Slow scope, fast scope...what does it all mean?

[ollypenrice]

17 hours ago, AstroMuni said:

I should read the timing of the posts too 🙂

Old post or not, this debate, and the misinformation it never fails to generate, is forever young! For instance, a few posts above this we see the assertion that two 6 inch scopes collect different amounts of light because they have different focal lengths and, therefore, different focal ratios. This cannot be true. When a six inch diameter beam of incident light arrives at the 6 inch objective of a telescope, how does it know the focal length of the objective it is about to pass through? And even if it discovers that the objective is a fast F2, how does it increase its photon count with this discovery? No, exactly the same beam passes into the objective whatever its focal ratio.

I think the confusion has a single and simple origin, the camera lens.  Regular photographers use the terms aperture and focal ratio  interchangeably.  They can do this because, when they do so, they are discussing a lens operating at a fixed focal length and they are changing the F ratio by changing the aperture.  (They open or close a diaphragm in front of the objective.) When they do this, it is perfectly obvious that they alter the amount of light arriving at the chip and so they alter the required exposure time. 

This is precisely what we are not doing when we place a six inch F5 scope next to a six inch F10 for comparison. We are not changing the aperture, so there is neither more light nor less joining the party. Sure, we are changing what we do with it but here we move into sampling rate, or how many pixels we place under the photons from an object - but, for now, let's just de-mystify F ratio.

Olly

[AstroMuni]

On 21/08/2021 at 08:26, ollypenrice said:

Old post or not, this debate, and the misinformation it never fails to generate, is forever young

That is so very true. But I guess thats what keeps this forum vibrant and alive 🙂

After all its a complex topic and I can appreciate that not everyone can comprehend the full implications. When my kids used to ask me questions, I used to find it hard to explain the why aspect, even though I knew how to solve their problem.