[AZ-Observing] Re: Refractor in and out of focus image
- From: Stan Gorodenski <stanlep@xxxxxxxxxxxxx>
- To: az-observing@xxxxxxxxxxxxx
- Date: Fri, 17 Jun 2011 14:31:48 -0700
Thanks, Chris. Your explanation makes sense.
Stan
On 6/17/2011 1:52 PM, Chris Schur wrote:
> 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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