Matching Resolutions in Astrophotography

Nov 18, 2024 by WO Team

Achieving high-quality astrophotography requires balancing camera resolution, telescope resolution, and atmospheric seeing. Misalignment among these factors can limit image detail and waste resources. This article simplifies how these elements interact and introduces key formulas for optimizing your setup.

1. Telescope Resolution

The telescope’s angular resolution is the smallest detail it can resolve, dictated by its aperture size and the wavelength of light (λ):

\text{Angular Resolution (arcseconds)} = \frac{1.22 \times \lambda}{d}

For visible light ( λ ≈ 550nm ) and a telescope aperture of 200mm, the theoretical resolution is 0.68 arcseconds. However, in practice, this is limited by the seeing conditions.

2. Camera Resolution: Pixel Scale

The pixel scale determines how much sky a single pixel on the camera covers. It connects the camera sensor and telescope focal length:

\text{Pixel Scale (arcseconds/pixel)} = \frac{\text{Pixel Size (μm)} \times 206}{\text{Telescope Focal Length (mm)}}

For example, a camera with a pixel size of 5 μm attached to a telescope with a 1000 mm focal length will have a pixel scale of approximately 1.03 arcseconds/pixel. This scale must align with the atmospheric seeing to avoid under sampling or oversampling.

3. Seeing: Atmospheric Constraints

Seeing describes the blurring of celestial objects due to atmospheric turbulence, measured in arcseconds. Even a high-resolution telescope cannot exceed the limits imposed by seeing. For example:

Good seeing	< 1 arcsec
Average seeing	1−2 arcsec
Poor seeing	> 2 arcsec

If seeing is 2 arcseconds, using a telescope with a theoretical resolution of 0.5 arcseconds offers no additional detail, as the atmosphere is the limiting factor.

Matching Resolutions for Optimal Results

To ensure a balanced setup:

Ensure the telescope resolution doesn’t exceed the limits of the seeing. For example, in 2-arcsecond seeing, using a telescope with a 0.5 arcsecond resolution is ineffective, as the atmosphere will blur details beyond 2 arcseconds.

Pixel scale should be within 1/2 to 1 times the seeing. For example, if the seeing is 2 arcseconds, aim for a pixel scale between 1 and 2 arcseconds/pixel.

If the pixel scale is too small (oversampling), you waste resolution and memory without capturing more detail.

If the pixel scale is too large (undersampling), you lose fine details, as the camera sensor cannot capture enough data.

▲ Under Sampling

▲ Suitable Sampling

▲ Over Sampling

Image from ZWO Website