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Equipment Discussions >> Reflectors

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azure1961p
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Re: Does your reflector give "refractor like" images? new [Re: ed_turco]
      #5773264 - 04/02/13 04:22 PM

Quote:

I disagree with the statement that no Newt can equal an equal aperture APO. But it will take a lot of very creative thinking and some hard work to do the job.

Some time soon, I will prove my point, period.

And judging from the bashing I got on the Refractor Forum a while back, no APO owner will ever admit it.

I do concede that an ordinary Newt, as currently envisioned, cannot equal an equal aperture APO. It just ain't gonna happen! And comparing a large Newt to a small APO is comparing apples to oranges.

As usual, I will add no further comments to this thread; I have work to do.


Ed




I'm interesed to see what you come up with Ed. I've appreciated your convictions here. I'd like to see what you've got in mind.

Pete


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Jeff Morgan
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Re: Does your reflector give "refractor like" images? new [Re: Darren Drake]
      #5773400 - 04/02/13 05:12 PM

Quote:

I think one of the main reasons people think that refractors are a step above newts is that they have better thermal management properties by design. The light path is protected from the observers body heat and the light only goes through the tube once. This in addition to the fact that most refractors are smaller apertures make for more steadily stable images so much more often than in newts.




Do you how you can tell a reflector owner is about to say "refractor like" image? He carefully looks over each shoulder first

But seriously, you hit one of the main weaknesses of the reflector - heat. The primary mirror sits close to the ground, where there is (typically) a sharp temperature gradient. And the light path traverses that gradient twice.

Add to this the refractor objective is usually spherical and probably much smoother than the parabola required of the Newtonian. One look through a Schiefspiegler, an all-spherical reflector design, will make you a believer in spherical surfaces.

While the spider vanes and central obstruction get lots of blame (much deserved), I have found that looking at fainter open clusters can effectively subtract that from the equation. The diffraction is still there, but too faint to register. On this type of target you can get a lot better comparison to a refractor.


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nevy
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Re: Does your reflector give "refractor like" images? new [Re: Jeff Morgan]
      #5773424 - 04/02/13 05:20 PM

I'm sure refractor owners can soup up their toys & get the views close to a good reflector , if they know what they're doing. ;-).

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David Knisely
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Re: Does your reflector give "refractor like" images? new [Re: jnewton]
      #5773522 - 04/02/13 05:42 PM

Quote:

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?




Such comparisons can sometimes become a little pointless if you let them go on long enough. You can't really compare a large aperture f/4 Dob to a small apochromat for a variety of reasons. Even the largest Apochromats are not readily available in apertures larger than 10 inches (and most that are often mentioned are in the three to six inch aperture range). In an old "rant" I once posted about some of the so-called 'debates' concerning this on sci.astro.amateur many years ago, I wrote:

"I recall one late summer evening in 1990, when a friend of mine brought his 6 inch APO to our annual picnic and star party. I almost drooled over the view it gave of Saturn sitting in the rich starfields of Sagittarius, with its dark wide field and image of the planet which was so sharp you could "cut your eye" eye on it. I looked with my 10 inch (which was just settling down after being set up), and the image didn't seem to have quite the quality of my friend's APO (slightly more scattered light and a slightly woolly appearance). Later, I again viewed Saturn in an old Criterion RV-6 Newtonian, and was surprised how good the image was (quite comparable to the APO). The APO clearly had a bit of an edge on performance, but considering the cost of the APO, I felt that the RV-6 owner got the "better" of the deal. Later on, I again looked at Saturn with my ten inch at 300x, and the seeing was perfect, showing a bit more detail than the APO did, although the view in the 6 inch APO was extremely pleasing, with incredible sharpness and contrast (and an incredible price tag too). Years later during one of the "Mars" opposition events at Hyde Observatory, I had a nice time showing the public Mars in my 10 inch Newtonian. Another friend had his five inch APO running not far away, so during a brief break, I went over to look at Mars in his scope, It was a very nice view, but as expected, my 10 inch showed more detail on the planet and showed it more easily. My ten also went fainter than the two smaller APOs did on galaxies, so once again, the larger aperture won."

My 10 and 14 inch reflectors each give very very good images, as I have optimized their optical quality as much as possible. I have seen things with them that I have NEVER seen in my 100mm refractor (or even in a four to six inch apochromat). One night last year in my 14 inch f/4.6 Newtonian, I got a view of Jupiter that to this day I have not seen equaled in my observing experience using my equipment, with incredible sharpness and detail (especially considering I was using a binoviewer much of the time). In my case (as with many others), the simple fact is that Aperture Rules (and, of course, optical quality). Other than that, 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. Clear skies to you.


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Project Galileo
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Re: Does your reflector give "refractor like" images? new [Re: nevy]
      #5773560 - 04/02/13 05:51 PM

Nevy,

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Starman1
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Re: Does your reflector give "refractor like" images? new [Re: jnewton]
      #5773620 - 04/02/13 06:19 PM

Quote:

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?



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.


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Scott BeithAdministrator
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Re: Does your reflector give "refractor like" images? new [Re: David Knisely]
      #5773973 - 04/02/13 08:44 PM

Quote:

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.


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GeneT
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Re: Does your reflector give "refractor like" images? new [Re: jnewton]
      #5774087 - 04/02/13 09:13 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.

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azure1961p
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Re: Does your reflector give "refractor like" images? new [Re: Scott Beith]
      #5774272 - 04/02/13 10:02 PM

Quote:

Quote:

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


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jgraham
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Re: Does your reflector give "refractor like" images? new [Re: GeneT]
      #5774276 - 04/02/13 10:03 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.


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Mike B
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Re: Does your reflector give "refractor like" images? new [Re: jgraham]
      #5774319 - 04/02/13 10:23 PM

Quote:

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.

Quote:

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



This addresses the matter quite well!


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Datapanic
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Re: Does your reflector give "refractor like" images? new [Re: Mike B]
      #5774653 - 04/03/13 02:48 AM

Wait a minute! Who has a 12.5" APO?

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David Knisely
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Re: Does your reflector give "refractor like" images? new [Re: Datapanic]
      #5774671 - 04/03/13 03:17 AM

Quote:

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!


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jpcannavo
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Re: Does your reflector give "refractor like" images? new [Re: Jon Isaacs]
      #5774724 - 04/03/13 05:46 AM

Quote:

Quote:

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




Maybe comparing apples to peas?

Jon




Fujis to crab apples


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jpcannavo
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Re: Does your reflector give "refractor like" images? new [Re: jpcannavo]
      #5774743 - 04/03/13 06:29 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

Edited by jpcannavo (04/03/13 07:01 AM)


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azure1961p
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Re: Does your reflector give "refractor like" images? new [Re: jpcannavo]
      #5774789 - 04/03/13 07:25 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.


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Galicapernistein
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Re: Does your reflector give "refractor like" images? new [Re: azure1961p]
      #5774816 - 04/03/13 07:59 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.

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Ed Jones
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Re: Does your reflector give "refractor like" images? new [Re: jnewton]
      #5774847 - 04/03/13 08:25 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.

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Darren Drake
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Re: Does your reflector give "refractor like" images? new [Re: Ed Jones]
      #5774851 - 04/03/13 08:28 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.

Edited by Darren Drake (04/03/13 08:28 AM)


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David PavlichAdministrator
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Re: Does your reflector give "refractor like" images? new [Re: Darren Drake]
      #5774901 - 04/03/13 08:51 AM

Quote:

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.




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

David


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