Am i retarded?

[vinny]

2 quick questions i am not sure if there is a simple answer or if i am being a complete plonker,

1:

I know if you place a 2x or 3x barlow between the eyepiece and the scope it makes the image darker,but if it magnifies the image by 2 or 3 times why does it not magnify the light by this amount as well?????

2:

Why has a scope got a maximum magnification and how is this worked out ? for example could i use a 5mm eyepiece and a 3.3x barlow on my scope (114mm X 1000mm reflector) and if so what sort of magnification would i expect??

Thanks in advance for any help anyone can give.

[Doc]

You are not retarded, these are good questions.

1) I would say the light has more glass to enter and get through, thats why the image appears darker.

2) You have to know your Focal length which in your example is 1000mm. So say you have a 10mm eyepiece you divide 1000 by 10 and you get a magnification of x100. Add a 2X barlow then your mag goes up to x200 because your 10mm becomes a 5mm.

[red dwalf]

don`t know about question one but as for question two,

if you have a 1000mm focal lenth scope with a 5mm eyepiece it will give you 200x magnification ( 1000 divided by 5 = 200 )times this by 3 i.e. the 3x barlow and you will have 600x magnification which is a lot

[SteveL]

the barlow makes objects darker because it is taking the object, with a certain amount of light, and magnifying the object size... so that same amount of light is now spread out over a larger area, hence why it is darker....

[brianb]

1) I would say the light has more glass to enter and get through, thats why the image appears darker.

That effect exists but is insignificant.

The cause is that the light is captured by the objective lens (refractor) / primary mirror (reflector) - the bigger the more light - but when you magnify the image more the available light gets spread out over a bigger area, so the brightness appears to be reduced.

Except for stars, which should remain a simple point up to a very high magnification.

[Merlin66]

The maximum usable magnification is related to the resolving power ( basically the aperture) of the telescope, generally is 50 x objective in inches ie for a 3" refractor you pushing things beyond about x150 magnification,

The usual example given is try to magnify a picture from a newspaper with a magnifying glass; as you increase the magnification the image "breaks down" into the dots they use to print the image; the same thing happens with the telescope and high magnification is called "empty magnification" as there's no more detail to be seen... it's just not there......

Hope this helps..

[hal2000]

A telescope doesn't have a maximum magnification as such, but will often have a recommended maximum. This is usually 50x per inch of mirror width, so a 4" mirror would have a maximum of 200x, a 6" 300x etc. It is possible to achieve higher magnifications than these under ideal conditions (ie a very calm atmosphere), but these conditions would be very rare under UK skies.

In reality, if you magnify too much, firstly, it will be very difficult to focus cleanly, and secondly, the object will shoot across the field of view very quickly.

Anyway, high magnification is not really the object of the exercise, it is better to have a nice well-focused object which will stay in the field of view for a while, and will be easy to follow across the sky.

[ollypenrice]

Another way to answer question one would be the inverse of SteveL's good answer; imagine looking at an eyepiece full of moon without the barlow. You are looking at a certain area of moonlit surface. Now put in the barlow. You increase the magnification and so reduce the field of view. Now you are looking at less illuminated surface so are getting less light.

Olly

[vinny]

Thanks to everyone for the help