Newbie Qn: About focal length and Aperture
Hmm, i read somewhere longer higher f/numbers, e.g f/10, f/12, give dimmer but broader views of objects in the sky, whereas, lower f/numbers, e.g f/4, f/5, gives narrower but brighter views of the sky. Its like squeezing light into either a bigger area for high f/number or into a smaller area for lower f/number, thus producing either brighter or dimmer objects viewed thru the ep. Thats y high f/number telescopes are for planets and lower f/number for deep sky objects... I corrack???
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weixing
- Super Moderator
- Posts: 4708
- Joined: Wed Oct 01, 2003 12:22 am
- Favourite scope: Vixen R200SS & Celestron 6" F5 Achro Refractor
- Location: (Tampines) Earth of Solar System in Orion Arm of Milky Way Galaxy in Local Group Galaxies Cluster
Hi,
Actually, the light grathering power of a scope is not rated to f-ratio at all - the aperture is the main factor.
When people said that f/5 give brighter image than f/10... that is just a misconcept. I think most people got this idea is because of camera lens. Below is the formula for f-ratio:
F-Ratio=(Focal Length of Len) / (Diameter of Len)
It is true in camera len that a "fast" lens will generate a brighter image than a "slow" lens - how?? In camera lens, the f-ratio is control by the aperture: the smaller the aperture, the "slower" the lens; the bigger the aperture, the "faster" the lens. Remember, a scope aperture is fix. Then you will ask: How come I need to exposure longer using a "slower" scope than a "faster" scope in astrophotography?? This is only true if you use prime astrophotography(no eyepiece). Before I explain this, remember this:
As Magnification Increase, Brightness Decrease.
So, when you doing prime astrophotography, actually there is some magnification even you don't use eyepiece. In camera lens, 50mm is consider as 1x, so a 1500mm focal length scope is actually having a (1500/50) 30x magnification when use without eyepiece. So a "faster" scope with a shorter focal length will generate a brighter image, since it has a lower magnification, eg. 500mm focal length telescope - 500/50=10x.
Also, a "faster" scope will normally give a brighter and wider Field of View than a "slower" scope - why? This is because a scope with longer focal length("slower" scope) will normally give a higher magnification, so dimmer image and narrower Field of View, than a scope with shorter focal length("faster" scope) when using the SAME EYEPIECE. To show my point, below is a comparsion between a "fast" scope and a "slow" scope (both will have same light gathering power) using a 40mm eyepiece with Apperant Field of View(AFoV) of 43 degree:
102mm (4") f/5 telescope
------------------------------
Aperture: 102mm
Focal Length: 510mm
Magnification: 510/40=12.75x
True Field of View (TFoV): 43/12.75=3.37degree
102mm (4") f/10 telescope
------------------------------
Aperture: 102mm
Focal Length: 1000mm
Magnification:1000/40=25x
True Field of View (TFoV): 43/25=1.75degree
My english is not very good..
Hope the above explaination is clear enough and you get my point... 
Have a nice day...
Actually, the light grathering power of a scope is not rated to f-ratio at all - the aperture is the main factor.
When people said that f/5 give brighter image than f/10... that is just a misconcept. I think most people got this idea is because of camera lens. Below is the formula for f-ratio:
F-Ratio=(Focal Length of Len) / (Diameter of Len)
It is true in camera len that a "fast" lens will generate a brighter image than a "slow" lens - how?? In camera lens, the f-ratio is control by the aperture: the smaller the aperture, the "slower" the lens; the bigger the aperture, the "faster" the lens. Remember, a scope aperture is fix. Then you will ask: How come I need to exposure longer using a "slower" scope than a "faster" scope in astrophotography?? This is only true if you use prime astrophotography(no eyepiece). Before I explain this, remember this:
As Magnification Increase, Brightness Decrease.
So, when you doing prime astrophotography, actually there is some magnification even you don't use eyepiece. In camera lens, 50mm is consider as 1x, so a 1500mm focal length scope is actually having a (1500/50) 30x magnification when use without eyepiece. So a "faster" scope with a shorter focal length will generate a brighter image, since it has a lower magnification, eg. 500mm focal length telescope - 500/50=10x.
Also, a "faster" scope will normally give a brighter and wider Field of View than a "slower" scope - why? This is because a scope with longer focal length("slower" scope) will normally give a higher magnification, so dimmer image and narrower Field of View, than a scope with shorter focal length("faster" scope) when using the SAME EYEPIECE. To show my point, below is a comparsion between a "fast" scope and a "slow" scope (both will have same light gathering power) using a 40mm eyepiece with Apperant Field of View(AFoV) of 43 degree:
102mm (4") f/5 telescope
------------------------------
Aperture: 102mm
Focal Length: 510mm
Magnification: 510/40=12.75x
True Field of View (TFoV): 43/12.75=3.37degree
102mm (4") f/10 telescope
------------------------------
Aperture: 102mm
Focal Length: 1000mm
Magnification:1000/40=25x
True Field of View (TFoV): 43/25=1.75degree
My english is not very good..
Hope the above explaination is clear enough and you get my point... Have a nice day...
Yang Weixing
"The universe is composed mainly of hydrogen and ignorance."
"The universe is composed mainly of hydrogen and ignorance." 
if one is f/6, u calculate the aperture by dividing the focal length by f/ratio.. meaning 1000mm / 6 = 166.66mm (aperture)
as for f/10, u will get 1000mm / 10 = 100mm (aperture)
IF you using the same eyepiece, the magnification is the SAME bcoz of the same focal length.. However, the FOV is different bcoz of the aperture size and whether your scope is a refractor or MCT.. refractor has a wider angle than MCT..
as for f/10, u will get 1000mm / 10 = 100mm (aperture)
IF you using the same eyepiece, the magnification is the SAME bcoz of the same focal length.. However, the FOV is different bcoz of the aperture size and whether your scope is a refractor or MCT.. refractor has a wider angle than MCT..
*~Stars AND Clear Sky always~*
Leslie
*Owner of Antares 93mm Achromat Refractor*
-10.11.03-
*Owner of Takahashi SKY 90 Fluorite Doublet Apochromat Refractor*
-25.03.06-
Leslie
*Owner of Antares 93mm Achromat Refractor*
-10.11.03-
*Owner of Takahashi SKY 90 Fluorite Doublet Apochromat Refractor*
-25.03.06-
-
weixing
- Super Moderator
- Posts: 4708
- Joined: Wed Oct 01, 2003 12:22 am
- Favourite scope: Vixen R200SS & Celestron 6" F5 Achro Refractor
- Location: (Tampines) Earth of Solar System in Orion Arm of Milky Way Galaxy in Local Group Galaxies Cluster
Hi,
If both have the same focal length, then the one with the "faster" f-ratio scope, in this case is the f/6, will have a larger aperture than the "slower" f-ratio scope, in this case the f/10. So the "faster" scope will produce a brighter and more detail image than the "slower" scope, because it got LARGER APERTURE.
By the way, normally a "fast" scope will generate a more fuzzy image at high magnification than a "slow" scope, unless the "fast" scope optics are of excellent quality..... Now you know why those excellent "fast" scope are so expensive...
If both have the same focal length, then the one with the "faster" f-ratio scope, in this case is the f/6, will have a larger aperture than the "slower" f-ratio scope, in this case the f/10. So the "faster" scope will produce a brighter and more detail image than the "slower" scope, because it got LARGER APERTURE.
By the way, normally a "fast" scope will generate a more fuzzy image at high magnification than a "slow" scope, unless the "fast" scope optics are of excellent quality..... Now you know why those excellent "fast" scope are so expensive...
Yang Weixing
"The universe is composed mainly of hydrogen and ignorance."
"The universe is composed mainly of hydrogen and ignorance." 
weixing is right abt the brighter image, the f/6 will have more aperture and therefore a brighter image. dimmer stars will be more visible in the scope wif the bigger aperture
as for the image quality, consideration has to be given to the type of design. but it's true for the same design, a faster scope will deliver slighty inferior images, for refractor is it more false colour and not as sharp. for reflector coma will be more visible at the edge of the field and also the image will be less sharp.
"IF you using the same eyepiece, the magnification is the SAME bcoz of the same focal length.. However, the FOV is different bcoz of the aperture size and whether your scope is a refractor or MCT.. refractor has a wider angle than MCT.."
magnification will be the same, but fov will oso remain the same. the formula for calculating the true fov is given by "apparent fov/magnification". apparent fov is the size of the field u see thru the eyepiece, for normal plossl eyepiece, the apparent fov will be ard 52 degrees. for more expensive wide field field eyepiece, the apparent field may be like 82 degrees for the nagler, maybe 68 degress for the panoptics.
working out the formula u will be able to get the true fov, which is the actual area which u will be seeing thru the scope/eyepiece.
usually mct will have far more focal length than a refractor, due to the focal ratio, ard f/12 for a typical mct but only f/6-f/8 for a typical refractor. therefore we usually say refractor will be able to have a much wider field than a mct/sct of the same aperture. wider field in this case means the tfov.
~MooEy~
as for the image quality, consideration has to be given to the type of design. but it's true for the same design, a faster scope will deliver slighty inferior images, for refractor is it more false colour and not as sharp. for reflector coma will be more visible at the edge of the field and also the image will be less sharp.
"IF you using the same eyepiece, the magnification is the SAME bcoz of the same focal length.. However, the FOV is different bcoz of the aperture size and whether your scope is a refractor or MCT.. refractor has a wider angle than MCT.."
magnification will be the same, but fov will oso remain the same. the formula for calculating the true fov is given by "apparent fov/magnification". apparent fov is the size of the field u see thru the eyepiece, for normal plossl eyepiece, the apparent fov will be ard 52 degrees. for more expensive wide field field eyepiece, the apparent field may be like 82 degrees for the nagler, maybe 68 degress for the panoptics.
working out the formula u will be able to get the true fov, which is the actual area which u will be seeing thru the scope/eyepiece.
usually mct will have far more focal length than a refractor, due to the focal ratio, ard f/12 for a typical mct but only f/6-f/8 for a typical refractor. therefore we usually say refractor will be able to have a much wider field than a mct/sct of the same aperture. wider field in this case means the tfov.
~MooEy~