Resolution vs magnification

[spaceboy]

What is the better at the end of the day? I ask this question as I have on more than one occasion found, under my skies at least that a 4" scope will knock the socks off any of my other scopes for sheer steadiness of viewing. It is like it is the magic bullet for shooting through all but the worst of seeing conditions.

It then got me thinking which would be the better, a smaller scope that can achieve much higher magnifications and so closer views to make seeing smaller moon / solar / planetary details or a larger scope with less magnification but with better over all resolution just smaller image due to the lower magnifications.

I understand that most will just bump up the magnification on a larger scope and settle for those fleeting moments of good seeing but it was more a curiosity question of what would at least on paper offer more. I have tried to find software that will give a resolution calculation to answer this but they all seem to be restricted to exit pupil, mag, fov, surface brightness, ppi, etc...

[feras281]

I have got a 4.75 inch refractor and an 8inch dobsonian, I find that the smaller aperture refractor can handle higher magnifications much better for example looking at the cassini divison in saturn rings, at around 200X I have noticed the larger aperture does blur even though its stated limit is 400X . From what I understand the larger the aperture the more vulnerable to atmospheric distortion but I am not too sure the exact formulae or relationship, the advanate of coure of the large aperture is looking at faint DSO at low power which would be more visible

[faulksy]

i have owned a few cats and now my dob, the best cat i had was the celestron 9.25 which was great but could never handle magnification as good as my dob, i can vertualy always push over 300x on globs and the same on planets most cases even higher, also consider the image scale with bigger aperature

[John]

My 4.7" ED refractor gets close to the image quality (resolution and contrast) that my 12" dobsonian delivers on the planets under average seeing conditions but the larger scope pulls away under better conditions. The refractor excels between 180x and 250x on the the planets wheras the 12" dob gets into its stride at around 250x-320x. The 12" scope shows details on the Moon that I've just not been able to see with the 4.7" refractor.

On double stars I prefer the nature of the view that the refractors deliver although the larger aperture scope will resolve tighter pairs. The 12" scope is the only one (so far) that has split Sirius for me.

My refractors do better compared to the larger scope than the substantial gap in aperture might suggest but ultimately the 12" scope will outperform them unless the conditions are really poor.

The seeing conditions are the most substantial factor affecting performace.

[ollypenrice]

First of all you have to compare optical quality with optical quality. The cost of an optical surface of a given quality goes with its area, so doubling the aperture means quadrupling the area and so quadrupling the cost of the figuring, more or less. Is the larger scope figured to the same standard as the smaller one? It perfectly well can be, but is it?

The you have to bear in mind that faster F ratios cost more to make than slower ones. Are these comparable in the two scopes on test? And faster systems are more sensitive to collimation.

Only then do you get down to the real business of aperture and seeing. They say small apertures beat bad seeing better than large and the theory sounds convincing to me - but without really comparable optics of different apertures I think it might be hard to say.

I sometimes compare a 5 inch apo on the planets with a 10 inch SCT. The apo usually wins but certainly not always and their optical finish is different, the small one being better - and the small one cost a lot more.

Olly

[spaceboy]

I know seeing can be a pain where I am. Apparently we in the UK sit between Ferrel and Hadley weather cells and add to that the jet stream and it's all a recipe for mush. It would appear that all this goes on over the West midlands for a good proportion of the year if my skies are anything to go by

[spaceboy]

First of all you have to compare optical quality with optical quality. The cost of an optical surface of a given quality goes with its area, so doubling the aperture means quadrupling the area and so quadrupling the cost of the figuring, more or less. Is the larger scope figured to the same standard as the smaller one? It perfectly well can be, but is it?

The you have to bear in mind that faster F ratios cost more to make than slower ones. Are these comparable in the two scopes on test? And faster systems are more sensitive to collimation.

Only then do you get down to the real business of aperture and seeing. They say small apertures beat bad seeing better than large and the theory sounds convincing to me - but without really comparable optics of different apertures I think it might be hard to say.

I sometimes compare a 5 inch apo on the planets with a 10 inch SCT. The apo usually wins but certainly not always and their optical finish is different, the small one being better - and the small one cost a lot more.

Olly

A valid point Olly but good or bad optics you can tell what is effected by seeing and what won't focus because of poor optics. I do get your point though.

[jetstream]

Dob vs frac?

[brantuk]

Dob vs frac?

DIY - don't involve yourself lol :grin:

[John]

Dob vs frac?

As a Bowl of Petunias once thought "Oh no, not again !!!"

[spaceboy]

Sorry I didn't intend this to turn in to a design debate. If anything my guess is the same rules would apply across any two apertures in any given design although for the record I have found closed tube designs to be less effected by seeing that open tube not to mention that the optics in refractors in particular seem to cool quicker than those employing mirrors in the light path.

For arguments sake then. Say I have a 4" refractor OR reflector and a 6" refractor OR reflector both of equal optical quality and focal length. As already agreed that smaller scopes do seem less susceptible to seeing conditions so can achieve slightly higher magnification. SO again for sake of argument the 4" achieves x120 where the 6" achieves x100, which out of the two is going to offer the better views in most peoples opinion ? The 4" with more mag allowing more of a close up view or the 6" with lower/ smaller magnification views but with higher resolution ?

[nightfisher]

Spaceboy I'm with you on this, my 4" f10 always seems to shine through unless the seeing is spot on then the 150 is the one to use

[spaceboy]

Your in part a reason why I went a head and posted this thread. I think we all know you like buying scopes but you always end back where you started with the 100RS. I think the combination of good sharp optics and the 4" aperture of the Tal makes it a keeper. I know I have come very close to owning a 100RS on a couple of occassions. As said in this post 4" under my skies seems uneffected by seeing 90% of the time where as my larger scopes seem to struggle. OK the 4" reaches it's magnification limit quicker so on those nights of good seeing a bigger scope comes in to it's own but for observing miles the 4" keeps giving.

[nightfisher]

Get a nice 100rs and you won't look back