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Noob question on chromatic aberration

refractor binoculars
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#1 RefractoryTWO

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Posted 17 April 2019 - 09:40 PM

When I look at the moon through my Steiner Military Marine 8x30 binoculars, which I’ve read are not the best when it comes to CA, I see a tiny bit of yellow on the rim. It’s not at all annoying to me.

 

Does this mean that I’d be just fine for visual only with a 5-inch archomat and that spending more on an APO would be a waste of money for my aging and slightly color-blind eyes? Thanks.


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#2 mistateo

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Posted 17 April 2019 - 09:46 PM

I had the Celestron Omni 120mm achromat and for visual observation, I did not find the CA all that objectionable.  AP was a different story though, YMMV.


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#3 aeajr

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Posted 17 April 2019 - 10:26 PM

When I look at the moon through my Steiner Military Marine 8x30 binoculars, which I’ve read are not the best when it comes to CA, I see a tiny bit of yellow on the rim. It’s not at all annoying to me.

 

Does this mean that I’d be just fine for visual only with a 5-inch archomat and that spending more on an APO would be a waste of money for my aging and slightly color-blind eyes? Thanks.

I have two 80 mm F5 achromats.  I barely notice any CA.   Doesn't mean it isn't there, I just don't notice it.  But if I shoot a picture through the eyepiece I see that color band around the moon.

 

The higher the focal ratio, the less CA you are likely to see.  So, a 5" F6 achromat will likely show more CA than a 5" F10 achromat and an F15 will show even less.

 

If you are not doing AP and your are not fussy or bothered by that slight fringe on some very bright targets than you should be fine.  But note, it is the focal ratio that is the predictor of the degree of CA. 


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#4 SeattleScott

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Posted 17 April 2019 - 11:54 PM

With just 8x magnification and just 30mm of aperture you won’t see much CA. What you describe actually sounds like lateral color, not CA. CA is mainly an issue for brighter objects and higher magnification. Up to reasonable levels, like say a 4” F6.5, CA will not really be an issue for low to medium DSO viewing. High magnification views will be a bit soft, and bright stars and planets will be colorful. An Apo gives you more flexibility for viewing DSO as well as planets with the same scope, zooming in with higher magnification on globular clusters and planetary nebulae, etc. Keep in mind there are about 100 different models of Apo refractors. If you exclude the cheap department store stuff, there are probably more Apo models than achro models, by a considerable margin. This would not be the case if Apos were a waste of money.

The other approach is to play the long game. As in a 4” F10 or 6” F12 achro, something like that. At those double digit F ratios the CA is tamed enough for pleasing views of the moon and planets. So you get more flexibility in targets but you have a long heavy tube to manage.

Scott
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#5 Jon Isaacs

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Posted 18 April 2019 - 12:10 AM

When I look at the moon through my Steiner Military Marine 8x30 binoculars, which I’ve read are not the best when it comes to CA, I see a tiny bit of yellow on the rim. It’s not at all annoying to me.

 

Does this mean that I’d be just fine for visual only with a 5-inch archomat and that spending more on an APO would be a waste of money for my aging and slightly color-blind eyes? Thanks.

 

The short answer is no, it doesn't mean spending more money on an APO would be a waste of money.

 

A longer answer:

 

Your binoculars are 8x30s,  A comparable view in a 120mm refractor would be 32x,  At such a low magnification, you would not see much false color.  If you increase the magnification to look at the planets and brighter binary stars, the chromatic aberration in standard commercially available achromat will be quite apparent and blur the image.  The fact that the colors are not focused reduces the contrast. 

 

Chromatic aberration is the inability to bring the colors of the spectrum to a common focus.  If the green is in focus, the blue will be out of focus.  What happens is that the ends of the visual spectrum are out of focus, the red and violet, and so you see this as a purple (red and blue = purple) haze surrounding the object.  

 

One of my scopes is a 120mm F/8.3 Achromat.  Last year I sold a 120mm apo.  At lower magnifications, there is little chromatic aberration visible in the achromat but when viewing Jupiter, Saturn and Mars, they views are not sharp and crisp the way they were in the 120mm apo.  

 

I am 71 so my sensitivity to color is weak.. 

 

Just this evening, I was looking at some bright stars with my 80mm F/11.3 achromat.  Because of it's smaller aperture and slower focal ratio, this scope has much better color correction than the 120mm F/8.3 but the chromatic aberration was quite visible.. 

 

I am quite sure that if a 120mm F/8.3 achromat were setup next to a 120mm ED/apo and both were pointed at Jupiter, Saturn or Mars, you would easily see the difference.

 

Jon


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#6 sg6

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Posted 18 April 2019 - 01:58 AM

CA "worsens" with a couple of small factors: A bit of a simplification but easy.

A faster scope shows it more and a wider objective shows it more.

 

So don't go buying a 150mm f/5 achro. Go buy say a 102 f/7 or 120 f/8 or something along similar lines. ES do an 80mm f/8 that should be good if an 80mm appeals more for ease of use. I have a 102 f/6 and like the OP accept whatever it delivers. There are a number of 90 f/10 refractors around that cover a lot of astro aspects.


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#7 Sky Muse

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Posted 18 April 2019 - 05:53 AM

When white light, from any object in the sky, or from a flashlight even, is refracted, it splits the white light into its component colours, as Newton had discovered and demonstrated with a prism...

 

http://3mana.com/wp-...8/05/newton.jpg

 

Of course, the prism cannot convert those separate beams of colours back into a single beam of white light; but a refractor can, but to varying degrees of accuracy...

 

https://starizona.co...cs/achromat.jpg

 

See that wayward beam of colour, represented in green?  It didn't quite make it to the focal point, and as other two did.  That wayward colour is what is seen through a shorter achromat; shorter than an f/15, I might as well say.

 

Achromats are the inexpensive refractors you find within the marketplace online.  Many are short, some are long.  The longer the achromat, the more accurately the colours are realigned into the white light emanating from an object in the sky.  Conversely, the shorter the achromat, the more false-colour is seen when viewing brighter objects, and it gets worse as the magnification goes up, as it becomes magnified along with the image; the intensity.  Achromats have been around since the mid-1700s, and up until relatively recently they were longer.  I cast my head into my hands, and bemoan, "If only we might've kept the 'idea' of downsizing of so many other consumables out of the astronomical market."  Everybody wants their playthings as small and compact as possible, and regardless of the performance-related consequences.  Short, "fast" achromats are a regression of Hall's hallowed design; certainly not a progression.  They are primarily for ergonomics, not optical performance, although something may be said for the low-power wide-field views that they provide, but only when observing the dimmer deep-sky objects and vistas.

 

You may not see that wayward beam of colour, but it's there, and damaging to the image.  For the lack of a better analogy, imagine that you're building a house, and it requires three truckloads of bricks, but one of those trucks never made it to the site.  Not to worry, as the house is built anyway, although incomplete as a result, and curious in appearance to boot.

 

With a single achromat, you'll have a choice of either ergonomics(shorter, more compact tube, softer images), or optical performance(minimal false-colour and sharpness). 

 

With a single apochromat, you can have your cake(short, more compact tube), and eat it too(minimal to virtually no false-colour, sharper images).

 

Now, with refractors(and Newtonians), what you see is what you get: a short tube = a short focal-length; a long tube = a long focal-length.  It's more difficult to reach the higher powers with a short focal-length, for meaningful observations of this sprite and that, given the general range of eyepieces from 4mm to 40mm.  It might be easier to carry, handle, and to mount on a tripod...but... 

 

A focal-length shorter than 650mm or so is not good for trying to reach the higher powers.  Even with 650mm and 750mm focal-lengths, a 2x or 3x barlow combined with said eyepieces is oft required.  Focal-lengths of 900mm and up, but not too much longer, play well with said range of eyepieces, and in observing most everything in the sky satisfactorily.

 

Keep in mind that a telescope, in the first place, is for seeing faraway objects up close, and a longer focal-length makes that possible, and easier.

 

Many are making a compromise with this one.  It's not too long, per its aperture, nor too short, and at f/8 the false-colour should be tolerable...

 

https://www.bhphotov..._4_7_120mm.html

 

For visual, the mount that's included, an EQ3-class, would be doable with a refractor of that size; if you don't mind the moment-arm effect(a shake and a wiggle here and there).  An EQ-5 would lessen said effect...

 

This... https://www.highpoin...r-ota-21090-ota

 

...mounted onto this... https://shop.opticsp...AUaAgwDEALw_wcB

 

That's an EQ5-class mount, and a bit sturdier.  If you'd prefer an  alt-azimuth, you may have to go with one that's more costly than that EQ-5 in order to support that refractor.


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#8 Jon Isaacs

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Posted 18 April 2019 - 06:42 AM

Many are making a compromise with this one.  It's not too long, per its aperture, nor too short, and at f/8 the false-colour should be tolerable...

 

https://www.bhphotov..._4_7_120mm.html

 

 

Alan:

 

A nice explanation.  However...  I wonder how much experience you have with the 120mm (4.7 inch) F/8.3 achromats? I have owned three of them, I currently own one that I have fitted with a SkyWatcher Pro 2 speed.  The chromatic aberration maybe "tolerable" but it definitely affects the contrast on the planets and softens the views.  As I said in my previous post, the views at high magnifications are not at all "apo" like.

 

Chromatic aberration does not depend on focal ratio alone, it also depends on aperture.  Chromatic aberration increases with increased aperture and decreases with increased focal ratio.  This means the level of color correction can be estimated for an achromat using what is often called the chromatic ratio:

 

CR = Focal ratio /aperture (inches)    

 

A 4 inch F/10 has a CR of 2.5, a 3 inch F/15, has a chromatic ratio of 5.  The 4.7 inch F/8.3 has a CR of 1.8.  An ST-80 is 1.6.  The 80mm F/11.3 discussed in my previous post has a CR of 3.6.  The 8x30 binoculars have a CR of about 3.3... 

 

There are couple of rules of thumb used to judge the color correction of an achromat.  The Sidgwick is based on what he calls "the greatest tolerable discrepancy.  The Sidgwick standard is a CR of 3 or greater.  The The 80mm F/11.3 discussed in my previous post has a CR of 3.6 and while the chromatic aberration is visible, the high power views are quite good.  

 

The Conrady standard is a CR of 5 or greater. Such scopes do exhibit chromatic aberration but it is difficult to see and the image is very APO like.  The 3 inch F/16, a scope quite common 50 years ago, has a CR of 5.2. Such scopes provide remarkable views.  

 

The difficulty is that to maintain the same level of color correction, the focal ratio needs to be increased in proportion to the aperture. The length is increased by the square of the aperture.   A 3 inch F/16 is already 4 feet long, the 80mm F/11.3 is 3 feet long.  A 5 inch that meets the Sidgwick standard, it needs to be F/15, the scope is over 6 feet in length and requires a serious mount.  And there is still significant chromatic aberration.

 

A 5 inch achromat that meets the Conrady standard would would be F/25, 10 feet long.  A 6 foot, a 10 foot scope, these are impractical.  No one manufacturers scopes like these.

 

-------------  

 

The 120mm (4.7 inch) F/8.3 is a compromise, the original question was whether not seeing much CA in the binos meant a 5 inch APO would be unnecessary. 

 

In my experience, there is no mistaking the planetary and double star views of a 120mm F/8.3 achromat with a 120mm F/7.5 Skywatcher ED.  The achromat shows the basics but the higher power views are far from sharp and crisp.  The APO provides views that are essentially perfect.. An achromat that provided similar performance would be something like 10 feet long and require a massive mount.  The APO is only 3 feet long so it much, much handier and less hassle and when the cost of the mount is considered, the APO would more than likely be cheaper.

 

Jon


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#9 gnowellsct

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Posted 18 April 2019 - 07:01 AM

Binoculars work as well as they do because of low magnification and small apertures. That's nice keeps costs down. Watch a bird soaring high in a bright blue sky you may see why people pop $2500 for Swarovskis.
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#10 Sky Muse

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Posted 18 April 2019 - 07:39 AM

Jon,

 

I'm not strongly suggesting that 120mm f/8 achromat, but I do know that it would be a better choice than Celestron's 150mm f/5 achromat(a kaleidoscope), or even the 150mm f/8, the "venerable" C6-R.  

 

 

In the end, I guess it will depend upon one wanting to spend $300 for a 120mm achromat, or well over $1500 for a 120mm apochromat.  There are alternatives however...

 

https://www.bhphotov...ft=BI:514&smp=Y

 

...although the focal length is a bit long, and comparable to a 100mm f/15 achromat, but the Maksutov would have better colour-correction.  It, too, would play well with said range of eyepieces.  You certainly wouldn't need a barlow for that one, unless for a special high-powered observance.

 

In addition, a Maksutov is the only mirrored design of telescope that has been described as "refractor like" in its performance. 

 

Or, if the OP might drop down an inch, actually less than an inch...

 

https://www.astronom...ractor-ota.html

 

An unobstructed 4" aperture is quite capable.  You have a 4" apochromat yourself.


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#11 Jon Isaacs

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Posted 18 April 2019 - 08:29 AM

An unobstructed 4" aperture is quite capable.  You have a 4" apochromat yourself.

 

I do and I enjoy the versatility of a 4 inch apo.  But they are expensive and for affordable excellent high magnification views, as you mentioned Newtonians are the best choice.  An 8 inch or 10 inch Dob is capable of planetary views that are superior to a 4 or 5 inch apo.  

 

Jon


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#12 RefractoryTWO

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Posted 18 April 2019 - 08:34 AM

A lot of good knowledge and great suggestions here. Thanks, all! I consider myself a little more educated.


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#13 Sketcher

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Posted 18 April 2019 - 09:46 AM

I have a 5-inch apochromat, (f-ratio not all that important) as well as 3-inch (f/5) and 6-inch (f/6.5) achromats, along with various other telescopes.

 

Will you be happy with a 5-inch achromat?  Unfortunately, none of us know.

 

I do know that any 5-inch achromat will be vastly superior to my first telescope.  I was happy with that telescope; but at the time, I would have been grateful to have any telescope.  I also know that I could happily use any 5-inch achromat today.

 

Don't get me wrong though.  As others have mentioned, the shorter the f-ratio and the greater the aperture, the greater the CA.  An apochromat should provide sharper views than any equal aperture, equal f-ratio achromat.  Yet, some are quite satisfied with the views offered by achromats.  There are others who wouldn't touch an achromat with a 10-foot pole!

 

An achromat will still show plenty of detail; but will it show enough to satisfy you?  It seems that most people are never satisfied with any telescope!  Just look at all the used telescopes that people buy and sell!  In my opinion, a 5-inch achromat can provide a person with an impressive start in this hobby -- without breaking the bank.

 

Also, not all achromats are made to equal standards; and not all apochromats are made to equal standards.  Differences exist within those telescope categories.

 

It may also be worth keeping in mind the price differences along with the general consensus (or at least the consensus among some of us) that an experienced observer using an achromat will see more than will a beginner using an equal-aperture apochromat.  It's not just the telescope.  One may also argue that it's better to learn about the do's and don'ts of telescope care without putting at risk a more expensive telescope.


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#14 SeaBee1

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Posted 18 April 2019 - 11:07 AM

I would have to agree with Sketcher. I started with a 4 inch achro, and never noticed the CA... until I had used it for a while and became a better observer. And that 4 incher showed me a bunch of stuff. But... when the CA became noticeable, I knew I didn't want to notice it... but it was there forever after. I decided to replace it with a SW 120ED... CA unnoticeable again... and happy again. It isn't a true APO, but close enough for now.

 

Is a true APO in my future? I don't know, but I don't think so. The 120ED is a significantly better scope both in CA control and aperture. The point I want to make is that, if you are really interested in this hobby, first scopes are rarely last scopes. And I still have my 4 inch achro...


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#15 gnowellsct

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Posted 18 April 2019 - 11:31 AM

 

 

It may also be worth keeping in mind the price differences along with the general consensus (or at least the consensus among some of us) that an experienced observer using an achromat will see more than will a beginner using an equal-aperture apochromat.  It's not just the telescope.  One may also argue that it's better to learn about the do's and don'ts of telescope care without putting at risk a more expensive telescope.

Well now this is the first time I've read that but maybe I haven't been paying attention.  My suggestion is that the beginner may not see as much looking *through* the apo because he is too overwhelmed looking *at* the apo:  Wow that little thing cost $3500?   Whuzzat?


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#16 Frisky

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Posted 18 April 2019 - 02:20 PM

My Meade, 120mm, f8.3 refractor shows almost no noticeable purple fringing. It will show some on the brightest stars, like Sirius, but not on the moon and planets. On the moon, I see a sliver of gold or yellow on the limb. When I use the built-in aperture mask, the purple is gone from Sirius and other bright stars! I say CA is greatly over-blown by people in this and other forums. They tend to be searching for optical perfection, so they look for CA. I've also noticed no CA at all on deep sky stuff. I advise the OP to buy a 120mm achro and not worry about CA. Here is my beautiful "Strehl One" 120mm achro!

 

Joe

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#17 penguinx64

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Posted 18 April 2019 - 02:20 PM

I notice CA looking at the moon with every pair of binoculars and with my achromat refractor too.  II notice more of it at higher magnifications.  I tried Baader Fringe Killer and Contrast Booster filters with my refractor, and they help reduce CA quite a bit.  


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#18 Jon Isaacs

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Posted 18 April 2019 - 02:50 PM

My Meade, 120mm, f8.3 refractor shows almost no noticeable purple fringing. It will show some on the brightest stars, like Sirius, but not on the moon and planets. On the moon, I see a sliver of gold or yellow on the limb. When I use the built-in aperture mask, the purple is gone from Sirius and other bright stars! I say CA is greatly over-blown by people in this and other forums. They tend to be searching for optical perfection, so they look for CA. I've also noticed no CA at all on deep sky stuff. I advise the OP to buy a 120mm achro and not worry about CA. Here is my beautiful "Strehl One" 120mm achro!

 

Joe

Joe:

 

As I have said, I currently own a 120 mm F/8.3 achromat with the same optics as yours. I have owned two others. The views have been essentially identical.

 

I posted a bunch of analysis, I can post some more but the point is to show that the chromatic aberration does exist, it affects the view of the planets. 

 

This is not to say that a 120 mm F/8.3 achromat does not provide enjoyable views of the planets, it can for sure. But I think that if you and I were out under the night sky with  a 120 mm F/8.3 achromat and a 120 mm F/7.5 ED APO, you would see a definite difference and you would come to see the purple fringing.

 

This is not to say the 120 mm F/8.3 is a bad scope, it's a good scope and a good balance between field of view, aperture, manageability, color correction etc, unlike the 120 mm F/5, this one does a reasonable job on the planets and on deep sky.. When discussing a scope, I want people to have reasonable expectations. If they see some purple haze around Jupiter, at 200x, they should not be surprised, it should be expected.

 

Jon


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#19 Jon Isaacs

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Posted 18 April 2019 - 03:10 PM

It seems that most people are never satisfied with any telescope!

 

 

I think some people are never happy with any telescope. They tend to buy and sell looking for the perfect telescope.

 

But there's another group that is happy with just about every telescope ever made. They may  still buy but only sell when there's no more storage space.

 

It's the cup half full versus the cup half empty. I like to think I can enjoy most any telescope, no matter how perfect or in perfect. They all have their strengths and weaknesses. I spent last night with an very good 80 mm F/11 achromat, viewing that the moon and double stars. I was using a set of entry level Plossls. I have a lot of fun.  Simple and easy.

 

I have "better" eyepieces and an 80 mm FD/APO that would have provided somewhat sharper cleaner views but I might not have had as much fun.

 

The original question posted was whether seeing only minimal color in 8x30 binoculars meant an 5 inch would be unnecessary and that a achromat would be fine.

 

I wrote that simple answer was NO.

 

The view through 8x30 binoculars cannot be generalized to a 5 inch refractor.

 

Jon


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#20 Frisky

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Posted 18 April 2019 - 03:54 PM

I think seeing is what determines sharpness of view. If you have good seeing, an achro will have great views! If you do see a little CA, it will be minimal and taken care of by a simple aperture mask. It's mind over matter folks. If you don't mind a little CA, it doesn't matter.

 

Joe


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#21 RefractoryTWO

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Posted 18 April 2019 - 06:31 PM

I have two 80 mm F5 achromats.  I barely notice any CA.   Doesn't mean it isn't there, I just don't notice it.  But if I shoot a picture through the eyepiece I see that color band around the moon.

 

The higher the focal ratio, the less CA you are likely to see.  So, a 5" F6 achromat will likely show more CA than a 5" F10 achromat and an F15 will show even less.

 

If you are not doing AP and your are not fussy or bothered by that slight fringe on some very bright targets than you should be fine.  But note, it is the focal ratio that is the predictor of the degree of CA. 

Thanks for the tip about focal ratio and CA.



#22 RefractoryTWO

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Posted 18 April 2019 - 06:35 PM

With just 8x magnification and just 30mm of aperture you won’t see much CA. What you describe actually sounds like lateral color, not CA. CA is mainly an issue for brighter objects and higher magnification. Up to reasonable levels, like say a 4” F6.5, CA will not really be an issue for low to medium DSO viewing. High magnification views will be a bit soft, and bright stars and planets will be colorful. An Apo gives you more flexibility for viewing DSO as well as planets with the same scope, zooming in with higher magnification on globular clusters and planetary nebulae, etc. Keep in mind there are about 100 different models of Apo refractors. If you exclude the cheap department store stuff, there are probably more Apo models than achro models, by a considerable margin. This would not be the case if Apos were a waste of money.

The other approach is to play the long game. As in a 4” F10 or 6” F12 achro, something like that. At those double digit F ratios the CA is tamed enough for pleasing views of the moon and planets. So you get more flexibility in targets but you have a long heavy tube to manage.

Scott

For the record: I didn’t mean to imply that APOs are a waste of money. Was wondering if one would be overkill for my eyes. But thanks.



#23 RefractoryTWO

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Posted 18 April 2019 - 06:40 PM

The short answer is no, it doesn't mean spending more money on an APO would be a waste of money.

 

A longer answer:

 

Your binoculars are 8x30s,  A comparable view in a 120mm refractor would be 32x,  At such a low magnification, you would not see much false color.  If you increase the magnification to look at the planets and brighter binary stars, the chromatic aberration in standard commercially available achromat will be quite apparent and blur the image.  The fact that the colors are not focused reduces the contrast. 

 

Chromatic aberration is the inability to bring the colors of the spectrum to a common focus.  If the green is in focus, the blue will be out of focus.  What happens is that the ends of the visual spectrum are out of focus, the red and violet, and so you see this as a purple (red and blue = purple) haze surrounding the object.  

 

One of my scopes is a 120mm F/8.3 Achromat.  Last year I sold a 120mm apo.  At lower magnifications, there is little chromatic aberration visible in the achromat but when viewing Jupiter, Saturn and Mars, they views are not sharp and crisp the way they were in the 120mm apo.  

 

I am 71 so my sensitivity to color is weak.. 

 

Just this evening, I was looking at some bright stars with my 80mm F/11.3 achromat.  Because of it's smaller aperture and slower focal ratio, this scope has much better color correction than the 120mm F/8.3 but the chromatic aberration was quite visible.. 

 

I am quite sure that if a 120mm F/8.3 achromat were setup next to a 120mm ED/apo and both were pointed at Jupiter, Saturn or Mars, you would easily see the difference.

 

Jon

Good to hear from a fellow observer who thinks his sensitivity to color is weak. Thanks. This was the gist of my question: Is an APO worth it for a person who looks up and seldom sees much color in the heavens?



#24 RefractoryTWO

RefractoryTWO

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Posted 18 April 2019 - 06:42 PM

CA "worsens" with a couple of small factors: A bit of a simplification but easy.

A faster scope shows it more and a wider objective shows it more.

 

So don't go buying a 150mm f/5 achro. Go buy say a 102 f/7 or 120 f/8 or something along similar lines. ES do an 80mm f/8 that should be good if an 80mm appeals more for ease of use. I have a 102 f/6 and like the OP accept whatever it delivers. There are a number of 90 f/10 refractors around that cover a lot of astro aspects.

I’ve also been considering something in the 102-115 range...

Thanks.



#25 RefractoryTWO

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Posted 18 April 2019 - 06:45 PM

When white light, from any object in the sky, or from a flashlight even, is refracted, it splits the white light into its component colours, as Newton had discovered and demonstrated with a prism...

 

http://3mana.com/wp-...8/05/newton.jpg

 

Of course, the prism cannot convert those separate beams of colours back into a single beam of white light; but a refractor can, but to varying degrees of accuracy...

 

https://starizona.co...cs/achromat.jpg

 

See that wayward beam of colour, represented in green?  It didn't quite make it to the focal point, and as other two did.  That wayward colour is what is seen through a shorter achromat; shorter than an f/15, I might as well say.

 

Achromats are the inexpensive refractors you find within the marketplace online.  Many are short, some are long.  The longer the achromat, the more accurately the colours are realigned into the white light emanating from an object in the sky.  Conversely, the shorter the achromat, the more false-colour is seen when viewing brighter objects, and it gets worse as the magnification goes up, as it becomes magnified along with the image; the intensity.  Achromats have been around since the mid-1700s, and up until relatively recently they were longer.  I cast my head into my hands, and bemoan, "If only we might've kept the 'idea' of downsizing of so many other consumables out of the astronomical market."  Everybody wants their playthings as small and compact as possible, and regardless of the performance-related consequences.  Short, "fast" achromats are a regression of Hall's hallowed design; certainly not a progression.  They are primarily for ergonomics, not optical performance, although something may be said for the low-power wide-field views that they provide, but only when observing the dimmer deep-sky objects and vistas.

 

You may not see that wayward beam of colour, but it's there, and damaging to the image.  For the lack of a better analogy, imagine that you're building a house, and it requires three truckloads of bricks, but one of those trucks never made it to the site.  Not to worry, as the house is built anyway, although incomplete as a result, and curious in appearance to boot.

 

With a single achromat, you'll have a choice of either ergonomics(shorter, more compact tube, softer images), or optical performance(minimal false-colour and sharpness). 

 

With a single apochromat, you can have your cake(short, more compact tube), and eat it too(minimal to virtually no false-colour, sharper images).

 

Now, with refractors(and Newtonians), what you see is what you get: a short tube = a short focal-length; a long tube = a long focal-length.  It's more difficult to reach the higher powers with a short focal-length, for meaningful observations of this sprite and that, given the general range of eyepieces from 4mm to 40mm.  It might be easier to carry, handle, and to mount on a tripod...but... 

 

A focal-length shorter than 650mm or so is not good for trying to reach the higher powers.  Even with 650mm and 750mm focal-lengths, a 2x or 3x barlow combined with said eyepieces is oft required.  Focal-lengths of 900mm and up, but not too much longer, play well with said range of eyepieces, and in observing most everything in the sky satisfactorily.

 

Keep in mind that a telescope, in the first place, is for seeing faraway objects up close, and a longer focal-length makes that possible, and easier.

 

Many are making a compromise with this one.  It's not too long, per its aperture, nor too short, and at f/8 the false-colour should be tolerable...

 

https://www.bhphotov..._4_7_120mm.html

 

For visual, the mount that's included, an EQ3-class, would be doable with a refractor of that size; if you don't mind the moment-arm effect(a shake and a wiggle here and there).  An EQ-5 would lessen said effect...

 

This... https://www.highpoin...r-ota-21090-ota

 

...mounted onto this... https://shop.opticsp...AUaAgwDEALw_wcB

 

That's an EQ5-class mount, and a bit sturdier.  If you'd prefer an  alt-azimuth, you may have to go with one that's more costly than that EQ-5 in order to support that refractor.

Note to self: Stay away from short achromats. Thanks for the schooling.




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