reflector vs. refractor
Posted 31 January 2004 - 08:23 PM
Posted 31 January 2004 - 09:56 PM
Local seeing has a lot to do with living in the desert. The temperature drops thoughout the night. Larger scopes often never reach thermal equilibrium, or do so just before dawn. The refractor wins in these situations, which predominate in desert viewing. I can be doing planetary or lunar work while the big guns are still suffering from the fuzzies, even hours later.
In the desert, condensation is almost never a problem. I've only had a scope dew up (Mak, SCT, or Refractor) one time in over three years of observing from my location. Well, it wasn't dew, it was ice from a freezing ground fog, but I've never seen it before or since.
Refractors, for the most part, don't need collimation. They travel well. Newts and Dobs need collimation. Many people don't collimate their scopes regularly, so their performance suffers relative to a refractor. Mis-collimation lowers resolution and contrast.
Mirror coatings oxidize and need to be redone every few years. Many people don't do this, and their views suffer as a result.
APO refractors often have sub-1000mm focal lengths. With 2" eyepieces, this makes them ideal for open clusters and the larger nebulae. Long focal length SCTs and Maks are especially hampered in this regard. Refractors also have longer focal ratios, often around f-8 to f-10, and eyepieces, regardless of design, perform better at the slower f-ratios.
In essense, Erik, you are correct. A properly collimated, thermally stablized, curved-vaned, well baffled/flocked reflector with a secondary of less than 20% of the diameter of the primary, with a clean mirror of good figure, and good seeing will give a better view of the planets than an APO refractor. IF you have all those things, you will have a killer planetary scope and location.
Almost everyone I know owns an SCT (big secondary, long cooldown, fairly portable) or a fast Dob (big secondary, long cooldown, often poorly collimated, still portable, inexpensive) or an achromat (chromatic abberation, good optics, very portable, inexpensive). Most club members prefer the view of the planets through my APO, though.
I'll guarantee this, though. If I lived in SF or in Florida, with the nice, smooth, laminar air flow off the ocean and fairly stable temperatures, I'd have a 6" or 8" Newt, baffled or flocked or otherwise contrast-enhanced, with an undersized secondary on a curved or single-stalk spider, a low-profile helical focuser, a full set of UO HD Orthos (parfocal eyepieces are a must with low-profile, limited throw focusers), and that would be my planetary rig. I might even have two spiders, with one for deep-sky work with a big secondary and the other with a small secondary for planets.
But since I live in the desert, and have a small car, I have an APO.
Posted 31 January 2004 - 10:41 PM
That's one hell of a reply, there absolutly nothing i can add. It even get's me to want an APO now.
Posted 31 January 2004 - 10:49 PM
Posted 31 January 2004 - 11:11 PM
Posted 31 January 2004 - 11:19 PM
BUt I have a modified 5" Newt. Before I could afford an APO, my wife would have to pack up and leave me flat.
Posted 31 January 2004 - 11:46 PM
I think we need to compare apples to apples - I'd rather have a 4" APO than a 4" Newt or Cat, but if I can have aperture I'll take that first.
Posted 01 February 2004 - 02:14 AM
Posted 01 February 2004 - 02:15 AM
Posted 01 February 2004 - 03:04 AM
gotta throw in my 2 cents here.
goin back to the original post , i must agree about
never seeing a small refractor show as pleasing a view or
as much detail as my 8 in. newt. now i can understand somewhat the photography aspects for a
refractor if thats how you make your living,
but for visual use how can anyone
justify the cost of a small apo vs a good quality 8 in newt.
seems more like toys for big kids and maybee bragging rights.
Posted 01 February 2004 - 08:01 AM
so i guess my question is, what advantages do refractors have besides being more portable and taking less time to reach outside temperature? am i missing something?
Can`t add much to all that Warped`s astutely pointed out except maybe that, (my apologies if I missed this somewhere in the middle of Warp`s excellent post), a reflector`s tube is open at the top which can produce swirling amalgamations of hot and cold air which can interfere with the contrast, sharpness, clearness, etc., of the image in many places around the country. All such advantages, most of which were pointed out by Warped, including that many are excellent for astrophotography, can function at their full potential most of the time thereby even further providing owners with their full money`s worth which isn`t true with larger newts because of seeing conditions, etc., most are maintenance-free, can operate in almost any climate immediately when many newts suffer from tube currents, etc., as well as cooldown issues in cooler climates, retain resell value much better than newts so owners can get most of their money back should they ever decide to get rid of it, have top-notch mechanics, pinpoint stars across the ENTIRE fov, provide the best performance/inch of aperture, are literally works of art, pride of ownership, asthetics, no cool-down which is extremely important in cooler climates, portability which is extremely important to some, (Eric mentioned these last 3), etc., etc., etc., ALL figure in to the relatively high price!
Please click here for more on how a refractor can be better than a reflector under the right circumstances.
Please pay attention to the grades that Astronomy Mag. gives the different sizes, types, etc., of refractors for each type of viewing.
But approaching this from a different angle and for those who didn`t read it, good 5" to 6" refractors can outperform numerous good 6"-7" and even larger reflectors on the Moon and planets, splitting binary stars and resolving globular clusters. Why? 1st, refractors show more low-contrast planetary details than many larger Newts, etc.
This shouldn`t be that difficult to understand since according to Physics, Modulation Transfer Functions show this as follows,
Let R = the Aperture of a Refractor,
Let N = the Aperture of an obstructed Newt
Let CO = the Central Obstruction
So, if R > N - CO, then the unobstructed refractor should show more low-contrast planetary detail than the obstructed newt as confirmed and explained further at all the following sites,
It should be pointed out that although the human eye may or may not be able to tell the differences in contrast between an unobstructed scope and one with a 20% obstruction in scopes of = apertures, the unobstructed scope proves that it can tell the difference by showing more low-contrast details.
But from Physics, there`s more to it than just this. Perhaps more importantly, as many experienced planetary observers know, Dawe`s Limit breaks down when it comes to viewing low-contrast planetary details as explained at Legault`s very informative site on the subject as follows,
"...The minimum size of the details that a given instrument can show must not be confused with its resolving power computed with the approximative formula:
R = 120/D (R in arcseconds, D in millimeters).
This value only works with double stars, it gives the approximative minimum separation of a couple of stars of equal magnitude that allows a given instrument to separate them visually. The fact that a planetary detail will be detected or not is very complex, it depends on many factors like its shape, color, intensity, contrast of the features on extended objects, observer`s location, seeing conditions, irradiation effects of bright objects reducing acuity of the eye, etc.
So, in good seeing, details whose size is under this stellar resolving power can be seen, etc. For ex., the size of the Cass. div. is about 0.7 arcsecond at the ends of the ring, and about 0.3 arcsecond where the ring reaches the edge of the planet. Regardless, a 9" Schmidt-Cass easily shows the Cass. div. despite the fact that it`s theoretical resolving power is only 0.55 arcseconds..."
In many newts, the CO is 20%-25%, (for ex., Eric`s is 23%.). So, sometimes R > N - CO and, in such cases, the refractor outperforms the newt on low-contrast planetary detail, etc. In addition to this CO issue, there`s a huge difference in light transmission. Unlike newts, refractors don`t have a multiple-reflection optical path causing light-scattering diffraction and internal reflections that brighten the sky background, reduce contrast, and smear images. In contrast, (Lol), refractors have the highest % of light gathered that reaches the eye. They can transmit well over 90% of the light collected while newts quite often transmit only 77%-80%.
The result of a refractor's lower diffraction and higher light transmission? In good seeing, refractors can show subtle lunar and planetary details with a more easily observed and wider contrast range with sharper images than in most newts.
Also, because the Moon and planets are all so bright, light-gathering isn`t as important as high powers. So the refractor`s smaller aperture is often an advantage for this type of viewing. Thus, it`s not uncommon to see good refractors handling powers of 80x/in., (like my C-102), and 100x/in., (like my Apos), in above ave. seeing conditions.
With all this in mind, I still have not seen a 5" newt, including high-end ones with Zambuto, etc., optics, show more planetary detail than my 4" Apo, or outperform it in any other way, except for showing a few more faint fuzzies, despite the fact that the refractor spotted them a full inch more of aperture. For that matter, I still haven`t met the 6" newt with Zambuto mirrors, etc., or otherwise that can outperform my 5" Apo in any way except for a few more faint fuzzies, despite the refractor spotting them a full inch of aperture. Many of these 6" newts couldn`t outperform the 4" Apo on planets, etc., either, despite being spotted a full 2" of aperture!
But please also keep in mind that money isn`t a telescope part. Thus, IMHO, it`s irrelevant to the telescope design. If one is trying to find out which design`s better, IMHO then, the fairest way is to compare the best scope of each design using the same apertures under the same conditions, etc. It`s then easy to see that the refractor, (other than possibly an off-axis newt), outperforms the newt in almost every optical way and in many cases, it`s not even close.
But then $$$ is mixed back in and each person must decide if a given scope is worth the money for what ever their preferences are.
That`s my opinion on it, anyway.
Please Note: Your results may vary depending on several different factors! If your results differ from those of mine and our other club members who helped with these tests, then so be it.
Well, that`s my $.02.
Good Luck to all and Clear, Steady Skies!
Posted 01 February 2004 - 08:51 AM
the problem here is that we are not really comparing appels with appels.We are comparing 4000$+ apo with newtons around 800$.
Ofcorse the apo wil perform bether.
I think that if you take a newton that is build to the same high quality standart that al the apo's are build to it will always be as good or bether.
What we are doing now is comparing a JVC amplifier costing 150$ with a high-end amplifier costing 10000$.
The difference in sound is also big.
I dissagree that you dont need light gathering power to look at moon and planets because it comes with more aparture and you need aparture to resolve smaller details.
Ofcorse again the quality of the objectief and the rest of the telescope has to be very good to so that you can use the high powers you need to see the smal details and if seeing conditions lets you use them.
We al know that there always be refractor and reflector people and this tread can go on and on buth there is nothing else to do with al this rain anyway.
Posted 01 February 2004 - 09:40 AM
I recently went for a refractor aside form all the reasons mentioned above. They look less threatening and more like a telescope. I was vacationing on a mountain overlooking a vally sometime ago with my 6" on a Dob mount. When a concerned patriot call for security becuase, they thought I was a terrorist about to fire a cannon into a town down in the vally. Bundled up in a black trench coat along with a ski mask for warmth, I was lucky not to get shot while observ. Pete
Pete that happen to me once, i was in a public park, i somehow wanted to show the stars to the people passing by, then people started to sit near me, 1,2,5,10 person were sitting there looking at me, i found this pretty weird and after a while someone approched and asked me when would the firework start!!!
Posted 01 February 2004 - 10:06 AM
When you place, say, $3,000 for an FS102 (2,000 naked ota + 1,000 to get pointed upwards), you are not only paying for performance, you are paying for the best performance available in a 4" scope, and the comfort of not worrying about anything: no miscollimated optics or optics that cannot be collimated in case of a fall, no under- or over-greased focuser, no flexing in the focuser drawtube, etc.
You will notice there are plenty websites devoted to getting your Autostar to work or your synta mount to track, but none about tuning up (meaning: getting it to work properly) a Takahashi refractor.
It's available only to the deep-pocketed, and even the deep-pocketed might not choose to put their money there, but it makes sense, provided you are not give the bad excuses for owning one (such as saying that it would beat a $3,000 8" reflector).
Posted 01 February 2004 - 10:27 AM
When you place, say, $3,000 for an FS102 (2,000 naked ota + 1,000 to get pointed upwards), you are not only paying for performance, you are paying for the best performance available in a 4" scope, and the comfort of not worrying about anything
Sure thing - aperture for aperture the Taks (and most of the other APO's) will give great, no-fuss performance. They'll even outperform a somewhat larger scope. I do question whether a $1000 mount will do the same; at that price level you are still firmly into the Synta and Synta clones or limited to altitude/azimuth mounts and will still have mount issues. I used to use an LXD55 mount for my 4".
I'm not against small refractors; I had a very nice 4" APO myself and will soon have one again. It's just that, like every other instrument out there, it'll be a compromise. There are times when giving up that much aperture won't make sense to me. There are also times when it's a good tradeoff in exchange for wonderful visual performance on brighter objects.
Besides, they make great finderscopes!
Posted 01 February 2004 - 12:02 PM
One of the great things about belonging to an astronomy club is this: everybody has a scope. Our "regulars" have 8", 10" and 12" SCTs, we have a couple of Dobs, a Newt, some achros, one guy has an 18" Obsession, and I have the APO. I seldom observe alone, and my observing partner has a 10" SCT, so between us we've pretty much got everything covered.
Posted 01 February 2004 - 04:04 PM
And it's wrong to say that money doesn't matter, or shouldn't matter. It *always* matters. Even with professional instruments it's no different.
They're all reflectors. The reason is that it would be fantastically expensive to build a refractor as large as the biggest reflectors. Imagine an 8 meter or 10 meter objective lens. Unlike a reflector, it couldn't very well be a mosaic. It would have to be one piece. Just the R&D you'd have to do to determine how the objective would be figured, and supported, to defeat sagging and adjust for changes in the scope's orientation would kill you. (You'd probably need some kind of hydraulically-activated, computer-controlled compression ring. Or maybe some super-slick adaptive optics corrector. Just guessing.)
Now, would the images be better than the 10 meter reflector? Probably. But for the money, you could probably build 100 10 meter refractors. Maybe even hook them up as some kind of super-duper interferometer.
Money always matters.
Posted 01 February 2004 - 09:39 PM
I dissagree that you dont need light gathering power to look at moon and planets because it comes with more aparture and you need aparture to resolve smaller details.
Although this is correct, light-gathering still isn`t as important viewing planets as it is, for ex., viewing dso`s, since the planets are so bright to begin with. Thus, mag. is more important when viewing planets since higher mags. are going to make all the details visible that the scope`s able to show.
The brightness of planets can`t be emphasized enough. An excellent ex. of all this was during the Mars opposition. Some details on Mars were washed out by the combination of how bright it was and the light-gathering ability of larger apertures. The planet was so washed out in some larger scopes, it actually looked pink, while their owners scurried around trying to find the right combination of filters, etc!
Money always matters.
Although money can always matter, it depends a great deal on what one is actually talking about.
For ex., if one is talking about what the best scope for the money is, they might get answers like the Orion ED80 refractor, the C-102HD, the XT-6 dob, the MN-56, the MK-67, the C-9.25, etc.
But if one now asks which is the best telescope out there, no one`s going to mention one of the scopes above.
Similarly, if one`s talking about the best optical performance at = apertures for the money, (strictly speaking amateur scopes 12" or smaller since TMB makes Apo refractors this large and even larger), then it certainly wouldn`t be a refractor.
But if one`s talking about the best optical performance at = apertures PERIOD!!!, then it would be the refractor.
this tread can go on and on
This is certainly true. I stated my piece, etc, etc., etc! So...
Clear, Steady Skies!
Posted 01 February 2004 - 09:42 PM
Posted 01 February 2004 - 09:57 PM
SCTs advantages are........well i guess they're more portable than newts, but they have a lot of downsides as well,
Yes, SCT's are a compromise - but the size and weight difference is quite substantial. I can't fit a 14" F/10 Newt (or my 10" F/7.5 or the 12.5" F/8) in my observatory and certainly don't want to pay for a german EQ mount that would handle either one but the 14" SCT sits in there very nicely on a compact mount. I do like Newts also but to keep the size reasonable many folks choose faster ones - and there's a compromise inherent there as well. One must choose what features to give up and what to keep.
Posted 01 February 2004 - 10:26 PM
Posted 01 February 2004 - 11:36 PM
I now own a 12.5" Mag1 Portaball (also with Zambuto mirror.) I'm still waiting for the night where the refractors demonstrate their clear superiority. I'm not holding my breath. Under crappy seeing conditions, I've seen the phenomena of a refractor providing what its owner called 'a more aesthetically pleasing view.' This is another way of saying when the seeing is bad, smaller aperture scopes don't see the bad seeing as well as large aperture scope. (In this type of condition, one can 'stop down' the larger scope and see the same sort of views that are seen by the refractor.)
When the seeing is good to excellent and when optical quality is excellent, aperture wins every single time. And dollar for dollar, high quality reflectors rule.
But don't take my word for it. Check out Gary Seronik's article "Four Infamous Telescope Myths" in the February 2002 issue of Sky and Telescope. You can also go to star parties and try a few experiments. Under good seeing conditions, take a look at the planetary views through a correctly collimated reflector equipped with a Zambuto, Royce, Swayze, Hall, etc. mirror. Then take a look at the views through a 6" refractor that's many times more expensive. I think that the results might surprise you.
Finally, consider this Mars image, made by Wes Higgins with a 14.5" Starmaster. In the past, when the optics in most large reflectors were mediocre at best, I believe that high quality refractors provided the best views. Now, with high quality optics readily available in large reflectors, I believe the situation has changed.
Posted 02 February 2004 - 12:16 AM
Posted 02 February 2004 - 01:20 AM
Posted 02 February 2004 - 11:06 AM