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CA - does it bother you?


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Recently I have been using my refractors more (Vixen 80mm f11 and SW 120mm f5) for grab and go sessions and looking at the moon, Jupiter etc. As a result of this is have become used to images which have a variable level of CA. Previously I have always used Newts and they are still my scope of choice where feasible. Last night I used my 6" f11 dob http://stargazerslounge.com/topic/159354-orion-optics-6-f11-planetary-dobsonian/ for the first time in ages and it was a sheer delight to see the moon and Jupiter with sharp lines and completely free of any CA. I'd forgotten how good this scope is. OK it takes an hour to cool properly but when it does it is stunning.

I realise now that CA bothers me more than I thought and I should be looking for an ED version some time in the future for grab and go sessions.

Does CA bother you and if you have changed to an ED from a slow frac, was the difference noticeable?

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Shane, it didn't bother me when I had only my ST80 clone to look through. It started to bother me a lot more when I got hold of my ED80, and the views were so much better. I found the same true on my bino's... my original pair were great, until I got an ED pair. 

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Hi Shane,

I've had a few achromats up to F/12 in focal ratio and they all showed CA to some degree, as you would expect. I suspect this is quite a personal thing but I prefer to see little or no CA if possible so, when I could, I did go for ED doublets. You still get a tiny bit of CA with those but you have to really search for it and it's confined to a faint splash of purple around the brightest stars.

I think also the practical matter of mounting the scope has influenced my decision. An F/15 4" achromat is practically free of CA but it's an unweildy beast to find a good mount for. My ED doublet Vixen 102 F/6.5 probably shows about the same CA as a good F/15 achromat of the same aperture but will sit happily on an AZ-4 mount or Vixen Porta II. When I remember to tighten the dovetail clamp properly that is ! :rolleyes2:

I reckon you will get varied reponses to this one though :smiley:

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Oddly, not a great deal, even though I have ED and apo scopes.

I seem to just either accept it or sort of filter it out and so look at the image as best I can and enjoy it.

Partially bothered if I rant against CA then one day I may tell some youngster that theirs is not worth looking through and put them off. They are not going to afford a £1k apo. So yes there may be CA but that is all. :eek: :eek:  Pretty harmless stuff really. Lets face it Newton made his name owing to CA. Didn't do him any harm. :rolleyes: :rolleyes:

You can actually have a conversation explaining the colours and how they occur. Kids seem then to accept that it is there and why, then get on with the important bit of just looking and enjoying it all. :grin: :grin:

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Personally I don't like it. I don't dislike it as much as astigmatism, but I would much prefer if it is not there. I don't think there is a problem with aberrations if you know what you are getting. A while ago, I owned a pair of binos Rev 15x70 which showed all kind of possible aberrations. These did not affect me because it was a ~£50 purchase, intentionally made because I was too unsettled to invest in something of better quality. Despite this I enjoyed the views a lot and those binos gave me the possibility to continue observing the sky. 

Actually, I have to say that I liked wide field views so much that I decided to invest in a tool that solved all those optical issues and that I could still take it wherever I wanted and mount it quickly. 

What I want to say is that having a tool with some limits/imperfections is not a bad idea at all, as it allows you to understand those limits/imperfections better and to think whether those limits are a concern or not for you. If you are asking this question, it seems to me that CA is a bit a concern for you. Surely it can be improved with an ED or a full Apo. Whether this is worth or not for you, I cannot answer. For me it is worth and I haven't ever regretted it. However, I also considered other factors such as: WF, compactness, lightweight, almost absent cooling time, easy to use, comfortable posture when observing, and use of different magnifications (the reason why I didn't upgrade the binos). For a tool that I can take anywhere, anytime, yes, this is really worth for me. Instead, if I could use it only few times, I would probably accept some aberration and don't look back. 

This is just my opinion based on my limited experience of course

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The first time I looked through an achro refractor I made the mistake of looking at a bright Jupiter low in the sky and through a scope that wasn't cooled properly. Colourful wasn't the word but I soon fell in love with refractors for the sharp views they give. I like me newts but coma and all the other aberrations that can come with eyepieces when used in the faster (more manageable) scopes can be more of a distraction than a bit of purple fringing....... that can, if so desired be filtered out "to some degree".

I have also owned a C100ED and while I'm not 100% sure it was one of the better examples out there it still done a good job of clearing up CA. The only problem I found is while the views were sharp and colour free I was still left wanting for aperture. I went from that to an AR127L f/9.5 and I now have an EVO150 f/8.3 and (I think I landed lucky with it as the CA is actually well controlled) it is a joy to use. What I have found is refractors don't seem to suffer seeing as bad as newts. The sharp sweet spot is huge where as with a newt you have to really feather the focuser to get it bang on. Doubles split with ease and in my eyes the moon seem to offer that bit more contrast even with a bit of fringing going on around craters. So I guess no, CA doesn't bother me but then I still have my newts if it did.

All said and done though I'm thinking I would really like the views through a slow newt like your 6" f/11.

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Hi Shane,

I can only tolerate it in the longer focal lengths. My F10 refractor is quite good, using it last night on the moon there was a very thin blue line around the edge of the lunar disc, hardly noticeable for me. I do however remember looking through a friends startravel 80 at the moon at a power of around x100 and the view truly shocked me, just loads and loads of blue everywhere, just horrible. Possibly the worst view I have ever had (am I being over dramatic here?).

Having said that, the little startravel put up a good view of the double cluster. I'm also thinking of adding an ST120 to my collection, but ONLY for DSO's....it won't even get a sniff of the moon or planets. Do you like your ST120 for widefield? I'd be interested to see what you think of that scope overall.

Cheers,

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cheers all, some interesting views as John predicted!

don't get me wrong, I really like my solar views through fracs although these are of course in narrow bands and therefore CA is not an issue. I do find it an annoyance though on moon and Jupiter so may have to rethink my grab and go in due course. 

I also agree that a view is better than no view and in truth I'd put up with CA or any other aberration for that matter rather than not see something.

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yes, the 120mm SW is quite superb at wide field, lower power observing. In truth if you point it at the moon and Jupiter it would wow an inexperienced observer - in fact it did the other night when I showed my dog minder the half moon through it - they were amazed. at low power though with 26mm Nagler and 2" diagonal it's a peach. it's also my main white light solar scope (I'll never go back to newts for solar) and with a Lunt wedge is truly excellent.

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I think it is a overblown topic. Most should just enjoy what they have. If you go looking for it, you probably will find it. Maybe I am just too old to to care or my eyes are old enough to not notice it much. If I feel someone is going to be bothered by it, I just stack a light yellow and light blue filter for them. I never hear them say much after that.

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cheers all, some interesting views as John predicted!

don't get me wrong, I really like my solar views through fracs although these are of course in narrow bands and therefore CA is not an issue. I do find it an annoyance though on moon and Jupiter so may have to rethink my grab and go in due course. 

I also agree that a view is better than no view and in truth I'd put up with CA or any other aberration for that matter rather than not see something.

Shane, you would do really well to have a look through a 127 maksutov for grab an go solar system, very good scopes, also the 150 pro mak is just superb and works on AZ/giro mount

If you fancied a browse with a coupe of Maks i might have a trip over, or you can pop up when i have got into new house with lovely dark views  :grin:

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I think most aberrations if they're expected, are acceptable in a scope - None of them are free of a compromise. It's as pointless complaining about a bit of false colour around Jupiter in an Achro, as it is wide diffraction spikes on the same subject when you point a Newt at it.

Now if you have a scope that is supposed to avoid a specific aberation and then you see some, that's worth getting worked up about.

Russell

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You have to remember that the atmosphere will introduce chromatic aberration (or more accurately prism-like dispersion) on any object below about 40° elevation - which is most planets most of the time in the UK. It gets more noticeable with bigger better scopes because they will generally support more magnification. One case that particularly sticks in my mind was viewing Mars about 15° elevation through my friend's superb newtonian and it looking like rainbow planet! Why? Because the disk was only five arc seconds across and the telescope and seeing allowed lots of magnification which really highlighted the atmospheric dispersion. How much these things bother you though I think depends a lot on what you like to observe. Personally I find CA a bit annoying on Jupiter but totally irrelevant on deep sky and star clusters. I guess one could have the same discussion about diffraction spikes or coma. Whilst we all strive for the best possible view, I think it can be a trap to become too focussed on optical perfection as it can easily become an obsession that gets in the way of actual observing. :)

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It is of course a matter for the individual.  I had my first telescope about 45 years ago and have enjoyed most types of telescopes in that time, warts and all in some cases.  I do wonder though how many people who seem completely intolerant of CA would have got on  before EDs and Apos were so commonly available. (other than using a reflector of course!)  After all, though some people may find it hard to believe, some great observers used nothing else but achromats for their life's work. (and they weren't all f15 or 18!)

It does amuse me sometimes how many people will spend thousands of pounds on various equipment and then spend endless time and effort in order to find some minor fault with it.  This seems most frequent with people buying 'high end' (or an least high priced) apos.  Don't get me wrong, at times I have done this myself, but my view is we should be enjoying using our equipment, however humble, rather than fretting about any possible shortcomings it may have.

I have observing notes going back for many years.  The most noticeable thing to me is that in all the wonderful things I have seen, the equipment used (if any) only gets a brief mention.  It's the observations I made that remain in the memory, not the identity of  the various telescopes I used.

I have an 80mm apo and a 152mm f5.9 achromat, I love using both of them and would not wish to be without either.

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We could have a similar discussion about coma, spherical abberation, astigmatism, diffraction spikes, etc ,etc. With a similar variety to the reponses too I reckon.

However, Shane asked about CA so thats what the thread has concentrated on.

On that topic, I find this table quite interesting:

post-118-0-05147800-1422908147.jpg

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Very interesting John thank you. Maybe I am a spoiled with my CA and diffraction spike free 6" f11 newt.

Pretty much coma free too at F/11 Shane with a collimation sweet spot of nearly an inch :smiley:

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CA in my Tal100RS doesnt bother me at all, I have looked through a Startravel and on starclusters its faboulous; but on Jupiter the CA is horendous.. I suppose a well corrected flourite glass acro will show less CA but still get you the wide fields of a shorter achro. As always these scopes come at a cost; so i suppose there is a compromise.

Ben.

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