Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

My brain is stuck on this one

  • Please log in to reply
4 replies to this topic

#1 Huangdi

Huangdi

    Viking 1

  • *****
  • topic starter
  • Posts: 544
  • Joined: 24 Jul 2019

Posted 30 May 2020 - 05:55 AM

I'm sort of confused right now, perhaps someone can help out?

If I have one 80mm Scope with 480mm focal length, that leaves me with a focal ratio of F6

If I have two 80mm scopes with 480mm focal length, I have two scopes at F6.

If I have a 160mm scope with 480mm focal length, I have one scope at F3

F3 gathers 4x as much light as F6, right?

But two F6 scopes only gather 2x as much?

It's probably plain and obvious, but I'd like some confirmation!

Edited by Huangdi, 30 May 2020 - 05:56 AM.


#2 ChrisWhite

ChrisWhite

    Fly Me to the Moon

  • *****
  • Posts: 5,829
  • Joined: 28 Feb 2015
  • Loc: Colchester, VT

Posted 30 May 2020 - 06:12 AM

Yes. 

 

Remember that it's not just the diameter of the scope that has doubled when you went from 80mm to 160mm, but that the area of the scope has quadrupled.

 

π*r^2

 

area of your 80mm f6: 5026mm

area of your 160mm f3: 20106mm

 

So two f6 scopes would give you 10052mm of area, which is still half of what the single 160mm f3 scope provides.


Edited by ChrisWhite, 30 May 2020 - 06:16 AM.

  • Huangdi likes this

#3 terry59

terry59

    Cosmos

  • *****
  • Posts: 9,567
  • Joined: 18 Jul 2011
  • Loc: Colorado, USA

Posted 30 May 2020 - 06:16 AM

I'm sort of confused right now, perhaps someone can help out?

If I have one 80mm Scope with 480mm focal length, that leaves me with a focal ratio of F6

If I have two 80mm scopes with 480mm focal length, I have two scopes at F6.

If I have a 160mm scope with 480mm focal length, I have one scope at F3

F3 gathers 4x as much light as F6, right?

But two F6 scopes only gather 2x as much?

It's probably plain and obvious, but I'd like some confirmation!

I'm going to avoid the "which way is better" perspective and focus instead on the difference between running one scope and having a dual setup. The latter has a lot of additional challenges to overcome to pull off. Unless your question is purely academic of course

 

smile.gif


Edited by terry59, 30 May 2020 - 06:17 AM.

  • Huangdi likes this

#4 ChrisWhite

ChrisWhite

    Fly Me to the Moon

  • *****
  • Posts: 5,829
  • Joined: 28 Feb 2015
  • Loc: Colchester, VT

Posted 30 May 2020 - 06:18 AM

I'm going to avoid the "which way is better" perspective and focus instead on the difference between running one scope and having a dual setup. The latter has a lot of additional challenges to overcome to pull off. Unless your question is purely academic of course

 

smile.gif

 

It might be less challenging to get two (or four) f6 scopes pointing at the same spot producing nice round stars than a single f3 scope! lol.giflol.giflol.gif


Edited by ChrisWhite, 30 May 2020 - 06:19 AM.


#5 Huangdi

Huangdi

    Viking 1

  • *****
  • topic starter
  • Posts: 544
  • Joined: 24 Jul 2019

Posted 30 May 2020 - 06:19 AM

Thank you for clarifying, I thought so!


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics