question about ideal match of eyepiece magnification and scope (1 Viewer)

[Jason Bugay Reyes]

i've been wondering since i looked through my friend's 60mm scope with 45x eyepiece fixed lens and what is the main reason why it is a bit dark compared with same scope but using, let say 20x or 30x eyepiece ?

in theory, is 60mm scope suitable using more than 30x eyepiece ?

and why when we use zoom eyepiece, started from 20-30x, the view is superb and bright, but from 40-60x, it gets darker and loss in detail too ? assuming using 60mm or 82mm scope.

why cant the brightness and sharpness of zoom eyepiece from 40x and above can be similar like from 20-30x mag ?

is it because of the element in the glass of the eyepiece or it has to do with the light theory ?

and what is the ideal fixed eyepiece magnification to match with the scope in terms of bightness and sharpness in the following sizes (exclude zoom eyepiece) :

1. 50mm scope
2. 60mm scope
3. 66mm scope
4. 77mm scope
5. 80mm and above scope

hope someone can help me with this

thanks :-O

p/s im using 82mm scope with 30x eyepiece and im happy with it hehehe :king:

 

[dahyon]

The brightness of an image depends upon a number of factors:

• Magnification applied to the image at the eyepiece;
• Intensity/quality of the light coming from the object being viewed
• Diameter size of the telescope objective as a light gathering lens.
• The type and quality of glass used for the objective lens and eyepiece.
• Transmission and reflection loss of light in passing through the instrument.


The exit pupil refers to the size of the circle of light visible at the eyepiece of a telescope in form of a disc. The larger the exit pupil the brighter and sharper the image.

To determine the size of the exit pupil, divide the objective diameter by the magnification:
60/20 = 3mm
60/30 = 2mm
60/60 = 1mm

RELATIVE BRIGHTNESS INDEX (RBI)
Relative brightness index or RBI is used as a comparison of image brightness but it does have limitations. It is determined by square of the exit pupil.

RBI = Diameter / Magnification = square of exit pupil.
Example :60D and 20x gives = 3 x 3 = 9 = relative brightness.


TWILIGHT FACTOR
Twilight factor is a measurement of viewing sharpness and image detail in low light conditions, this is also considered a flawed measure. The larger the twilight factor, the more efficient the low light performance. To calculate the twilight factor, take the square root of the magnification multiplied by the objective lens diameter.

Example on a 80D 20x:
TWILIGHT FACTOR 80x20 = 160 v (square root) = 40 = twilight factor
This formula takes magnifications more into account than the relative brightness index and is a better indication for the quality of brightness. When observing in greater detail a distant object you will seek to obtain a higher contrast and therefore will have to compromise between the magnification and the objective lens diameter.

Essentially your choice of equipment depends upon what you are looking at, how far away it is and the quality of light available not forgetting that your eyes will also behave differently with varying quality of light.

GOOD LUCK

 

[iporali]

what is the ideal fixed eyepiece magnification to match with the scope in terms of bightness and sharpness in the following sizes (exclude zoom eyepiece)

Hi horukuru,

In my experience the point when the image starts to look not-so-bright and not-so-"enjoyably"-sharp is when the exit pupil diameter shrinks to roughly 2mm (or less). This means magnification of 25x on a 50mm scope, 30x on a 60mm and so on (with binoculars this is maybe 2.5mm). Of course this is just my opinion and in spite of darkness I often find higher powers very useful for ID'ing, but on my 80mm scope I often get the same feeling at powers of 50-60x as you did with the 60mm scope at 45x. I think it is just that if the eye pupil is significantly larger than the exit pupil of the scope/eyepiece, the eye feels disappointed .

Best regards,

Ilkka

 

[Jason Bugay Reyes]

thanks a lot now i understand hehehe