Spherical projection

[none]

Hi,

I use PhotoShop (CS3) for working with astronomy images. I use various
types of blending to combine many shots of the same thing.

Once I have the final product, I'm often curious about what some of the
more faint spots are. Sometimes I'll check Wikipedia and compare to their
various sky charts, and sometimes I'll use Google Sky.

The problem is, everyone has different ways of "flattening" their images.
The full sky, as viewed by an observer on Earth, is a sphere (or
hemisphere, since about half of it is blocked by the Earth). It's pretty
much the same problem as trying to make a map of the Earth fit into a
rectangular or square shape. Mathematically, I believe that it is actually
impossible to project from spherical to "flat" in a way that conserves
area. In other words, you can't take the surface of a sphere and flatten
it out into a rectangular shape without parts of it becoming distorted in
some way.

Google Sky images, for example, use a projection that causes the top and
bottom to become highly stretched -- kind of like the maps of the world
that make Greenland and Antarctica look really huge. The charts on
Wikipedia are generated from a program called PP3 that apparently uses a
different type of projection.

The bottom line is that neither of those two images line up with an actual
picture of the sky. And it's not a simple matter of using the "transform
(ctrl-T)" command in Photoshop. I guess you could say that some sort of
"non-linear" transformation would be required to convert between the three
different types of projections.

Anyone have an idea how to convert between the three? I understand that it
won't be trivial, because it all depends on where the "poles" are and so
on... Maybe there is a plug-in to help?

Thanks in advance.

[Rob]

I use CS4. Not sure if this is available in CS3

from bridge I open the selected image files into PS layers.

select all layers - them under edit, auto align layers there are
options to join the images and one is spherical.

next auto blend layers.

alternate is to get a panoramic program which also has these options
like , Autopan or PTGui Pro

[none]

Rob wrote:

>
> I use CS4. Not sure if this is available in CS3
>
> from bridge I open the selected image files into PS layers.
>
> select all layers - them under edit, auto align layers there are
> options to join the images and one is spherical.

Well, CS3 doesn't have the "spherical" option here, but I don't think it
would help. For example, here is what one of my images would look like
(Big Dipper, not my image, but very similar):
http://www.spacetelescope.org/images/screen/heic0716d.jpg

And then here is the chart of that same area in the sky:
http://en.wikipedia.org/wiki/File:Ursa_Major_constellation_map.png

They have very different content.

[Rob]

Get a copy of PTgui it has many items which will assist like control
points. Many modes of stitching etc.

[none]

Rob wrote:

> Get a copy of PTgui it has many items which will assist like control
> points. Many modes of stitching etc.

Looks like it does what I need. I see some examples of astronmical images.

Thanks.