Guide cameras compared: image scale, field size and guide-scope focal length

Guide cameras are judged by different numbers than imaging cameras. Field matters because the software needs stars to lock onto, and image scale matters because it sets how finely the guider measures star motion. Both come from the same two inputs as any camera: pixel size and focal length. The table below uses guide-scope focal lengths of 120 to 240 mm plus 400 and 600 mm for larger guide scopes.

Focal length is the number people get wrong

The PHD2 manual asks for the unbinned pixel size and the guide focal length so it can work out the image scale, which it uses for baseline guiding settings, calibration checks and performance reports. It warns that focal length is a common mistake: a 50 mm finder scope is likely around 150–175 mm focal length, a 60–80 mm refractor guide scope in the 240–500 mm range, and an off-axis guider uses the main telescope’s focal length. The PHD2 best practices also advise judging guiding in arcseconds, not pixels, which only works if those two inputs are right.

Guide scale against main-camera scale

scale ratio = (guide pixel ÷ guide focal length) ÷ (main pixel ÷ main focal length)

As an example, take a main camera with 3.76 µm pixels on a 400 mm refractor: 1.94″/px. The guide cameras here would give:

Guide pixel size	120 mm	180 mm	240 mm
2.9 µm (IMX462, IMX662, IMX290, IMX585)	4.98″ (2.6×)	3.32″ (1.7×)	2.49″ (1.3×)
3.75 µm (ASI120MM Mini)	6.45″ (3.3×)	4.30″ (2.2×)	3.22″ (1.7×)
4 µm (ASI220MM Mini, QHY5III200M)	6.88″ (3.5×)	4.58″ (2.4×)	3.44″ (1.8×)
5.86 µm (ASI174MM Mini, QHY5III174M)	10.07″ (5.2×)	6.72″ (3.5×)	5.04″ (2.6×)

PHD2 does not publish a target ratio. Its binning notes say that binning is unlikely to be a problem when the unbinned guide scale is below 1″/px, that guiding may suffer if the unbinned scale is already above 2.5″/px, and that results also depend on the main camera’s image scale. On these numbers, every camera at 120 or 180 mm is already coarser than 2.5″/px, so PHD2’s note argues against binning there. The 2.9 µm sensors cross below 2.5″/px at 240 mm, and all of them do by 400 mm except the 5.86 µm bodies (3.02″/px). The guiding math guide goes through the ratio in more detail.

Field size and guide stars

Sensor area decides how much sky the guider sees. At 240 mm:

• ASI120MM Mini: 68.8′ × 51.6′, about 1.0 square degree.
• 2.9 µm 1920–1944-pixel sensors (ASI462MM, QHY5III462C, Ceres 462M, Mars-M II, GPM662M, GPCMOS02000KPA): about 80′ × 45′, also about 1.0 square degree.
• SC2210 (ASI220MM Mini, QHY5III200M): 110′ × 62′, 1.9 square degrees.
• IMX249/IMX174 (ASI174MM Mini, QHY5III174M): 2.71° × 1.70°, 4.6 square degrees.

PHD2’s multi-star mode picks up to 12 stars and uses up to 9 of them at once, refining the measurement with the secondary stars, according to the manual. A sensor that covers four times the sky gives that algorithm more candidates to choose from; a small sensor on a sparse field may leave it with one. The advanced settings page adds that using subframes switches multi-star guiding off.

Off-axis guiding and small pixels

Through an off-axis guider the guide camera works at the main focal length. On an 8-inch SCT at 2032 mm, 2.9 µm pixels give 0.29″/px. PHD2’s profile wizard shows a warning icon when very small pixels meet a long focal length and suggests binning; 2×2 here gives 0.59″/px. The image scale calculator shows the binning steps.

Practical differences

Back focus ranges from 7.5 mm (Ceres 462M) and 8.5 mm (ZWO Minis, GPM662M) to 12–17.5 mm on the other bodies; ToupTek publishes none for the GPCMOS02000KPA. According to ZWO, the 120 Mini comes to focus where an eyepiece would; the ZWO planetary and guide page covers the Mini series. The QHY5III462C and GPCMOS02000KPA are color; the rest are mono. The QHY miniCAM8M is not a guide camera at all: it is a cooled IMX585 camera with a built-in filter wheel, included as a size reference.