Focal Lenght/Focal Ratio

[Revan]

what's focal lenght and focal ratio? If a telescope has a higher focal ratio does that mean it has more zoom? But zoom is only effective if it has a large aperture?
I have a 10" dob and nexstar 4se both with focal lenght of 1200mm. Does that mean that they have same zoom but the 10 inch will be brighter/have more detail, or since they have different focal ratios f/4.5 vs f/12, does that mean the 4se has more zoom.

[bojan]

Wikipedia and Google are your friends....
https://www.google.com/search?client...&bih=832&dpr=1


https://en.wikipedia.org/wiki/Focal_length

[OzEclipse (Joe Cali)]

Hi Revan,

Focal length in a Newtonian/dob telescope is the distance from the surface of the main mirror in a reflecting telescope to the point of focus. You can measure the distance from the mirror to the diagonal and add the distance from the diagonal to the focal point. In your 4SE, the folding of the optics magnifies the image to create a longer equivalent focal length and can't be measured linearly.

Experienced observers will tell you that it's not all about the magnification, you are referring to magnification as zoom. Mirror diameter does not affect magnification. If you put the same eyepiece in two telescopes with the same focal length, the magnification will be exactly the same in both telescopes.

The resolution is proportional to the diameter of the telescope mirror or lens. A 4" scope can theoretically resolve 1.1 seconds of arc, a 10" scope can resolve 0.45 seconds of arc. A second of arc is 1/3600th of a degree.

Atmospheric turbulence (seeing) limits the highest magnification you can use.

Large deep sky objects are sometimes best observed at low magnifications. The dilation of your pupil limits the lowest magnification you can use and that pupil dilation decreases with age. If you use a magnification too low, not all of the light exiting the scope will enter your eye and you will see objects of similar brightness to that seen through a smaller aperture instrument.

Cheers
Joe

[Matthieu]

Hey Revan,
In addition to the above, a combination of Chat GPT and the "Backyard Astronomer's Guide" have really supported me in my learning. Chat GPT being interactive allowed me to ask heaps of stupid question and as I understood things better, I validated information in the Backyard Astronomer's guide.

Clear skies!

Matt

[AlexN]

Zoom is not a word I'd use as it means something very different.

Think of it this way.

Focal length = Aperture of telescope in mm multiplied by focal ratio.

if you have a 100mm f/12 and a 200mm f/6 a 254mm f/4.7, they will all have a focal length of or very near to 1200mm.

if you put a 15mm eye piece in each telescope, you would see the same amount of sky in the eyepiece, however the 254mm and 200mm would show a brighter images than the 100mm This is because focal ratio, is essentially 'how fast' a telescope gathers light.

a 65mm f/6.4 scope has a focal length of 420mm, so does a 150mm f/2.8... However, the difference in light gathering power between a 150mm aperture and a 65mm aperture is quite immense, this is why you typically see 'faster' telescopes and camera lenses (f/2 ~ f/3.5) quite a bit more expensive than an f/6 of the same optical design...