Comment 2 for bug 604975

On 13/07/2010 21:36, Fred_76 wrote:
> Public bug reported:
>
> In the CCD.cpp file, here is a formula that is used to calculate the
> field of view of the CCD sensor :
>
> float CCD::getActualFOVx(Ocular *ocular)
> {
> float FOVx = (chip_width * 206.265) / ocular->getEffectiveFocalLength();
> return FOVx;
> }
>
> float CCD::getActualFOVy(Ocular *ocular)
> {
> float FOVy = (chip_height * 206.265) / ocular->getEffectiveFocalLength();
> return FOVy;
> }
>
>
> There are two errors :
> 1) The factor 206.265 is not adequate. It is adequate to calculate the sampling angle of one pixel by using the following formula :
>
> sampling (in arc. sec) = 206.265 * pixel width (µm) / telescope focal
> length (mm)
>
> but it is not suitable for the FOV of the whole sensor.
>
> The right exact formula for the field of view of the CCD shall be :
>
> FOVx (arc. min) = 2 * atan(chip_width (mm) / (2* telescope focal length (mm)) * 180 / pi() * 60
> FOVy (arc. min) = 2 * atan(chip_height (mm) / (2* telescope focal length (mm)) * 180 / pi() * 60
>
> This formula can be simplified :
>
> FOVx (arc. min) = 6875.5 * atan(chip_width (mm) / (2* telescope focal length (mm))
> FOVy (arc. min) = 6875.5 * atan(chip_height (mm) / (2* telescope focal length (mm))
>
> It can even be further simplified if chip size<< telescope focal
> length, but to allow the use of DSLR lenses (with short focal length)
> the above formula would be better.
>
> FOVx (arc.min) = 3437.7 * chip_width (mm) / telescope focal length (mm)
> FOVy (arc. min) = 3437.7 * chip_height (mm) / telescope focal length (mm)
>
>
> 2) The formula in the CCD.ccp file is using the "ocular" focal length.
> It is not correct. The focal length of the telescope shall be used. Most
> astronomers are using their CCD in the focal plane of their telescope,
> and not in line behind an ocular. Even behind an ocular the use of the
> ocular focal length only is not adequate.
>
>
> Therefore the right code in CCD.cpp should be :
>
> float CCD::getActualFOVx(Telescope *telescope)
> {
> float FOVx = 6875.5 * atan(chip_width / telescope->getEffectiveFocalLength() / 2);
> return FOVx;
> }
>
> float CCD::getActualFOVy(Telescope *telescope)
> {
> float FOVy = 6875.5 * atan(chip_height / telescope->getEffectiveFocalLength() / 2);
> return FOVy;
> }
>
> or something like that...
>
> Fred
>
> ** Affects: stellarium
> Importance: Undecided
> Status: New
>
>
As mentioned a several days ago, I was working on a patch for Oculars.
It includes several bug fixes (including this one) and some new
options. The problem is, patches being submitted are not being accepted
so I wasn't exactly rushing to finish this one...

@Timothy: I wanted to get this finished properly and then contact you to
discuss, but as this is the second bug report that will cause
over-lapping code changes that are a real pain to merge, I've pushed my
patch to LP for review (see ~scrupeus/stellarium/oculars). There is
still a little cleaning up to do but the main components are there. I
have some other ideas but they can wait.
Check the bzr log for changes made. If you want this patch, let me know
and I'll finish it properly. If not, I won't bother with it but you can
cherry-pick my fixes.
Peter