461 replies to this topic

[sonny.barile]

Yes I’m exaggerating    but why is this a thing in so many refractor discussions on the web?

Why is It that so many will do a comparison of a 4 inch APO and an 8 inch SCT or Newtonian and think that they see more in the APO? Limiting magnitude and resolution will surely be better in the larger scope. Am I missing something?

 

 

 

 

Edited by sonny.barile, 31 December 2017 - 12:03 AM.

[treadmarks]

A good question. Let me answer it by saying, if a refractor guy offers you some Kool Aid, don't drink it.

[Codbear]

Sorry Sonny, I've been busy watching my AP130 kick my 16" Teeter's butt and haven't had the chance to respond! 

 

I think (or hope) what some of my fellow refractorites are alluding to is that, particularly with very high quality - and very expensive - optics, stars do exhibit excellent pinpoitedness and planetary detail is extremely sharp, particularly compared to the often "soft" images that many SCTs exhibit, as well as inferior and/or poorly collimated reflectors.

 

But what cannot be disputed is the fact that an 8" SCT at the same magnification as a 4" apo, on a night of good quality seeing, will show greater separation of double stars, though the view may be a tad "blobby" compared to the 4", if it could split the pair at all! The view in the 8" will be brighter as well...as you alluded to, it's physics.

 

So, while the physics cannot be argued, certainly the sharpness of the image can, and that's where the refractorians will gladly chime in. 

[Jon Isaacs]

So, while the physics cannot be argued, certainly the sharpness of the image can, and that's where the refractorians will gladly chime in.

 

 

So there I am with my 120 mm ED/apo and my 10 inch F/5, $240 on Astromart both pointed at at an 0.83" double..  The 10 inch is of course cooled with a fan and properly collimated. The seeing is excellent.. 

 

The 120 mm provides a slightly elongated star image.  The 10 inch makes a clean split. 

 

It takes a careful eye to see the slight elongation.  A refractor fan might say the refractor was sharper,  I say no way Jose. It's not sharper..  It's duller..  Unable to resolve the image.  

 

The same is true viewing the planets..  Viewing globulars..  How can it be sharper,  you can't resolve it? 

 

Jon

[Astrojensen]

 

Why is It that so many will do a comparison of a 4 inch APO and an 8 inch SCT or Newtonian and think that they see more in the APO?

Because, let's face it, there are A LOT of 8" SCT's and newtonians out there with poor quality mirrors, that lack collimation and are not properly cooled down, so people simply DO see more, often a lot more, in the 4" apos. Some also observe under conditions that simply will not allow the SCT or larger reflectors to come even close to its theoretical performance on most nights. I know, I am one of those observers. 

 

I had a C8 with very good optics. On exactly TWO nights in the several years I had it, did it show what it really was capable of, with sharp airy disks at 800x. Most of the time, it was a fuzzy mess at even 150x, no matter how long I tried to cool it down. It was regularly soundly and thoroughly beaten on planetary details by a humble 5" f/9.4 achromat and my 6" f/8 $200 newtonian walked all over it. I never did a direct comparison between the C8 and the 100mm f/8 TMB apo I had on loan, but I almost always saw things in the TMB I had never seen in the C8, whenever I looked at the same targets. 

 

I knew the optics were good, so I considered rebuilding the C8 with active cooling and all that jazz, but in the end, I decided to just sell it. 

 

Oh, before you get going, I know how to collimate an SCT and *always* checked it before an observing session and tweaked it as needed (it had Bob's Knobs installed). When I tried using the scope regularly, it also lived in a box out in the shed, so that cooldown time was supposedly minimized. It was still a fuzzy mess most of the time, while both my 5" achromat and my 6" f/8 newtonian took just a twenty minute to half-hour of cooldown from the warm living room and then soundly outperformed the C8 for the rest of the night on planets and other high-resolution objects. 

 

 

Clear skies!
Thomas, Denmark

Edited by Astrojensen, 31 December 2017 - 03:52 AM.

[gfstallin]

If I spent $8,000 on a 5-inch refractor, I too would expect that it would best an 11-inch SCT costing 1/3 as much.

 

Joking aside, I think it all depends on what a person finds important. People who get worked up over the refractor vs. reflector thing are just looking for an excuse to argue. My 80mm semi-apo offers nice, sharp views almost as soon as I take it outside. However, it does not begin to compete with the resolution of my used, well-collimated 8-inch SCT, which cost roughly the same price. Now, that C8 probably has been sitting outside for two-and-a-half hours before I even put it on a mount, with active cooling from a Lymax for the last half hour. And of course, my 8-inch SCT cannot really do justice to wide fields, and stars in any part of the field aren't as sharp as they are in the refractor no matter what I insert into the optical path. Since I live under skies that are as bright as they come, I'm not entirely concerned about grand vistas. The reason I bought a small refractor is precisely because there are just some things my SCTs just cannot do as well as a small refractor when those vistas are available at dark sites. For that matter, other than resolution and limiting magnitude, there is only one thing my C8 cannot do as well as my C11: motivate me to lift weights at the gym so that I can continue using that C11 and its required heavy EQ6 mount when I am older. 

 

Some people will just find the view more pleasing in a refractor. Who could blame them? The view is nice. However, I find my planetary imaging results in the C8, C9.25, and C11 to be orders of magnitude better than anything I will see with my refractor. Even visually, my one time side-by-side comparison of the 80mm refractor with the C8 was no contest. Though the lunar wide fields offered by the refractor were more aesthetically pleasing, I could see lunar detail with the C8 that was not even faintly visible with the refractor. That does not mean the C8 or a similarly-sized reflector with okay mirrors and coatings is "better" than the 80mm semi-apo refractor, it just means the C8 was better at resolving lunar details than a refractor. That is it. Maybe I find more absolute value in that resolution than a refractor guy, and a refractor guy values the aesthetics and/or convenience of a refractor more. Our preferences are irrelevant in judging optics from a quantitative point of view, but they mean everything in terms of what we are willing and able to use on a regular basis. 

 

Whether it is a $10,000 refractor, $6,000 reflector or a pair of cheap binoculars one's grandmother gifted him in 1987, the optics that make us happiest are always the best optics. It does not matter what other people say or other people see. I'm color blind and never understood what people complained about with "spurious color" in apochromatic refractors. "Does that really bother you, or do you just like to whine?" Once two telescopes of different designs are cooled and collimated, things get a lot more subjective provided the optics in them are of decent quality. 

 

George

Edited by gfstallin, 31 December 2017 - 05:21 AM.

[beanerds]

I once owned and loved a C102hd  , great scope that kept up with my wife's 150mm f8 Dob from dark sky but from our light polluted back yard the 6 inch was better on most objects .

 

Fast forward 2 years and I now owned a very good Saxon 150mm f8 refractor ( C6r ) and observed with people that owned nice C8's and Meade 8 inches and the same applied here as with the old C102 , the C8's went deeper but the C6r was sharper in town , out in the country in dark skies the difference was slight with the edge to the 8 inch CAT's , sorry to say that by this time even great 8 inch f6 Dob's where lkeft in the dust , not optically of course but user friendly-ness  , ever drove a Dob at 400x ?: that's the problem so no black mark there .

 

Today ? hmmm  .., I have and love a C9.25 and iStar 127mm f8 that share equal time on my CI700 mount , thats the difference , quality optics mounted on a rock of a mount  , like a good Dob but driven so 500x aint a problem ( pain at midnight ) ,, the older you get you learn that optics are a big part but ,,,, user friendless counts .

 

Beanerds 

[Erik Bakker]

Given an experienced observer, what a scope can show of it's full potential depends very much on local conditions. Temperature swings, wind, local heat sources, seeing, jet stream. To name but a few.

 

Those living in places with great conditions tend to see much more of a scopes' potential, especially big scopes. Where I live, smaller scopes tend to do better. But on nights of great conditions, the big ones leave all others in the dust.

 

Thus those with good conditions tend to prefer larger scopes, wondering what all the fuss about small refractors is about. Those with lesser conditions tend to see it the other way around.

 

And then there are personal preferences of course.

Is the primary focus on solar, lunar, double star or deep sky observing? What is a realistic budget one is willing to spend now? What weigh/bulk scope can one comfortably lift to house or transport to a remote site? How much observing or cooling time do you generally have?

 

As we can see, there is no one perfect scope. Now or in the future. That's why there are so many and we can learn which scope best suits us now or in the future.

[OleCuss]

I once went to a star party which turned me into a bit more of a refractor guy (still have more of other optical designs, however).

 

The wife and I were not a part of that star party but we were there when they started setting up for a fairly high-altitude public viewing star party.  They set up nice and early and had more than one hour to achieve thermal equilibrium.

 

Well, when the Sun was down people started viewing.  I was fortunate enough to view through 5 scopes in pretty rapid sequence before we had to go.  4 of the scopes were pointed at Mars and one was on Saturn.

 

The 6", 8", and 15 or 16" (can't remember for sure which at this time) Dobs had literally horrible views of Mars.  Not minimally bad but literally horrible.

 

Went to a 130mm StarFire and the view was great and you actually got detail.

 

Went to a Swarovski 80mm spotting scope and got a beautiful view of Saturn (tiny but great nonetheless).

 

The refractors just worked.  Cool-down was not the big problem those wonderful Dobs had to deal with.

 

Two of the Dobs had black tubes which meant that they were being significantly heated when the Sun was still up and the bigger Dob was truss-tube but still had the big old mirror to cool down.  So even though they were set up long before sundown they still had to struggle for thermal equilibrium when the Sun set and we had rather rapid cooling in a somewhat thinner atmosphere.

 

The A-P had a white tube and presumably had less heating.  The spotter just worked as well.

 

Since I don't usually have a whole lot of time for viewing, a refractor wins hands down most of the time.  No question that my 8", 10", 11", and 12" scopes should be able to show me more, but most of the time they can't because I don't have the time to do the set up and cool-down so the 80mm Swarovski spotter I got (used) after that star party is my most-used scope.

 

Oh, and esthetically?  My most enjoyable views of M31 have been with 80mm refractors at high altitude with dark skies.  They give the right FOV to frame it almost perfectly!

 

Yup, I'm sort of a refractor guy even though if given plenty of cool-down time and time for setting up I agree that I'd see most of my targets better with my bigger scopes.

 

 

 

[Astrojensen]

Given an experienced observer, what a scope can show of it's full potential depends very much on local conditions. Temperature swings, wind, local heat sources, seeing, jet stream. To name but a few.

 

Those living in places with great conditions tend to see much more of a scopes' potential, especially big scopes. Where I live, smaller scopes tend to do better. But on nights of great conditions, the big ones leave all others in the dust.

 

Thus those with good conditions tend to prefer larger scopes, wondering what all the fuss about small refractors is about. Those with lesser conditions tend to see it the other way around.

 

And then there are personal preferences of course.

Is the primary focus on solar, lunar, double star or deep sky observing? What is a realistic budget one is willing to spend now? What weigh/bulk scope can one comfortably lift to house or transport to a remote site?

 

As we can see, there is no one perfect scope. Now or in the future. That's why there are so many and we can learn which scope best suits us now to in the future.

Yes, and a lot of the observers with big scopes who live in more favorable climates seemingly cannot possibly comprehend how it is to live in a challenging one. Quite a lot of my observing are unplanned, surprise half-hour to one-hour sessions between showers or low-pressure fronts. Under such conditions, the small refractor reigns supreme and allows me to get the best of the preciously few available photons. Some large-scope observers have even told me, that they would give up and find another hobby, if they were forced to live where I do. Yeah, but I didn't give up, I just found the best tool for the job and try to make the best of it. 

 

 

Clear skies!
Thomas, Denmark 

[Axunator]

Yes, and a lot of the observers with big scopes who live in more favorable climates seemingly cannot possibly comprehend how it is to live in a challenging one. Quite a lot of my observing are unplanned, surprise half-hour to one-hour sessions between showers or low-pressure fronts. Under such conditions, the small refractor reigns supreme and allows me to get the best of the preciously few available photons. Some large-scope observers have even told me, that they would give up and find another hobby, if they were forced to live where I do. Yeah, but I didn't give up, I just found the best tool for the job and try to make the best of it. 

Add living in this kind of climate to living in a small downtown apartment with limited storage space, and suddenly a small, quality APO makes a lot of sense - and will show more than a larger and more cumbersome scope that wouldn't get used at all...

 

I realize that this is not what OP exactly meant, but still partially explains why some owners of small fracs may come across as very happy users of their gear.

 

And I must confess there's an esthetic quality to a view through a nice APO that I have not experienced with other types of scopes (admittedly more run-of-the-mill variety in my case). I'd describe it as "sharpness" and "contrast" because I cannot think of better words - agreeing to disagree with Jon's sentiment above 

Edited by Axunator, 31 December 2017 - 07:24 AM.

[Axunator]

And while I am not the one to make a silly claim that 4" APO will have better resolution and/or light gathering ability than 8" Newt or SCT when the conditions are appropriate for the bigger scope to perform up to its potential, I do seriously suggest that a quality APO will much better handle higher magnifications per aperture than similarly sized, obstructed, reflective designs. This is partially explained by simple physics (better MTF curve due to lack of obstruction, better light throughput), and - possibly - partially by the phenomenon that typical reflective optic scopes of typical APO apertures (think 4-6") are more likely to be mass produced, at cheaper cost and with tighter time constraints, and therefore superbly performing examples are fewer and further between. 

[CHASLX200]

Yes I’m exaggerating    but why is this a thing in so many refractor discussions on the web?

Why is It that so many will do a comparison of a 4 inch APO and an 8 inch SCT or Newtonian and think that they see more in the APO? Limiting magnitude and resolution will surely be better in the larger scope. Am I missing something?

No SCT can compare to my Taks when it comes to dead sharp 100X+ per inch star images or low power views for star sweeping. No SCT can close to my many high end Newts with 600 to 1100x view of the planets. I have owned around 60 SCT's and only 4 were very good and one super sharp C8 that was one in a million. All the other SCT just offer up so so lack luster high power views of the planets.  Ya just can't beat the Zambuto Newts or the old time 8" F/8 Cave's , Edmunds and such on the planets.  Just that one in a million 1984 black C8 would come close to my 8" F/8 Newts.  It is much harder to make a near perfect SCT that are mass produced for so cheap.

 

Now a 4" APO vs a C8 on say M42, well the C8 will show much more, but that APO will win in contrast everytime.

[bobhen]

Yes I’m exaggerating    but why is this a thing in so many refractor discussions on the web?

Why is It that so many will do a comparison of a 4 inch APO and an 8 inch SCT or Newtonian and think that they see more in the APO? Limiting magnitude and resolution will surely be better in the larger scope. Am I missing something?

Seeing limits all telescopes resolving power. The resolving power of a telescope is only its POTENTIAL resolving power not its guaranteed resolving power. And seeing can limit a larger scope’s potential resolving power to that of a smaller scope.

 

So. If seeing limits a 10-inch telescope to the resolving power of a 5-inch telescope and that 5-inch telescope is of better quality or a more favorable design or both, then it is rather easy for the smaller telescope to deliver better views.

 

Bob

[Erik Bakker]

I happen to have a rare 1983 black Super C5 and it too is marvelous. Kind of like the black C8 from that era you describe Chas. But even that one cannot take on the incredible FS102 on anything but user friendliness and bulk 

 

[CHASLX200]

I happen to have a rare 1983 black C5 and it too is marvelous. Kind of like the black C8 from that era you describe Chas. But even that one cannot take on the incredible FS102 on anything but user friendliness and bulk 

I love that rare black C5 you have. Always wanted one.  The FS 102 is a power house than can do 100x per inch.

[Erik Bakker]

To get an idea of it's compactness: my Super C5 on fork mount and tripod vs FS102 NSV/EM10 equatorial/EM200 tripod

 

[Spikey131]

Best of both worlds

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[rmollise]

Yes I’m exaggerating    but why is this a thing in so many refractor discussions on the web?

Why is It that so many will do a comparison of a 4 inch APO and an 8 inch SCT or Newtonian and think that they see more in the APO? Limiting magnitude and resolution will surely be better in the larger scope. Am I missing something?

 

Well, it's an apples to oranges comparison...but maybe not quite the way you think. Both telescopes have their place and strengths the other lacks. For example, I believe you will prefer the look of M31 and NGC 7000 in a small refractor. And it will certainly do a much better job imaging these objects...

[CHASLX200]

To get an idea of it's compactness: Super C5 on fork mount and tripod vs FS102 NSV/EM10 eqautorial/EM200 tripod

 

C5 and FS102 (1).jpg

That had to be one of the first black C5's made.

[RGM]

Best of both worlds

x2

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[Axunator]

A small APO can also be a wonderfully versatile scope. My 4-incher can provide glorious 3.6 degrees at 18x and splendid 250x on bright solar system targets, i.e. it's seeing limited most of the time in my abysmal weather. In an 8-incher or larger the image is usually just brighter (which is often unnecessary for solar system and double star observing) but does not show much more detail because of atmosphere.

 

In my 4 inch Mak, 150x approaches eye torture for me - in addition to becoming a bit soft, the image is getting dim enough that floaters are as or more distracting than in the 4 inch APO at 250x. No mystic there, mostly physics (two additional, non-enhanced reflective surfaces and 34% central obstruction).

 

Where larger aperture (regardless of optical design) is much more effective even here in the Arctic, are faint and small DSOs.

[treadmarks]

 

So, while the physics cannot be argued, certainly the sharpness of the image can, and that's where the refractorians will gladly chime in.

 

 

So there I am with my 120 mm ED/apo and my 10 inch F/5, $240 on Astromart both pointed at at an 0.83" double..  The 10 inch is of course cooled with a fan and properly collimated. The seeing is excellent.. 

 

The 120 mm provides a slightly elongated star image.  The 10 inch makes a clean split. 

 

It takes a careful eye to see the slight elongation.  A refractor fan might say the refractor was sharper,  I say no way Jose. It's not sharper..  It's duller..  Unable to resolve the image.  

 

The same is true viewing the planets..  Viewing globulars..  How can it be sharper,  you can't resolve it? 

 

Jon

 

The problem with the whole refractor vs. bigger reflector debate is that it's assuming we're only talking about low contrast features, which is the most favorable terrain for a refractor. Jupiter is dominated by low contrast features. With other planets, it's a mix of high and low. Jupiter is a cool object. One of the best. I wouldn't actually blame you if you bought a scope just to look at it. On low-contrast targets like Jupiter, a perfect 5" APO will go about even with an average 8" SCT. But I haven't seen Jupiter since last summer. And I probably won't be looking at it again until March or so.

 

So I've been looking at other things with my telescope, too. And with most other objects, as you hint at, it's no contest - the refractor won't even be able to detect or resolve the things the bigger scope is looking at.

[Peterson Engineering]

Hi Sonny,

 

As Jon points out, the distinguishing performance difference between an SCT and a refractor is one of contrast.  Inch per inch a refractor will provide superior contrast due to the fact that it doesn't have an obstruction.  However rule of thumb is that aperture improves contrast, and to provide to provide the same contrast as a refractor the SCT must have 2-inches more aperture.  So as far as contrast is concerned an 8" SCT matches the performance of a 6" refractor.

 

For planetary work where seeing usually limits magnification, smaller apertures typically behave better in bad seeing.  But contrast is critical.  So a refractor will usually outperform an SCT for planetary viewing.

 

Regards,

 

Pete

[Daniel Mounsey]

I've always found that the ones who don't understand the attributes of each optical system are the ones who lack experience in the broader arena of deep sky observation. Even Ray Charles could see that.