In astrophotography, success is not about “more magnification”, but about correct sampling. In this guide we explain how focal ratio, pixel size and effective focal length work together, helping you choose the right telescope and camera without ending up with an overly demanding or inefficient setup.
Why focal ratio and sampling really matter
Two telescopes with the same aperture can deliver very different results. The key difference is often the image scale (arcseconds per pixel) and how well it matches seeing, guiding accuracy and mount stability.
- Too much focal length for the seeing → soft, noisy stars (oversampling)
- Too little focal length → loss of fine detail (undersampling)
- Balanced setup → tighter stars and more usable detail
Focal ratio: what it is and what it changes
The focal ratio is expressed as f/ and calculated as:
f/ = focal length / aperture
For deep-sky imaging, a “faster” system (e.g. f/4–f/5) collects signal more quickly than f/7–f/8, but only if the sensor and image scale are properly matched.
Sampling (arcseconds per pixel): the key formula
Image scale is calculated using a simple formula:
Scale (arcsec/pixel) = 206 × pixel size (µm) / focal length (mm)
- Pixel size: physical size of one sensor pixel
- Focal length: effective focal length including reducer or Barlow
How to interpret the value
- Low value (~0.6–1.0”/px): higher resolution, higher demands on seeing and guiding
- Medium value (~1.2–2.5”/px): often an excellent real-world balance
- High value (>3”/px): wide field, more tolerance, less fine detail
Oversampling and undersampling explained
Oversampling
When the sampling is finer than the seeing allows, typical results include:
- bloated or soft stars
- persistent noise even with long integration times
- increased stress on mount and guiding performance
Undersampling
When sampling is too coarse:
- stars appear unnatural or blocky
- fine detail cannot be resolved
- small objects look overly compact
Practical reference ranges for deep-sky imaging
- ~1.2–2.5 arcsec/pixel: balanced and relatively easy to manage
- <1 arcsec/pixel: demanding, requires good seeing
- >3 arcsec/pixel: ideal for wide-field nebulae
Optimizing your setup without changing the telescope
Focal reducer
Reduces effective focal length, widens the field of view and increases tolerance for deep-sky imaging.
Barlow lens
Increases effective focal length, useful mainly for lunar and planetary imaging.
Pre-purchase checklist
- Define your main targets (nebulae, galaxies, planets)
- Check the camera pixel size
- Calculate the effective focal length
- Apply the sampling formula
- Avoid extreme values whenever possible
Not sure about your setup?
Contact us with your telescope, focal length and camera details. We’ll help you verify your sampling and suggest only the accessories that really make sense.