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reflector vs. refractor

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#26 Rhadamantys

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Posted 02 February 2004 - 12:13 PM

Below is a (poorly translated I am afraid) part of an article from David Vernet. I suggest that you read it carefully. I tend to trust David Vernet's opinions, for three different reasons: he doesn't sell anything to amateurs, his constatations are easy to verify and he is a master mirror maker (Texereau School of Optical Quality) who is working for Labeyrie (Interferometery, adaptative optics, etc.). The complete (french) article is available here http://astrosurf.com...az/diametre.htm

Vincent



Why does one usually see large telescopess, which for planetary work or double star observing give worse images than smaller telescopes?

There are many reasons for this:

- The larger the telescope, the more difficult it is to obtain a good optical quality. Small telescopes, those smaller than 4", are quite easy to build with a good optical quality, especially for industrially built telescopes. The larger the telescope, and the more, for money issues, one is tempted to reduce optical quality. For instance, it is the case on large dobsonian equipped with second rate optics made in the USA. [Translator's Note: The author is pointing his finger at Torus here. I have also heard of a french amateur who received a 15" Obsession with a poorly figured primary mirror: bad move, he was a member of the Societe Astronomique de France, where Foucault is reverred as God and Texereau as his Prophet. In France Obsession's ethics have now become questionable among serious amateurs.]

Plus there are unscrupulous "opticians" who keep alive the idea that large telescopes are more sensitive to turbulence than small ones, and that it was therefore normal to observe degraded images... This idea, perfectly wrong, is unfortunatly widely spread among the amateur community and permits to those dishonnest people to justify the poor planetary results obtained with such poor large optics, even when sold as "exceptional".

All in all, this conspires to create a comparison between small high quality telescopes and large and poorly figured telescopes. In such conditions, especially if those large telescopes' optical defects are very important, it is normal to see better images in small telescopes.

- The comparison is often made between well collimated refractors and poorly collimated reflectors.

Central obstruction reduces image contrast, especially on the planets. Modest refractors, because they are unobstructed, give better images. But this gain quickly find its limitation, for an important difference in apperture will erase what gain the absence of central obstruction gives on a smaller telescope. A comparative test on jupiter between a 8" f/4 Newtonian, optically beyond reproach, with 37% central obstruction, shows that one obtains the same images as in a Astrophysics 155 EDF refractor.

- Internal turbulence is usually greater in large telescopes than in small telescopes. If this "telescope turbulence" is greater than atmospheric turbulence, especially because the tube elements are more massive, the mirror thicker, a small telescope can have the upper hand.

- Large telescopes' optics are more sensitive to mechanical issues. The poor adjustment of an astatic cell, may create astigmatism. In extreme cases, a smaller telescope can give
better images.

- There are also very subjective judgments. One has seen that a refractor or small telescope create quite large diffraction patterns, easily visible and not very sensitive to turbulence. This give flattering images which are, for the beginner, easier to interprete than the brownian movement of the spekles in a large telescop. Double stars are thus seen as a nice couple of diffraction pattern, and the planets, even poorly detailed and quite fader than in a large telescope, will look much more stable. Some amateurs consider those flattering images to be more aesthetic, even when they objectivly lack the informations a larger optic can give. This most subjective sentiment, backed by the sometime astronomical price of those modest apperture telescope and the necessity to justify this astronomical price, is used to justify that the images given by a small apperture telescope are "of course" of better quality than those given by a large apperture telescope.

In spite of all those difficulties, an experienced amateur, with a good mastery of his equipment and owner of a large telescope with quality optics, will favor the images of this large instrument when compared to a smaller one most of the times and regardless of turbulence.

Thus, regardless of the location, the larger the apperture, the better the images.

Of course, this is to be modulated by the price of the telescope, its dimensions, its ease of use, the location, etc. which will have everyone find a limitation to his telescope.

#27 jmoore

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Posted 02 February 2004 - 12:27 PM

Several of you guys talk about the unparalleled viewing experience through large Zambuto mirrors. No doubt this is true, but it turns the discussion to issues of quality, rather than issues of scope design or aperture per se.

I realize that you can't ignore quality in such a discussion, but for those of us with $$ pockets of very finite depths, how would the discussion change if we talked about Orion or Celestron scopes?

You say a 6" refractor (presumably even the very high-end ones) can't touch a 14.5" Starmaster. Well, let me pose the similar question, but how about comparing a 6" achromat, or even a 4-5" high-end APO (Astrophysics, Takahashi, or whatever), to say, a well-collimated 10" Orion Dob.

Does aperture still win...not just in terms of light-gathering, but in terms of image quality? I'm hoping the answer is yes ;)

#28 LivingNDixie

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Posted 02 February 2004 - 12:36 PM

warped hits a good point in his first posting... Car size is an important factor...

Now I personally think a 10 f/6 or 12 f/6 is the best scope to buy because I believe in aperature and the more you got the better. I like slower scopes b/c they are easier to collimate and therefore easier to get consistently good views through...

But to each their own...

LivingNDixie

#29 eric moerman

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Posted 02 February 2004 - 01:14 PM

I think the 10" will still beat the achr.and apo's when it is of decent quality and has no mayor flaws.
If you would aske how an 8" would do than i would have my doubts.
Im not saying that Orion dobs are bad,there are great value for the money.If you should want to compare an 8" with the apo's you will have to get everyting right to let it work for 100%and then it maybe will be bether but it will be closer than with a 10".
A 10" will defenetly beat them if you get to work it right. :waytogo:

eric

#30 Jarad

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Posted 02 February 2004 - 01:24 PM

Jeff -

A 10" dob of decent quality should be able to beat a 4-5" APO. When you are talking about an Orion or Hardin dob, they usually have decent optics (not 1/20th wave, but probably close to 1/8th, with some samples being significantly better or worse....). The limiting factors become colimation and cooldown. Most of those mid-range dobs could benefit a lot from a fan - spend $30 to buy a nice quiet 4" pancake fan and a battery pack, and spend some time on collimation, maybe a bit of tube flocking, and you can get some really nice performance from an Orion, Hardin, Celestron, etc. dob. They should blow away a 4" apo, and pretty closely match a 6" in terms of quality (of course they win hands down on deep sky stuff...).

Jarad


#31 Guest_**DONOTDELETE**_*

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Posted 02 February 2004 - 01:27 PM

LivingNDixie got it exactly right, at least in my case. The question was why would anybody spend thousands of dollars on an APO when a bigger, better Newt can be had for less money. In my circumstances, I CANNOT have a bigger, better Newt. Well, I could, but I couldn't move it. If we set aside environmental issues, maintenance issues, and portabilty, the Newt wins. Factoring in my environmental issues and my (lack of) transportability (I'd keep my optics cleaned, collimated, and in good order) the APO is superior for my uses. I could go spend $20,000 to get a big car to carry a big scope, or even $10,000 to get a hatchback to carry a big scope, but that would still be $7,000 to $17,000 more than my APO rig. What sense does that make? Circumstances warrant that I have a small scope, so I have the best (in my view) small scope available. And I'm happy with it.

Having a small, quick-to-cool scope that is very portable, of excellent optical quality, that I can take to the mountains to view at 8,000 feet is better, for me, than any Newt that would be stranded on my patio at my condo where I'm surrounded by dozens of street lights, dozens of security lights, and tile roofs that radiate heat all night and muck up the seeing.

Regarding asthetics, I find the views in a refractor more asthetically pleasing. Since I am a visual observer, asthetics are important. If I don't enjoy the view, I won't view.

As I stated, if my situation were different, I'd make a different choice, but it's not, so I didn't.

Clear skies.

#32 Jarad

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Posted 02 February 2004 - 03:13 PM

Hey Warped, I have nothing against small refractors. I have an ED-80 myself, which I love and use more than any other scope. There is definitely something to be said for portability, quick set up and quick cool down. But, the ED-80 doesn't show nearly as much as my big scopes. It gets used more often because I am willing to take it outside for a quick 10-15 minute session on almost any night the sky is clear, which I am not willing to do with the bigger scopes. But if I have a full night to observe, I am willing to put in the extra time and effort to set up the big scopes, and they can blow the little ED-80 away on planets and deep-sky.

Also, I think small APO's are portable, but they get big quick. A 6" APO on a GEM will weight as much or more than a 12" dob, and take longer to set up. The reason people like these isn't for portability, it's for the wide flat fields for photography (not that there's anything wrong with that!).

Jarad



#33 jmoore

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Posted 02 February 2004 - 03:29 PM

Ah, come on Warpd...Newts aren't THAT un-transportable. The Truss tubes break down to a couple boxes that fit in the trunk of your car.

My 8" Newt also fits easily in my little Subaru...and a 10" Newt wouldn't be much bigger. My 8" takes me about 15 min max to set up.

So, I can't debate your other concerns (cool-down, maintenance, aesthetic preferences, etc.), but I don't really buy the $17K auto-mobile argument. You've been taking too many lessons from Johnnie Cochran! :winky:

#34 LivingNDixie

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Posted 02 February 2004 - 04:59 PM

warped (and others),

One of the first things I ask someone when they are looking at scopes is, "how big is your car?". I think alot of us really don't think about how were going to transport our scopes when were shopping arround for our scopes. If I could afford it, I woud go with a Nextstar 11 for deepsky and portibilty and then have a 5inch APO for planets from my house. Unfortuntely cost does not allow for that setup, so I did what many of us do, picked my priorities carefully and then measured my wifes SUV. I think the Discovery 10 f/6 PDHQ is prolly one of the best unknown scopes out there (much like C9.25 is for SCTS). Everyone who looks through my scope can hardly believe that its a Discovery mirror....

LivingNDixie


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Posted 02 February 2004 - 05:38 PM

Please, nobody misunderstand this thread. I'm not angry, or bitter, or mad at anybody or anything like that. I was reluctant to even reply to the original message, but I thought it deserved an honest answer, seeing that I have an APO. And, frankly, I wanted to see the responses generated.

For the record, I have a 12.5" f-6 Dob in Colorado with my folks. I could move it from their garage to my garage, but it would still sit in the garage. And I prefer observing with my APO anyway, so I've not relocated it yet. Does that shock anybody? Am I just too lazy to have a big scope? Maybe.
:jump: :roflmao: :jump:
Clear skies! :praying:

#36 Tom L

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Posted 02 February 2004 - 05:49 PM

You have a big scope...an LX200 10"! :D ;)

#37 eric moerman

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Posted 02 February 2004 - 05:52 PM

hello,

i think that whit a litle bith of carreful desing you can go a long way in transporting a (big or long)newtonian.
My scope is a 12.5" f7 on a eq fork mounth and everyting still fits in my car.
im driving a nissan sunny and thats a small car to american standarts i supposse :thinking:
Ofcorse if your buying a ready made telescope your stuck with the dimensions.

eric

#38 Guest_**DONOTDELETE**_*

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Posted 02 February 2004 - 06:07 PM

TomL,
Shhhhh! You'll ruin my fun! Though I think I mentioned that earlier. :lol:

I don't need a bigger scope! I don't want a bigger scope! :shrug: Help! Help! I'm being repressed!

#39 Barry Fernelius

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Posted 02 February 2004 - 06:09 PM

jmoore writes:
---
I realize that you can't ignore quality in such a discussion, but for those of us with $$ pockets of very finite depths, how would the discussion change if we talked about Orion or Celestron scopes?
--- but then he also queries:
Well, let me pose the similar question, but how about comparing a 6" achromat, or even a 4-5" high-end APO (Astrophysics, Takahashi, or whatever), to say, a well-collimated 10" Orion Dob.
---

If $$ are important, realize that you're making a strange comparison when you look at both the high-end APOs and the 10" Dob. I'll give a flip answer: buy the 10" Orion Dob now, and put yourself on the waiting list for a 6" Astrophysics refractor. By the time you'll be able to take delivery on the refractor, you will have enjoyed the heck out of the Orion Dob, and you will have decided whether a high-quality Newtonian like a Starmaster (or whatever) is worth it to you. Then, in some undetermined future, you can make a direct comparison between the APO and the Orion Dob (or its successor).

jmoore wisely says:
---
Ah, come on Warpd...Newts aren't THAT un-transportable. The Truss tubes break down to a couple boxes that fit in the trunk of your car.
---

Here I agree with you totally. My primary telescope is a 12.5" f5 Mag1 Portaball with a lovely Carl Zambuto mirror that has a Strehl ratio that makes me blush. I can set my observing system up in about 10 minutes, including collimation. I transport my entire mobile observatory (and a friend or two) in a sporty yellow VW New Beetle.

And the whole shabang cost considerably less than a high-end refractor OTA and a high-end mount. (In fact, the cost difference would almost pay for a used VW New Beetle! :grin:)

#40 Charles

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Posted 02 February 2004 - 06:36 PM

Quick question being we always seem to be trying to state that cheaper can beat quality preformance, i.e., a 10inch reflector of good quality can beat a i.e.. TAK or AP 4, 5, or 6 inch refractor.

First question is, what determines a good 10 Reflector?

Second Question is, what price is a good Reflector?

Last time I looked at GOOD Reflectors, i.e, the astrograph series of TAK, the Epsilon 250, which is a 10 inch will cost you roughly 9,300. dollars. I do not see the cost savings between a great Reflector over the refractor, but I will agree the Epsilon will beat the Refractor hands down!

Heck the TAK CN-212 will cost you $4,500.

So are we stating that because you can beat these prices by a great deal that one is wasting there money on these Newts/Reflectors/Cassegrains?

Charles

#41 eric moerman

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Posted 02 February 2004 - 07:08 PM

Charels

to answer your first question,i think a good quality 10" is a telescope where there is puth as much effort in as they do whit apo refractors.
Same quality opticaly and mechanicly.
Onfortunetly there are not so much manufacterers out there that do this at the moment.
2 question;im building a loth of telescopes to (newtons) and i try to make them work as good as possibel(baffling,quality optics,ventilation,...)so i know a litle bit how much effort one telescope needs.
I think the price Parrallax instr. is asking for there 10" newt.(4350$)is verry sharp!
I cant make it for that and still make a living out of it(if i should need to live from it) and i think they are VERY GOOD reflectors.
Its also one of the reasons that there are not more manufacterers of high quality newtons,most people are not prepared to pay so much for "only"a newtonian.
The "cheap" 10 newtonians will beat the apo's buth only because they have more apparture and if your lucky that you have a "good"one.I think its not a really fair to compair a 10" newt.whit a 5 or 6" apo,an 8"newt.and a 6" apo would be more fair and they will give a bether vieuw on howe they compare.
So i dont think people waste there money if they buy a "good"newtonian or apo.
You simply have to pay if you want bether quality and bether performance.
A Toyota and a Mercedes do bassicly the same to,they just drive,buth they do drive differently,its the same with telescopes.

eric


#42 Charles

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Posted 02 February 2004 - 07:28 PM

Thanks Eric, I was just curious if this thread was going like so many other threads of similiar subjects.

I keep seeing people think that they can beat APO optics with a less expensive scope and I am of the opinion you can not. I could see more planetary and nebula detail with my old Meade 12 than I could see with my 5" APO, but that does not mean it beats an APO's optical quality. I think comparing Reflector aperture to APOs of lesser size aperture is comparing apples and oranges. If one want to campare a APO to a reflector, comparing the same size optics would be a more fair comparison and in that catagory these less expensive reflectors would lose. It all boils down to quality and you pay for what you get. It is a given, that a 10 inch less expensive reflector might see a larger image than a smaller APO just due to magnification, but that is a not a true test of the quality of the same detail all things being equal.

I also do believe great reflector truely exceed APO optical quality, but to do that you have to spend a lot of money and you are back to paying the same prices if not higher than true APO scopes.

Charles :)

#43 eric moerman

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Posted 02 February 2004 - 07:53 PM

I agree Charels,

If you want high quality you will pay for it no meather what kind of telescope you want.
If you should want to compare newt.and apo i think an 8"newt.and a 7 apo should be close.
You have to consider the loss of surface by the sec.mirror.
In the end the best telescope is still the one that fits your bill and the one that lets you enjoy the sky and that can be a 100$ wallmart telescope as wel as a 20000$ apo or newt.

eric

#44 Guest_**DONOTDELETE**_*

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Posted 02 February 2004 - 08:34 PM

Hello All,

Since the fact that seeing conditions affect larger apertures more adversely has been questioned, let me present what expert authors such as Terence Dickinson, Alan Dyer, James Muirden, etc., as well as numerous noted amateur astronomers, astronomers throughout history, etc., etc., etc, ALL have to say on the matter based on their vast experience at the ep. Btw, I`d take the word of ALL these guys over the word of anyone who posts in these forums, (no offense :D )!

1st, here`s what Dickinson and Dyer state in, "The Backyard Astronomer`s Guide",

"Blurry Atmosphere: The Earth`s atmosphere is always in motion distorting the view through the telescope. Some nights are worse than others. At low power, the effect is usually not noticeable. But at high power, it can blur the image badly. Increasing the magnification only makes things worse; it becomes impossible to see any more detail. Instead, the image becomes fuzzier and fainter. SINCE BIG TELESCOPES HAVE TO LOOK THROUGH A LARGER COLUMN OF AIR THAN DO SMALL TELESCOPES, THEY ARE OFTEN MORE AFFECTED BY ATMOSPHERIC TURBULENCE, (ASTRONOMERS CALL THE CONDITION, "POOR SEEING").

"Many amateur astronomers find that about 300x and 1-3 arc sec. is the practical upper limit for any size telescope on most nights."

"ATMOSPHERIC TURBULENCE

On nights of poor seeing, turbulent air churning above the telescope can turn the view into a boiling confusion. When this happens, don`t bother testing or collimating. BECAUSE THEY LOOK THROUGH A LARGER VOLUME OF AIR, LARGE TELESCOPES ARE AFFECTED MORE BY THIS PROBLEM THEN SMALL ONES, MAKING IT DIFFICULT TO FIND A GOOD NIGHT TO TEST BIG INSTRUMENTS."

"On an otherwise wonderful, clear night, the deep sky observer can be plagued by rotten seeing that bloats star images and smears galaxies...the result is a loss of detail and a cutback of as much as a magnitude in the faintest stars visible. The faintest deep sky objects can be seen only when skies are clear and seeing is good."

And this is what expert author James Muirden states,

"Bad seeing affects different apertures in different ways, being more serious the larger the telescope."

"In bad seeing, there is greater general steadiness of the image in a small telescope."

"When the seeing is bad, the image will maintain it`s general outline rather better with a small aperture."

And here`s just a sampling of what notable amateur astronomers, web sites, pages, etc., have to say about it from ALL over the internet,

http://www.cloudynig...owto/seeing.htm

http://www.enerdynet...cs/bigger.shtml

http://www.astronomi...1&t1=3&cat_id=3

http://homepage.ntlw.../seeing2002.htm

http://uk.geocities....8/pickering.htm

http://webspace.oane...stro_seeing.htm

For those who didn`t read ALL the excellent info in Jack Schmidling`s article, (from right here on Cloudynights), here`s just a few excerpts,

"In general, the higher up in altitude, the better the seeing because there is less atmosphere to see through. This is one reason professional observatories are usually located high up on mountain tops. However, some sea level locations (southern Florida for example) can be nearly as good at certain times and some locations such as the Midwestern U.S. are nearly always bad."

"It is also important to know what the seeing is when evaluating a telescope. Many new scopes and their manufacturers get a bad rap for "lousy optics" when, in fact, the best optics on Earth could not produce a better view because of poor seeing."

"One fact little understood by purchasers of new telescopes is that the effects of poor seeing increase dramatically as the size of the telescope is increased. This is simply because a small telescope has to look through a much smaller column of air than a large one. A fairly good night with a small scope might be not worth taking out a large one. Pickering established his system using a 5" diameter telescope and his scale would have to be fudged when used with a scope of larger or smaller aperture."

"I have yet to see anything better than P-7 here which points out the fact that the 16" has yet to be used to it's fullest capability."

"when viewing the surface of Mars or the Moon, for example, no more detail can be seen on a poor night with a larger scope."

"The following is a summary of the seeing conditions at my location about 50 miles NW of Chicago since Oct 1997

Total Nights................ 213
Clear Nights............... ..71

P-1.....0
P-2.....9
P-3....26
P-4....21
P-5.....7
P-6.....4
P-7.....3
P-8.....1
P-9.....0
P-10...0"

P-1 to P-3 is very bad, P-4 and P-5 is poor, P-6 and P-7 is good and P-8 to P-10 is excellent. Thus; 35, (49%), of these nights were very bad; 28, (39%), were poor; 7, (10%), were good; only 1, (1%), was excellent.

His results are typical of our club`s and my viewing experiences here, at the places we`ve visited in the Northwest, Midwest, Northeast, middle and central Eastern parts of the country, etc.

Hope ALL this Helps. :D

Clear, STEADY Skies!



#45 jrcrilly

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Posted 02 February 2004 - 09:00 PM

SINCE BIG TELESCOPES HAVE TO LOOK THROUGH A LARGER COLUMN OF AIR THAN DO SMALL TELESCOPES, THEY ARE OFTEN MORE AFFECTED BY ATMOSPHERIC TURBULENCE, (ASTRONOMERS CALL THE CONDITION, "POOR SEEING").


Sorry; don't buy it. The column of air is the same thickness for all of us. What perpetuates this is the fact that larger scopes tend to hit magnification limits imposed by seeing long before they hit magnification limits imposed by their optics. Smaller scopes, which run out of available magnification much sooner, are able to reach their optical limts more frequently. Of course, on the same night, the larger scope will easily achieve the same magnification while also bringing in more light and resolution.

We just need to remember that larger scopes, even when limited to the lower magnifications imposed by seeing, are still larger.

#46 jrcrilly

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Posted 02 February 2004 - 09:29 PM

Quick question being we always seem to be trying to state that cheaper can beat quality preformance, i.e., a 10inch reflector of good quality can beat a i.e.. TAK or AP 4, 5, or 6 inch refractor.

First question is, what determines a good 10 Reflector?

Second Question is, what price is a good Reflector?


Hi, Charles.

I had to give my 4" TV APO plus cash for my 10" reflector but I think it's a good one and I'm glad I did it. The refractor was MUCH faster (F/5.4) and easier to mount for imaging but for visual use there's just no comparison.

A 5" APO (such as yours) would be very cool indeed but you've already seen what a poor imager I am - if I were gonna invest in something like that plus an imaging mount I'd spend the bucks on a huge, exquisite Dob instead and use it visually.

#47 erik

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Posted 02 February 2004 - 09:30 PM

i agree completely with what john said, it's easy to say that your refractor has a nice crisp image, if your only using 120x mag.and a larger newt might have a softer image at 300x, but during moments of good seeing, you'll see much more detail with the reflector.even assuming that a high end refractor can handle 75x per inch, that would mean a 4 inch refractor would top out at 300x, which is certainly impressive, but then it too would be affected by the seeing. and anything smaller than a 4 inch would be limited by the aperature, meaning you couldn't use as high a magnification.(also, from what i've seen, 75x an inch is pushing it on any scope, regardless of type or aperature). where i live, the highest useful magnification i use on planets is 240x.however, in some areas of the country i've been able to use a lot more. my 120mm achromat, by the way tops out at about 140x. -erik wilcox

#48 Guest_**DONOTDELETE**_*

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Posted 02 February 2004 - 09:43 PM

Hello John,

But if bad seeing is limiting scopes to 3 arc secs. and around 150x for ex., (which is often the case in many places, but many still find it worth going out to sketch, view dso`s, etc.), then wouldn`t it be true that both scopes will not be able to show any details smaller than 3 arc secs. no matter how much mag. up to around 150x is used? And wouldn`t the same thing be true with limits of 2 arc secs. and around 200x mag., etc., assuming of course both scopes are capable of reaching such resolution and mag. limits?

Besides ALL the experts, etc., and I were talking mostly about the image quality being better in the smaller scope which we`ve found to be the case at numerous places around the country as explained above.

Clear, STEADY Skies! :D

#49 jrcrilly

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Posted 02 February 2004 - 10:07 PM

Hello John,

But if bad seeing is limiting scopes to 3 arc secs. and around 150x for ex., (which is often the case in many places, but many still find it worth going out to sketch, view dso`s, etc.), then wouldn`t it be true that both scopes will not be able to show any details smaller than 3 arc secs. no matter how much mag. up to around 150x is used? And wouldn`t the same thing be true with limits of 2 arc secs. and around 200x mag., etc., assuming of course both scopes are capable of reaching such resolution and mag. limits?

Besides ALL the experts, etc., and I were talking mostly about the image quality being better in the smaller scope which we`ve found to be the case at numerous places around the country as explained above.


Hi, Dude.

I'll accept the possibility of seeing being so poor it limits both magnification and resolution to something within the limits of the smaller scope - of course, that still gives the smaller scope no advantage, just makes 'em equal for that moment. EXCEPT, of course, for light gathering.

Your second comment is more to the point. Some folks just like the way things look in refractors - that's subjective and I'd never argue with it. It's not necessary to justify it with rationalizations about smaller being better. After all, the serious APO guys I know use 'em up to 7" - I don't think they'd do that if they thought a 4" worked better.

Subjectively, I find the images in a 4" dim badly at 150X or more; perhaps my sight is just too poor for smaller scopes.

#50 erik

erik

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Posted 02 February 2004 - 10:35 PM

john, you brought up a good point about "the way things look" in refractors. there definitely is a difference in views, besides the obvious things you can point out and put your finger on. it's the same reason that i dont like the views in smaller SCT's or maks- to me the image just isn't as vibrant and "alive."saturn, for example, in my newt is tack sharp and almost white. the coloring is very subtle. i find that because the color is so subtle, it's easier to pick out details that are lost in my refractor, because in my refractor ,the color is much more obvious, and to me, too much so. saturn has a dark yellowish hue to it that seems to blend the details together.even obvious things like the cassini division aren't brilliant like they are in my newt.also,saturn in my newt has a 3-D quality thats definitely lacking in the refractor. of course, my refractor isn't an APO, and its f/5, so its not really a planetary scope, but i've noticed similar differences in other comparisons as well. i guess everyones eyes are different. -erik wilcox


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