Can somebody explain please?

[emadmoussa]

The Sky-watcher Evostar 80ED has a maximum practical power of 160x (Sky-watcher website)- while on FLO website it's 240x (potential).

I pushed this telescope to higher magnification than 160x and it was OK - so I tend to agree with FLO - but not quite sure why the difference in specs?

While I'm at it --

What advantage does the SW 120ED have over the 80ED in practical terms?

What advantages does the SW 120ED have over a 6" (152/900) refractor -- except for the colour correction of course?

[John]

I have an ED120 and I used to have a few 6" F/8's and I've owned a few ED80's. Over the 80mm ED the 120 captures a lot more light, handles more power, shows higher resolution on the planets, better views of DSO's and splits tighter doubles. I notice the 120's advantage over my 102mm ED so it will be even more so over the ED80. The downside is that it's a larger, longer and heavier tube of course but it's a lot more compact, lighter and easier to mount than a 6" refractor.

I've found the ED120 to equal the detail I could see on planets and the moon with the 6" refractors but aperture wins on DSO's so the 6" fracs win there. It depends on your interests I guess.

Visually the ED120 shows no false colour around the Lunar limb, Saturn, Jupiter and Mars. It does show a little around Venus and the brightest stars but it's pretty well controlled for an F/7.5 12cm refractor.

Re: the difference in max magnifications with the ED80, it all depends on conditions and subject being viewed. Under excellent conditions on binary stars or possibly the Moon, 240x may be possible. 160x-180x would be a more regular max power though, I reckon.

[emadmoussa]

I have an ED120 and I used to have a few 6" F/8's and I've owned a few ED80's. Over the 80mm ED the 120 captures a lot more light, handles more power, shows higher resolution on the planets, better views of DSO's and splits tighter doubles. I notice the 120's advantage over my 102mm ED so it will be even more so over the ED80. The downside is that it's a larger, longer and heavier tube of course but it's a lot more compact, lighter and easier to mount than a 6" refractor.

I've found the ED120 to equal the detail I could see on planets and the moon with the 6" refractors but aperture wins on DSO's so the 6" fracs win there. It depends on your interests I guess.

Visually the ED120 shows no false colour around the Lunar limb, Saturn, Jupiter and Mars. It does show a little around Venus and the brightest stars but it's pretty well controlled for an F/7.5 12cm refractor.

Re: the difference in max magnifications with the ED80, it all depends on conditions and subject being viewed. Under excellent conditions on binary stars or possibly the Moon, 240x may be possible. 160x-180x would be a more regular max power though, I reckon.

Very good answer. Thanks!

[AlexB67]

Nothing more to say, all that John said is what counts, only to add to take the practical limit is a very basic guide, all sellers use it. You can see it in a few ways, as a rough rule 2 X aperture or the point at which the exit pupil becomes 0.5 mm. Anything above that limit things will just become blurrier ( exceeding the diffraction limits of the scope ) , and become darker and therefore reveal no more detail. More often, as already pointed out, other criteria as to what is a useful magnification for an object in question must also be considered, for example, the power per inch criterion can be useful also.

A very brief write up here at cloudy nights I had in my list.

http://www.cloudynig...sb/5/o/all/vc/1

it points out the essentials without having to read a book

[ronin]

Compatibility between the scope and the eyepiece will be a factor.

I have 2 identical small scopes and the image quality is different between the 2 with the same eyepiece, moved between the 2 scopes. Small but one works better then the other with the same eyepiece.

The TMB and BST eyepieces from Sky the Limit are the same cost but one will be better then the other.

An 8mm TV plossl will I suspect be better then an 8mm GSO plossl, so if a good barlow is attached to get to 160x I would expect one good and one not so good.

This is the problem of saying something like "I can get 200x", you might with the items you have, another person may have a barlow and eyepiece that are specified the same but get a different view. Would you expect the same quality image from a 3.2mm TMB at £47 as from a 3.2mm Ethos (or whatever) at £500.

Another factor is simply do I consider the image you got good or not? If you do and I don't then was it a good image? Neither of us are right and neither are wrong.

So a lot comes into it.

[JamesF]

It also seems quite common for manufacturers to state the maximum magnification as twice the aperture in millimetres when in some circumstances it may be less and in others better might be possible (and then there are those who are in dreamworld).

It may be that if you start from Rayleigh's equation for the limit of resolution that twice the aperture in millimetres falls out as a good approximation of the magnification beyond which nothing further can be resolved or something like that (I'm guessing here -- I'm too tired to think about the maths at the moment), but that doesn't necessarily mean you won't get a nice steady clear image if you push it a bit further, particularly if the scope is well-corrected.

James