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Can eye pieces remove chromatic aberration

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#1 Stillhere

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Posted 14 May 2021 - 12:28 PM

Ok, I bought a Skywatcher Startravel achromatic 120 f:5. It is not apo, so chromatic aberration is really bad on bright objects like moon and planets. I got a fringe killer, is ok but I  am wondering whether some good eye pieces like Tele vues will reduce aberration problems by the complex glass setups. I also made a mask to turn this scope into a 75mm x 600 f:8 scope to look at moon and bright planets. Any ideas much appreciated, really annoyed at the aberration. I should of realised a fast scope would do this and not being apo.


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#2 Mitrovarr

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Posted 14 May 2021 - 12:33 PM

No (at least not eyepieces you can actually get). The only device that could really reduce the chromatic aberration of a scope was the Chromacor, which isn't produced anymore.


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#3 rblackadar

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Posted 14 May 2021 - 12:43 PM

Just as an aside, do you know about atmospheric dispersion? (I didn't, starting out.) Some -- or perhaps a lot -- of the color you see may be due to that, and you *can* correct for it with an ADC.



#4 Mitrovarr

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Posted 14 May 2021 - 12:55 PM

He has a 120mm F/5 refractor. I don't think atmospheric dispersion is the problem. I like those telescopes, but they have loads of CA, no avoiding it.

My advice would just be to use the 120mm F/5 for the wide field deep sky applications it is good at, and later get a more appropriate telescope to use on planets and the moon.
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#5 Stillhere

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Posted 14 May 2021 - 12:56 PM

Thanks, looks like I have to keep masking the appreture, f:8 seems the minimum and seems to eliminate it. So defiantly produced at main lens and not existing eye pieces. Haven't looked at dso's yet cause of weather, hope is ok. Not much on line about this scope.



#6 Mitrovarr

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Posted 14 May 2021 - 12:57 PM

There is a very similar scope called the Orion ST120. You might find a lot of information under that name (search for ST120).
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#7 Supernova74

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Posted 14 May 2021 - 01:05 PM

If anything some eyepieces can make achromatic aberration even worse to be honest,I’ve mainly noticed this on longer focal length equivalents.



#8 Stillhere

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Posted 14 May 2021 - 01:25 PM

Thanks guys, I also read somewhere that the 2.5x powermate reduces abberation by extending the focal length of the telescope. I wonder if anyone has witnessed this. I have seen chromatic abberation in eyepieces when you look off centre in them. I do like the look of this scope, looks like it belongs to nasa with it black and white metallic look. We only have one telescope shop in my state, so if I ask them for advice they will give you a bum steer just to make a sale.



#9 ButterFly

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Posted 14 May 2021 - 01:27 PM

Almost all eyepieces show lateral color.  That is, they also show CA, but growing as the field angle grows.

 

Planetary observers know this and place the planet at the "right" part of the field.  The eyepiece's lateral color may cancel out some of the scope's color.  The worse it starts off, the luckier you have to be to correct it with the eyepiece.

 

Same with atmospheric dispersion.  Find the right spot in your field.  As I started to use an ADC, it became apparent that it would be worthless without tracking.  As soon as I move away from the center of the corrected field, the eyepiece's colors start showing.



#10 bobhen

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Posted 14 May 2021 - 01:33 PM

Thanks guys, I also read somewhere that the 2.5x powermate reduces abberation by extending the focal length of the telescope. I wonder if anyone has witnessed this. I have seen chromatic abberation in eyepieces when you look off centre in them. I do like the look of this scope, looks like it belongs to nasa with it black and white metallic look. We only have one telescope shop in my state, so if I ask them for advice they will give you a bum steer just to make a sale.

A Powermate or Barlow will not reduce CA. The optics in those devices do not but the unfocused light caused by CA back into the light path. The fast F5 120, 102 and 80mm achromats are not recommended for high power use. They are great for low power, wide field deep sky observing from a dark sky.

 

Bob


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#11 Stillhere

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Posted 14 May 2021 - 02:15 PM

Thanks bob, I have been on this crazed destructive course of trying to decide to spend big dollars to get this scope to do what it's not made to do. I do own a 70mm f10 slow scope I was at least trying to mimic as well so I only have to take the one scope to dark skies. My area is mosquito infested so scoping around home is no fun very quickly. That is why I chose this scope for portability.



#12 epee

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Posted 14 May 2021 - 02:51 PM

There are filters that suppress CA. Although I'm not sure if they actually sharpen the image or simply remove the halo.



#13 ButterFly

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Posted 14 May 2021 - 03:34 PM

Thanks bob, I have been on this crazed destructive course of trying to decide to spend big dollars to get this scope to do what it's not made to do. I do own a 70mm f10 slow scope I was at least trying to mimic as well so I only have to take the one scope to dark skies. My area is mosquito infested so scoping around home is no fun very quickly. That is why I chose this scope for portability.

It's still a great scope for what it's good at.  Most objects show very little color.  An 8" dob is still quite portable, great for planets and the moon, and you have eyepieces anyway.  A good 8" will cost much less than a TV eyepiece.  The scopes would pair well together if the focal lengths aren't too close.  I lilke my 120mm APO for lunar/planetary, but the dob is always better for planets, except when the seeing is not that good.  Ask your local club whether anyone has an 8" they are willing to let go.  Try the ones a few towns over as well - 8" dobs are common.


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#14 teashea  Happy Birthday!

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Posted 14 May 2021 - 07:21 PM

It would be nice if there were an eyepiece that could do this but unfortunately not.


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#15 Stillhere

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Posted 14 May 2021 - 09:04 PM

Thanks for that, the mirror scopes worry me that I would have to get the polished aluminium coated mirror re-coated every few years. The refractors at least seem more stronger and are nearly a complete sealed system from dust and debri, except when changing eye pieces. That naked mirror is starring up all the time.

#16 Mitrovarr

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Posted 14 May 2021 - 09:07 PM

You don't have to recoat mirrors every couple of years. Even people who recoat fairly aggressively usually don't do it more than once every 10-15 years.


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#17 sevenofnine

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Posted 14 May 2021 - 09:24 PM

If a standard Dob with it's "naked" shocked.gif  mirror bothers you then why not a Maksutov? A 5" Mak is great for planets and the Moon and because of it's mirrored design...Zero CA!  It's a very rugged design that rarely needs collimation. Very compact and easily portable too. Good luck! waytogo.gif



#18 Stillhere

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Posted 14 May 2021 - 09:34 PM

Thanks for that, the mirror scopes worry me that I would have to get the polished aluminium coated mirror re-coated every few years. The refractors at least seem more stronger and are nearly a complete sealed system from dust and debri, except when changing eye pieces. That naked mirror is starring up all the time. I think I will mask to a 75mm f:8 when looking at moon and gas giants. I should be able to get to 140x without blurring. I am only interrest in Jupiter and Saturn in the planets anyway, brings back childhood memories of seeing them in my neigbours scope.

#19 Stillhere

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Posted 14 May 2021 - 10:46 PM

There we go made my mask made from a recycle bin, ha! literally a recycle bin. Plastic should last the elements.

#20 PatrickVt

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Posted 14 May 2021 - 10:52 PM

Thanks for that, the mirror scopes worry me that I would have to get the polished aluminium coated mirror re-coated every few years. The refractors at least seem more stronger and are nearly a complete sealed system from dust and debri, except when changing eye pieces. That naked mirror is starring up all the time. I think I will mask to a 75mm f:8 when looking at moon and gas giants. I should be able to get to 140x without blurring. I am only interrest in Jupiter and Saturn in the planets anyway, brings back childhood memories of seeing them in my neigbours scope.

I, myself, prefer refractors but there are different tools for different jobs.  You mention being "only interested in Jupiter and Saturn" yet a short to mid-range refractor is not the tool for the job in this case.  You would want a long focal length refractor at a minimum.  This particular job is where the longer focal length cassegrains shine.  Those are definitely the tools for the job unless you have the space and energy for a 6-10" f15-20 refractor.  And, again, I actually prefer refractors but they are not the ideal tool for planetary work. 

 

This is the second time you mentioned recoating mirrors "every few years" as a drawback to all scopes with mirrors.  It was already pointed out that "every few years" is a gross exaggeration.  There certainly are some drawbacks to mirrors but this really isn't among them.  There are different tools for different jobs and each of these tools has its pros and cons.  Recoating mirrors doesn't ever get mentioned unless you have no intention of caring for your telescope in the usual ways.

 

About eyepieces and CA...   an eyepiece can certainly introduce more CA.  Any lens added to the objective lens can add aberrations.  This means that if you are seeing CA with a low cost eyepiece in multiple telescopes, it is possible you can eliminate some of the CA that you are seeing when using the lower-end eyepice by purchasing a higher grade eyepiece.  A prism diagonal can introduce CA as well in faster scopes.  An eyepiece won't fix CA introduced by an objective lens cell though.  Higher grade eyepieces can get you back to the native optical quality of the objective lens though when compared to lower end eyepieces with the same objective lens cell.  

 

Patrick


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#21 Stillhere

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Posted 15 May 2021 - 03:32 AM

Sorry for mentioning about mirrors twice, I used a back page which brought up my question un posted. Glad to know about the prism diagonal. Saturn and Jupiter aint the only thing I am interrested in, just the only planets. Mostly because you would need such huge magnifications to see the others clearly, except perhaps venus which can get quiet big and close. I read somewhere that someone recoats their mirror every three years, maybe they just had ocd.

#22 PatrickVt

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Posted 15 May 2021 - 08:47 AM

Sorry for mentioning about mirrors twice, I used a back page which brought up my question un posted. Glad to know about the prism diagonal. Saturn and Jupiter aint the only thing I am interrested in, just the only planets. Mostly because you would need such huge magnifications to see the others clearly, except perhaps venus which can get quiet big and close. I read somewhere that someone recoats their mirror every three years, maybe they just had ocd.

If someone re-coats their mirrors every three years, they either have the facilities and equipment to make that job a quick and easy job or, as you said, they may have some sort of OCD.  In most cases, I've seen mirrors after 20 years (always stored safely) present no major problems.  A closed tube Mak, which would be one of the better tools for all planetary work, should last even longer.  Honestly, I would not worry about re-coating mirrors unless you plan to abuse your telescope(s).  

 

It doesn't matter which planets in our solar system that you may be interested in for observing and/or imaging....  the longer the focal length, the more it is the right tool for the job.  A 600mm focal length is not the tool for the job.  You can view planets in such a short scope but the view will be tiny.  Going beyond around 100x magnification in such short scopes isn't usually a good idea either.  Short focal length scopes are really meant for wide-to-medium field views.  For planets, ideally, you want as narrow as you can get.  All the planets in our solar system are tiny compared to most objects in the sky so, although they are closer to us, they are so tiny that you really need much more magnification than you would think.  I think the proximity of our planets makes this a bit deceiving at first.  A telescope in the 1500mm focal length range would be a good place to start.  To complicate things there is a bit of a trade off, just remember that the longer the focal length is, the narrower your view is which means it becomes more and more difficult to accurately find your targets.  Once you find your view, you'll be very happy you have all that extra focal length.

 

Back to your original topic though...  a 120mm f5 scope is going to be more and more colorful with CA the more you push the magnification.  If you stay wide with less magnification, you'll have less CA.  Some people add a Baader Contrast Booster for cutting some of that CA.  Here is a good article about the topic.  Don't expect miracles though.  These filters will help to cut CA but won't eliminate the CA completely especially in such a fast scope as the 120mm/f5.

 

As I mentioned previously, better eyepieces might help with this problem if your current eyepieces are on the low end and perhaps introducing some CA.  Same with your diagonal if it is a lower end diagonal especially if it might be a prism diagonal.  But, with such a fast widefield scope, you will have CA.  It is just a matter of how much you can effectively cut or simply accept.  

 

For solar system objects, a Mak-Cass or SCT in the 1500mm+ range is the way to go though.  First, there will be no CA which was the original point of this thread.  The views will be larger and the scope will support more magnification.  There are two trade-offs though...  this will be a heavier scope requiring a better quality mount and you'll need extra time for the scope to acclimate to the ambient temperature.  And, of course, the far narrower views will make it harder to pinpoint your targets in the scope.  I'm a refractor guy so if a refractor guy is advising that a mirror is the better tool for the job and will help resolve your stated issue, that should carry some weight. 

 

On a side note, I myself would like a better scope for planetary views and imaging too.  The easy route would be a cassegrain of some sort.  I keep oscillating back and forth between a cassegrain and a large, extra long refractor though.  The cassegrain would be the easier route but I'm not known for ever taking the easy route.  There is a part of me that wants to build a refractor in the 1500-2000mm focal length range.  Storage, the mount and my extensive spinal injuries are all obstacles to this plan though.

 

Patrick


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#23 Stillhere

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Posted 16 May 2021 - 06:33 PM

Thanks for your info, much appreciated. I too have back injures, so I get it.
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#24 Sarkikos

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Posted 18 May 2021 - 08:00 AM

Keep your 120mm f/5 achro for grab-n-go deep sky.  I have an ST120 and have seen hundreds of DSO with the scope.   It is an excellent grab-n-go scope for deep sky.  But not so great for planet/lunar due to the CA.

 

For planets and the Moon, think about a Newt or Cat.  No CA there.  A C90 Mak is a great little Moon scope.  The view of planets will be limited, but at least there won't be CA.   An 8" f/6 Dob is another option, which would be good for the Moon, planets and deep sky.

 

Or just keep your 70mm f/10 achro for planet/lunar, and the 120mm f/5 achro for deep sky.  You already have a good combo there.

 

Mike


Edited by Sarkikos, 18 May 2021 - 08:09 AM.

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#25 Stillhere

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Posted 21 May 2021 - 01:47 PM

Ok I got a Saxon 2x barlow and a Baader zoom 8-24 ep. And has dramatically helped this scope with the already Baader semi-apo, fringe killer moon & skyglow. This has really improved things that I am now happy, while some ca is present at real high mag it is finer. It was really thick before with cheaper eye pieces and no name barlow so they seem to contribute also to ca. Happy with moon, Jupiter and Saturn and I was checking them out through a dirty glass window. I now don't feel I made a terrible decission getting this scope. I made a 75mm mask that helps on daytime terrestrial viewing at high mag when full sun hitting white surfaces causing some ca, cleaned up nicely, not needed all time, just a another option. So I have had to spend more on this scope to get it up to scratch, but I guess that is typical of everything these days.
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