Stargazing Problem about calculating focal length of a telescope

AI Thread Summary
The discussion revolves around a formula for calculating the focal length of a telescope, specifically focusing on a step in the derivation that involves the term -(1+b). The user seeks clarification on how this term is obtained from the previous equation, expressing confusion over a sign discrepancy in the second term. They understand the substitutions made but request a detailed walkthrough of the mathematical steps involved. The conversation highlights the importance of precise mathematical manipulation in optics. Overall, the thread emphasizes the need for clear explanations in complex formula derivations.
Blacky&Imy
Messages
3
Reaction score
0
Hello!
I am preparing an exam about optics and I can't understand a formula.my problem is at the end of the second page (the explanation is in Italian, but the formulas are easy to read for everybody).
I can't understand how in the last step

f=R/2 - (1+b) * r^2/4R etc they found -(1+b)

can somebody help me please?
 

Attachments

  • page 1.png
    page 1.png
    11.2 KB · Views: 512
  • page 2.png
    page 2.png
    6.2 KB · Views: 525
Astronomy news on Phys.org
Are you asking how they mathematically got from the previous equation to the last one?
 
yes. I understand what is substituted in what, but I get a different sign in the second term, and I'd like to see all the passages...
 

Thread 'How feasible is home radio astronomy?'

Is a homemade radio telescope realistic? There seems to be a confluence of multiple technologies that makes the situation better than when I was a wee lad: software-defined radio (SDR), the easy availability of satellite dishes, surveillance drives, and fast CPUs. Let's take a step back - it is trivial to see the sun in radio. An old analog TV, a set of "rabbit ears" antenna, and you're good to go. Point the antenna at the sun (i.e. the ears are perpendicular to it) and there is...
View full post »

Thread '3I/ATLAS, Interstellar Comet'

3I/ATLAS, also known as C/2025 N1 (ATLAS) and formerly designated as A11pl3Z, is an iinterstellar comet. It was discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS) station at Río Hurtado, Chile on 1 July 2025. Note: it was mentioned (as A11pl3Z) by DaveE in a new member's introductory thread. https://www.physicsforums.com/threads/brian-cox-lead-me-here.1081670/post-7274146 https://earthsky.org/space/new-interstellar-object-candidate-heading-toward-the-sun-a11pl3z/ One...
View full post »

Similar threads

Stargazing Telescope Design: Calculating Total Distance from Primary Mirror to Eyepiece
Replies
2
Views
2K
Stargazing Telescope optical path length vs focal length
Replies
2
Views
5K
Stargazing Fixing declination runaway on a Meade LX200 telescope
Replies
6
Views
4K
Stargazing Is the INTEY Ultra-Clear Telescope a Good Choice for Beginner Astronomers?
Replies
10
Views
5K
Stargazing Problem in making a small refracting Telescope
Replies
3
Views
4K
I How to calculate focal length?
Replies
8
Views
3K
Calculate a radius of a circle on a screen
Replies
3
Views
1K
Calculating focal length from a number of focal ratios
Replies
5
Views
3K
Stargazing Telescope Illumination: Mirror Diameter & Focal Length
Replies
1
Views
3K
How Do You Calculate the Focal Length of a Telescope's Objective?
Replies
2
Views
2K

Hot Threads

Recent Insights

Back
Top