Jump to content


Photo

Does your reflector give "refractor like" images?

  • Please log in to reply
72 replies to this topic

#1 jnewton

jnewton

    Mariner 2

  • -----
  • Posts: 293
  • Joined: 20 Dec 2007

Posted 02 April 2013 - 05:31 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? :question:

#2 azure1961p

azure1961p

    Voyager 1

  • *****
  • Posts: 10475
  • Joined: 17 Jan 2009
  • Loc: USA

Posted 02 April 2013 - 06:11 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

#3 Jarad

Jarad

    Fly Me to the Moon

  • *****
  • Posts: 6389
  • Joined: 28 Apr 2003
  • Loc: Atlanta, GA

Posted 02 April 2013 - 06:36 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

#4 azure1961p

azure1961p

    Voyager 1

  • *****
  • Posts: 10475
  • Joined: 17 Jan 2009
  • Loc: USA

Posted 02 April 2013 - 06:46 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

#5 Darren Drake

Darren Drake

    Mercury-Atlas

  • *****
  • Posts: 2823
  • Joined: 09 Oct 2002
  • Loc: Chicagoland

Posted 02 April 2013 - 07:44 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.

#6 kfrederick

kfrederick

    Gemini

  • *****
  • Posts: 3030
  • Joined: 01 Feb 2008

Posted 02 April 2013 - 08:12 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 .

#7 Cotts

Cotts

    Just Wondering

  • *****
  • Posts: 4966
  • Joined: 10 Oct 2005
  • Loc: Toronto, Ontario

Posted 02 April 2013 - 08:48 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

#8 RogerRZ

RogerRZ

    Whatta you lookin' at?

  • *****
  • Posts: 3140
  • Joined: 09 Jan 2006
  • Loc: West Collette, NB, Canada

Posted 02 April 2013 - 09:43 AM

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

#9 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 44804
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 02 April 2013 - 10:17 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

#10 Jeff Porter

Jeff Porter

    Vostok 1

  • *****
  • Posts: 192
  • Joined: 03 Sep 2010
  • Loc: Utah

Posted 02 April 2013 - 10:17 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. :shrug: :bangbangbang:

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

#11 Aquatone

Aquatone

    Ranger 4

  • -----
  • Posts: 357
  • Joined: 23 Mar 2006
  • Loc: California Bay Area

Posted 02 April 2013 - 11:00 AM

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

#12 ed_turco

ed_turco

    Apollo

  • *****
  • Posts: 1391
  • Joined: 29 Aug 2009
  • Loc: Lincoln, RI

Posted 02 April 2013 - 11:08 AM

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. :D


Ed :crutch:

#13 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 44804
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 02 April 2013 - 11:41 AM

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



Maybe comparing apples to peas? :)

Jon

#14 Galicapernistein

Galicapernistein

    Mariner 2

  • -----
  • Posts: 251
  • Joined: 24 Sep 2007
  • Loc: Detroit Michigan

Posted 02 April 2013 - 12:07 PM

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



Maybe comparing apples to peas? :)

Jon


:lol:

#15 Mike B

Mike B

    Starstruck

  • *****
  • Posts: 10383
  • Joined: 06 Apr 2005
  • Loc: shake, rattle, & roll, CA

Posted 02 April 2013 - 12:18 PM

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


I will very much look forward to this, Ed! :waytogo:

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! :lol: 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. :shrug:

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.
:ubetcha:

#16 Mike B

Mike B

    Starstruck

  • *****
  • Posts: 10383
  • Joined: 06 Apr 2005
  • Loc: shake, rattle, & roll, CA

Posted 02 April 2013 - 12:19 PM

Maybe comparing apples to peas?



Or, perhaps, comparing apples to appease?
:smirk:

#17 rgconner

rgconner

    Explorer 1

  • -----
  • Posts: 62
  • Joined: 17 Mar 2013
  • Loc: Sacramento, CA

Posted 02 April 2013 - 12:29 PM

Does your F350 perform like my Porsche?

Depends, are we racing, or hauling rocks?

#18 Mike B

Mike B

    Starstruck

  • *****
  • Posts: 10383
  • Joined: 06 Apr 2005
  • Loc: shake, rattle, & roll, CA

Posted 02 April 2013 - 12:37 PM

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

Excellent analogy!

#19 kfrederick

kfrederick

    Gemini

  • *****
  • Posts: 3030
  • Joined: 01 Feb 2008

Posted 02 April 2013 - 12:53 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 .

#20 dan_h

dan_h

    Surveyor 1

  • -----
  • Posts: 1985
  • Joined: 10 Dec 2007

Posted 02 April 2013 - 01:55 PM

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

#21 azure1961p

azure1961p

    Voyager 1

  • *****
  • Posts: 10475
  • Joined: 17 Jan 2009
  • Loc: USA

Posted 02 April 2013 - 03:22 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. :D


Ed :crutch:


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

#22 Jeff Morgan

Jeff Morgan

    Fly Me to the Moon

  • *****
  • Posts: 5795
  • Joined: 28 Sep 2003
  • Loc: Prescott, AZ

Posted 02 April 2013 - 04:12 PM

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 :grin:

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.

#23 nevy

nevy

    Viking 1

  • *****
  • Posts: 625
  • Joined: 07 Feb 2012
  • Loc: UK

Posted 02 April 2013 - 04:20 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. ;-).

#24 David Knisely

David Knisely

    Hubble

  • *****
  • Posts: 15724
  • Joined: 19 Apr 2004
  • Loc: southeastern Nebraska

Posted 02 April 2013 - 04:42 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:


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.

#25 Project Galileo

Project Galileo

    Viking 1

  • *****
  • Posts: 843
  • Joined: 14 Nov 2007
  • Loc: Jefferson County, Colorado

Posted 02 April 2013 - 04:51 PM

Nevy, :lol: :roflmao:






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics