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What can I see with a 80mm refractor?

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#51 CollinofAlabama

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Posted 27 June 2013 - 10:36 PM

The Focal Ratio is the ratio of the primary (in this case an 80mm lens) and the number of times the primary's diameter the particular scope build needs to bring the image to focus. 80 x 7.5 = 600mm where the focal length of 600mm divided by 7.5 yields 80, of course, thus an F/7.5 instrument. One critical point not mentioned above is that the scope I recommend, the Celestron 80ED, uses Extra Dispersion (ED) glass in its lens design, rather than the 18th century crown and flint design of the achromats. The achromats CANNOT bring all the rays of light to focus at the exact same plane. This causes something known as chromatic aberration, or CA for short. The practical effect of this is a purple haze, particularly around bright objects like planets, brighter stars, and the moon. This purple haze can be mitigated somewhat by making the focal ratio higher, so that an 80mm F/5 achromat has an almost kaleidoscopic amount of purple haze, an F/8 something less, an F/11 somewhat less still, and an F/15 even less

ED refractors, however, do much, much better at aligning all the various wavelengths of light to the same point. They're not perfect, but they are awfully close to perfect, and much better than ANY achromat, F/15 and especially below.

But besides throwing up much less artificially colorful images, consequently representing the target in "true", magnified color, ED scopes achieve this at much shorter focal ratios. As a former Maksutov owner (a type of catadioptric that is essentially a reflector-type, but by design with long, F/13+ like focal ratios), I can tell you that finding a target, especially a beginner without a go-to mount, is much more difficult, due to the absolute lack of corresponding field of view. The F/ratio of any telescope is intimately tied to its True Field of View. They are inverse, the higher the F/ratio, the lower the TFOV. And the absolute focal length of the instrument is critical to TFOV, also. There's simply no way to compare the experience of panning about the Saggitarian Star Cloud, or the Virgo-Coma Markarian Chain in instruments at F/15 as opposed to one with 600mm of focal length (in the C80ED's case F/7.5). But even finding Uranus, e.g., in an F/15 scope without go-to is a difficult task for an experienced observer, but for a novice? The 600mm focal length instrument, here the C80ED, will not be a cakewalk, but it will be about 100x easier.

And then we get to the ergonomic issues of tube length, wind sail, looking at zenith, etc. And for the record, I have ALWAYS employed chairs in my basic observation gear. Rather than a fancy adjustable one, I own two distinct chairs: one that sits rather low, and one that sits about "normal" height. But I have also had to employ my derrière on the ground when using F/9 and longer refractors, and only such long tube refractors have ever required such uncomfortable groveling in my decades of astronomical observations.

Again, all these are my preferences, and there are clearly Maksutov and long focal ratio refractor owners who don't feel this way (of course, most Mak owners would agree with me about the ergonomic issues since Maks tend to be even shorter than ED refractors). However, achromatic refractor owners would have to concede that my preferences are certainly the more popular ones. That most telescope owners have: newtonians, which in their incredibly popular dobsonian incarnation eliminate the straining near zenith issue with their focusers positioned near the top of the scope; ED refractors; and Maks, as opposed to long tube refractors, should demonstrate something. Perhaps your preferences, mrelliot, are more like mine, more like the majority's, and not like the strained neck, narrow field loving long tube refractor crowd. Just something to consider.

I will concede they are attractive to look at, a long tube refractor, almost like a work of art. Sadly, art doesn't help much when you're trying to put a target near zenith in your eyepiece, and the narrow field makes putting ANY target, regardless of its orientation, into the eyepiece that much more difficult.

So, given all this, I stand by my original recommendation of the C80ED for your first true astronomical telescope. It's a very good place to start. Buena suerte.

#52 Astrojensen

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Posted 28 June 2013 - 02:31 AM

Hi Colin

While I totally agree on your analysis of false color, I must respectfully disagree with you on a number of other points:

But even finding Uranus, e.g., in an F/15 scope without go-to is a difficult task for an experienced observer


This is absolutely not true by any stretch of the imagination. First, finderscopes are invented for a reason, second, a long-focus refractor is often long enough to act like a sort of pointer on its own. I've used a 85mm f/19 (1600mm focal length) refractor for many, many years and have always been able to find what I wanted to see (if it was within the reach of the scope, of course). For most things, I just aim along the tube, like down a rifle barrel. This more than suffice for bright stars and planets, and for dimmer targets I starhop.

And then we get to the ergonomic issues of tube length, wind sail, looking at zenith, etc. And for the record, I have ALWAYS employed chairs in my basic observation gear. Rather than a fancy adjustable one, I own two distinct chairs: one that sits rather low, and one that sits about "normal" height. But I have also had to employ my derrière on the ground when using F/9 and longer refractors, and only such long tube refractors have ever required such uncomfortable groveling in my decades of astronomical observations.


Funnily, I have only very occasionally needed to sit on the ground, while observing with one of my refractors, while on the other hand, I always find myself down in the grass, when I want to observe something low in the sky with my 12" dobsonian. And for most of the sky, I can't use my normal chairs with the dob, as the eyepiece gets too high in the sky. The dob needs a fancy, tall chair, the refractors don't. The dob is my most unergonomical scope by FAR. Compared to my 6" refractor, I find it very tiring to use, and I honestly consider selling it and getting myself a nice 8" f/12 refractor. It is also by far the most wind-sensitive scope I own, even de-shrouded.


Clear skies!
Thomas, Denmark

#53 Niklo

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Posted 28 June 2013 - 04:00 AM

Hi Colin,
I have not compared it myself but I heard from other people that the chromatic aberration of an cheap 80 ED and a Vixen 80L f/15 is quite similar. The ED should have a little less chromatic aberration but the difference should not be much.

I read from a good Zeiss AS 80/1200 which was tested as a half APO.
http://www.astro-for...n-echter-Hal...

Of course the Zeiss AS 80 is better than the Vixen 80L but it is not a ED refractor, too and has an achromatic design.

Nevertheless ED refractors have advantages.

Kind regards,
Roland

#54 Astrojensen

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Posted 28 June 2013 - 05:36 AM

A Zeiss AS is close to being an ED, as it already have twice as good color correction as an achromat of the same focal ratio.

The Vixen 80L has good color correction, but the 80mm f/7.5 Synta ED's are better. That doesn't neccesarily make them better planetary telescopes, however, as there is more to a good planetary telescope than just color correction.


Clear skies!
Thomas, Denmark

#55 CollinofAlabama

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Posted 28 June 2013 - 12:47 PM

Thomas, how much does the mount for your 6" scope weigh? Also, how high does all that weight stand? The Vixen Portamount, like Kenny has pictured above, is under 12 lbs and stands somewhere below yours, taking a wild guess. I believe if mrelloit finds a 10" dob off-putting, the large and heavy mount of your 6" refractor will not look appealing to him, either.

Additionally, I don't buy Thomas' apologetics of long tube achromats regarding the lack of TFOV in these scopes, especially as pertains to the use by a newbie. Of course, some will do better than others, but ALL would do better with an instrument with more TFOV, like the C80ED. There is no object you can name, even the moon, that will be easier to put into the eyepiece in an F/11+ 80mm refractor than the Celestron F/7.5 ED scope. Basic mathematics, no opinion involved. This is especially true for the newbie. If you've never seen the Eskimo Nebula in Gemini in a finder, you're long tube "gun barrel sight" is going to be much less useful than a shorter tube "gun barrel sight" with a wide field eyepiece in the focuser for that "wait a second" moment of catching the Eskimo for the first time in a low power eyepiece.

It is important for we more experienced astronomers to listen to the OP. He started the post. Let's try and address his needs above our own predilections.

#56 Astrojensen

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Posted 28 June 2013 - 01:29 PM

Hi Collin

The mount for my 6" achro is an EQ-6, on a 115cm tall Baader tripod. It's quite heavy, yes. However, I didn't mention it because I wanted the OP to run out and buy one, but as an example, compared to my 12" dob. Both are big scopes and I guess the 6" is actually the heavier, but I haven't weighed them, so I am not sure. I simply mentioned both, because I felt that your comment that a dob was always ergonomically superior to a long refractor was far too generalized and those were the scopes I had experience with and they were about equal in physical size. With my 6" refractor on its EQ-6 mount, I can survey most of sky, sitting comfortably on a normal chair. The dob needs a special, tall chair, and the eyepiece end moves around A LOT. I need to constantly move around, when I use the dob, while I only occasionally need to move the chair, when I observe with the refractor.

The lack of TFOV in long focal length instruments is both real and not. It is real, if we think in terms of what TFOV any given eyepiece will give, but if we think magnification, then the lack of TFOV is not there, once we are above a certain threshold. A 80mm f/15 at 30x with a 40mm 70° eyepiece will give a TFOV 2.33° across. A 80mm f/7.5 with a 20mm 70° will do exactly the same. And an awful lot of our telescopic observing is done at more than 50x. A short focus scope CAN give more TFOV, but at the magnifications we typically employ, it doesn't do so.

The perceived difficulties of aiming a long-focus scope are similar. Suppose you want to find Neptune. It's not visible with the naked eye from anywhere. With any scope of, say, 1000mm focal length or more, you'll want some kind of finderscope on it. Finding Neptune with the 7x50 finderscope on a 80mm f/15 scope is arguably easier than finding it with a 80mm f/7.5 that doesn't have a finderscope. If the 80mm f/7.5 also has a 7x50 finderscope, there'll be no differences. And in the main scope at any magnification, Neptune will look completely identical. You just need very different eyepieces to get to the magnification you want.

And yes, there is no object that is easier to put into a long f/ratio scope than a shorter one, but that is why we attach small, short f/ratio scopes to our long f/ratio scopes, to act as finders.

Physical size differences aside, a long f/ratio scope really is no more difficult to observe with, compared to a short one, and may have intrinsic advantages, making their use well worth the added hassle of a bigger mount and taller tripod.


Clear skies!
Thomas, Denmark

#57 csrlice12

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Posted 28 June 2013 - 02:22 PM

My dobs heavier then my refractor + EQ mount/tripod, but for quick "pull it out and look"...the dob will be up and running while I'd still be polar aligning the refractor. The weight differential isn't that much, and truthfully, speed of setup is more important for a quick look in the back yard then a few pounds weight difference. If I'm going to be in the back yard for awhile though, I'll use the refractor as it has tracking and I mostly view planets and the moon from my white zone back yard......

#58 Astrojensen

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Posted 28 June 2013 - 03:11 PM

My polar "alignment" consists of carrying the mount and tripod outside, plunk it down, roughly aimed at Polaris and that's it. I normally track manually, as if it was a sort of equatorial dobsonian. It's much easier to manually track with an equatorial than with an altaz. It's all I need at up to almost 400x.


Clear skies!
Thomas, Denmark

#59 mikey cee

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Posted 28 June 2013 - 05:40 PM

Hone your eyesight and observing technique for several years is my advice! Then when you finally get your 10" refractor you'll swear you can see canals on Mars!! :p Mike

#60 mrelliot

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Posted 28 June 2013 - 07:18 PM

Is it easier to learn the night sky with a dob or an 80mm refractor?

#61 azure1961p

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Posted 28 June 2013 - 08:13 PM

. If you've never seen the Eskimo Nebula in Gemini in a finder, you're long tube "gun barrel sight" is going to be much less useful than a shorter tube "gun barrel sight" with a wide field eyepiece in the focuser for that "wait a second" moment of catching the Eskimo for the first time in a low power eyepiece.

It is important for we more experienced astronomers to listen to the OP. He started the post. Let's try and address his needs above our own predilections.


I'm not sure that's a strong point. A low power eye piece for finding small planetary nebula? I'm more on board with at least 60x as a finder ocular and I never but never actually sight in on a planetary with my finder. I locate the star field, star hop at 60x-70x which even at that magnification still leaves many challengingly small. There's a lot to be said for star hopping skills and on invisible objects where only a field is known for finder purposes. I fear GOTO has a backlash effect of astronomers with weak skills in these areas. I also cannot sign onto the notion that Uranus is difficult for any experienced observer with any telescope. Nor Neptune for that matter. I like a lot of your points but have exception with a couple.

Pete

#62 CollinofAlabama

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Posted 28 June 2013 - 09:38 PM

On July 3rd, Venus buzzes the Beehive cluster. Sky and Telescope recommends a 30x view to start. The C80ED with a 25mm Plossl yields 24x in just over a 2* TFOV. In short, it'll be excellent, mrelliot, and you won't need anything else but the scope and a mount. Now you'll certainly need to add a 2" dialectic diagonal and more eyepieces over time, but there'll be time enough for all that later. Get the scope and get started. There are many places to start. But given your own predilections, I'd say get started wih the C80ED and a Portamount. You'll be glad you did. Good luck.

#63 BigC

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Posted 28 June 2013 - 10:17 PM

Is it easier to learn the night sky with a dob or an 80mm refractor?

The refractor has the advantage that you need merely to lift your eye from the ocular to see the same part of the sky ,alkbeit reversed in the scope view.I wonder if that is why many Japanese refractor users apparently didn't use diagonals?Note also many old pictures of refractors have no diagonal.

Although most ,including myself, do use diagonals these days(or nights actually;I have a correct image lens for daytime).

Perhaps a trifecta of 80mm f15 OTA carrying a 9x50 finder plus a Telrad ?

If I had to settle on just one refractor telescope I think my C6R with 8x50 finder because aperture rules!(Well,really gold rules,..but that is another story.)

#64 REC

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Posted 29 June 2013 - 11:11 AM

Great description and my 80ED is my most "used" scope. Look for one used for under $500.

Good luck and have fun!

Bob

#65 CollinofAlabama

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Posted 29 June 2013 - 11:48 PM

Great description and my 80ED is my most "used" scope. Look for one used for under $500.

Good luck and have fun!

Bob


I agree Jay Bird did a good job sizing things up earlier in the thread. Now, one note, REC writes of finding an 80mm ED scope used for under $500, but the Celestron 80ED I linked to in my first post in this thread is for $349 shipped, brand new with a Celestron 1 year warranty to boot! I have no affiliation with Celestron or this vendor. Just saw this great deal and thought I'd pass on the love. Happy 4th, mis estadounidenses.

#66 REC

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Posted 30 June 2013 - 07:30 AM

Wow, if you can find that Celestron 80ED for new at $350 shipped, I'd grab that. I paid that for mine used, great performer!

Bob

#67 stevenf

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Posted 30 June 2013 - 03:06 PM

I thought that Celestron 80ED was discontinued (I would love to have one and have been looking for a used one)so I'm just a little suspicious of this ad.

edited to add: ha, but not so suspicious that I didn't order it anyways after looking into it more :) Awesome deal!

#68 mrelliot

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Posted 30 June 2013 - 04:16 PM

On Celestron's website, it says that the C80ED is discontinued.

#69 CollinofAlabama

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Posted 30 June 2013 - 04:23 PM

Stevenf,

I think you're right, it has been discontinued, but Astronomers Without Borders (never heard of them before now) is indeed selling them for $349 shipped, new! That's why I wrote its the best deal in refractors right now, cause it is. I think Celestron did some deal with them regarding excess, discontinued inventory, because they're not being produced any more, but they do come with a manufacturers warranty and are one fantastic deal. This is why I've recommended it so to mrelliot.

#70 stevenf

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Posted 30 June 2013 - 04:32 PM

Thanks very much for pointing out that deal Collin. My first ED refractor! And free shipping to Canada! I already can't wait to get it into my hands :)

#71 CollinofAlabama

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Posted 30 June 2013 - 04:45 PM

Mrelliot,

See my above post. You're right, they are no longer being produced and are, in fact, a discontinued item. For that matter, my AT80ED is a discontinued item, too, but every night I take it out, do you think it performs more poorly because of that fact? The C80ED was a great scope and it's still being sold while supplies last by Astronomers Without Borders, with a Celestron warranty. Once they're gone, they shall not be sold new anywhere else again.

However, the CN sponsor, Astronomics, does sell the AT72ED, with a 2 year warranty, and it's still in production as far as I know. Although this one may not have quite the aperture grasp of the C80ED, it will have an even greater TFOV and comes with a hard aluminum carrying case, which is nice. And it only costs $30 more, so still a good deal if you prefer something still in production.

In early 2008, my wife had just gotten her PhD, a new management job, and was looking for a new car. We accidentally drove into the local Mercury dealership (she was absolutely convinced she wanted a Nissan, Toyota, Mazda, etc, because "Japanese were better"). We had a Corolla budget, and we test drove a used Camry with 60000 miles on it. But, finally, the salesman got her to try a 2007 model Mercury Milan (same as the Ford Fusion), they had on the lot because it was a standard and we live in a lazy part of Texas where the locals don't want to be bothered by such, but my wife and I are completely comfortable driving. She fell in love with it. As we test drove the Corolla, Sentra, Hyundai-style cars in this class, she was struck by the price and features of the last year's model Milan. Five years later, she is convinced that when we replace my car, it should be with a Mercury! The C80ED from AWB is an excellent, rare-find deal.

#72 mrelliot

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Posted 30 June 2013 - 05:00 PM

I don't really care if a scope is in production or not, there just harder to find then new ones.

#73 CollinofAlabama

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Posted 30 June 2013 - 05:50 PM

Stevenf,

Glad I could help. I remember a few years back when Meade blew out their 80mm doublet line for a similarly low priced offer -- I told all our Club members about it and the President picked one up. He's been very happy with it since. These deals come along once in a blue moon. Fortunately Astronomics has the AT72ED so even if someone misses the killer deal, this one is always available, and it has its own charms, indeed. Glad things have worked out for you, tho, Stevef.

#74 pdxmoon

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Posted 01 July 2013 - 02:18 PM

Kinda like this one,Collin? :shocked:


Kenny, what is this scope?

#75 pdxmoon

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Posted 01 July 2013 - 03:08 PM

Can you point me to some links as to what I'd need to mount the The C80ED on a tripod? I'd rather use an altar than an eq. I'm interested in this package as a lunar grab and go scope. (Also I'm a newbie, so links and pictures help!)






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