RigTonightGuide scope or off-axis guider? Getting the guiding ratio right
Autoguiding software such as PHD2 or the guiding module in your capture software measures a guide star many times a minute and nudges the mount to keep it still. For that to work, the guide camera has to see movements that matter at your imaging scale.
The guiding ratio
Work out the image scale of both cameras with the usual formula, 206.265 × pixel size (µm) ÷ focal length (mm):
- Imaging scale: main camera on the main telescope
- Guide scale: guide camera on the guide scope, or on the main telescope if you use an off-axis guider
The ratio is guide scale ÷ imaging scale. Guiding software measures star positions to a fraction of a pixel, so the guide camera can be coarser than the imaging camera. Up to about 4× is comfortable, 4–6× is workable with good polar alignment, and beyond that guiding struggles to keep up.
Worked examples
Askar FRA400C + Player One Ares-C Pro, guided with the Askar 32 mm f/4 guide scope and a Player One Ceres 462M
- Imaging: 206.265 × 3.76 ÷ 400 = 1.94 ″/px
- Guiding: 206.265 × 2.9 ÷ 128 = 4.67 ″/px
- Ratio: 2.4×, fine
EdgeHD 8 at 1422 mm + Player One Poseidon-C Pro, same guide scope
- Imaging: 0.55 ″/px
- Ratio: about 8.5×. The guide scope can't see the errors that blur the image.
Same rig with an off-axis guider
- The guide camera now looks through the main telescope: 206.265 × 3.75 ÷ 1422 = 0.54 ″/px
- Ratio: about 1×, ideal
The trade-offs
| Guide scope | Off-axis guider | |
|---|---|---|
| Finding a guide star | Easy: wide field | Harder: a small prism at the edge of the field |
| Flexure between guide and main optics | Possible | None |
| Back focus | Unaffected | Uses some (e.g. 37.5 mm for the William Optics Elite Drawer OAG Kit, which includes a filter drawer) |
| Best for | Up to roughly 600–800 mm | Long focal lengths and SCTs |
If you switch to an OAG, remember its thickness comes out of your back-focus budget. The builder accounts for it automatically. See Back focus explained.