What prime focus means
Prime focus is the point where a telescope’s main lens or mirror brings light to a focus. In prime focus astrophotography, the camera sensor sits at that point with no eyepiece or extra lens between. The telescope effectively acts as a long lens, and its focal length sets the image scale.
Prime focus compared with other methods
- Prime focus: The camera attaches directly using adapters. This is the common method for deep-sky imaging.
- Afocal imaging: A camera or phone looks into an eyepiece (phone guide). It is simple and cheap but less precise.
- Eyepiece projection: The eyepiece projects an enlarged image onto the sensor, giving high magnification. It is less common today.
- Focal reducer or Barlow at prime focus: A reducer shortens the effective focal length for a wider, faster field (see fast vs slow telescopes). A Barlow lengthens it, often for planets.
What you need
- A camera and matching T-ring, for DSLR or mirrorless cameras, or the correct adapter for an astronomy camera
- A T-adapter or nosepiece fitting the telescope’s focuser
- Optionally a field flattener or reducer, with the correct spacing
- A mount that can carry the load and track
See astrophotography with a telescope and DSLR for connecting a camera step by step.
Prime focus by telescope type
Refractors (refractor vs reflector). Usually reach focus easily and are a popular choice for prime focus imaging, often with a flattener to correct the edges of the field.
Schmidt-Cassegrain and Maksutov telescopes (pros and cons). Focus is adjusted by moving the mirror, and the long focal length gives high magnification. A focal reducer is often used for deep-sky targets.
Newtonian reflectors. The focal plane sits just outside the tube, and some reflectors do not have enough inward focuser travel to reach focus with a camera. Imaging-oriented Newtonians are designed with this in mind.
Dobsonians. These are Newtonians on alt-az bases, with the same backfocus concerns plus tracking limits. See Dobsonian astrophotography.
Backfocus and reaching focus
The distance from the last optical surface or adapter to the sensor must match what the optics expect. Flatteners and reducers usually specify a required spacing, and incorrect spacing can cause distorted star shapes at the edges. If the camera cannot reach focus:
- Use a shorter adapter or low-profile focuser configuration
- Check whether the focuser can be racked farther in or out
- Consider a Barlow, which can move the focus point outward on some setups
Image scale at prime focus
Image scale depends on the focal length and the camera’s pixel size. A longer focal length magnifies more and narrows the field. A shorter one gives wider views of large nebulae. Many imagers use an online calculator to compare a telescope and camera before choosing targets. The best telescope for astrophotography discusses focal length choices.
Tracking requirements
Because prime focus images are not magnified by an eyepiece, long exposures still need tracking. An equatorial mount with good polar alignment is the basis, and guiding extends the exposure length. For a broader overview, see astrophotography with a telescope.
Common problems
- Camera will not focus: A backfocus problem, as above.
- Stars misshapen at the corners: Check flattener spacing and tilt.
- Vignetting: Check that the image circle covers your sensor, and use flat frames.
- Image sag or shift: Tighten fittings and check balance, since the camera’s weight can flex a loose focuser.