achromatic vs apochromatic

[marcusk]

Hi

I have read the technical diference between these two types of refractor, but do you think the difference would be noticeable to a beginner/untrained eye?

Would a 80mm Apo give some super crisp contrasty wide field views that maybe a 120mm achromatic may miss and while the latter will have more light gather it won't be as good quality visually - but will I notice?!

A few things to note - this is my first scope and ultimately I will be looking at imaging over visuals (although appreciate that may not be on this scope) and I have a 'good' eye and I'm fussy over image quality (work in graphics and animation production so I know good quality when I see it!)

Cheers

Marcus

[Capricorn]

Owing to spherical and chromatic aberration the achro will not produce the quality that the apo will. A 120 will produce both CA and SA. So if quality is paramount then really the apo is required.

Equally why is it so important?

Working in graphics might mean that you can tell a good image from a poor one, doesn't mean that it bothers you.

I work in optics, nothing at all to do with graphics, and also know what optical quality is, measure down to 1/10 green wavelength which your graphics gets no where near.

[Bizibilder]

I own and use both an Achromatic and an Apochromatic 120mm refractor and when used visually there is a slight difference. However this is only really noticeable on very bright objects like Venus. I am quite happy with the Achromat for visual work - usually the Moon. For imaging I can't tell them apart for white light Solar imaging with a DSLR - but for DSO long exposure imaging they are a world apart!! The APO winning hands down every time.

As a beginner I would go with the achromat and see how you get on with it. You can get filters designed to reduce any colour fringing that you get - but as I said, I personally don't find them necessary - its a decision that you have to make for yourself.

The final decision may well come down to cost!

[sologuitarist61]

I own two achromat refractors and in the end you get used to working around the problems. A lot can be done with a minus violet filter and post processing - the main point of course is that the Achromat are a darned sight cheaper, so if your budget doesn't stretch to an Apo then the Achromat is a good choice.

[DirkSteele]

As already mentioned above, colour fringing (the main issue with Achromatic Refractors) is really only an issue with bright objects such as the Moon and planets. And whether that fringing is a problem or not (for visual use) is very much a personal thing. I know many observers who have no issue with it all, where as others find it very objectionable. I fall in the second category which is why both my refractors are Apo, despite being a visual only observer.

The difference is more apparent in astrophotography.

Quality of image also has a lot of other contributing factors, other than colour correction. Not all scope optics are created equal, regardless of their colour correction characteristics. An argument could be made that in general Apo's produce better overall images (talking visual again) because extra effort has most likely gone into their design and manufacture. But in reality, that is not really the case.

Good advice would be to look through examples of each, and make a personal assessment of whether the substantial increase in cost is worth eliminating the colour fringing around bright objects. It can be very expensive to eliminate though, both my 4" class Apos cost in excess of £3,000 each.

[NGC 1502]

Another point to bear in mind is that a short focus achromat will perform differently to a long focus achromat.

Short focus achromats give great low power wide views, but go downhill at medium to high power.

Long focus achromats don't give the very low power wide views but perform better at medium to high power. In fact at F10 and above can give a lovely sharp image.

Regards, Ed.

[Vlad]

Point to note.

Achromatic telescopes do not have spherical aberration. If it does, that is a fault in manufacture. Send it back. Chromatic aberration yes, they have that, but the amount depends on focal RATIO, not focal length, so an F/15 will show so little as to be hardly noticeable, in amateur sizes, but at F/5 it will be noticeable. Spherical aberration, Never!

[Marki]

I have an 80mm Apo f7, and 100mm f10 and 125mm Achromat f9. I only really notice colour on the brightest objects, and even then not too much. The achromats I own are both TAL, and are noted for good quality optics (especially for their price). To a considerable extent longer focal length will reduce any chromatic aberation and if the optics are good enough and the focal length long enough, CA will be pretty much undetectable. Obviously the trade off is a much longer scope, and something much less useful for ccd/other imaging.

My 80mm apo gives nice sharp views across most of the field, with no colour issues. My TAL100rs achromat give sharp images across pretty much the whole field, with colour only really noticeable (to me) on the fuill moon, Venus and Jupiter and the brightest couple of stars. The TAL125r is also an achromat and will resolve more, with brilliantly sharp and contrasty images, with a similar amount of colour to the TAL100rs. Both will give nice wide field views with suitable wide angle eps. I much prefer the views through the achromats to the 80mm apo - warmer, more contrasty, and sharper across the whole field.

By way of contrast to Dirk's kit, the TAL100rs costs £250.

However, beauty is definitely in the eye of the beholder. I might have very different opinion if I looked through Dirk's kit, and if I was an imager too.

[John]

Point to note.

Achromatic telescopes do not have spherical aberration. If it does, that is a fault in manufacture. Send it back. Chromatic aberration yes, they have that, but the amount depends on focal RATIO, not focal length, so an F/15 will show so little as to be hardly noticeable, in amateur sizes, but at F/5 it will be noticeable. Spherical aberration, Never!

Most chinese refractors have a degree of spherical aberration, usually a touch of undercorrection. Thats why the most popular type of Chromacor (a CA / SA corrector made in the Ukraine) was the one that added 1/6 wave over correction - combining it with the usual undercorrection gives a virtually null corrected scope as well as one that shows 90% less CA.

In practical use a modest amount of SA does not spoil the performance too much - if it did around 80% of chinese refractors would have been returned to their sellers !

The focal ratio needed to make CA unnoticeable increases with aperture. A 4" F/15 achromat will look more or less CA free much of the time but a 5" needs to be slower to get the same effect - more like F/17.

[ollypenrice]

Don't get involved in DS imaging with an achromat. The camera reaches deeper into the short violets than does the eye and the different wavelngths have different foual points. SInce focus is everythibng (or nearly!) in DS imaging... it is vital to have an apo.

Olly

[marcusk]

Excellent comments, thanks folks; again, plenty of food for thought but I get the sense that particularly for a beginners scope, a good acromatic may be best... I'll look through both if I can.

As a side note I must say this forum is incrediably active, knowledgable and positive compared to the forums I usually view. Nice one all

One point:

Equally why is it so important?

Working in graphics might mean that you can tell a good image from a poor one, doesn't mean that it bothers you.

I work in optics, nothing at all to do with graphics, and also know what optical quality is, measure down to 1/10 green wavelength which your graphics gets no where near.

A bad image does bother me!! I would way rather view a crisp view of a cluster that was smaller in the eyepeice than a large ever-so-slightly fuzzy or purple tinged cluster. Just me I'm afraid

Thnaks again all...

[marcusk]

Saying what I said, here's another interesting viewpoint....

Don't get involved in DS imaging with an achromat. The camera reaches deeper into the short violets than does the eye and the different wavelngths have different foual points. SInce focus is everythibng (or nearly!) in DS imaging... it is vital to have an apo.

Olly

But its also made me think (and I found a thread about exactly this) of what I may be dangling off the end of my scope... I have a Canon 7D and it is rather heavy. The Skywatcher Apo's seem to have a rep for not being able to hold heavy cameras, so may need to spend a lot more to get a rear end that is good enough...

[Marki]

Maybe you could get a Skywatcher 80mm ED or something and fit a better focusser? Like a moonlite (do they do those for 80mm?).

[marcusk]

Maybe you could get a Skywatcher 80mm ED or something and fit a better focusser? Like a moonlite (do they do those for 80mm?).

Yep, good call, I was just looking at exactly that... seem good quality scopes but would need to spend a bit more for the focuser.

[John]

I have a Moonlite on a Skywatcher ED120. Great combination

I'm not into imaging though, visual observing only but my heaviest eyepiece / diagonal weighs around 3 lbs and the Moonlite copes with that easily enough.

The trend seems to be back towards quality rack & pinion focusers now though.

[Earl]

I'm not so sure the Skywatcher 80ed Pro is as good a bargain as its portrayed.

For imaging you need a flattener / reducer, in my experience you have to get the matched one to save lots of hassle with spacing.

You need to upgrade the focuser if you want to put any weight on it other than a DSLR as it slips to easily.

By the time you have done this, the £'s have added up and its not looking as good as other options out there.

Dont get me wrong its a good piece of glass for the price, but if you want to take it a bit seriously the issues do start to come up.

[marcusk]

I'm not so sure the Skywatcher 80ed Pro is as good a bargain as its portrayed.

For imaging you need a flattener / reducer, in my experience you have to get the matched one to save lots of hassle with spacing.

You need to upgrade the focuser if you want to put any weight on it other than a DSLR as it slips to easily.

By the time you have done this, the £'s have added up and its not looking as good as other options out there.

Dont get me wrong its a good piece of glass for the price, but if you want to take it a bit seriously the issues do start to come up.

Ok, fair point, so what could you buy instead then, based on what you say (and going off thread...)

[Earl]

I think you will be hard pushed to get a better scope for its price than one of these with reducer and gubbins to connect your camera,

http://www.altairast...&cat=270&page=1

the price looks big vs the Skywatcher, but when you replace the focuser and get the flattener to bring it up to match this scope, I think this will still be a better choice.

Olly Penrice is testing one of these currently and so far has good things to say, also have look through this thread http://stargazerslou...atic-refractor/

Im sure badgers will be happy to tell you how he is getting on with the scope.

[marcusk]

I think you will be hard pushed to get a better scope for its price than one of these with reducer and gubbins to connect your camera,

http://www.altairast...&cat=270&page=1

the price looks big vs the Skywatcher, but when you replace the focuser and get the flattener to bring it up to match this scope, I think this will still be a better choice.

Olly Penrice is testing one of these currently and so far has good things to say, also have look through this thread http://stargazerslou...atic-refractor/

Im sure badgers will be happy to tell you how he is getting on with the scope.

Thanks Earl, looks good. Pricey, but I feel I will have to save the pennies and invest for this hobby anyway...

[Earl]

Thanks Earl, looks good. Pricey, but I feel I will have to save the pennies and invest for this hobby anyway...

I have spent a small fortune on various scopes, and could of bought the scope I currently have twice over.

Spend once and make it count is the best advice I can give.

[marcusk]

I have spent a small fortune on various scopes, and could of bought the scope I currently have twice over.

Spend once and make it count is the best advice I can give.

Wise advice, thank you.

I was just looking and reading about the Altair you linked to above; I noticed the Starwave ED80 was significantly cheaper. Is that simply because it is a doublet and not triplet lens design? Not igoring you advice in the above post just querying...

[Earl]

Wise advice, thank you.

I was just looking and reading about the Altair you linked to above; I noticed the Starwave ED80 was significantly cheaper. Is that simply because it is a doublet and not triplet lens design? Not igoring you advice in the above post just querying...

AH HAA !! the raw nerve.

The Starwave is a Semi APO the Build Quality is excellent however the optics are not an Astrograph. I had one of these and was disappointed but only due to it been advertised as an astrograph. As a Semi APO it is very good.

[laser_jock99]

AH HAA !! the raw nerve.

The Starwave is a Semi APO the Build Quality is excellent however the optics are not an Astrograph. I had one of these and was disappointed but only due to it been advertised as an astrograph. As a Semi APO it is very good.

The photo's in this album were taken with Earl's old Star Wave ED80:

http://s48.photobucket.com/albums/f206/laser_jock99/ASTRONOMICAL/ALTAIR%20ASTRO%20STARWAVE%20ED80/?start=all

As Earl points out there is always a bit a blue fringing which needs processing out. The build quality can't be faulted- has a better focusser than my Equinox 120ED. Good beginers scope- but a triplet should be considered for serious imaging.

[marcusk]

haha, yes I couldn't help but spot the cheaper one

Lovely pics there, really nice.

[Peter Drew]

Anyone seriously bothered by CA for visual use might like to consider a Newtonian reflector, these are true APO's and cost a fraction of the price of an APO refractor.