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Equipment Discussions >> Reflectors

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jnewton
sage


Reged: 12/20/07

Does your reflector give "refractor like" images?
      #5772204 - 04/02/13 06:31 AM

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?

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azure1961p
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Reged: 01/17/09

Loc: USA
Re: Does your reflector give "refractor like" images? new [Re: jnewton]
      #5772239 - 04/02/13 07:11 AM

Well that's a loaded question. A ten inch f/4 is going to have a larger CO than a 20" and this will affect contrast. Next, at twenty inches there's no apo that can compete with it.

I think the short answer is longer focus reflectors can give refractor like images but you still kno you re looking through a reflector. Thermal cooling is needed and if you want to be a stickler, so are curved spiders. There's some contention here but I don't think any reflector will ever be the equivalent of a good apo of like sized aperture in the sizes commercially available.
Pete

Edited by azure1961p (04/02/13 07:12 AM)


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Jarad
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Reged: 04/28/03

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Re: Does your reflector give "refractor like" images? new [Re: azure1961p]
      #5772256 - 04/02/13 07:36 AM

Depends on what you mean by "refractor like".

The one thing that refractors tend to do very well that reflectors can't is show really bright objects against a pure velvet black background. In a reflector, you are going to have some type of diffracton effect (an even glow from the CO, plus spikes from a straight spider or more even glow from a curved spider).

I can get that "refractor-like" look on things like M13, because the diffraction is faint enough that I can't see it. But on Venus, Jupiter, Sirius, or the moon reflectors can't quite get that razor sharp blackness.

That being said, the extra aperture does let a good reflector show more detail than a refractor. So yes, Jupiter throws some glow around it in my 14.5" that isn't there in my 4" apo, but there is no comparison in the amount of low-contrast surface detail on the planet itself - the 14.5" blows the small apo away.

Jarad


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azure1961p
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Reged: 01/17/09

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Re: Does your reflector give "refractor like" images? new [Re: Jarad]
      #5772264 - 04/02/13 07:46 AM

I think Jarad makes a key point here, refractor like or not, a large enough f/4 will handedly outdo all commercial apos currently sold. It might be served a little differently but the angular resolution gets to a point the 4" or even a 7" can't make up for the difference on contrast alone.

Pete


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Darren Drake
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Reged: 10/09/02

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Re: Does your reflector give "refractor like" images? new [Re: azure1961p]
      #5772338 - 04/02/13 08:44 AM

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.

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kfrederick
Carpal Tunnel
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Reged: 02/01/08

Re: Does your reflector give "refractor like" images? new [Re: Darren Drake]
      #5772394 - 04/02/13 09:12 AM

http://www.dgmoptics.com/index.htm That pic says it for me . Stars do not have spikes/ The background darkness are two easy to see in a unobstructed telescope . In the HST images just think of all the detail hidden by those spikes . Just my thoughts .

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Cotts
Just Wondering
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Reged: 10/10/05

Loc: Toronto, Ontario
Re: Does your reflector give "refractor like" images? new [Re: kfrederick]
      #5772458 - 04/02/13 09:48 AM

I unabashedly claim that my Intes 6" f/8 Mak Newt, with a 16% central obstruction and no spider to smear light gives a totally 'refractor-like' view on ANY object, bright or dim.

And it only cost $1000....

Dave


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RogerRZ
Whatta you lookin' at?
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Reged: 01/09/06

Loc: West Collette, NB, Canada
Re: Does your reflector give "refractor like" images? new [Re: Cotts]
      #5772562 - 04/02/13 10:43 AM

I have a 6.5" f/5.5 MN65 that does a fair job at impersonating a refractor (except for the eyepiece position ).

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Jon Isaacs
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Re: Does your reflector give "refractor like" images? new [Re: RogerRZ]
      #5772641 - 04/02/13 11:17 AM

A few thoughts:

Does refractor-like mean large Airy disks with a healthy dose of field curvature and maybe even some chromatic aberration? Does it mean relatively small exit pupils and relatively slow focal ratios?

Or does "refractor-like" mean a essentially perfect view, one free of field curvature, coma, astigmatism, chromatic aberration, with Airy Disks that are impossibly small?

I think under the right circumstances, essentially perfect views are possible with a Newtonian, I think most refractors do not give "refractor-like" views. Even the NP-101 with it's seemingly perfect color correction and flat field is too underpowered.

In terms of perfect views, I think a 12.5 inch F/6 tube Newtonian in the mild San Diego climate fitted with good fans and a careful observer does a good job... The 31mm Nagler + Paracorr + 12.5 inch F/6 + dark skies... clean and sharp across the FOV, no field curvature... coma undetectable. Contrast... can be amazing.

I no longer use the term "refractor-like" because refractors have their own set of aberrations and most are underpowered. I use "near-perfect" to describe those pristine, totally amazing views..

Most scopes are able to produce near perfect views of some set of objects and are not so hot on others.

Jon


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Jeff Porter
super member
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Reged: 09/03/10

Loc: Utah
Re: Does your reflector give "refractor like" images? new [Re: RogerRZ]
      #5772643 - 04/02/13 11:17 AM

Only when I set it up in such a way that it looks like a refractor. Unfortunately I can only see the grass in this configuration.

In all seriousness all three of my reflector scopes give great views. To achieve this you need good optics to reduce light scatter for a black background, solid collimation, and thermal control. At that point only the atmosphere will limit your views.

Jeff P


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Aquatone
sage


Reged: 03/23/06

Loc: California Bay Area
Re: Does your reflector give "refractor like" images? new [Re: Jeff Porter]
      #5772744 - 04/02/13 12:00 PM

If in terms of "refractor like" one means tight pin-point stars against black with a high degree of contrast with no scatter or softness, then I have recently undergone a conversion. I now have no doubt that a well designed (and collimated!) reflector can produce at least a comparable visual experience. The sharpness, clarity of detail, and lack of scatter, in my Lockwood 24" F/3.3 (22.9% obstruction) under the right viewing conditions is easily comparable to any of my refractors, and obviously superior in terms of angular resolution and light gathering ability. I do take the time to properly collimate to a high degree of precision but the rewards are high. Of course there is no comparable refractor in this size range so a strict comparison is not quite fair or valid. However the question is about the aesthetics of the view in < F/4 large Newtonians.

I am second to none in my love for refractors and own both an AP130 and AP160, but the latter is mainly used as an imaging instrument these days. I now use my refractors as much for their "grab and go" convenience as I do for the aesthetics of the views.

Chris


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ed_turco
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Reged: 08/29/09

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Re: Does your reflector give "refractor like" images? new [Re: azure1961p]
      #5772764 - 04/02/13 12:08 PM

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


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Does your reflector give "refractor like" images? new [Re: ed_turco]
      #5772852 - 04/02/13 12:41 PM

Quote:

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




Maybe comparing apples to peas?

Jon


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Galicapernistein
super member


Reged: 09/24/07

Loc: Detroit Michigan
Re: Does your reflector give "refractor like" images? new [Re: Jon Isaacs]
      #5772902 - 04/02/13 01:07 PM

Quote:

Quote:

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




Maybe comparing apples to peas?

Jon






Edited by Galicapernistein (04/02/13 01:17 PM)


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Mike B
Starstruck
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Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: Does your reflector give "refractor like" images? new [Re: Aquatone]
      #5772939 - 04/02/13 01:18 PM

Quote:

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




I will very much look forward to this, Ed!

Quote:

If in terms of "refractor like" one means tight pin-point stars against black with a high degree of contrast with no scatter or softness...



That's my impression of what is typically being compared, and i'd agree that, with some effort, one can get fairly close with a Newtonian. My Starsplitter Dob does a fairly good job at this, generally putting a nice point to stars!

But i sometimes observe with a fellow who owns an AP155, and IT has the "refractor-like views" that are being discussed here! So the view aesthetic in each system is going to be different... and "different" doesn't mean bad. Just different. As Jon has said, a Newtonian isn't aimed so much at achieving a "refractor-like" aesthetic- why would it be? That's not where it works best. It's designed to collect more photons in a manageable & affordable package. The goal is to let it do so, while optimizing it's system... let the views become what they are, naturally.

I s'pose, if one were inclined to do so, one could always fit an aperture mask on their Newt- bypassing the CO & vanes so as to obtain the almighty "refractor-like" views. But then ya got a 4" scope.

When seeing conditions (ground & aloft) are poor, the "refractor-like" abilities of a larger Dob are not gonna be there- oh yeah, it'll still go deeper & resolve (most things) finer than a smaller Apo, in general, but you will KNOW you're looking thru a reflector... by the aesthetics (or lack thereof, if you will ) of the view.

There are objects & conditions i've seen where the fine planetary detail (Jupiter is a good example) is better portrayed in the Apo, yet more often i've seen where the Dob's view of Jupiter is at least equally detailed to the Apo's, while also bigger & brighter.

But when the air is good- in/around the Dob, as well as above it- even the view's aesthetics can be mighty close. And what can be seen & resolved... not close at all; the Dob just walks off with it... especially on deepsky.


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Mike B
Starstruck
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Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: Does your reflector give "refractor like" images? new [Re: Galicapernistein]
      #5772943 - 04/02/13 01:19 PM

Quote:

Maybe comparing apples to peas?




Or, perhaps, comparing apples to appease?


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rgconner
member


Reged: 03/17/13

Loc: Sacramento, CA
Re: Does your reflector give "refractor like" images? new [Re: Mike B]
      #5772963 - 04/02/13 01:29 PM

Does your F350 perform like my Porsche?

Depends, are we racing, or hauling rocks?


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Mike B
Starstruck
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Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: Does your reflector give "refractor like" images? new [Re: rgconner]
      #5772989 - 04/02/13 01:37 PM

... or racing where? Highway or off-road?


Excellent analogy!


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kfrederick
Carpal Tunnel
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Reged: 02/01/08

Re: Does your reflector give "refractor like" images? new [Re: Mike B]
      #5773001 - 04/02/13 01:53 PM

For sure a perfect newt can make anyone happy . The problem is spherical lens surfaces are not that hard to mass produce. The parabolic surface is Harder to have perfect .

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dan_h
Carpal Tunnel


Reged: 12/10/07

Re: Does your reflector give "refractor like" images? new [Re: kfrederick]
      #5773098 - 04/02/13 02:55 PM

Quote:

The problem is spherical lens surfaces are not that hard to mass produce. The parabolic surface is Harder to have perfect .




True, a spherical lens surface can be machine finished, to a certain degree. But there is more to a lens than a single spherical surface. As a minimum there are four surfaces, two thicknesses and wedge errors, one air space, and four close tolerance radii to be controlled. All of these are interdependant to a point and so the final overall precision may actually be more difficult than a single parabolic mirror surface.

The fact is, if you want top notch performance from either a lens or a mirror, you need a qualified optician to provide the final touches.

dan

Edited by dan_h (04/02/13 03:22 PM)


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azure1961p
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Reged: 01/17/09

Loc: USA
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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Reged: 09/28/03

Loc: Prescott, AZ
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
professor emeritus
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Loc: UK
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
scholastic sledgehammer
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Reged: 11/14/07

Loc: Jefferson County, Colorado
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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Reged: 06/24/03

Loc: Los Angeles
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
SRF
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Reged: 11/26/03

Loc: Frederick, MD
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
Ely Kid
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Reged: 11/07/08

Loc: South Texas
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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Reged: 01/17/09

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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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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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City Kid
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Re: Does your reflector give "refractor like" images? new [Re: Darren Drake]
      #5775240 - 04/03/13 10:58 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.



Now that would be a fun thread to follow!


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Re: Does your reflector give "refractor like" images? new [Re: jnewton]
      #5775263 - 04/03/13 11:08 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

Edited by dscarpa (04/03/13 04:09 PM)


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nirvanix
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Re: Does your reflector give "refractor like" images? new [Re: jpcannavo]
      #5775336 - 04/03/13 11:29 AM

Quote:


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.


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GOLGO13
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Re: Does your reflector give "refractor like" images? new [Re: jnewton]
      #5775353 - 04/03/13 11:34 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.


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GeneT
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Re: Does your reflector give "refractor like" images? new [Re: jgraham]
      #5775380 - 04/03/13 11:46 AM

Quote:

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.


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GeneT
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Re: Does your reflector give "refractor like" images? new [Re: Darren Drake]
      #5775382 - 04/03/13 11:47 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.




Do it!


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kfrederick
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Re: Does your reflector give "refractor like" images? new [Re: GeneT]
      #5775411 - 04/03/13 12:01 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

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Darren Drake
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Re: Does your reflector give "refractor like" images? new [Re: GeneT]
      #5775448 - 04/03/13 12:26 PM

Quote:

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.




Do it!




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


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Galicapernistein
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Re: Does your reflector give "refractor like" images? new [Re: kfrederick]
      #5775521 - 04/03/13 01:02 PM

Quote:

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.


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kfrederick
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Re: Does your reflector give "refractor like" images? new [Re: Galicapernistein]
      #5775573 - 04/03/13 01:27 PM

funny but true

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FirstSight
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Re: Does your reflector give "refractor like" images? new [Re: GOLGO13]
      #5775781 - 04/03/13 03:07 PM

Quote:

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 agree that a 10" to 15" reflector and a really good 4" refractor are the perfectly complementary combination. I often set up my TV NP-101 alongside my 12" reflector, and bounce back and forth between them all night, sometimes comparing images but much of the time soaking in the panoramic widefield view in the NP-101 plus some 150x-ish view of planets, and individual DSOs in the 12" reflector.


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Sean Puett
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Re: Does your reflector give "refractor like" images? new [Re: kfrederick]
      #5775884 - 04/03/13 04:04 PM

I would answer yes for the most part. Paracorr helps make my reflector coma free. Decent eyepieces don't introduce flaws of their own. As long as collimation is dead on, I would say it is very refractor like. It is NOT a refractor. It still shows spikes on the brightest stars. I find some beauty in the spikes anyway. There is no field curvature. My reflector has perfect color correction. There is no hint of false color, focused or not. My reflector has a big problem though showing so many extra stars. Globs look bright and contain individual stars, not like a grey smudge. Talk about diamonds, a nicely resolved glob under dark skies, that is diamonds on black velvet.
So, why do I still own and enjoy a refractor? 4.5*tfov and nice flat field. I use it to help locate dso that I am not sure how to find yet. Scanning the milky way with a 4"f5.4 refractor is one of the best ways to spend part of a summer night. When you start to get tired but, you are not yet resigned to the zero gravity chair and binoculars. Carefree pointing of the refractor I mentioned will help give you a second wind. Once that runs out it is time for the chair and binoculars before bed...


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nevy
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Re: Does your reflector give "refractor like" images? new [Re: Sean Puett]
      #5776208 - 04/03/13 07:04 PM

I had a refractor once , didn't like it , sold it , I like dobs.

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buddyjesus
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Re: Does your reflector give "refractor like" images? new [Re: azure1961p]
      #5776347 - 04/03/13 08:18 PM

Quote:

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.




I think you and I have the same solution in mind Pete. a small scope to complement a big one.


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Thomas Karpf
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Re: Does your reflector give "refractor like" images? new [Re: Starman1]
      #5776578 - 04/03/13 09:55 PM

Quote:


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.





The fun part about any sort of comparison is the idea of 'apples to apples'.

By cost ($1000 max)? An 8-10" dobsonian vs a 4-5" achro on an inexpensive mount. Dobsonian wins hands down.

Aperture? 8" reflector vs 8" refractor, or 4" reflector vs 4" refractor. Refractor wins, but who the heck really wants to try and transport and mount an 8" refractor, or wants to use a 4" reflector?

Tube length? A 2' long SCT is a 12" diameter scope, which beats everything else.


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azure1961p
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Re: Does your reflector give "refractor like" images? new [Re: David Pavlich]
      #5776627 - 04/03/13 10:15 PM

Quote:

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




Yeah but it would've been funny till it got locked.

Edit: Ok I took the plunge and posted it in the refractor forum.



Pete

Edited by azure1961p (04/03/13 10:22 PM)


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GOLGO13
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Re: Does your reflector give "refractor like" images? new [Re: Thomas Karpf]
      #5776721 - 04/03/13 11:07 PM

I think one thing that is often overlooked is comparing apples to apples quality levels between reflectors and refractors. For instance, a lot of people seem to compare a 4 inch Televue or other top end company to an Orion 10 inch dob. I'm not saying there is anything wrong with an Orion 10 inch dob (love mine), but it's not a high end dobsonian. A better comparision would be a dob which has been optimized design wise and has a high quality mirror. You could even compare those in terms of cost. A high end 10 inch dob can cost as much as a high end 4 inch apo.

People really should look through as many scopes and designs as they can to appreciate what type of scope they would like. A friend of mine has a Maksutov Newtonian which provides absolutely awesome views. It's not your normal scope design talked about on these forums, but it's a great scope design. Aperture is great, but there is also something to be said for grab and go.

A lot of it comes down to what are you using the scope for. There are many objects where a small 4 inch APO would be the best scope for the job. There are many other objects (faint DSOs) where a very large newt would be the best option (especially for us folks with limited funds). A small refractor is a nice travel scope.

Personally I like all scope designs and wouldn't mind owning one of each. I have much more of an appreciation for refractors now than I did when I began observing. But I still understand the value of aperture. Best views of planets I have had were through a 30 inch F3.6ish dob in need of a new coating. We often debate aperture from 6 inches to 8 and from 8 inches to 10. The big differences are going from 8 to 16. That's when you really get an appreciation for aperture.


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Datapanic
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Re: Does your reflector give "refractor like" images? new [Re: GOLGO13]
      #5776960 - 04/04/13 02:30 AM

Talk about stirring up trouble! Those that like the Reflectors, Refractors, SCT's and Classics forums are going to catch on to the conspiracy rather quickly! But meanwhile...



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buddyjesus
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Re: Does your reflector give "refractor like" images? new [Re: Datapanic]
      #5777014 - 04/04/13 05:18 AM

haha

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jpcannavo
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Re: Does your reflector give "refractor like" images? new [Re: GOLGO13]
      #5777015 - 04/04/13 05:19 AM

Quote:

I think one thing that is often overlooked is comparing apples to apples quality levels between reflectors and refractors. For instance, a lot of people seem to compare a 4 inch Televue or other top end company to an Orion 10 inch dob. I'm not saying there is anything wrong with an Orion 10 inch dob (love mine), but it's not a high end dobsonian. A better comparision would be a dob which has been optimized design wise and has a high quality mirror. You could even compare those in terms of cost. A high end 10 inch dob can cost as much as a high end 4 inch apo.

People really should look through as many scopes and designs as they can to appreciate what type of scope they would like. A friend of mine has a Maksutov Newtonian which provides absolutely awesome views.




I think this is an excellent point. It's one thing to ask "does the typical reflector on the field match the typical refractor". Its quite another to ask how the best examples of each compare. I think this bears on the whole Mak-Newt performance thing as well. Setting aside the differences in thermal issues - height above ground, open vs. closed tube etc - there is no intrinsic reason why the on-axis performance (specifically the modulation transfer function) of a spherical mirror with a meniscus corrector should exceed that of a single paraboloid. Yes, central obstruction is often mentioned, i.e. the "tiny 20% obstruction". But lots of dobs have similarly "tiny" obstructions. What about spiders? Well, apart from the aesthetics of spikes, the impact on the MTF with thin veins is minimal. I think the real difference comes down to overall quality. these scopes are typically made - as are Apos - to a far higher standard than a similarly mass produced dob. And this is true in all respects: optical surfaces, tube baffling etc. And, in the case of spherical surfaces, high optical standards are more easily achieved.

Edited by jpcannavo (04/04/13 05:19 AM)


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David PavlichAdministrator
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Re: Does your reflector give "refractor like" images? new [Re: GOLGO13]
      #5778243 - 04/04/13 06:07 PM

Quote:

I think one thing that is often overlooked is comparing apples to apples quality levels between reflectors and refractors.




Very common. I always question when someone wants to compare a basically handcrafted refractor to an off the shelf Newt or SC and do it in an apples to apples context. And 9 times out of 10, it's comparing planetary views. That's fine if all you do is look at Jupiter and Saturn and the Moon, but that 10" off the shelf Newt is a big winner when you compare it to a 4" top shelf refractor when looking at NGC891.

David


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Daniel Mounsey
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Re: Does your reflector give "refractor like" images? new [Re: jnewton]
      #5778749 - 04/04/13 10:55 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?




In some cases they actually can, but if you want a view that's more often like a refractor, then you'll just need to get a refractor. BTW, that's the short version.


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Daniel Mounsey
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Re: Does your reflector give "refractor like" images? new [Re: Starman1]
      #5778773 - 04/04/13 11:14 PM

Quote:


Nah, 12.5" will ALWAYS show more.








Oh really, how about Collinder 70 Don.


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okieav8rAdministrator
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Re: Does your reflector give "refractor like" images? new [Re: David Pavlich]
      #5778789 - 04/04/13 11:23 PM

Quote:

Quote:

I think one thing that is often overlooked is comparing apples to apples quality levels between reflectors and refractors.




Very common. I always question when someone wants to compare a basically handcrafted refractor to an off the shelf Newt or SC and do it in an apples to apples context. And 9 times out of 10, it's comparing planetary views. That's fine if all you do is look at Jupiter and Saturn and the Moon, but that 10" off the shelf Newt is a big winner when you compare it to a 4" top shelf refractor when looking at NGC891.

David




I agree David. Due to a reflector's low cost per inch of aperature, I'm going to employ the light gathering capability of a big newt every time when it comes to looking at faint objects like distant galaxies. Observing such objects are where big reflectors really shine. On the other hand, I love the views of open clusters and rich field views of the Milky Way that I get with my 4" refractor. Different uses for different scopes.

Edited by okieav8r (04/05/13 09:58 AM)


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Jon Isaacs
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Re: Does your reflector give "refractor like" images? new [Re: jgraham]
      #5778808 - 04/04/13 11:37 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.




It's F/8... if your NEwtonian were F/8...

Jon


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Starman1
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Re: Does your reflector give "refractor like" images? new [Re: Daniel Mounsey]
      #5778890 - 04/05/13 01:20 AM

Quote:

Quote:


Nah, 12.5" will ALWAYS show more.








Oh really, how about Collinder 70 Don.



Or Kemble's Cascade, or Collinder 399 (The Coathanger), or..., or..., or...
So THAT'S why they made the NP101!


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Sean Puett
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Re: Does your reflector give "refractor like" images? new [Re: Starman1]
      #5779075 - 04/05/13 07:58 AM

NP 101 was made for guys with large reflectors. It is the perfect companion to a large reflector. It does well everything that a large reflector does not. The np127 is even better but, twice the price or so.

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azure1961p
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Re: Does your reflector give "refractor like" images? new [Re: Sean Puett]
      #5779081 - 04/05/13 08:02 AM

I agree . A refractor capable of low power wide fields is the perfect compliment to a large scope . In my case, large is 8" so 70mm fits the bill perfectly

Pete


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Ed D
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Re: Does your reflector give "refractor like" images? new [Re: azure1961p]
      #5779235 - 04/05/13 09:40 AM

I couldn't agree more. In my case it's a 6" f/8 Dob and a 72mm f/6 refractor , and throw in my 8x56 binos for good measure .

Ed D


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Mike B
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Re: Does your reflector give "refractor like" images? new [Re: Ed D]
      #5779343 - 04/05/13 10:42 AM

Quote:

NP 101 was made for guys with large reflectors. It is the perfect companion to a large reflector.




In my case they're a 15" & a WO ZS66 'fractor .



Fine companions!


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bherv
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Re: Does your reflector give "refractor like" images? new [Re: Mike B]
      #5779535 - 04/05/13 12:31 PM

My reflectors: 16" f4.5, 8" f/6 and 5" f/5. My refractors: 4" f/9.8, 80mm f/5.6 and a handful of 60mm's.
Barry


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jgraham
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Re: Does your reflector give "refractor like" images? new [Re: bherv]
      #5779596 - 04/05/13 12:55 PM

"It's F/8... if your NEwtonian were F/8..."

Hmmm, wellll, I've made dozens (yes dozens) of Newtonians from f/3.7 to f/10. My pair of biggos are a 16.5" f/6.5 and a 16" f/4.5. What I wasn't expecting from my 6" f/8 achro is how well it performs as an RFT. Prior to this my favorite was my trusty olde 6" f/4. The little f/4 is still one of my favorite compact scopes. I often sweep the sky with it craddled in my arms.

What a neat time to be an amateur astronomer!


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Sean Puett
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Re: Does your reflector give "refractor like" images? new [Re: jgraham]
      #5779937 - 04/05/13 03:39 PM



What a neat time to be an amateur astronomer!




This is what I say. Has there ever been a time with so much affordable quality astronomy gear? No. Scopes that some of us complain about would have cost a fortune 100+ years ago. And before that, you had to be nobility to afford a telescope. Now we reflector vs refractor and apo vs achro to death. Sorry for sidetracking the thread but, we are the most fortunate amateurs and some of us do not realize it.


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