CA - does it bother you?

[Moonshane]

That's where I am coming from really Gaidis. It's not so much the blue halo on the limb but the general fuzziness caused by CA that bothers me.

[Dude_with_the_tube]

That's where I am coming from really Gaidis. It's not so much the blue halo on the limb but the general fuzziness caused by CA that bothers me.

completely agree Shane, no issue with the halo around the limb and i could actually live with that halo,but i do dislike the fuzz it causes and i feel like being robbed

[cotterless45]

I get very little ca with my old Vixen 102M, it just gives lovely contrasty views.

However , I suffered a bit using my 150 f8 achro. After reading several reviews and asking around, I tried a Baader semi apo filter.

Just fitted it into the 2" diagonal. Now in theory it shouldn't affect sharpness of the view. With my eyesight, contrast and fine details are much improved and there's greater distinction in close binaries.

LeeB came up one night when the seeing was extra good. We had little trouble observing the centre rill of the Alpine Valley.

I can really recommend this filter , certainly in my case as a resolution booster !

Nick.

[Moonshane]

is there any affect on the general colour cast?

[John]

That's where I am coming from really Gaidis. It's not so much the blue halo on the limb but the general fuzziness caused by CA that bothers me.

I tend think this way as well. I suppose I'd like as much of the light gathered by the objective to be focused in the right place, ie: within the airy disk. I believe that some of the faster achromats struggle to achieve diffraction limited performance (ie: around 0.80 strehl) because of the levels of CA and SA.

When I was playing around with achromats and chromacors I was quite surprised how much the overall image quality, not just the CA visible, improved when the SA was corrected as well as the CA. The scopes performance without the chromacor was quite respectable but with the correction I realised how much was missing.

I did try a William Optics VR-1 filter as well and that reduced the visible CA by around 50% but it did add a yellowish tint and did not sort out the SA of course. Filters remove light of course - ideally it would be put back into the airy disk, which the chromacor could do.

[Dude_with_the_tube]

the only issue John,try and find chromacor as they are not produced anymore,and the ones what become available demand a bit silly money.

What really leaves only couple of options if you want an almoust CA free image:get a long focal length Achro (i am talking F12-15 or even F18) but this makes the OTA very long and troublesome for mounting,in some cases also the weight of the thing will become issue.Option two and three: Get a nice ED or better option get a nice juicy Fluorite and you will have pure image perfection thats related to refractors,or option 5>get a nice F6-8 dobsonian for planetary.

or last option:just accept the image as it is and live with the CA and other issues.

[John]

They are expensive and very hard to find now Dude but I managed to find 3 over a period of a couple of years of careful searching - one was an very rare Chromacor II. All have gone abroad to other owners now.

I agree they are not a serious option now but they did illustrate to me the potential of 6" refractors when 90% of the CA and the SA are properly corrected and gave me very close to ED doublet performance at a fraction of the cost of a 6" ED doublet.

[DeejayP999]

I conclude that I find CA less bothersome than Diffraction Spikes and Coma.

Maybe I just prefer 'fracs, CA and all.

That said, apart from Apos, I've always gone for long focal length scopes.

[nightfisher]

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.

I have found the CA on TAL100`s to be barely noticeable and SA non existent, i have been very kindly loaned an older 100r TAL with the purple lens to run a comparison old and new, it has been said the older ones show less false colour, i guess i will find out for myself  

[John]

I have found the CA on TAL100`s to be barely noticeable and SA non existent, i have been very kindly loaned an older 100r TAL with the purple lens to run a comparison old and new, it has been said the older ones show less false colour, i guess i will find out for myself  

That will be a very interesting comparison Jules

I have a copy of the Neil English "Astronomy Now" (I think ?) review of the TAL 100 which covered both scopes as I recall, or at least had photos of both types in it. I'll have to dig it out again !

[Mr Spock]

I moved from a ST80 to an 80ED and the difference was huge. Stars are a lot cleaner and sharper plus you actually notice the star colours more.

For a long time I had just a 120mm achro and a 140mm Mak. The 120 achro would have similar detail to the 140, but was hazy and smeared due to CA. The clean image and solid blacks of the Mak won every time; despite the long cool down time of the Mak it was my preferred choice.

[Moonshane]

I have found the CA on TAL100`s to be barely noticeable and SA non existent, i have been very kindly loaned an older 100r TAL with the purple lens to run a comparison old and new, it has been said the older ones show less false colour, i guess i will find out for myself

Mine shows no false colour other than red as it has a DERF permanently fitted for solar ha use :0)

[nightfisher]

I moved from a ST80 to an 80ED and the difference was huge. Stars are a lot cleaner and sharper plus you actually notice the star colours more.

For a long time I had just a 120mm achro and a 140mm Mak. The 120 achro would have similar detail to the 140, but was hazy and smeared due to CA. The clean image and solid blacks of the Mak won every time; despite the long cool down time of the Mak it was my preferred choice.

I remember that 140 Mak well, there has been a good few times i wish i still had it, may even be tempted with another one day

[Mr Spock]

I was just thinking I wish I still had it...   Best scope I've had for double stars.

[nightfisher]

I was just thinking I wish I still had it...   Best scope I've had for double stars.

Yes, Maks are very good for doubles and a great many other targets, i will have to meet up with Shane and let him try the 150 pro mak against his 150 f11 newt

[kev]

Not sure im bothered by ca. Im more bothered about the contrast and how that is affected by the sum of all the aberrations.

A st80 at f5 is bound to have a large degree of ca, and other abbs which are apparent at high mag. A 80ed is well corrected in ca and it shows too. Its also a longer f ratio than a st80 so the other abbs should be less. At the other end a f11 6" newt has very few aberrations at all, so should be nicely contrasty. Also of course as f/ gets higher the tolerance in focal point gets proportionally more.

Remembering incidentally that by using a barlow at eg x2 makes the focal 1/mag (eg 1/2) much tighter).

Me. I use a st80 (with a sa filter) as a dump it in the boot working away from home. A st102m vixen for sun work (generally filtered green), but also as a quick view in gaps scope, and a 8" f6 as the main scope. I find f6 to be a min easily useable without being too picky about focus.

Sent from my iPhone so excuse the typos!

[Donkeiller]

Small amounts of CA, like I get with my 102mm f/6 Kson ED, do not bother me.

The truck-loads I got with a SW 120ST f/5 achromat did.

[mikeDnight]

Hello moonshane

This is my first post on the forum. Even though I've been an active observer for 35 years I've rarely ever got involved in debates about telescopes. Yet your question "Does CA bother you? led me to join your forum debate.

It has to be said that a 6" F11 Newtonian will take some beating. As a lunar and planetary instrument it will likely be able to give any top class refractor achromatic or apochromatic a run for its money. It won't however be able to match your F5 refractor with regard to width of field and while sweeping star fields the CA won't be much of a distraction. Like tools of a tradesman i suppose each telescope design can be thought of as having its own speciality.

Twelve years ago I was able to pit three refractors against each other in a side by side comparison. They were a Helios 150mm F8 achromat, a 100mm F10 Tal achromat and a 102mm F9 Vixen fluorite apochromat. Each scope was trained on Saturn, which was low in the eastern sky. Each scope at this point gave similar views and I was supremely confident that the 150 achromat, because of its supposed superior resolution, would thrash the other two scopes as the planet climbed higher. How wrong I was! As Saturn climbed high in the sky the little fluorite quite literally demolished the competition. I was stunned! I also drove home that night wondering where on earth I was going to find the £2,200.00 I'd need to buy my own Vixen fluorite. Then came another worrying thought "How am I going to get this one past the wife?"

To this day I am convinced that the reason the achromats couldn't compete with the apo was because of their subtly destructive CA. Even at F10 they are short focus refractors. Pit the Vixen against a F15 of the same aperture and the scope would doubtless have met a worthy opponent. Having said all this, the 150mm F8 greatly outperformed the 102mm fluorite on the deep sky. Horses for courses!

Since that first night with the Vixen I have bought several apochromatic refractors, the largest being a 152mm fluorite doublet by Takahashi. The smallest is a Equinox 80. I've spent a small fortune on my insane love of refractors, so I suppose in answer to your question "Does CA bother you?", the answer has to be " Yes!"

mikeDnight

[John]

Welcome to the forum Mike - interesting post

A Vixen FL102 is a scope I'd love to own. My Vixen ED102SS is the closest I've got .... so far !

[hunterknox]

CA is the last thing that bothers me, as in it does bother me, but it's the last thing on my list of things a scope has to get right. Any kind of perceivable softness (spherical aberration), coma, diffraction spikes usually bug me more.

I've tried various ED refractors and have been impressed every time. They do enough of a job on CA that I'd definitely consider them before a Newt or catadiatropic of some kind, but then I'm a refractor guy at heart.

[Moonshane]

Great post Mike, thank you.

I suppose you hit on the reasons I made the post in the first place:

• Compromise is always a factor in telescope choice (be it cost, optical quality or bulk).

• Different scopes suit different objects. 

• Different scopes (and aberrations) suit some people better than others.

• People will rarely agree.

In my case I seem to prefer to concentrate on what I am actually looking at rather than the periphery. This I think is why CA bothers me more than say SA as the latter tends to materialise in the edges of field. CA, however, to my eyes at least manifests itself all over the field other than directly on axis.

For example, my 120mm f5 Skywatcher frac is pretty bad for CA on the moon and also has some SA. However, I bought this scope mainly for white light solar use and the aperture makes detail much more apparent in the central field which is what I am looking at, CA is not an issue in such a narrow band (I use a solar continuum filter) and I don’t even notice the SA as I am looking at the middle of the field and sketching what I see. Also the other main use (wide field low power observing) does not really reveal either aberration unless you really look for it.

My 6” f11 newtonian has a curved spider fitted which has removed entirely the diffraction spikes and it works beautifully on planets and the moon. It’s also superb on doubles but there is a niggling flare off the spider on doubles which means for some reason it is not as good as when I had the four vane spider. I’ll address this in due course somehow once I establish which bit of the spider is causing the flare (if I can be bothered! As it’s not a big issue and it is still a wonderful double star scope).

All this said, my 12” and 16” f4 dobsonians have very faint diffraction spikes even on Jupiter and the detail when seeing is good to great is truly spectacular. Masked to e.g.110mm or 165mm they also produce wonderful views. I am clearly becoming more proficient in justifying the scopes I have (6)  just in case the Spanish Inquisition arrive unexpectedly.