Is bigger better?
Posted 27 January 2014 - 04:07 PM
Posted 27 January 2014 - 04:11 PM
Posted 27 January 2014 - 04:29 PM
I think it was more a problem of mechanics and thermal effects than optics. There was also probably some influence of 33% central obstruction.
My other former Newton 250/1600mm (again Orion Optics, lambda/8) was however always visibly better than 100mm ED scope. Here the aperture ruled. However, even with this scope there was certain class of DSO objects where 100mm ED refractor was giving better results - typically low surface brightness objects, like globular cluster NGC5033. This was probably due to sight light from all the nearby street lights.
Also, funny thing, I was able to push on stars the 100mm ED refractor below magnitude 14.0. While the faintest star that I saw through 250mm Newton was just 14.7. Theoretically, I should be able to go with 250mm telescope about 2 magnitudes lower than with 100mm one. I attribute this mostly to a better comfort I had with ED100. I was observing sitting and with mounts with fine motion and I could effectively shield my eyes. While with N250 I was observing standing, it was not easy to observe effectively at high magnifications.
Posted 27 January 2014 - 06:14 PM
Great point! Comfort affects perception; or, as Mister Spock might say, "So... Human."
Posted 27 January 2014 - 06:36 PM
Posted 27 January 2014 - 06:45 PM
I'd say my unknown 10" newt reigns supreme in my collection. Nothing can touch it. I can see things with it direct vision that are invisible in my really excellent Optical Craftsmen 8" newt and I don't think the mirror in the 10 is anything special, its just a lot bigger.
Posted 27 January 2014 - 07:08 PM
Posted 27 January 2014 - 07:52 PM
On the other hand, resolution and light gathering are aperture dependent. While I may present a very sharp and aesthetically pleasing view of Jupiter with a 4 inch refractor to other viewers who come by for a look, they are not going to see the detail at 160X that they see at 500X with someone else's C11 at the star party. The same goes obviously for a look at M31 with the 4 inch refractor compared to M 31 with a 14 inch dob. Routinely though the 4inch F11 produces nicer images than the C8s etc on the field. All this however is highly subjective and arbitrary.
Posted 27 January 2014 - 09:01 PM
Yes it depends on the quality of the optics.
I recall one evening the AP 180 f/9 and setup nearby the AP 155 EDFS f/7 and about 30 feet away the 10in Intes Micro Maksutov-Cassegrain (not sure which model) by if memory serves it was an f/15 system.
Anyway all 3 telescopes were observing Saturn and by chance we first walked up to the AP 180mm then the 155mm and finally the 10in Intes.
The images of Saturn through the AP's were excellent however our group of 4 all agreed that the planetary images of the Intes were "better" than the 2 AP refractors.
We kept going back and forth (to confirm) for a good 20 minutes or so.
Another night I remember observing Saturn with the 7in f/9 Astro Physics refractor and then with a 12.5in NGT spit ring EQ Newtonian and the views with NGT were better than the AP.
Not saying the AP was bad - only the NGT was better.
So if we are comparing premium quality optical systems then the bigger will also be better.
Posted 27 January 2014 - 09:39 PM
However, even with this scope there was certain class of DSO objects where 100mm ED refractor was giving better results - typically low surface brightness objects, like globular cluster NGC5033. This was probably due to sight light from all the nearby street lights.
NGC-5033 is a galaxy, NGC-5053 is a globular cluster, is that what you were thinking of? I generally don't think of globular clusters in terms of surface brightness, I think of them in terms of resolution and can be resolved with a 4 inch scope while a 10 inch is much more capable in this regard.
As far as the question, "Is bigger better", this is complicated and there is no one answer. If one is going to step back from the seeing issue then the assumption would need to be that the seeing is good enough that the telescope's optics are the limiting factor and not the seeing.. That means excellent seeing.
The next issue is the thermal management and the handling of stray light. Like seeing, these are often gremlins that plague larger scopes. Thermal issues can be addressed but they still can limit the performance of a large scope, stray light, not an issue with planetary, not an issue if the skies are dark. Both these factors depend on the scope as well as the operator.
The final factor is the target in question, the factors that are important when viewing the planets are quite different than those that affect the deep sky. My experiences basically go like this:
The original poster assumed excellent seeing and I am going to assume, because it is something I pay careful attention to, a properly cooled and aligned scope. I am not one to roam star parties, my experiences are my own with my own scopes.
Under these conditions, my experience has been that bigger is better when it comes to viewing the planets and double stars. Contrast and resolution are a function of aperture and if the seeing permits a large aperture scope to perform, it will do so. It doesn't take exceptional optics for a significantly larger scope to outperform the smaller scope, just good optics. A 10 inch or 12.5 inch has such an advantage over a 4 or 5 inch, a small scope can only do what it can do.. The other night the seeing was quite excellent, the temperature had remained at 50F for several hours, the views of Jupiter were among the finest I have ever seen, the contrast and detail, like a photo.
Typically this particular scope is not my favorite for the planets, it has a 2 inch thick mirror and has a mirror that is large enough (25 inches) that typically the seeing will be causing serious issues. But that particular night, these were not problems and the scope was performing admirably and indeed bigger was most certainly better.
As far as deep sky objects, again it does depend on the object but resolution and optical quality are less important because the eye simply cannot see the details that are there at the focal plane, the dark adapted eye has poor resolution. Small objects are more easily seen in large scopes. If I am hunting down faint galaxies, I find that the larger the aperture, the more I see.. If I am looking at the details in a bright planetary nebulae like the Eskimo nebula, the larger aperture provides the superior view, essentially independent of optical quality. The same can be said of globular clusters and open clusters for that matter.. M30 in a top notch 4 inch is shows only the barest details, in a 12.5 inch, I can resolve many of the stars and in an even bigger scope, even more so...I never really saw M7 until I looked at it in my 16 inch. In a 4 or 5 inch scope it's a beautiful bright open cluster but in a significantly larger scope, one can see a multitude of stars and even other clusters both open and globular hidden within M7.
So, ignoring seeing and assuming a properly setup and cooled scope, in general, bigger is better.. There are some objects that are better seen in a smaller scope because the simply won't fit in the eyepiece of a larger scope. Large, low contrast gradient objects are better suited for smaller, shorter focal length scopes.
It is my experience that it is the seeing, the thermal management, the stray light control, that can result in a large scope not performing up to snuff. Larger scopes require more care and effort by the observer to get the most out of them.. In the few public outing I attend, I just don't quite see others paying the attention to the details that are critical in getting their scopes to perform up to snuff.. At a serious star party, this would not be the case...
That's how it works for me... If I want the best possible planetary views and the seeing is excellent, (which it can be in San Diego), I will startcooling either the 10 inch F/5 or 13.1 inch F/5.5 and hour before sunset and figure it will be at least an hour after sunset before the scope is ready...
Posted 27 January 2014 - 10:21 PM
I had a Vixen A102Mwt(Japan) a good scope. But the 3.25" Jaegers gave similar or better planetary images. The Vixen just didn't have as smooth or accurate figure. Still a good scope but the smaller Jaegers was just as good.
3" scope outperform a 6" ?? only if there are problems with the 6" or the seeing is terrible. It happens often though. My 4" Jaegers gave better images than a 10" dob and a C8 at a public event. The 10" dob wasn't cooled down and the collimation wasn't spot on. The C8 looked well collimated, but it wasn't well cooled. The others had potential, but had issues that really reduced performance. Terra's comment was spot on. Saturn is sooo sharp! was the oft comment. Under non optimal conditions, smaller refractors can perform better than a larger scope.
Posted 27 January 2014 - 11:04 PM
Posted 28 January 2014 - 12:24 AM
But quality, with additional difficulty, can also be scaled.
All that matters then is the degree one copes with difficulty ... to obtain the desired, scaled, quality.
Some difficulties, like instrument optics/rigidity/stability, can be dealt with by obtaining better ones comprehensively.
Other difficulties, like seeing and thermal issues, cannot. One must absent them for quality to be present.
Compromise occurs when we're not aware of the flaws that injure quality. Which when scaled, happens more readily.
When bigger isn't better is when scaling fails to deliver quality.
Posted 28 January 2014 - 08:50 AM
I was going to start a thread on 'What factors go into your telescope purchases?' but I think it can tie-in with this topic - especially when you think of all the choices we have with this hobby!
So: What guides your Classic Telescope purchases??
Posted 28 January 2014 - 09:36 AM
"The optical quality of both the larger and smaller scopes must be similar for aperture to rule."
This certainly depends on the object in question. For faint, small objects, the eye is the limiting factor, it cannot resolve the details that exist at the focal plane so that quality of the optics is not particularly important.
In terms of objects like planets, fine scale contrast and resolution are proportional (actually inversely proportional) to aperture so a larger scope need not be of the same optical quality to provide greater resolution and contrast... The optics in my 10 inch GSO Dob are almost certainly not of the same quality as those in my 4 inch apo refractor nor at they as good as those in my 60mm F/13.3 Asahi-Pentax. But for viewing the planets, for splitting double stars, it is by far the better scope because on an absolute scale, the optics are better.
Posted 28 January 2014 - 09:37 AM
It is not important that it is globular. I meant it as a typical member of class of objects with low surface brightness. It was just a first one that came to my mind.
Posted 28 January 2014 - 10:42 AM
Learning to test optics is not hard and in many ways easier then many of the mechanical restorations present here. Also the types of errors I see are not small ones so your not going to have to worry about splitting hairs to see if you have 1/10 vs 1/12 optics. It is going to be that your "1/10" wave optics are 1/2 wave and the zones and other errors are going to jump out at you. So it is only going to take one look to tell that there is no way that your optics can be anywhere close to diffraction limited.
Posted 28 January 2014 - 11:30 AM
Posted 28 January 2014 - 12:24 PM
Jon gives good answers to the unasked question.
So much depends on the condition of the telescope, not just the aperture. The lack of thermal equilibrium will destroy an image in any telescope. Bigger telescopes take longer to reach that condition.
I have set up the 9" Clark next to larger telescopes many times. Sometimes the Clark gives a better view, i.e., Jupiter, Moon, double stars and sometimes it doesn't.
I don't mind seeing something better than what my 9" can produce.
Posted 28 January 2014 - 03:19 PM
Bottom line is, alot of larger scopes don't have great optics and/ or have cool down and tube current issues.
Posted 28 January 2014 - 03:32 PM
Posted 28 January 2014 - 05:30 PM
Good viewing of the planets is largely dependent on image scale. The longer the focal length along with larger optics is always a good winner. That is why so many F15 systems provide such excellent views of the planets. The C14 with it's 140 inch focal length is a good example. When you get a C14 with good smooth optics and it is dialed in correctly and the seeing is good the view is pretty good. However, no matter what you say a large refractor with good optics providing those pin point stars on a velvet back ground is about as close as to driving a 911 Porsche. There is no substitute.
Personally I don't think there is an answer to you original question. There are too many variables and qualifications. I remember a night where I had one of my best views of Jupiter thru a C14. Next in fairly dark skies, M33. Very dim view the galaxy which filled the eyepiece field even tho it was long focus. The contrast was not impressive. So I move to my 4 inch f/5 refractor and look at M33. Perfectly framed, spiral structure much more evident and contrast superb. Your question is like asking the better of apples and oranges. But I enjoy the thread.