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SW 120ED - Expecting too much?

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#1 brisguy

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Posted 01 January 2021 - 06:10 PM

I recently purchased a SW Evostar 120ED and have been trying it out. After reading some reviews, I have to say I am a bit disappointed. I live in a highly light polluted area, so I have stuck to planets and the moon so far and have only used the stock equipment (EP and diagonal). However, I read reviews that stated how low the chromatic aberration (CA) is and for instance how much detail the reviewer could see of Mars (like polar caps). I can't see any detail on Mars, using the 5mm EP (180 X), but I do see plenty of CA. I would say it takes up 5-10% of the object diameter with the red on one side and blue on the other.

 

I could provide pics, but I only have a cell phone adapter and phone camera to use right now. So they a pretty blurry.

 

On the plus side, on good nights I can see bands on Jupiter and Saturn and make out the dark band in the rings. I also tried viewing without the diagonal since I read that is is not top-notch and I did see a slight improvement in the CA.

 

I realize this is a fuzzy question, but am I expecting too much from this 'scope?


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#2 junomike

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Posted 01 January 2021 - 06:15 PM

Mars is not an ideal object (neither is Jupiter or Saturn currently). You need to hit it near opposition to see the greatest detail.

Also, Red on one side and Blue on the other is most likely Atmospheric Dispersion which is not the blame of the telescope.


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#3 Orion92

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Posted 01 January 2021 - 06:19 PM

Stan -

 

I have the smaller sibling to your scope, the 100ED. The entire ED line by Sky Watcher is quite solid. I have not heard of much CA in your scope but it is a faster scope than my 100 ED so it's possible to get a little. Red on one side and blue on the other sounds like atmospheric dispersion to me and not CA.... Mars is far past opposition/prime viewing. It is very tiny right now, do not have high expectations. Saturn and Jupiter are also very low and caught up in the lower atmosphere with poor seeing. You will need to wait until late Spring/Summer for those two to be in better positioning. 

 

Besides the planets, the Moon, double stars, and brighter deep sky objects are your best bets for urban observing. Take a look at the Astronomical League's Lunar 1 program. I just completed it a couple of months ago and had a blast getting to know our closest satellite. In fact, I no longer loathe the Moon, but welcome its' return.

 

Clear skies,

 

KP


Edited by Orion92, 01 January 2021 - 06:20 PM.

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#4 Jethro7

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Posted 01 January 2021 - 06:29 PM

Hello Stan,

Junomike, posted first, and is absolutly correct. Dont blame the scope these SW Evostar scopes are well thought of. As far as expecting too much that all depends on what you are trying to view. Lunar, planetary, bright DSO's, bright Starclusters and splitting double stars are what refractors do best and refractors are my favorite genre of scopes. You will need aperture to go after DSO's. Many contributors here on Cloudy Nights have a combination of telescopes to cover their viewing needs. On your next session take a trip to our Moon and M42 in Orion. I think you will like it. There are lots of objects that are with in reach of your telescope. The real beauty of a nice refractor is that they still shine when sky conditions are not good enough for the light buckets.

 

HAPPY SKIES AND KEEP LOOKING UP Jethro


Edited by Jethro7, 01 January 2021 - 09:05 PM.

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#5 martym

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Posted 01 January 2021 - 06:31 PM

How old are you?  You have to remember that your eye is part of the optical system. When you are trying to view something through a .5mm exit pupil you are not going to get the same satisfaction

that you may have had in your early 30's.  Your eye is just as much a part of the optics as the rest of the scope.



#6 John Huntley

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Posted 01 January 2021 - 06:38 PM

I saw some detail on Mars a couple of nights back with my ED120. It's well past it's best now though. The south polar cap has shrunk to the point of being very difficult to discern at all just now. Back around the opposition (mid October) the ED120 gave me some excellent and detailed views of the surface features over a number of nights. The scope could easily handle 250x - 280x on this target.

 

The only false colour that I could see was being created by atmospheric dispersion as noted by previous posters.

 

Jupiter and Saturn have been OK here (the UK) but very low down so contrast and detail is often scrubbed off by having to view though more of our atmosphere. The ED120 does the best it can but does not perform miracles with the planets in these positions.

 

Give your ED120 more time to impress - I'm sure it can, and will !


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#7 Jeff B

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Posted 01 January 2021 - 06:55 PM

I suggest looking at some of the brighter stars that are at least 60 degrees above any horizon with your 6mm eyepiece at focus and slightly in and outside of focus and tell us what you see.

 

Jeff


Edited by Jeff B, 01 January 2021 - 06:57 PM.

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#8 CHASLX200

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Posted 01 January 2021 - 06:55 PM

My SW120ED would do 350x easy on the planets. No planets to look at at this time other than a smaller Mars.  Jup and Sat are way to low now. Venus is low in the AM.


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#9 Jeff B

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Posted 01 January 2021 - 07:15 PM

My SW120ED would do 350x easy on the planets. No planets to look at at this time other than a smaller Mars.  Jup and Sat are way to low now. Venus is low in the AM.

Uranus is very well placed right now!  grin.gif



#10 CHASLX200

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Posted 01 January 2021 - 07:20 PM

Uranus is very well placed right now!  grin.gif

I don't think i have viewed that planet more than 3 times since 1977.  Wished i could swap planets around and place it where Mars is.  That would be a big blue disk.


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#11 KTAZ

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Posted 01 January 2021 - 07:43 PM

I suggest looking at some of the brighter stars that are at least 60 degrees above any horizon with your 6mm eyepiece at focus and slightly in and outside of focus and tell us what you see.

 

Jeff

Jeff beat me to it.

 

The moon is very bright and will ride pretty high close to midnight. I suggest you take a look along the edges of the nearly full moon when it is high, near the meridian, and check for CA. Living in a highly light polluted area doesn’t help, but any object viewed a lower elevations will show the classic signs of atmospheric dispersion.

 

Check out the Pleiades when it is high as well...you will be impressed.


Edited by KTAZ, 01 January 2021 - 07:43 PM.


#12 aa6ww

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Posted 01 January 2021 - 07:46 PM

Just learn the constellations and whats in them. The few planets and the moon will have very very very little **** appeal once you know whats in the constellations.

The most difficult part of this hobby isn't the equipment, it isn't the condition of the night skies, its knowing whats out there to look at.

Finding any object is easy, especially with GoTo technology, but you can only find what you know is out there. 

I run into numerous people at star parties who have thousands of dollars in equipment, then when the skies turn dark, they have no idea what to look for.

Keep a notebook handy and make a list of whats in each constellation as you find them and learn about them and before you know it, you can easily spend an entire night just looking at all the objects in one constellation.

 

Ralph

 

 

 

I live in a highly light polluted area, so I have stuck to planets and the moon so far and have only used the stock equipment (EP and diagonal).

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Edited by aa6ww, 02 January 2021 - 05:13 AM.

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#13 BillP

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Posted 01 January 2021 - 08:42 PM

However, I read reviews that stated how low the chromatic aberration (CA) is and for instance how much detail the reviewer could see of Mars (like polar caps). I can't see any detail on Mars, using the 5mm EP (180 X), but I do see plenty of CA. I would say it takes up 5-10% of the object diameter with the red on one side and blue on the other.

Hmmm.  If on-axis, the planet is showing different colors on either side of it, then that is likely not axial chromatic aberration from the main optic.  If it were you should only see one color around the rim of the planet, most likely a dim blue/purple.  What you explain sounds more like either atmospheric dispersion or a poor eyepiece.


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#14 drd715

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Posted 02 January 2021 - 01:01 AM

I suggest looking at some of the brighter stars that are at least 60 degrees above any horizon with your 6mm eyepiece at focus and slightly in and outside of focus and tell us what you see.

Jeff

When you do this "star test" as close to straight up as practical the star should be as nearly one single color - visually white. If you see red on one side and blue on the other side it is possible that the two lens elements in the objective could have shifted (not on center with each other) and become miss aligned during hard handling shipping. While this is a possibility it is not a common problem. The star test will also show imperfections in the optics and columation deficiencies. Look up star testing an astronomical telescope online for help interpreting what you see.

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#15 Wildetelescope

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Posted 02 January 2021 - 09:48 AM

Isn’t that also what you would see with atmospheric dispersion? Red on one side blue on the other? Also intermittent flashes of color could be seeing induced. For me, a constant purple or green rim on the limb of the moon or planets, or in high contrast regions on the moon are what I look for. If I do not see color there, it is not worth worrying about for visual use for me.

Jmd

When you do this "star test" as close to straight up as practical the star should be as nearly one single color - visually white. If you see red on one side and blue on the other side it is possible that the two lens elements in the objective could have shifted (not on center with each other) and become miss aligned during hard handling shipping. While this is a possibility it is not a common problem. The star test will also show imperfections in the optics and columation deficiencies. Look up star testing an astronomical telescope online for help interpreting what you see.

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#16 Reid W

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Posted 02 January 2021 - 10:01 AM

Stan- what is the diagonal you are using?



#17 drd715

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Posted 02 January 2021 - 10:46 AM



Isn’t that also what you would see with atmospheric dispersion? Red on one side blue on the other? Also intermittent flashes of color could be seeing induced. For me, a constant purple or green rim on the limb of the moon or planets, or in high contrast regions on the moon are what I look for. If I do not see color there, it is not worth worrying about for visual use for me.

Jmd


Atmospheric dispersion is primarily a low angle of viewing phenomenon. When testing using a star nearly straight overhead any "color" is most likely the optics however bad "seeing" flickering dancing stars will show rapidly changing color effects due to a turbulent atmosphere.

Colors around a bright star such as an even puple halo are optics related and particularly notable in an Achromatic refractor. The thin "green" rim on the edge of the moon is usually an artifact of a lower focal ratio Achromatic design. Higher quality ED or APO scopes should not show this effect.

If a lens system is decentered - the optical center of each separate lens not lined up with the corresponding lens, this can cause a star to show lop sided color side to side (all the stars in the image would show this effect across the field).

Hopefully modern lens cells are better than in years past (with centering adjustment screws). Some years ago when a sxope arrived and it showed lots of color on a star the manufacturer recommended slightly loosing the retaining ring on the objective cell and tapping the cell to recenter the lens elements (scarry to me - not reccomend) in the hopes it would come into better alignment. Kind of hit or miss.

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#18 Jeff B

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Posted 02 January 2021 - 10:53 AM

Stan, ship it to me and I'll test it for you.  If interested, private message me.

 

Jeff



#19 brisguy

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Posted 02 January 2021 - 12:26 PM

Thanks all for the great responses. This makes me feel a lot better. I DO know my way around the night sky (or at least I used to) and I have a sky view app for my phone to jog my memory when needed. The light pollution is so bad (Bortle class 7) that it can be hard to get oriented though. I do know enough to know that I can't expect much from objects near the horizon. I first looked at Mars in November when it was almost as big as Saturn and I think it was pretty high, but can't remember. It certainly is now - almost directly overhead - but smaller. I did not realize that atmospheric dispersion could cause those effects but suspect that must be the issue. I did not mention it before, but I do not notice the blue/red blur on stars or the moon. I have taken a few moon pics (with the cell phone setup) and only notice a slight purplish halo at the edges, and then only when zooming in on the picture, so I am guessing that is the residual CA, which is perfectly acceptable.

 

The sky conditions here vary greatly and it is hard for me to tell when viewing will be good or not. But I have noticed some nights are much better than others once I peer through the eyepiece.

 

It sounds like some are suggesting I check the collimation using a star test. I have not tried that yet, but will when the skies clear up. Someone asked about the diagonal, and all I can say is that it is the one that came with the scope and is a mirror, not a prism. As mentioned, I have tried viewing with this removed and saw a slight improvement in detail.



#20 Jon Isaacs

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Posted 02 January 2021 - 12:52 PM

Stan:

 

I don't know where you're located, that would help gauging seeing and expectations.

 

What I can say is that I have an Orion 120 mm Eon, it uses the same Skywatcher 120 mm F/7.5 FPL-53 doublet as your scope, it's just in a different tube assembly.

 

For me, it does not disappoint.  When the seeing allows it, and in San Diego quite often it does,  it's an excellent planetary and double star performer. I see no CA though I'm older so my blue sensitivity has diminished. 

 

This photo is an example of atmospheric dispersion on Venus with another scope. I'm not sure which scope I used but I believe it was a Newtonian which is free of CA.

 

3790232-Venus Through the Eyepiece.jpg
 
Jon

 


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#21 Jeff B

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Posted 02 January 2021 - 01:34 PM

Stan, a few points and cautions, most of which can be easily checked off with observing.

 

1. I always caution people against using the moon's edge to judge CA.  People typically place the edge well off axis, towards the edges of the FOV.  In that case you will see a color fringe at the moon's edge, even with a newtonian, but fer-sure with an ED doublet (and even triplets for that matter).  What you're seeing is lateral color error, where the various colors, while perhaps still in focus with each other, have spread out from each other, much like what you saw a prism do back in high school science class.  ED doublets have lateral color off axis.  So do eyepieces, which use refractive glass too.  So does your eye, big time (for example, go ahead and put the moon's edge out close to the field edge, see the fringe, then move your eye around.  You will see that fringe change in intensity and color.  

 

2. I consider atmospheric dispersion (AD) a form of lateral color.  Again, on Mars, move your eye around a bit and you can "modulate" the AD to a degree.

 

3. A bum mirror diagonal will show up as astigmatism, oval star shapes that flip 90 degrees as you sweep through focus at high power.  If bad enough, you will see a "+" pattern to the image at focus.  Check for that.

 

4. If an otherwise well made and put together objective is out of "collimation" and/or the focuser too, you will be looking off of the axis of "best optical correction".  With an objective lens, you will see astigmatism and maybe some lateral color too but astigmatism will be the obvious one if your collimation is/are really bum.

 

5. If you see coma at focus in the star test, that is typically a dead give away that the two lens elements are not properly centered on top of each other.  Check for that.

 

So, this is why I suggest you have a look at some stars at high power and tell us what you see.  I hope you see "nothing".  But if you do see "something" associated with the above, tell us about it.

 

Jeff  


Edited by Jeff B, 02 January 2021 - 01:37 PM.

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#22 belgrade

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Posted 03 January 2021 - 06:42 PM

A common misconception in amateur astronomy which leads to likely thousands of unused, dusty and forgotten telescopes is that views will rival pictures... or our dreams. Never happens - even with a 25”+ dobs in pristine skies of Davies Mountains and Big Bend.

There’s nothing wrong with your scope - btw, I have the same - it’s one of the best and reasonably affordable ED/APO doublets ever. Enjoy it as prior posts suggested. Try lunar, solar with a filter, and quite a few brighter deep sky objects as well as planets where positioned well. And don’t stress, you’ll do just fine!

#23 Gary Riley

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Posted 03 January 2021 - 09:52 PM

I have that same scope and it gives excellent views when sky condition allow. As has already been said, Mars is getting too small now to show much, if any, real detail. Jupiter and Saturn are getting lower and lower in the western sky and this will greatly affect your views. Use that fine scope on the Moon, brighter open clusters, double stars, and the brighter nebulae. It is a keeper IMO.
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#24 Thomas A Davis

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Posted 03 January 2021 - 10:15 PM

I've owned a Skywatcher 120ED for about 5 years now, and found it to be an excellent scope.  In the past I've owned two excellent AP refractors (130EDT and 180EDT), and while not up to that level of correction, I have not been the least disappointed with the performance of the scope.  It has a bit of bleed in the red end of the spectrum, but not very much.  It does not show any blue fringing at all, even at very high magnifications.  Compared to a 1985 vintage C102 Fluorite I owned, I find the 120ED to be at that level of performance.  I have had good success with it for both planetary with a ZWO I224MC and deep sky imaging with the .85X reducer/corrector and my Canon  6D camera.  Bad seeing can make even the best of scopes seem to perform poorly.  I remember the first time I used the C102F, Jupiter looked like it was under water.

 

Just as a reality check, here is an image of Mars with mine, which shows about as much as I could ever expect from a 120mm scope.

 

Tom

 

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#25 Jared

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Posted 03 January 2021 - 10:21 PM

I recently purchased a SW Evostar 120ED and have been trying it out. After reading some reviews, I have to say I am a bit disappointed. I live in a highly light polluted area, so I have stuck to planets and the moon so far and have only used the stock equipment (EP and diagonal). However, I read reviews that stated how low the chromatic aberration (CA) is and for instance how much detail the reviewer could see of Mars (like polar caps). I can't see any detail on Mars, using the 5mm EP (180 X), but I do see plenty of CA. I would say it takes up 5-10% of the object diameter with the red on one side and blue on the other.

 

I could provide pics, but I only have a cell phone adapter and phone camera to use right now. So they a pretty blurry.

 

On the plus side, on good nights I can see bands on Jupiter and Saturn and make out the dark band in the rings. I also tried viewing without the diagonal since I read that is is not top-notch and I did see a slight improvement in the CA.

 

I realize this is a fuzzy question, but am I expecting too much from this 'scope?

if you are seeing red on one side of the planet and blue on the other, that is mostly an atmospheric effect, at least assuming Mars is near the center of the field of view and there is nothing wrong with the eyepiece. Chromatic aberration in the telescope--longitudinal chromatic aberration--will typically show as a colored halo with the same color all the way around the planet or star. 

 

Try rotating the eyepiece in the diagonal to see if the chromatic aberration moves. I suspect you will find it doesn't. Also, try looking at a bright star near zenith to check for chromatic aberration. 

 

While I suspect you are mostly seeing atmospheric effects, I'm not super confident. the effects you are describing are most common when the planet is close to the horizon, and for mid-latitude northern hemisphere observers Mars is pretty well placed in the evening skies these days. How high up was Mars when you were observing?


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