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Any chromatic abberation would only become noticeable on the brighter stars, and it wouldnt be obtrusive. Also with a refractor you'd get a wonderful, piercingly sharp, wide field that a Newtonian could not match, with less coma and no spider diffraction or central obstruction. Central obstructions in reflectors limits how low you can go with magnification as you get a blind spot in the centre of the field at very low power, but not so with a refractor. Diamond dust on black velvet is what the rich field frac will offer to advantage!

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However, if you take a top quality long focus achromatic, with very good figure and polish (as the Tak has, but in  a shorter FL), the difference will be much less. One such was the D&G (USA) 5" F15 achromatic I used to own: the contrast on that scope was very close to that of my FS128, and the long focal length meant that you could use very high magnifications - which again aid contrast when viewing fainter DSOs, helping them to stand out more). But the long (2metres!) tube was a real pain to mount, unlike the Tak.

I've owned both and obviously the APO is better but 4k better ummm I currently have the 152 starwave and so far very impressed with it deep space and planetary are very good even on the moon the CA was very low and not at all as obvious as previous f8 refractors at 6" I've owned if it's a one scope for life I'd say get the APO but if your going to own several scopes  I'm happy with my current choices

Of course there were differences in exit pupil and magnification and a long focal Achromatic will perform really close to the FS as you know from real data.

But most DSOs in any 5" scope in most UK skies will never be really impressive..if you crave those sort of views, get an 8-15" Dob!??

Having said the above, I will say that the 6” can hold on its own as a great telescope. It is a fantastic RTF which can be pushed to high magnification, but there is a reason why one sells for a multiple of the other.

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Long answer: A cherry picked 6” F5 TS Achro which I owned and sold to a friend against a 5” Tak FS-128 resulted in my friend purchasing a Tak FS128.

I had a chance to compare a 6" f8 Celestron achromat against a TEC 140 once, and at low powers with a 31 Nagler it was very similar. So in a way yes, if you're looking at big dim things in wide fields such as M31, to my eyes it was 95% of the performance for 10% of the cost. At higher magnifications the difference was more obvious though, stars not just quite such tight pinpoints in the achromat and chromatic aberration around the brighter ones. Small deep sky objects were still pretty good in the achromat, but didn't just quite have the edge of the apo. Likewise, Jupiter was quite well resolved in the achromat but again, there's the purple halo which does detract a bit. The achromat was still a pretty good telescope especially considering it only cost about £300! If you happened to  have a couple of grand sitting in your pocket a 6" apo would definitely be noticeably better as an all rounder, but for many people there are other priorities like feeding your family! In which case the achromat would still give a lot of enjoyment

Thanks to all the responses. I was thinking an achro would do just as good with deep space objects. Most deep space objects (DSO) are in black and white, was my reasoning.

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I prefer to forego other “luxuries” (keep my car for a few years more, skip on buying so many clothes etc) but after learning how to truly see, I cannot unsee.

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Having said that, I would still be equally happy to have the 6” Achro. Just to be out there and enjoy the hobby - that’s what it’s all about.

A 6" newtonian would be a better choice I feel for faint objects. Especially as it gives a nice wide field with no chromatic aberration. It's also a lot lighter and easier to handle and mount than a 6" achromat. All that said, I tend to prefer my 120ED for most objects now unless I need more aperture with my 12" and 16" dobs.

Thanks to all the responses. I was thinking an achro would do just as good with deep space objects. Most deep space objects (DSO) are in black and white, was my reasoning.

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Plus reflectors typically have 92% - 94% reflectivity. Two surfaces therefore a loss of light of 12% - 15% and it is noticeable when everything is added together with the central obstruction.

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I would suggest that you wait until SkyWatcher comes out with the new 6” F8 ED - of undisclosed ED element so it is not FPL-51 or 53 but SkyWatcher does know how to make brilliant scopes (ED100 and ED120 and Esprit series for example), so my money is that it will be an excellent performer in the sub €2,500 category.

Side by side testing with same eyepieces on same objects indicated much better contrast on the Tak which more than made up for the increased light gathering of the 6” Achro. Significant difference on scatter control, snap to focus and contrast. Significant is to the eye of the beholder of course. After so many years of testing various scopes and training the eyeballs to see minute detail, what is significant to me may be something of a non-issue to another person.

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The TS F5 is designed mainly for low to medium power wide views, which will give, by definition, brighter backgrounds, which aren't ideal for DSOs.

Nicos pretty much hit the nail on the head! The high index glasses used in modern apo's allows for better light through- put than achromatic glass types, with fluorite at the top of the hill. Also, the level of optical correction on these high end refractors means sharper images, even at high power. Yet even between apo's there can be a noticeable difference. Some years ago in a S&T apo review, three top 5" apo's were put to a side by side test over several nights. Just to make things interesting, there was also a 7" Intes Mak Newt' trialed alongside the refractors. Two of the 5" apo's were Top class triplets by AP and I think TMB. The third apo was the Tak fluorite doublet FS128. After several good nights of observing from a dark site in the US the majority of experienced observers felt that, although close, the FS128 gave the brightest images of DSO's. Interestingly the 7" Mak Newt', despite its greater aperture, gave the dullest DSO views.

In part it depends on the scopes compared to each other. The ones you compared were an F5 achromatic (designed to be a low power wide field scope) and a Tak FS128 (one of which I am lucky enough to own), which is an F8.1 fluorite scope with one of  the best objectives in its' aperture, ever, and known for its' superb contrast. So, not really a fair comparison.

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