jones

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Posted 03 April 2013 - 07:25 AM

Some might take that as a put down, refractors have CA. I've only seen a few APOs that would qualify a reflector like images.

#39 Darren Drake

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Posted 03 April 2013 - 07:28 AM

If it was April 1st again I might have the courage to goto the refractor forum and start a "Does your refractor give reflector like images?" thread. :lol:

#40 David Pavlich

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Posted 03 April 2013 - 07:51 AM

If it was April 1st again I might have the courage to goto the refractor forum and start a "Does your refractor give reflector like images?" thread. :lol:


You would have probably gotten a Moderator Alert for stirring up trouble. :roflmao:

David

#41 City Kid

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Posted 03 April 2013 - 09:58 AM

If it was April 1st again I might have the courage to goto the refractor forum and start a "Does your refractor give reflector like images?" thread. :lol:

Now that would be a fun thread to follow! :lol:

#42 dscarpa

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Posted 03 April 2013 - 10:08 AM

I looked at Jupiter and DSOs through Jon's 12.5" F/4 Discovery with Paracorr and found the view refractor like. I don't usually notice the coma in my regular C9.25 but the using it the next night there it was. David

#43 nirvanix

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Posted 03 April 2013 - 10:29 AM

But assuming the best possible newtonian optics - excellent substrate surface, clean, and highest quality coating - do this:
Buy a large dob, say 16+ inches, with premium optics. Build a 6 inch off-axis mask, and knock yourself out enjoying extra-diffraction-free (Mak-Newt like? ;)) views of planets and bright stars when the mood hits you. And when you've had enough, crank up the aperture and resolution and do some serious observing!
Joe


So very well said Joe. I have a 4" aperture mask that when properly positioned on my 10" dob allows the light to fall uninterrupted by spider or CO onto a good mirror, turning my newt into a chromatic error free 4" f/12.5 that surpasses the most expensive 4" refractor in performance. I use if for double star splits mostly as it removes the vane scatter that gets in the way on tight doubles.

#44 GOLGO13

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Posted 03 April 2013 - 10:34 AM

I like both scope designs. If my time is limited, I am a big fan of a 3-4 inch apo refractor. Very short cool down, super sharp images, grab and go portability. They are not cheap, but they are very nice.

If I have a lot of time I like to setup both a refractor and my 10 inch dob. Dob for DSOs (and planets when seeing is good)...and refractors for wide fields and planets.

I think the harder call is if I could only have one scope what would it be. Of the scopes I have it would be the 4 inch refractor. If I had a better newt like a high quality truss dob, I'd probably choose the newt. However, there is a point to where the size of the newt is too big for me. My opinion is a 15 inch F4.5 is as big as I would go. I'd probably prefer a 12.5 inch F5. Gonna have to save up for some time to get that in the future.

#45 GeneT

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Posted 03 April 2013 - 10:46 AM

Hmmmm, sorry, my experience has been no. My "big" refractor is a 6" f/8 achro while my reflector cut a broad swath from 4.5" f/4 to 16.5" f/6.5, Newtonians, MCTs, and SCTs. The image through the unobstructed refractor is just so darned clean. You can also easily max-out the exit pupil without having to dodge around the shadow of the secondary.
I luv my reflectors, but my biggo refractor is a special piece o'kit.


My point is not which give the better images. My point is that the different light characteristics of various types of telescopes will produce different flavors of images. The quality of an image is a different issue than the flavor. I like both Whopper and McDonald's quarter pounder cheese burgers. They may be of the same quality, but their flavoring characteristics are different. :grin:

#46 GeneT

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Posted 03 April 2013 - 10:47 AM

If it was April 1st again I might have the courage to goto the refractor forum and start a "Does your refractor give reflector like images?" thread. :lol:


Do it! :grin:

#47 kfrederick

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Posted 03 April 2013 - 11:01 AM

Funny how people who have nothing in there lives perfect sure worry about the stars to be perfect.Bet most do not get there eyes checked or blood presure. but the stars need to be sharp . Guess I am one of those .Ha Ha

#48 Darren Drake

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Posted 03 April 2013 - 11:26 AM

If it was April 1st again I might have the courage to goto the refractor forum and start a "Does your refractor give reflector like images?" thread. :lol:


Do it! :grin:


Ah I just started one about achros vs SCTs. That should be good for a while lol.

#49 Galicapernistein

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Posted 03 April 2013 - 12:02 PM

Funny how people who have nothing in there lives perfect sure worry about the stars to be perfect.Bet most do not get there eyes checked or blood presure. but the stars need to be sharp . Guess I am one of those .Ha Ha


My doctor told me I suffer from Dimimageitis, and that the only cure is a large dob.

#50 kfrederick

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Posted 03 April 2013 - 12:27 PM

funny but true






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