Spherical projection

B
Posted By
Bishoop
Jun 15, 2009
Views
924
Replies
4
Status
Closed
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.

Powered by Creative Market

R
rob
Jun 16, 2009
none wrote:
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.

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
B
Bishoop
Jun 16, 2009
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_m ap.png

They have very different content.
R
rob
Jun 16, 2009
none wrote:
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_m ap.png
They have very different content.

Get a copy of PTgui it has many items which will assist like control points. Many modes of stitching etc.
B
Bishoop
Jun 16, 2009
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.

Related Discussion Topics

Nice and short text about related topics in discussion sections