A Quick Question About The F Word

[John P]

I'm looking at buying a 120mm refractor.

There are two choices. One is f5, the other f8.

What is the difference?

[John]

The F here means focal ratio, that is the relationship between the diameter of the objective lens or mirror and it's focal length (the point at which light comes to a focus). So the 120mm F/5 has a focal length of 600mm and the F/8 (F/8.3 I expect) is 1000mm.

The F/5 is good for low to medium power viewing, ie: deep sky objects etc while the F/8.3 is better for higher power viewing, ie the planets, the moon etc. Both scopes can do both things but each has their strengths and weaknesses.

Achromat refractors produce something called chromatic abberation (CA) which is where not all wavelengths of light can be bought to focus at exactly the same point. This means you will see some purple fringing (false colour its sometimes called) around the moon, brighter planets and stars. This is more pronounced in the F/5 scope.

Thats some info for starters anyway !.

[John P]

Thanks for that John. Problem solved.

I'm looking initially for something good on planetary detail, so the f8.3 it is