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Telescope power?


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Hello people.

Just wondering here, whats the story with the focal length vs aperture? im after a 'goto' scope and need help. The telescope I have now has 130 aperture with 650mm focal length. The telescope im looking at is 127 aperture with 1500 focal length.

I thought aperture is light gathering power, so will I see much more with a longer focal length with slightly less aperture?confused.gif

Here is the difference between the two:

This is my present one:

Type Reflector

Type of build Newton

Aperture (mm) 130

Focal length (mm) 650

Aperture ratio 5

Resolving capacity 0,88

Limit value (mag) 12,4

Light gathering capacity 345

Max. useful magnification 260

And the new one:

Type Reflector

Type of build Maksutov

Aperture (mm) 127

Focal length (mm) 1500

Aperture ratio 11,8

Resolving capacity 0,91

Limit value (mag) 12,3

Light gathering capacity 329

Max. useful magnification 254

If anything the one I have now is a wee bit better is it not? Whats that aperture ratio about? If anything should I save myself about €150 and get a 'goto' scope with the same focal and aperture specs as my present telescope. All I really want is more detail and a 'goto' scope together.

cheers lads!smile.gif

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I doubt you'd notice much difference between the two scopes visually, the maksutov has the disadvantage of having a larger central obstruction, blocking some of the light coming in, but the longer focal ratio means it will need less maintenance and will be more forgiving of cheaper eyepieces.

The longer focal length also means you'll get greater magnification with a given eyepiece.

Personally, i think if you want a goto scope, the easiest thing to do would be to just buy a mount and put your old telescope on it!

however, if you want to increase the detail and brightness of the objects you're looking at, you will need to increase your aperture. all depends on your budget and preferences

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Your present Instrument has a focal ratio of f5, which is simply its focal length, divided by it's aperture. T he telescope can be rated as fast when it come to using it as a photographic lens so to speak. The relative short focal length, also translated into a wider field of view for any given eyepiece. The image size, for example, Jupiter in your telescope using a 25mm focal length eyepiece, will be quite small in size.

The Maksutov is a longer focal length, with a slightly smaller mirror.

It is a lot slower in respect of being used for imaging, but for the brighter planets and the moon, it is a much more suitable instrument, both visually, and photgraphically.

As an example If you divide the 650 focal length by 25 mm, the magnification of your scope will be x25. Using the same eyepiece in the Mak it would be. X60

Hope this helps you a bit.


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Magnification is deceided by the telescope focal length divided by the eyepiece focal length.

So taking a 10mm eyepiece and putting it in your current scope yield a magnification of 650mm divided by 10 = 65 so it magnifies x65 bigger.

The new scope has a focal length of 1500mm so 1500 divided by 10 = 150 so x150 magnification from the same eyepiece.

BUT - its not all about pure magnification because there is a limiting factor of a maximum magnification due to the size of the primary optic (either the mirror in a refelecting telescope like a Maksutov or Newtonian or the main lens in a refractor). The limiting factor is often give as 2x the primary optic mm - so if you main lens is 130mm then the largest magnification would be x260. The largest the Maksutove could run to would be x2 127mm giving x254 magnification.

In theory that all holds true but typically refelectors seldom run at close to their theoreritiocal maximum with 60% of maximum being most likley. In any event sky conditions usually limit mag in the UK to around x250.

The aperture argument is about light grasp rather tan phure magnification and light grasp is rather harder to define so readily. IN a nutshell a bigger mirror or lens will get more light and so dimmer objects show up but also crucially the image quality becomes better and so can survive better magnification.

As the focal ratio goes up (focal ratio is the ratio between the scopes length and the width of its largest optical element (so for your existing scope 650/130= f5)) then some things happen; The scopes field of view (how much sky it can see) gets narrower, the view gets darker and the contrast increases.

Broadly speaking thats why refelectors work well on deep sky targets which dont have much contrast and are very dim to start and often very spread out - refelectors have a wide field of view, biggest aperture per £ spent and dont really need high contrast as the target has none to start with whereas refractors/maks have a high focal ratio which makes them better for compact bright objects like planets, binary stars etc where light gathering isnst quite so crucial because the target is bright anyway but contrast may be needed more.

Thats a very rough and ready explanation but I hope it helps - thats all as regards pure optics but people choose Maks or Newts on other reasons as well - maintenance(or lack of), portability etc etc and obviously cost because a Mak is almsot always the most expensive type of scope for its aperture.

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