Astrophotography Equipment: Autoguiding
Our equipment introduction for Astrophotography has now discussed telescopes, cameras, and mounts. That is enough to get you started and most beginners probably think that is a complete list of all the equipment you need. In fact, while that equipment will do for short-exposure work, if you wish to explore long-exposure imaging of deep sky objects, you will come to need equipment for one more capability: Autoguiding.
Autoguiding is arguably beyond the "beginner equipment" boundary, and I considered omitting it from this article. A beginner just entering Astrophotography can and should omit this additional complication, but I decided to provide this overview of the technique so that, when you reach the limits of what you can do with unguided photography, you will remember that this additional help is available.
What is Autoguiding?
What Problem Does it Solve?
Even the best mounts, made with high tolerances, well aligned, and running Periodic Error Correction, will have some residual errors in the gears, which will result in small motions of the target being imaged, especially when imaging at high magnification. The error results from imperfections in the drive gears and imperfections in other machining of the mount such as the degree of perfection of the 90-degree angle between the Declination and Right Ascension axis, or the play in the ball bearings that permits their rotation.wide-field refractor, the error may be insignificant but if you are imaging through a long focal-length scope such as an SCT, with exposure times in minutes, you will find this residual error turns your stars from pinpoints into little streaks, no matter how carefully you align the mount.
Autoguiding is a technique to reduce this small residual error in an otherwise well-performing mount to an acceptably small level.
Note that autoguiding is useful only for long exposures.
Depending on your telescope, autoguiding starts to become useful at exposure times of 30 seconds to a minute, or longer.
How Does It Work?
- During the long exposure of your main target, a separate camera is trained on a single star, called the “guide star”, and takes continuous short images of the star and a small area around it.
- Control circuits in the camera, or software in a computer, carefully measure the position of the guide star in each exposure to determine if there is any drifting.
- If any drifting is detected, correction commands are sent to the mount to move it back where it belongs.
The result is that any drifting of the image is corrected within a few seconds, and is not permitted to accumulate. On average, the guide star, and therefore the primary target, is kept stationary over a long period.
What Problems Doesn't it Solve?
It's important to emphasize that autoguiding is the “finishing touch” on a mount that is already performing very well. It is not a shortcut, and it does not correct
- Bad optics, bad focus, or bad seeing;
- Field rotation (Autoguiding an alt-az mount does not make it into an equatorial mount.);
- Poor polar alignment (All the stars except the guide star will appear to rotate around the guide star.);
- Large errors due to low-quality gears or motors;
- Large sudden motion caused by shifting equipment when the scope crosses the meridian (“mirror flop” on an SCT, or shifting across the RA gear backlash on any mount);
- Large sudden motions due to wind or catastrophes such as tripping over the mount or bumping your head on the telescope.
Four Requirements for Autoguiding
To use autoguiding, you need to solve four problems:
- Getting Light From a Guide Star
- Selecting and Attaching a Guide Camera
- Making Guiding Decisions
- Sending Corrections to the Mount
It will take a moment to realize these are new problems - your main telescope and the attached camera are busy collecting your target image, and they are not available for guiding. You need to find independent ways to collect and analyse the light of a guide star.
Let's review the options for achieving each of those.
Getting Light From a Guide Star
The first step is to locate a guide star and capture its light to the location of your guiding camera. You might do this with a small separate telescope, with a little mirror to steal some of the light from the main imaging path, or by placing a second camera chip near the main camera chip in the main imaging path.
Let's discuss those options, and one other highly specialized case, in a little more detail.
Separate Guide Scope
The guide scope can be a less sophisticated instrument than the main scope. Its only requirements are that it be able to focus the target stars cleanly, and that it be mechanically sound enough that there is no shifting of the image during use. Small refractors in the 480-600mm focal length range are probably the most common guide scopes.
Note: in the early days of astrophotography when guiding was done by manually watching a guide star and applying corrections, there was a rule of thumb that the guide scope should be close to the same focal length as the main scope. With modern autoguiding software, this is no longer the case, because the software mathematically calculates the centre of the star, and can make up for any loss of precision due to the difference in focal lengths.
This "guide scope" must be mounted and held rigidly so that it tracks with precisely the same accuracy and error as the main scope.
- "Piggyback" mounting places the guide scope on the main scope - usually by having a dovetail bar attached to the top of the scope and the guide scope holder mounted to that dovetail bar.
- "Side-by-side" mounting places the guide scope beside the main scope - usually by having a dovetail plate with two side-by-side female dovetail slots.
In both cases, the guide scope itself is usually held in a pair of rings with 3-point screw mounts, so that it can be aimed independently of the main scope by adjusting the mounting screws.
Advantages of Separate Guide Scope
There are a number of advantages to this system, resulting in it being very popular:
- It is the simplest of the systems, requiring only the mounting of the second scope. Pointing and focusing the guide scope, and attaching the guide camera, are straight forward.
- Being separate from the light path in the main scope, it does not affect the focus distances of the main scope, so it can be used with any camera arrangement.
- Being separate from the main scope, the guide scope can be unfiltered, providing the brightest possible star field for locating a guide star, allowing shorter exposure times with the guide camera.
- Since the guide scope can be pointed independently, and the guide camera can be rotated arbitrarily, there is a lot of flexibility in being able to locate and centre a suitable guide star.
Disadvantages of Separate Guide Scope
There are a few disadvantages to this system, including:
- The guide scope, being separate from the main scope, can move slightly, independently of the main scope. One moment they may be pointed to parallel places in the sky, then a moment later they may point in slightly different directions. For example, the guide scope may move slightly in its mounting rings. Or the plate holding the guide scope may flex slightly, changing its aim with respect to the main scope. This differential flexure is not important when imaging at low magnifications, but at high magnification it can result in the autoguiding system trying to correct for motion that wasn't present in the main imaging scope, introducing error into the image.
- The additional telescope adds weight and size to the mount, requiring a sturdier mount, making balancing more challenging, and requiring more clearance room around the system.
Because separate guide scopes are best when matched to similar focal length main scopes, and because they do not add any length to the main scope focusing path, they are most often used with main scopes that are refractors or Newtonian reflectors.
The usual setup is a "T-connector" that has suitable connectors to be inserted between the telescope and camera, with a small mirror at the edge of a right-angle portal that is fitted to take 1.25-inch eyepieces or cameras. With most systems this mirror can be moved slightly, either in and out, or rotated on its stalk, or rotated around the diameter of the adaptor. All of these motions are to permit "steering" the view of the guiding light to aid in locating a suitable guide star.
Advantages of Off-Axis Guider
- OAGs avoid the problem of differential flexure since there is no separate guide scope on an independent mount.
- OAGs result in one less telescope to be handled by the mount, and require a simpler mounting system overall.
- Placed before the filter wheel, the OAG deflects unfiltered starlight, providing bright stars and allowing shorter exposure times with the guide camera.
Disadvantages of Off-Axis Guider
- OAGs lengthen the optical path, requiring a scope that can move the focuser through a greater range. Most Newtonian reflectors, and some refractors, have insufficient focuser travel to accommodate an OAG. Even SCTs, with their large focus range, can have trouble reaching focus if additional items such as filter wheels are part of the setup.
- Although flexure between the main scope and guide scope is eliminated, flexure can be introduced because of the long path of devices attached to the main scope, which can sag as the scope tracks.
- OAGs require that a suitable guide star be visible in the part of the main telescope's light path that is sampled by the "pick-off" mirror. In some parts of the sky, especially at long focal lengths, it may be difficult to locate a suitable guide star.
- When imaging with a very large chip, the pick-off mirror may eclipse part of the image.
Because off-axis guiders avoid the differential flexure problem associated with long focal length main scopes, and because they add some length to the main scope focusing path, requiring a scope that has a large range of focus adjustment, they are most often used with catadioptric telescopes such as SCTs and MCTs.
Many modern astronomical CCD cameras are labelled "self-guiding". This usually means that the camera contains two CCD chips, side-by-side. The main imaging chip is in the centre of the field and, mounted immediately next to it, a small second chip acts as the guide camera. The camera and telescope are connected in such a way that light from the telescope falls both on the main imaging chip and the guide chip.
Advantages of Dual-Chip Camera
- The most obvious advantage is simplicity: there are no extra devices attached to the imaging train at any point.
- There is no differential flexure - being part of the same camera housing, you are guaranteed that the only motion the guide camera sees is motion affecting the main camera.
- There is no extra focus distance consumed. If you can bring the main camera into focus, the guide camera will be in focus too.
Disadvantages of Dual-Chip Camera
- The main disadvantage is that the guide chip is in a fixed location relative to the main camera chip. If there is no suitable guide star on the chip, the only way to find one is by rotating the camera, changing the framing of your photograph.
- Since the chip is mounted inside the camera, it is behind the filter wheel, so you are guiding on filtered light. If you are using narrowband filters, you may not be able to find guide stars, or they may be so dim that long exposures are necessary to be able to image them - too long for effective guiding.
Dual-chip cameras are most often paired with catadioptric telescopes, for the same reasons given for OAGs. In situations where the in-camera guiding chip is not a good choice, such cameras can still be used with a separate guider.
Special Dual-Sampling CCD
Finally, I will mention one special case. Some cameras (e.g. the SXVF-series cameras from Starlight Xpress) use a feature of certain CCD chips to read some pixels at a faster rate than others. On the SXVF cameras, special software allows every other line of pixels to be read quickly for guiding, while the alternate lines of pixels are left "on", gathering light in a long exposure. Such chips can truly be said to be "self-guiding".
The advantages and disadvantages are very similar to those listed for the Dual-Chip cameras above, with the additional advantage that the entire main chip is a potential source of a suitable guide star, although resolution of the entire chip is reduced by half when used in this way.
Selecting a Guide Camera
In some of the options discussed above, the guiding chip is built-in to the main camera. However, when using a separate guide scope or an OAG, a separate guide camera must be installed to image the guide star.
Ideally, the guide camera should have the following features:
- High sensitivity so short exposures can be used;
- Monochrome. Colour cameras can be used for guiding, but there is no advantage to doing so, and there is a loss of resolution and processing time;
- Low noise level, or at least very predictable noise that can be effectively cancelled with dark frame subtraction;
- No "hot pixels" (pixels that are "on" all the time due to electronic faults), because the guiding software may mistake them for stars and try to guide on them;
- Small and light, to minimize additional load on the mount and telescope;
- Self-powered (e.g. from the USB connection) to minimize dangling wires;
- Fast download (e.g. USB-2 interface) to facilitate rapid exposures, and to reduce the need for obsolete connectors and adaptors.
- Built-in relays to provide ST-4 guiding signals (discussed below) are a nice extra, provided in most dedicated guide cameras, and further reducing cables and connectors.
Any "mainstream" astrophotography CCD camera can be used as a guide camera, but that may be an expensive solution, since you would probably be paying for features you don't need. For this reason, most imagers use either a special guide camera, or an obsolete camera that has been "demoted" to guiding use after an upgrade of the main camera. Some webcams can also be used as guide cameras if they are sensitive enough to the dim light of guide stars.
Making Guiding Decisions
Once you have the light of a guide star falling on the chip in a guide camera, you need to have the star's motion tracked and converted into an error estimate, and suitable corrections calculated. Certain guide cameras can do this calculation themselves, with built-in circuitry, but most require connection to a computer, with the decisions being calculated by software.
- The original amateur-level autoguider was the ST-4 from Santa Barbara Instruments Group (SBIG). This was a completely stand-alone autoguider involving a small, cooled, CCD camera and a large clunky control box. They are still available on the used marked and work very well, after you get over the steep learning curve and complex user interface.
- A decade later, SBIG released the ST-V system, which improves on the ST-4 with a dedicated video screen and simple menu interface.
- Now, SBIG has announced a new stand-alone autoguider, the SG-4, which has a modern user interface.
Cameras such as these have the advantage of working without a computer. DSLR users, who may not be using a computer, can then autoguide DSLR images without having to add a computer to their setup.
While the stand-alone cameras above are convenient for DSLR users imaging without a computer, CCD users will have a computer on hand anyway, so the autoguiding system can be simplified by letting the computer do the calculations. This results in a cheaper system, a far simpler user interface, and value-added features such as the ability to graph the guiding corrections over time as a way to measure system performance.
Such systems have two components, the guide camera-to-computer connection, and the control software on the computer.
Connecting Guide Camera to Computer
Some dedicated guide cameras, when matched to a main camera of the same manufacturer, are connected, via a short cable, to the main camera, instead of to the computer. Then the guide camera's signals go on the main camera's USB connection. This reduces the number of cables running to the computer by one. (Reducing cables is always a good thing, since tangling cables in the dark can end an imaging session in a spectacular fashion.)
Personally, I use the autoguiding features built in to Maxim DL most of the time, since I am using that software anyway. I have also had good, but limited, experience with PHD. For a beginner, I would recommend starting with one of the simple and free packages like PHD, possibly replacing it some day if you invest in one of the comprehensive (and expensive) multi-function control programs.
Sending Corrections to the Mount
The final step in the autoguiding formula is that, once the computer or stand-alone guider have decided the mount needs to do a minor pointing correction, the correction commands need to be sent to the mount. Most mounts designed to be compatible with autoguiding have a special input connector for this purpose, while some mounts without this connector, but that allow computer control, can also be autoguided.
Mount with Autoguider Connector
This is usually called an "ST-4" guiding connector, since the protocol was first used on the ST-4 autoguider. This is not a data connector - it is not designed to be connected directly to a computer. Rather, it receives simple analog signals, represented by voltages on the control lines, to activate the mount's motors at slow speed. The wire connected to this terminal must come from a special circuit that produces the appropriate signals.
|Shoestring Astronomy is one example.|
Mounts with No Autoguider Connector
If your mount has no autoguider connector but still has the ability to be connected directly to a computer, you can usually instruct the guiding software to send appropriate commands over the serial link to guide the mount. However, I suspect this may not be worth the effort - any mount I have seen without an autoguider connection is unsuitable for astrophotography anyway.
So, in summary, what you need for autoguiding is:
- A camera with a built-in "self-guiding" chip or a separate camera for autoguiding mounted on either a separate guide scope or an off-axis guider;
- A computer running guiding software unless you have one of the rare stand-alone guiders; and
- A way to produce ST-4 guiding signals, either from a guiding port on a camera or from a dedicated guiding adaptor.
You will also need some practice time. Setting up autoguiding, calibrating the software, camera, and mount, and tuning the many parameters are all complex tasks that will take some time to master. You will, however, find it's worth it when you develop the ability to take multiple-minute exposures with no drifting of your target.
The next topic in our introduction to astrophotography equipment will be a discussion of the computer and software you will need to control your camera and acquire and process images.
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