## Re: [William-Optics] Re: ZS80FD - use at close range

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• Hi Timm I should have really used the term focal surfaces not planes, since they are curved surfaces not planes.Sorry about the misuse of terms. Now about
Message 1 of 17 , Feb 28, 2007
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Hi Timm

I should have really used the term focal surfaces not planes, since they are curved surfaces not planes.Sorry about the misuse of terms.

Now about those surfaces(spheres I believe). If you look at your photos from Hawaii you will find that the stars in the center of the FOV are sharp and round. When you look at the stars near the edge of the FOV they are blurred and elongated. The width of the star image in both the long and the narrow directions are wider than a star of equal brightness in the middle of the FOV. This is due to the field curvature. If you were to take a series of shots where for each shot you moved the focus "in" a small amount, you would see the following: The star images near the edge would start to become smaller in both directions and the image brighter. This would continues until the size in what was the smaller direction stopped getting smaller. This would be the best focus for that direction. As you continued to focus inward what was the long direction will continue to get smaller and the other direction will now start to get larger. At the best focus for the average of the two the star image will be round but not as dense or as small as it was in the center. The best focus in what was the long direction will cause a star image that is elongated but 90 degree to the elongation seen when the scope was focused at the center of the FOV. Therefore changing the focus can greatly improve the sharpness of an off axis star, but it can not make it as good as the image in the center with the scope focused at the center.

The above example shows what one will see due to the two different focal surfaces formed by a doublet. These two surfaces lie on the same point at the center of the FOV. They are called the Tangential focal surface and the Sagittal focal surface. For a doublet the Tangential surface is the most curved. It is what determines the sharpness in what was called the long direction above.

I hope that help. Sorry again for the misuse of the term planes.

Scott Walker
----- Original Message -----
Sent: Wednesday, February 28, 2007 12:21 PM
Subject: [William-Optics] Re: ZS80FD - use at close range

Hi Scott,

Can you explain further what you mean by "at the edge, the two loanes
of focus are not in focus at the same point"?

I can easily try to take a few photos of my 1x1 cm grid in the
basement, at different focus points. This might show what is being
sought.

Thanks,

Timm

--- In William-Optics@ yahoogroups. com, "Scott Walker" <sdwalker@.. .>
wrote:
>
> One can improve the focus on the edge but it is limited by the
astigmatism that most telescope have off the center of the FOV. At
the edge the two planes of focus are not in focus at the same point.
>
> Scott Walker
> ----- Original Message -----
> From: Acme Zenith
> To: William-Optics@ yahoogroups. com
> Sent: Tuesday, February 27, 2007 5:15 PM
> Subject: [William-Optics] Re: ZS80FD - use at close range
>
>
> --- In William-Optics@ yahoogroups. com, "Scott Walker" <sdwalker@>
> wrote:
> >
> > The image at the edge of the FOV should be limited by the field
> curvature.
>
> Yep, I gathered this, but it should still be possible to bring the
> edges to focus at the expense of the centre. As I understand field
> curvature if one took a photo of the inside of a sufficiently
large
> sphere(oid) all of it could be focus, but this is not the case
with
> spherical aberration. Hence my suggestion in testing to focus on
an
> edge.
>
> But now you also have me wondering what, for example, the .8x
field
> flattener does to spherical aberration!
>
> >
> > Scott Walker
> > ----- Original Message -----
> > From: Acme Zenith
> > To: William-Optics@ yahoogroups. com
> > Sent: Monday, February 26, 2007 7:19 PM
> > Subject: [William-Optics] Re: ZS80FD - use at close range
> >
> >
> > --- In William-Optics@ yahoogroups. com, "Scott Walker"
<sdwalker@>
> > wrote:
> > >
> > > Per the ATMOS model a 80mm aperture 555mm focal length scope
with
> > perfect spherical aberration in green at infinite focus
> > .1580 wave peak to valley when focused at 5550mm and to .06
waves peak
> > to valley at 11100mm. The close focusing causes under
correction.
> > These numbers are all at the center of the FOV. I think the
> > is much worse for faster systems.
> >
> > Thanks for this - I can stop torturing Timm.
> >
> > Any idea how to get ATMOS to run the numbers for what will be
the edge
> > of the field?
> >
> > >
> > > Scott Walker
> > > ----- Original Message -----
> > > From: Acme Zenith
> > > To: William-Optics@ yahoogroups. com
> > > Sent: Friday, February 23, 2007 9:59 AM
> > > Subject: [William-Optics] Re: ZS80FD - use at close range
> > >
> > >
> > > --- In William-Optics@ yahoogroups. com, "Timm Bottoni"
<t.bottoni@>
> > > wrote:
> > > > the original full shots, JPEG format, from the basement
> charts,shot
> > > > about 35 feet away.
> > > >
> > > I really appreciate your entertaining my questions on this.
I'm just
> > > going to ramble with some possibly fuzzy math for a second.
> > >
> > > Based on a f/l of 555mm I make your horizontal angle of view
> with the
> > > 22mm wide sensor as 2.3 degrees. As you say the test chart is
35
> feet
> > > away. tan(2.3/2) * 2 * 35 * 12 * 2.54 = 42.8. So I figure your
> > > horizontal field there is 42.8cm.
> > >
> > > 428mm / 22mm equals an image magnification of about 20:1, the
> maximum
> > > for this telescope without extension. And it looks good, so
this is
> > > very useful info.
> > >
> > > I'd like to know how things looks if you drop in enough
extension to
> > > make a magnification of, say, 10:1. You'll probably need a
new test
> > > chart, to keep some pixel level detail (as in the chart that
> went all
> > > the way to extinction at the bottom of your image).
> > >
> >
>

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