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ED120 - Will It Outclass Bigger OTA's?


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Just wondered, would a quality doublet such as the ED120 outperform a larger aperture scope of medium performance on DSO's?

I already know that the ED120 is rather good on lunar and planetary subjects.

But will the high quality of the objective result in a brighter and more defined DSO image when compared to say a 150mm Newtonian?  Or is physics physics and the larger aperture will always win?

I might end up with just one telescope in the future and I was thinking about this refractor.

Thanks for looking - Steve

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all threads like this confirm is that different scopes suit different folks and for different reasons. enjoy the one you have and don't worry about the one you haven't.

I'm no expert and have had absoluetly no experience with the ED120 but I imagine the 4.7" apochromatic frac would give a 6" newt a good run for its money. With no central obstruction the apos performa

My experience with the ED refractors vs. achromatic refractors is that you can always push the ED over the limit and still get a decent result in terms of sustained resolution and magnification. For i

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My experience with the ED refractors vs. achromatic refractors is that you can always push the ED over the limit and still get a decent result in terms of sustained resolution and magnification. For instance, the 80ED - while stated by the manufacturer it has a practical magnification limit of 160X, I pushed it to over 200X and the result was satisfactory. On the other hand, the ST80 soon got blurry once you hit the 150x, even with lower mag the resolution wasn't near the ED.

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I'm no expert and have had absoluetly no experience with the ED120 but I imagine the 4.7" apochromatic frac would give a 6" newt a good run for its money. With no central obstruction the apos performance would probably be better on most objects such as tight doubles, planets, lunar work, open clusters and globs. If your main interest in astronomy was lunar, white light solar and planetary observation, within reason this would be a very beautiful instrument to own.

But for the same price as the ED120, for example, you could get a 10" to 12" dob and I feel these instruments are going to show more total detail than anything in the 5-6" range. There's no fairy dust that comes with refractors, for it just seems to boil down to basic math - all things being equal, aperture will rule.

So, in my mind what you're looking at is not aperture but other factors when coming to choose. My own 4" frac is lighter than the 10", cools down fast, never needs to be collimated and has less coma due to the slower F ratio so I don't need to buy premium eyepieces to get a flat field of view. On the other hand, the 10" is easy to collimate, shows so much more detail on everything I look at and doesn't place me in strange and uncomfortable positions when viewing from it.

Having a 4" refractor and a 10" reflector, I prefer the views through the reflector, they're brighter, more pronounced and offer up so much more detail. But the refractor is convenient. On those nights when the moon is out and when the sky is not dark enough to take full advantage of the 10" it is the telescope of choice. When I want to view the Sun which happens almost everyday, again, it is the telescope of choice. And if I happen to suddenly wake up in the early morning and fancy a quick peek at the night sky, well, the frac, again, is the telescope of choice.

If it were me, I'd try to get both scopes :p . One for performance when you travel out to dark skies and one for convenience and those sessions around home.

Edited by Qualia
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A 6" newt (especially at F/8 or slower) is better than a 120mm apo at DSOs. Figure of the mirror, reflectivity of the coating do play a role, but a 150 mm aperture catches 56% more light than a 120mm one, and that is hard to compensate for. My 6" F/8 newt had slightly under 20% central obstruction by aperture, equalling less than 4% by area. It had 1/10th lambda optics, and would give any 5" apo a very good run for its money, even on planets and the moon.

My 8" SCT trounces a 5" apo on DSOs visually. I can see far fainter galaxies with it (down to mag 13.3 so far, and a 13.9 quasar). Faint DSOs primarily need lots of light. It helps to concentrate the light properly, but the first concern is the size of the "flux bucket", i.e. the aperture.

Where smaller, faster apos shine in DSO work is in wide field objects. My views of the Veil, Pleiades, M24, M31, and M33 through the little 80mm APM are stunning. M8, by contrast, was way better in the C8 than in the 80mm. Markarian's Chain, is also a region which needs aperture to really appreciate it. I would love to see that through a 20" Dob like Olly's "Sir Isaac". The optics might not be anywhere near the class of his TEC 140, but the views of faint DSOs are just amazing.

BTW: I traveled with both scopes (plus the Lunt, plus the bins, long live the Espace :D), because the C8 packs 8" aperture into an unbelievably small and light package.

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I have a 102ED and 152mm Achro (improved version), and considering the aperture of the latter you'll certainly notice a difference. Like Qualia said, a 4" refractor is pretty useful if you want a  compact set up that combines good views, portability and convenience. I had an 8" reflector and 11" SCT, I can say that the reflector somehow provided slightly brighter images than the SCT. having said that, at high magnification, considering the size of aperture and seeing condition I found that the 6" refractor showed sharper stars. That's just my experience.

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Where smaller, faster apos shine in DSO work is in wide field objects. My views of the Veil, Pleiades, M24, M31, and M33 through the little 80mm APM are stunning. M8, by contrast, was way better in the C8 than in the 80mm. Markarian's Chain, is also a region which needs aperture to really appreciate it. I would love to see that through a 20" Dob like Olly's "Sir Isaac". The optics might not be anywhere near the class of his TEC 140, but the views of faint DSOs are just amazing.

I have to agree with the views above. But also curious, Michael, have you compared a 8" Newtonian with 8" SCT at all?? 

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I have to agree with the views above. But also curious, Michael, have you compared a 8" Newtonian with 8" SCT at all?? 

Visually they are on a par for DSOs, except that the faster newt can show more of the sky at once. My SCT will show 1.34 deg at most, whereas I could easily get 2.11 deg out of an 8" F/6 Newt with 31T5 Nagler. The smaller central obstruction of the Newt theoretically gives it the edge over the SCT, but that amounts to about 0.055 magnitudes, which is not much. On planets there should also be a small difference with the Newtonian having the edge. This is a bit more noticeable, but for me does not outweigh the size issue.

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As an owner of an ED120 I'd agree with most of the comments made here. On the moon, planets and binary stars the ED120 performs as well as a centrally obstructed 6" scope including a 6" mak-newtonian which has a very small obstruction and I had the opportunity to compare the ED120 with at some length.

On deep sky objects aperture rules it's as simple as that. My ED120 has shown me DSO's very nicely for it's aperture and even managed to pick up a mag 12.3 supernova in a faint galaxy (though that was a point source of course rather than an extended object) but an 8" dob will show quite a lot more of most DSO's as a rule.

If your viewing is mostly lunar and planetary with just the occasional bit of DSO browsing I guess the ED120 could serve well as an only scope but my strong personal preference would be to have a 8"-12" dob to run alongside it :smiley:

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Thank you everyone for your excellent replies.  Very helpful, I understand that the laws of physics cannot be bent or broken.

To be honest, I'm thinking now (as the ED120 is expensive) that there's no need for me to get one.  I have a 90mm refractor (albeit an achro) for grab n go, then I have my 6" F8 Newtonian for lunar and planets.  So all I need now is a large aperture dobsonian...or perhaps a large aperture SCT.....hmm....

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Regarding the comparison between a 6" F/8 Newtonian and an 8" SCT on planets: the 8" shows finer detail, because it has more aperture, but the 6" F/8 has a bit more contrast for a given exit pupil (say when comparing an 8mm EP in the 6" to a 10mm EP in the F/10 SCT). Likewise, I much rather use my 8" SCT on planets than my 80mm frac. The difference is similar (but much bigger), in that at a given exit pupil, the 80mm gives a much crisper, but much smaller image that the C8. At the same magnification (OK, 119x vs 120x) the balance started to shift towards the C8, which gave the much brighter and sharper image, and at 200x there is no contest (under good skies). In the same vein, I would expect the 120 ED would be outperformed on planets by a 12" dob (assuming good figure of the mirror) by a similar margin.

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Since most of what has been said agrees with this little article. I'll post it again. Not the first time I posted it, but ever so useful. I almost think that article would be useful as a sticky somewhere in the getting started general help  and advice  section, since this type of question comes up often, or what can be seen in scopes in general  :smiley:

https://www.astronomics.com/what-can-you-expect-to-see-in-a-telescope_t.aspx

Edited by AlexB67
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Since most of what has been said agrees with this little article. I'll post it again. Not the first time I posted it, but ever so useful. I almost think that article would be useful as a sticky somewhere in the getting started general help  and advice  section, since this type of question comes up often, or what can be seen in scopes in general  :smiley:

https://www.astronomics.com/what-can-you-expect-to-see-in-a-telescope_t.aspx

Thats generally a useful piece  :smiley:

It's worth noting the caveat used at the beginning of the section on the 10" or larger instruments:

".....during infrequent excellent seeing conditions...."

I reckon as a result of the above, the ED120 gets surprisingly close to the 12" dob on quite a lot of occasions on lunar and planetary detail and binary star resolution. It's certainly not "blown out of the water" despite the 12"'s massively larger light grasp and potential resolution.

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Agreed. I suppose it would be better structured if it was mentioned at the beginning of that article, making a little note on the influence of atmospheric conditions and how it affects larger aperture, and then sort of generalising to ideal observing conditions for all different aperture sizes.

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You certainly need good atmospheric conditions to get the most out of aperture. The difference between the potential differences between a 120 ED and a 12" scope can be seen in planetary imaging, where there is no question about which scope shows more detail. Whether the human visual system can extract that detail is another matter entirely, especially with our prevailing seeing conditions.

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In retrospect, I have always thought I should have bought a Celestron C8. lol. But the "journey" has been interesting. :p

The only thing I always wonder about, is the variation and quality of SCTs. On CN particularly, there was always talk by people who had had been through three or four... until they got a "good one". Who got lumbered with their "seconds"? It is anecdotal that SCTs are only about 1/4 wave though? But then the USA is a different world - Dealers happily exchange telescope on a user whim, money no object to SOME people etc. [teasing] ;)

I often wonder how a top quality (certified optics) F10 Russian Maksutov would stand up against all the rest of these. Granted cool down time, but in an observatory context? Maybe one day, I'll sell my soul (there is little else left. lol!) for an Intes Alter M703

http://www.teleskop-express.de/shop/product_info.php/info/p2509_Intes-Alter-M703-DeLuxe---180-1800mm-Maksutov-Cassegrain.html

Edited by Macavity
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no doubt the frac will provide tighter stars and nice wide field views but other than that the 6" newt will be better on all other objects. anything larger will have an increasingy narrow field but give brighter images of faint objects with more resolved detail/stars. if you are looking at a newt of 12" or more than an aperture mask can create the same kind of images as a good refractor - my 12" can accommodate a 110mm f12 'dobfractor' which gives stars just as tight and planetary / lunar detail just as contrasty as an average to good quality refractor of similar aperture

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Just wondered, would a quality doublet such as the ED120 outperform a larger aperture scope of medium performance on DSO's?

I already know that the ED120 is rather good on lunar and planetary subjects.

But will the high quality of the objective result in a brighter and more defined DSO image when compared to say a 150mm Newtonian?  Or is physics physics and the larger aperture will always win?

I might end up with just one telescope in the future and I was thinking about this refractor.

Thanks for looking - Steve

Steve , as said here many times , aperture rules for deep sky , that's it .

But as a few here have said a  good 120mm ( 5 inch )  refractor  will beat a 150mm  Newtonion easily and give a 200mm  a run for its money on the moon and the planets on bad nights .

How do I know this ?

I own both a sweet Istar 127mm f8 refractor and a beautiful Takahashi Mewlon 210 DK , and yes the 210mm's of the Tak goes deeper , with ease , but fine details are seen in my 127mm refractor  on the moon and planets  that are easily seen in the Tak , but , then  the 210mm shows then easier , just !. :cool:  , confused ?

Its a hard call , on a bad night the 127mm rules , but a good night the 210mm is easily better , so its a toss up .

Aperture does rule  on deep sky ,and that's a fact .

But for lunar Planetary viewing , that's a different kettle of fish . :grin:  .

Brian.

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If it helps (unlikely) I just sold a 10 inch OO DOB to fund a new ED120 , I have no regrets, none at all. I tried masking down my DOB in a vain attempt to turn in into a pseudo refractor, it doesn't work, the final straw for me.

Ultimately you like what you like when all is said and done.

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Not too sure about the merits of the 120mm ED having never owned one but I can comment on how a range of optics compare as I own a 10" Dob that's F6.4 so it's no slouch on the planets and takes magnification well, I have a nice early 4" Tal which is very nice on the planets and shows very little CA except on the very brightest objects and finally the best of the bunch my 7" Intes Mak-Newt which is as contrasty as any frac i have used and is no slouch on deep sky either. The views that the Intes throws up once it has fully accliamtised is pure brilliance, have you considered a Russian lovely when considering the ED120 as the focal length of mine is only slighter over a metre and with its Moonlite focuser it is proving a decent performer (for a total novice) for planetry AP.

Just my tenpence worth! :grin:  :evil:

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In retrospect, I have always thought I should have bought a Celestron C8. lol. But the "journey" has been interesting. :p

The only thing I always wonder about, is the variation and quality of SCTs. On CN particularly, there was always talk by people who had had been through three or four... until they got a "good one". Who got lumbered with their "seconds"? It is anecdotal that SCTs are only about 1/4 wave though? But then the USA is a different world - Dealers happily exchange telescope on a user whim, money no object to SOME people etc. [teasing] ;)

I often wonder how a top quality (certified optics) F10 Russian Maksutov would stand up against all the rest of these. Granted cool down time, but in an observatory context? Maybe one day, I'll sell my soul (there is little else left. lol!) for an Intes Alter M703

http://www.teleskop-express.de/shop/product_info.php/info/p2509_Intes-Alter-M703-DeLuxe---180-1800mm-Maksutov-Cassegrain.html

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Not quite!  In my view, dark sky rules for DSOs, though aperture certainly helps

Exactly. If we can transport our 120ED to a dark sky, but we cannot move a large Dob away from LP. The frac will outperform it.

Even very modest apertures from dark skies will outperform very large ones under these conditions. I've used a 120ED from a dark site and can say it gave way better results from a dark site than my 10" does from my hideously LP back yard.

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In retrospect, I have always thought...

And then... [no comment] :p

Please put my previous response down to past frustrations. I do think there is a LOT to

be said for thinking about purchases beforehand. Reading as much as possible too...

Noone gets (stays) "rich", buying / selling (modding) stuff, after but notional usage etc. ;)

Things (scopes, eyepieces, mounts) sometimes really do GROW on you. After a while.

What "works" is [iMO] much more governed by physical / financial fitness. Location etc. 

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