Why do pictures of stars have 4 cardinal points?

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Discussion Overview

The discussion revolves around the phenomenon of diffraction spikes observed in images of stars, exploring their causes, implications, and variations across different telescopes and camera systems. Participants delve into technical aspects, personal observations, and artistic considerations related to astrophotography.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning
  • Experimental/applied

Main Points Raised

  • Some participants suggest that diffraction spikes are created by the internal support structure of telescopes, while others note that they can also be influenced by the observer's eyesight, particularly in cases of uncorrected astigmatism.
  • There are questions about the technical reasons for having 4 versus 6 diffraction rays, with some attributing this to the number of supports for the secondary mirror and the brightness of the star.
  • Participants mention that all stars produce diffraction spikes, but they are only visible for bright stars due to contrast with the background.
  • Some argue that spikes are a distinguishing feature of point sources like stars, while others challenge this by stating that planets and galaxies are not point sources and thus complicate the argument.
  • There is a discussion about the use of star filters in photography, which can create diffraction patterns for aesthetic purposes, and how different lens designs can affect the appearance of spikes.
  • Participants express differing opinions on the desirability of diffraction spikes in astrophotography, with some preferring to avoid them while others embrace their artistic potential.

Areas of Agreement / Disagreement

The discussion contains multiple competing views regarding the causes and implications of diffraction spikes, and no consensus is reached on the best practices for managing or utilizing them in astrophotography.

Contextual Notes

Participants note that the visibility of diffraction spikes can depend on various factors, including the design of the telescope or camera, the brightness of the stars, and the specific optical characteristics of the equipment used.

  • #31
Yes the spikes are produced before the image gets saturated, but the final result is an extended image which could be confused for a potential cluster member if it wasn't for the spikes...
 
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  • #32
pinball1970 said:
Top of pops was the usual one as there were always visible studio lights.
The rays in the TV camera images are caused by internal reflection/diffraction off of the lens's iris. Closely related to the telescope images, but of course there are no mirror supports in the TV camera.
 
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  • #33
James Demers said:
The rays in the TV camera images are caused by internal reflection/diffraction off of the lens's iris. Closely related to the telescope images, but of course there are no mirror supports in the TV camera.
Thanks the guys gave a few examples, I always liked them (spikes) in older 1970s studio shows
 
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  • #34
Astro telescopes do not have adjustable apertures so the whole length of the optical assembly is circular cross section. That minimizes diffraction effects. Smaller aperture scopes use lenses sothere’s No need for a web to hold a secondary mirror.
 

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