How does an F15 become an F2?

[Jarvo]

I’m bored and waiting for clear skies so my mind is wandering. 

One for the technically minded amongst us. 

So I’ve always wondered if you have an F15 SCT eg a Celestron and add a Hyperstar camera to the front end, how does the set up become so fast. I realise the light’s only bouncing off one mirror but still why so fast?

Many many prizes are not on offer for the correct answer but would be interesting to find out  

Thanks for looking and any answers incoming.

Jarvo

 

 

[Star101]

https://en.wikipedia.org/wiki/F-number

Realise that the FOV for said Celestron also changes. See image

 

[Bizibilder]

The primary of an f/15 SCT is an f/2 mirror.  Take that away and replace with the hyperstar/camera and you have an F/2 set-up.

[catburglar]

It’s easy- you don’t just add a hyperstar, it actually remove the secondary mirror, which usually multiplies the focal ratio of the primary mirror by x5, hence you’re left just with the primary which is typically around f/2 in most SCT’s