A telescope described as "150/750" or "D150 F750" is telling you two numbers: an aperture of 150 mm and a focal length of 750 mm.
Aperture (D)
The diameter of the main lens or mirror. It is the most important number:
- Light: a 150 mm telescope collects about 2.8 times the light of a 90 mm one (light grows with the area).
- Detail: larger apertures separate finer detail, up to the limit set by the steadiness of the air.
Focal length (F)
The distance over which the mirror or lens brings light to focus. With a given eyepiece, a longer focal length gives more magnification:
Magnification = telescope focal length / eyepiece focal length - so 750 mm with a 25 mm eyepiece gives 30x.
Focal length also sets the tube length of refractors and simple reflectors: a 900 mm Newtonian is close to a metre long.
Focal ratio (f/number)
Focal length divided by aperture: 750 / 150 = f/5.
- Low f/numbers (f/4-f/6) give wider views - good for star clusters and nebulae - and shorter tubes, but need more careful collimation.
- High f/numbers (f/8 and above) are more forgiving with simple eyepieces and suit planets and double stars.
For visual observing, f/number does not make an image "brighter" in the way it does for cameras; aperture and magnification decide that.
Putting it together
| Telescope | Aperture | Focal length | f/ratio |
|---|---|---|---|
| 76/700 reflector | 76 mm | 700 mm | f/9.2 |
| 114/900 reflector | 114 mm | 900 mm | f/7.9 |
| 150/750 reflector | 150 mm | 750 mm | f/5 |
| 203/1200 Dobsonian | 203 mm | 1200 mm | f/5.9 |