Telescope power?

Plug

Well 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?
cheers.

peterako

This and more in scovered in Slade-x's FAQ.

I'm not an expert...but this is my understanding:

Aperture equates to the light gathering power.

Basically the bigger the apperture the more light the scope can gather.
The more light the scope can gather the more detail that can be displayed on a given object.

An anaology would be looking at an object in bright daylight compared to viewing the same in twilight. Daylight being gathering a lot of light, twilight low gathering. (Same effect as apperture difference)

Focal Length of Scope/Focal Length of Eyepiece = Magnification

I may have the terminology wrong here, but an example:

The 130mm, 650mm Focal Length scope would give the following maginifications:

20mm Eyepiece: 650/20 = 32.5
10mm Eyepiece: 650/10 = 65
.
.
.
2mm Eyepiece: 650/2 = 325

BUT...there's a catch.

The aperture will dictate how useful the magnifaction is!

The useful magnifaction of a scope is approx 2 x the Aperture in mm as per Slade_x's excellent FAQ: http://www.boards.ie/vbulletin/showthread.php?t=2055203633

I have a 130/650mm Scope nad it is great up to about 160x
.

Often the seeing conditions on a given night are the limiting factor (how 'clear' the atmosphere is).

I also have a 200mm scope with 1000mm focal length which can give great viewing at magnifaction that exceed 300x but on average seeing nights will struggle to give clear images at 200x and on poor nights is no better than the 130mm scope.

And, of course, for the last 6 weeks there's been NO magnifacation possible...Blinking CLOUDS!

Hope this helps,
Peter

backboiler

A long focal length telescope means that, for a given magnification, you can use a longer focal length eyepiece. This is an advantage because typically a 25 mm one is much easier on the eye than a tiny 4 mm one.
Now that's an extreme; it'd need a telescope focal length a factor of 6 greater to have equivalent magnification but the principle holds with smaller differences.

Plug

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.

The focal length does not really have anything got to do with detail then just magnification and if they have roughly the same useful magnification does it really matter? Bottom line will I see the same or better?
cheers lads!:)

djhaxman

Aperture ratio is focal ratio - it's the focal length divided by the aperture, so in your scope it's 650/130 = a focal ratio of 5.