RigTonightImage scale and sampling: matching your camera to your telescope and seeing
Two numbers decide how much sky you capture and how much detail you can resolve: your field of view and your image scale. Both come from just three figures: focal length, pixel size and sensor size.
Image scale
Image scale is how much sky one pixel covers, in arcseconds:
image scale (″/px) = 206.265 × pixel size (µm) ÷ focal length (mm)
The 206.265 converts radians to arcseconds (206,265 ″ per radian), combined with the µm-to-mm conversion.
Examples:
| Rig | Focal length | Pixel | Image scale |
|---|---|---|---|
| Askar SQA55 + Player One Ares-C Pro | 264 mm | 3.76 µm | 2.94 ″/px |
| Askar FRA400C + Player One Poseidon-C Pro | 400 mm | 3.76 µm | 1.94 ″/px |
| EdgeHD 8 with 0.7× reducer + Player One Poseidon-C Pro | 1422 mm | 3.76 µm | 0.55 ″/px |
Field of view
field (degrees) = 2 × atan(sensor size ÷ (2 × focal length)), converted from radians
A Player One Ares-C Pro's 11.3 mm square sensor at 264 mm covers about 2.45° × 2.45°: enough for the whole North America and Pelican complex. The same camera at 1422 mm covers about 0.46°, which suits small galaxies and planetary nebulae. How to calculate your field of view has more worked examples and the sizes of popular targets.
Sampling: matching scale to seeing
Atmospheric seeing blurs every star into a disc a few arcseconds across, measured as FWHM. A star needs to cover a few pixels to look round and to give you all the detail the sky allows:
- Oversampled: pixels much smaller than the blur, below about a third of the seeing. You gain no detail, spread the light over more pixels, and lose signal-to-noise. Fix it by binning 2×, which doubles the scale.
- Well sampled: two to three pixels across the seeing disc, the Nyquist criterion. That's an image scale between a third and half of the seeing.
- Slightly undersampled: between half the seeing and the seeing itself. It's very common and usually fine; dithering and drizzle recover a little detail.
- Undersampled: a pixel bigger than the seeing. Stars can look slightly blocky. That's normal and fine for wide fields, and drizzle integration helps if you dither.
With 2″ seeing the ideal scale is 0.67–1.0 ″/px. The SQA55 rig (2.94 ″/px) is undersampled, which suits a wide-field rig. The FRA400C rig (1.94 ″/px) is slightly undersampled, and only becomes well sampled when seeing is around 4″. The EdgeHD rig (0.55 ″/px) is oversampled; binned 2× it becomes 1.09 ″/px, which is close to ideal.
Set your seeing honestly
Most backyards get 2–3″ on an average night. Set the seeing buttons in the builder to your typical conditions, not your best night, and the sampling check adjusts.