Autoguiding a telescope without a computer is possible by the use of Stand-Alone Guiders. You can also opt for the DSLR which has the capability of performing an auto-guide without the help of a computer. The auto-guide is advantageous for making photography on target objects simpler, especially when you have deep-sky objects that are too faint to be seen visually.
In comparison to the past, astrophysicists today use digital telescopes that have fantastic potential in capturing detailed images of objects in the deep sky.
However, while it is much easier to produce good wide-field constellation pictures with nothing less than a DSLR, an exposure of up to half a minute, and a fixed tripod, capturing detailed images of specific objects in the sky, is quite involved.
You will require longer exposures and lenses with a focal length of at least 200mm. Therefore, you need to attach your telescope to an equatorial mount precisely polar aligned to track the stars as they move across space accurately.
Otherwise, you will have a trailing image in the end.
How Does a Telescope Autoguide Work?
The concept behind auto-guiding is straightforward. During the prolonged exposure to your target, a separate telescope is trained on a nearby star (the Guide Star) and takes continuous short images of the principal and its surroundings.
The control circuits in the telescope accurately measure the position of the guide star on any exposure to determine if there is any drifting. If there happens to have any drifting, correction commands are then sent to the mount to move it back to where it belongs.
The advantage of auto-guide is that image drift is collected within a few seconds and can never accumulate. Therefore the primary target (GuideStar) is kept stationary for an extended period.
Astrophysicists can determine the perfection of auto-guiding when there is less than one pixel of object wandering in the image. For instance, when you are photographing a galaxy ideal for 500 pixels wide in your vision, the accuracy of your telescope is when the result does not exceed 501 or 502 pixels wide.
With such a result, it is hard to notice minor blurring on your target.
Procedure/ Requirements for Autoguiding
There are few requirements that you need to meet before performing auto-guiding. They include:
- Get light from the GuideStar
- Selecting and attaching a guide camera
- Making guiding decisions
- Sending corrections to the mount
Let’s get an insight on each.
Get Light from the GuideStar.
The first step in auto-guiding is locating a guiding star and capturing its light as you direct it to the location of your guiding camera. That is possible using a separate small telescope, pairing a little mirror that steals some light from the main imaging path.
Or you can place a second camera chip near the main camera chip in the main imaging path. So, let’s have a look at all three options.
- Separate Guide Scope
Attaching a smaller second telescope onto your system is the simplest way to observe a guide star. It is a less sophisticated instrument as compared to the main scope.
The only requirement is that it can focus on the target star. It should also be mechanically sound enough to ensure no shifting of imaging during operations.
You will often find small reflectors of 480-600mm focal length range commonly used as the guide scopes. Mounting guide scope must apply and ensure rigid attachment to track precisely the same accuracy and error as the main scope.
You can mount the guiding scope in two main ways:
- Piggyback- you will place the guide scope on the main range by having a dovetail bar attached to the top of the range and have the guide scope holder mounted to the dovetail bar.
- Side-by-side- mount the guide scope beside the main range. You will need to have a dovetail plate with two side-by-side female dovetail slots.
Note that the guide scope is always held in a pair of rings with 3-point screw mounts. That makes it in both cases independent of the main scope by adjusting mounting screws.
- Off-Axis Guider
It is also one of the simplest methods. All it consists of is inserting a tiny diagonal mirror or prism into the light path of the main telescope.
It will then reflect some light from a small portion of the image from sideways, through a port, and into the guide camera.
The main setup in the off-axis guider is the T-connector that comes with the ideal connectors to pair between the telescope and camera. It comes with a small mirror at the edge of a right-angle portal fitted to take about 1.25-inch of the eyepiece or camera.
Most telescope systems allow slight mirror movements, either in and out, or rotation on its stalk, or rotated around the diameter of the adapter. The rotations allow steering the view of the trapped guiding light as it aids in locating a suitable guide star.
- Dual Chip Camera
It is the use of a side-by-side camera where the main imaging chip is centrally positioned. The second chip placed immediately next to the main act as the guide camera.
The telescope and camera connection must be in such a way that light from the telescope falls on both the guide chip and the main imaging chip.
Selecting a Guide Camera
A guide camera should consist of the following features:
- Highly sensitive to short exposures
- Low noise levels, or to its slightest very predictable noise, are easy to eliminate with dark frame subtraction.
- No hot pixels (on pixels due to electronic faults) since it can guide software mistakes.
- Small and light to reduce the additional load on the mount and telescope
- Self-powered to reduce dangling wires
- Fast in downloading to facilitate huge exposure as it reduces the need for obsolete connectors and adaptors
- Built-in relays that provide ST-4 guiding signals.
Making Guiding Decisions
After having the light of a guide star falling on the clip of a guide camera, you now need to have a track of the star’s motion. The motions are then converted into error estimates and later used to calculate suitable corrections.
However, different guide cameras can calculate themselves with built-in circuitry, while some may need computer connections.
The stand-alone guides are genuinely the stand-alone guide systems on the market. They have the advantage of not needing a computer.
Meanwhile, DSLR users who do not use a computer can auto-guide DSLR images without using a computer in their setup.
Sending Correction to the Mount
It is the final step of auto-guiding once a stand-alone guide decides that the mount requires a minor pointing correction. Therefore, correction commands are sent to the support.
It is common to find out that most mounts are designed to be compatible with the auto-guiding and have an additional input connector for that purpose.
Conclusion
Autoguiding requires a built-in self-guiding chip or separate camera mounted on a different guide scope or an off-axis guide. However, for better results and performance, the astrophysicist needs to practice setting up auto-guiding, mounting, cameras, tuning, and calibrating the software.
In the end, you will fluently develop the ability to take multiple-minute exposures with zero drift off your target.
Please be careful and use at your own risk
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