[AZ-Observing] Re: Refractor in and out of focus image
- From: Chris Schur <comets133@xxxxxxxxx>
- To: az-observing@xxxxxxxxxxxxx
- Date: Fri, 17 Jun 2011 13:52:17 -0700 (PDT)
Stan, from what I understand (Tom correct me if Im wrong) in a vacuum, the
inside and outside will look the same. In an atmosphere, outside of focus sees
closer to the telescope than infinity, so you will be focuing on atmospheric
waves and ripples that are nearby. so if the ground turbulence is bad, they
extrafocal image will be full of wavy bands.
Clear Skies,
Chris
Schur's Web Portal: http://www.schursastrophotography.com
--- On Fri, 6/17/11, Stan Gorodenski <stanlep@xxxxxxxxxxxxx> wrote:
From: Stan Gorodenski <stanlep@xxxxxxxxxxxxx>
Subject: [AZ-Observing] Refractor in and out of focus image
To: "AZ-Observing" <az-observing@xxxxxxxxxxxxx>
Date: Friday, June 17, 2011, 1:13 PM
I have been told that some of the criteria to evaluate optics quality by
comparing the inside out of focus image to the outside out of focus
image does not apply to refractors. However, it seems some of it should
apply. Assume the outside out of focus image looks very good and
circular and looks like the text book example of good optics in Star
Testing Astronomical Telescopes, by Suiter, pages 12 and 13. Assume the
inside out of focus image also looks the same (circular and like the
text book example), but it is reduced in brightness and tends to
sometimes be difficult, depending on the star color, to see because of
atmospheric turbulence. For some star colors the atmospheric turbulence
is not very evident and the the two images look almost identical. Is
this still evidence of good APO optics, even though the inner out of
focus image is not exactly identical (i.e., dimmer) to the outer image,
and it seems to be more prone to atmospheric turbulence?
Stan
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