Why do we need to guide?
With long exposure astrophotography, we need to track the motion of night sky perfectly as the Earth rotates to get sharp images.
While this seems simple in theory, in practice, small discrepancies can spoil the perfect tracking we need. Since tolerances are in microns, deviations from perfect polar alignment, flexure of telescope optics and mechanics, and other factors all cause errors in the tracking rate. We correct for these by “guiding” the mount.
Why not a guidescope?
It is possible to guide the mount with a guidescope mounted on top of the main imaging system, but this solution is not always accurate. While the guide camera thinks its tracking the sky correctly, there is no guarantee it’s seeing the exact same errors the main imaging camera and telescope are. Some of the main limitations of using guidescopes are:
- Differential Flexure
The guidescope is controlling the entire system, but many of the other parts can move without the guider noticing. Connections between the guide and main scope can flex, as can (and likely will) either of the focusers of the scopes. With reflectors, the mirrors are not really fixed, and they tend to shift as the telescope tracks the sky. Everything may feel solid to you, but it isn’t on a microscopic scale.
- Inadequate Resolution
With the longer focal lengths of large telescopes, a small guidescope won’t have the resolving power to see the tiny movements that affect the main telescope and camera. Modern software can compensate for that to some extent, but in the end, you can’t cheat the physics.
- Extra weight
Placing another scope and its mounting hardware on top of the main imaging telescope adds extra weight to the payload in the worst place, making the mount work harder.
What Is an Off-Axis Guider?
The OAG uses a small prism as a mirror to deflect part of the incoming light from the telescope from the edge of the field (outside the image frame of the imaging camera) to an independent guide camera.
Why an Off-Axis Guider?
An off-axis guider provides far more accurate tracking than with a guidescope. Because it is attached securely right next to the main imaging camera there is very little chance of anything flexing between the two. The OAG has the full resolution of the main scope since it’s using the same optics, and it typically weighs very little (less than 0.5lbs). Prices start at about $150.
How to Setup an OAG
Setting up an OAG isn’t complicated. Here are the steps:
- Back Focus
You need to ensure your optical configuration has enough back focus, i.e., enough space to allow you to place an OAG in front of the camera, after the focuser or the last optical element (reducer, corrector, or flattener). You will find that most telescopes are designed to allow for an OAG that is from 10mm to 25mm thick.
- Mono Camera
If you have a filter wheel, always place the OAG in front of the filter wheel. It’s best to not filter out any of the incoming light so you can detect as many stars as possible.
Make sure the flat side of the prism is facing the telescope and the sky.
Many sensors are rectangular, so it’s best to position the prism adjacent to the long side of the sensor where there is more room available. The example below shows that in this orientation the prism can be much lower in the field without obstructing the main imaging sensor, allowing use of more of the field to find a guide star.
- Prism Position (Height)
Visually inspect the prism to make sure it’s not covering part of the sensor. It’s best to get the prism as close as possible to the sensor since telescopes usually produce the sharpest and brightest star images near the center of the field.
A good method to fine tune the position (up/down) is to check with flat calibration frames. You will start to see the shadow of the prism if it’s too close to the sensor.
In the first flat calibration image below, the shadow of the prism is detected by the main imaging sensor. The second flat frame below is after correctly adjusting the height of the prism.
This is where first-time users are often confused. You need to ensure that the distance from the prism to the main camera sensor is exactly equal to the distance from the prism to the guider sensor.
1. Start by measuring the distance from the prism to the main camera sensor (either with calipers or drawings) – RED arrow in illustration above.
2. Position the guide camera so its sensor is the same distance from the prism – BLUE arrow in illustration below.
3. Slew to a star and focus the main telescope and camera.
4. With autoguider software (e.g., PHD2), take continuous exposures of the stars.
5. Use the OAG guider’s helical focuser to fine tune its focus without touching the main telescope’s focuser.
6 Calibrate your OAG (PHD2 will do this for you with a useful “guiding assistant” tool as well).
- Can’t Find Stars?
If you slew to a target and can’t find bright stars to guide on, as sometimes happens with longer focal lengths, you can:
- Try longer exposures; if 1-3 seconds doesn’t register any stars, try doubling the exposure.
- Bin 2x to increase the signal to noise ratio.
- Rotate the angle slightly; a bright star may be just outside the current view. Be sure you rotate the imaging train before the OAG so the prism location doesn’t change. An ASCOM-compatible rotator will register the angle, so you don’t need to recalibrate the guider. If you rotate it manually, you will need to recalibrate the guider.
- Move the mount a small amount in any direction; a bright star may be just outside of the current view.
I prefer an OAG with a larger prism whenever possible. A 12.5×12.5mm prism has over 250% the area to capture stars compared to the more common 8×8 prism.
I also prefer guide cameras with larger sensors. For example, the Sony IMX174 has close to 300% the sensor area of the popular Lodestar camera, and 500% the area of the ASI120MM.
While a guidescope can work for short focal length refractors and camera lenses, an OAG is a much better solution for long focal lengths and reflector telescopes (as well as small refractors).
For an OAG, the main requirement is that your optical configuration have sufficient back focus. Other issues can be easily dealt with as laid out in the steps above. Even at 2500mm, my setup has no difficulty finding a suitable guide star.
An OAG allows you to better correct for tracking errors and get sharper data capture, making it well worth the small effort in setting it up!
PHD2, excellent free guiding software: https://openphdguiding.org/
Images taken by the author using OAGs: https://www.astrobin.com/users/Rouzbeh/
Editor’s note: See a review of low-cost off-axis guiders from Rouzbeh Bidshahri: