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Selecting a Guide Scope and Autoguiding Camera for Astrophotography

By: Brian Ventrudo
July 20, 2017

1. Overview - The Need for Autoguiding

A good equatorial mount is a must-have for astrophotography. But even the best equatorial mount will have some mechanical imperfections, resulting in tracking errors that lead to slightly elongated star images.In many mounts, the predictable periodic error in the right ascension motion can be manually corrected to some degree. However, imperfections in the gears, slight deviations in perpendicularity between the RA and declination axes, and mechanical play in the bearings can all contribute to additional errors that lead to elongated stars and smeared images of extended objects like nebulae and galaxies.These errors are more pronounced in mid-range equatorial mounts than in more expensive mounts, but even the finest mounts available to amateur astronomers will face these tracking imperfections.

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Figure 1 – Tracking errors during an image of the globular cluster M15 (Credit: Brian Ventrudo)

That's where autoguiding comes in. Autoguiding, or guiding in general, involves the application of small corrections to the position of an equatorial mount during long-exposure imaging. During the length of the exposure, the astro-imager follows the motion of a star—the guide star—and, when the star appears to move slightly because of errant motion of the mount, applies a corrective signal to the mount to move the star back to its original position. In the earliest days of imaging in the late 19th century until the 1990s, amateur astronomers guided manually by visually tracking a guide star over the course of a long exposure and nudging the mount mechanically or electrically back onto the right path. With the availability of computers and sensitive digital cameras, guiding is now accomplished automatically. A camera and specialized software monitors the position of a star and nudges the mount automatically to keep the position of the guide star on track. This is autoguiding.

Autoguiding is only necessary when taking exposures of at least 30-60 seconds. If you'retaking short images or video clips of bright objects like the Moon or planets, or you're taking sub-frames ('subs') of deep-sky objects of less than 30 seconds, you may not need to guide during your images. And autoguiding only offers a small but critical 'tweak' to the motion of an equatorial mount that's already properly aligned and correctly operating. It cannot correct for bad optics, poor polar alignment, large and sudden errors in tracking by a poor-quality mount, sudden errors caused by wind or bumps, or field rotation cause by tracking with an alt-azimuth mount.

2. Options for Autoguiding

During an exposure, your main telescope and camera are busy collecting photons from your main photographic target such as a nebula or galaxy. So to autoguide your images, you need a way to collect light from a guide star and get it onto a digital camera, then use the signal from the camera to correct the position of your mount. There are four main ways to collect light from a guide star:

  • A separate guide scope and guide camera
  • An off-axis guider that splits off light collected by the main telescope, but which directs the light to a separate guide camera that detects a guide star somewhere in the same field of view as the main object that's being imaged
  • A dual-chip CCD camera on the main scope with one chip for imaging the main subject and one for the guide star
  • A specialized dual-sampling CCD with some pixels designated for guiding and some for imaging the main target.
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Figure 2 – A small refractor guide scope piggybacked with mounting rings on a Newtonian reflector (Credit: Rawastrodata/Wikipedia Commons)

There are pros and cons to each guiding method, but in this article, you will learn about the first option, the guide scope, and how to select the right guide scope and guiding camera (also called an autoguider) for your imaging needs. Using a separate guide scope and guide camera is the simplest option, ideal for beginning imagers, and works well when used with a main telescope with a focal length up to about 1000mm to 1500mm. With a guide scope, you have the flexibility of choosing and following a guide star outside of the field of view of your main telescope. While the main scope may use narrowband filters during imaging, the guide scope is usually unfiltered so it may produce star images that are brighter than those rendered by the main scope. And, with a guide scope, you can also follow objects like comets that move at a rate slightly different than the stars.

3. Aperture and Focal Length of a Guide Scope

Several factors and trade-offs make for a good guider camera, including:

  • A sufficiently large aperture to detect faint guide stars in a nearly any field of view anywhere in the sky and to minimize autoguider exposure time, yet a small enough size and weight to prevent taxing the mount
  • A long focal length to discern small displacements in the position of a guide star, but a sufficiently short tube to prevent flexure relative to the main scope and cause guiding errors (a problem called differential flexure)
  • A wide field of view (and hence a short focal ratio or focal length) to have a wide field of view to detect a large number of potential guide stars
  • A sufficiently solid focuser to hold the autoguider camera without sagging.
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Figure 3 - A guide scope and guide camera mounted on an imaging refractor. Credit: Terry Hancock.

The simplest and most cost-effective guide scope for most amateur imagers is a small refractor. You don't need anything fancy or expensive with an ED or apochromatic lens. A well-figured achromat with a solid focuser that produces a clean, tight image on the guide camera sensor makes for a good guide scope. The more sensitive the guide camera, the smaller the objective can be, but something in the range of 50-80mm is a good place to start. Anything bigger than 80mm gets too heavy for most affordable equatorial mounts for astrophotography such as Sky-Watcher or Orion EQ-5 or EQ-6 class mounts, the Celestron AVX, or Losmandy GM-8 or GM-11. With more sensitive (and more expensive) CCD or CMOS-based guiding cameras, as mentioned below, flyweight guide scopes with apertures as small as 30mm can work well when guiding images with short focal length refractors as the main scope.

How about focal length for the guide scope? In the old days of manual guiding, when the 'detector' of the guide scope was the human eye, a relatively long focal length was needed so the eye could detect small deviations in the position of the guide star. As a rule of thumb, the focal length of the guide scope needed to be at least 1/3 the focal length of the imaging scope when guiding visually. But modern guide cameras and the software that monitors the position of the guide star on the camera sensor can determine the position of the star with much more precision than the human eye, often to within 1/10thof a camera pixel or less. So the focal length of the guide telescope can be much shorter relative to the main scope.

As an informal figure of merit for a guide scope and guide camera, you can think about the ratio of the imaging scale of the guide scope and the main imaging scope. Let's call this figure of merit F:

F = Image scale of the guide scope / Image scale of the imaging scope

Now each image scale depends on the pixel size of the camera and the focal length of the telescope:

Image scale (arc-seconds/pixel) = 206 x S / FL

where S is the size of the camera pixels in microns and FL is the focal length of the telescope in millimeters. So in these terms, the 'F' factors works out to

F = (Sgs x FLis) / (Sis x FLgs)

where Sgs is the pixel size of the camera on the guide scope, Sis is the pixel size of the camera on the imaging scope, FLgs is the focal length of the guide scope, and FLis is the focal length of the main scope.

When guiding by eye, which is the same 'instrument' used to look at the stars in the final image, the image scale for the guide scope and main scope are the same, which means if you see the guide star move visually with your eye in the guide scope, you will see the stars streak on the final image on the main scope. So ideally, the 'F' factor should be 1, although you can get away with F=3 roughly and still achieve good results. That's where the rule of thumb mentioned above comes from, that is, you should choose a guide scope with a focal length of at least 1/3 the focal length of the main scope when guiding visually.

With guide cameras and guiding software, which can detect apparent deviations in the guide stars of, say 0.1 pixels, the F-factor can be larger. For example, assuming a 0.1 pixel sensitivity of the guide camera and software, and assuming you want to ensure a tracking error of less than 1 pixel in the main image, the 'F' factor should then be 10 or more if the guide camera and imaging camera have the same pixel size. So the rule above suggests the focal length of the guide scope should be at least 1/10th the focal length of the imaging scope. If the focal length of your imaging scope is 1500mm, then you need a guide scope with a focal length of at least 150mm to give sufficient resolution for effective autoguiding. This is not a hard and fast rule, but it is helpful to understand what focal length you need.

4. Mounting a Guide Scope

Mounting a guide scope correctly is critical for accurate tracking during imaging. There are two common mounting schemes. The scope can piggyback on the main imaging scope, usually on a dovetail rail attached to the tube of the main scope. Alternatively, the guide scope and the main scope can each sit side-by-side on a dovetail bar so the scopes are pointing in the same direction in the sky.

In either case, the guide scope must be held rigidly so that it points with the same precision and stability as the main scope. If the guide scope changes its pointing direction slightly relative to the main scope because of mechanical flexure, the autoguiding software will detect a mount error where none exists and make an unnecessary correction. So minimizing this 'differential flexure' is critical when choosing a mounting scheme. One way to minimize flexure is to choose a guide scope with a short tube, which is why many guide scopes are short focal-ratio refractors of f/4 or f/5. It's also important to mount the guide scope using a pair of solid and high-quality mounting rings, rather than on a simple dovetail rail and bracket used for many finder scopes. The mounting rings themselves can mount to a solid rail that slips into a dovetail mounted on the main scope.

NOTE: In principle, guide scopes can be used for imaging with large, long focal length telescopes. But since longer focal length guide scopes are required, and since longer focal lengths lead to flexure, many expert imagers turn to off-axis guiders when working with imaging scopes with a focal length longer than about 1500mm or 2000mm.

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Figure 4 – A guide scope mounted with solid mounting rings to minimize flexure. Credit: Terry Hancock.

5. Some Recommended Guide Scopes

If a compact size is important, you can choose from among many small 50mm achromatic scopes on the market, most of which are designed with autoguiding in mind. These smaller-aperture scopes work well with sensitive cameras, yet still have enough focal length to guide images with a telescope up to about 1500mm focal length:

Agena StarGuider II 50mm Finder/Guide Scope. This scope has a weight of 1.5 lbs, a focal ratio of f/3.2, and a focal length of 162mm.The scope includes a helical focuser, but does not have mounting brackets included.

Agena StarGuider II 50mm Finder/Guide Scope with Bracket. This is the same scope listed above, but it includes a single bracket and Vixen/Synta dovetail. A bracket is included rather than rings to minimize cost, but the tube length is short so differential flexure is minimized.

Agena 50mm Deluxe Straight-Through Finder/Guide Scope with Helical Focuser. An upgrade from the StarGuider II, this scope has a focal ratio of f/3.8 and a focal length of 190mm. It weighs just under 1.5 lbs and includes mounting rings on a dovetail rail.

William Optics 50mm Right Angle Correct Image Finder/Guide Scope Kit with Bracket. A unique f/4 scope works as a straight-through guide scope or can be converted to a right-angle finder. As a guider, the weight is less than 1 lb without rings.

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Figure 5 – A 60mm f/4 guide scope with helical focuser

If you're using an older autoguider camera that's less sensitive, or you want to make sure you always have an accessible guide star, or you prefer a slightly longer focal length to improve autoguiding performance, you can choose a larger-aperture 60mm guide scope such as:

Agena 60mm f/4 Precision Straight-Through Finder/Guide Scope with Helical Focuser. This scope detects stars about 0.4 magnitudes fainter than a 50mm refractor which offers many more potential guide stars in each field of view. It weighs about 2 lbs so it still goes easy on your mount.

ZWO 60mm f/4.6 Guide Scope with Helical Focuser. This ring-mounted scope includes a dovetail with five ¼-20 mounting holes for additional flexibility. It's designed to work with ZWO cameras, and it can also work as a lens for a DSLR camera with an optional brand-specific T-ring. Total weight is 22.6 oz.

If you want a guide scope that can double as an excellent small imaging scope in its own right, you can choose a small 60-70mm ED refractor from manufacturers such as William Optics, Stellarvue, or Astro-Tech, for example. These scopes are considerably more expensive and heavier than the simpler scopes mentioned above, however, so make sure your mount can handle the extra weight.

Or you can move in the other direction and consider a very small guidescope such as the miniGuideScope by QHYCCD. This tiny cigar-sized scope has an aperture of just 30mm and a focal length of 130mm. But when used with sensitive autoguiding cameras with small pixels, this little scope can still pull in enough starlight to make it easy to detect a guide star.

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Figure 6 – The miniGuideScope by QHYCCD has an aperture of just 30mm

6. Cameras for Autoguiding with a Guide Scope

Before you purchase a guide scope, give some consideration to choosing your guide camera. The two components are not entirely independent of each other. For example, a more sensitive guide camera can work with a smaller-aperture guide scope to detect the same number of potential guide stars in a field of view in a shorter exposure time. But sensitive cameras are more expensive. A larger-aperture guide scope means you can get away with a less sensitive and less expensive camera, but larger guide scopes are heavier and may strain your mount. Or you can go with a smaller guide scope and less sensitive camera and give up choice in guide stars and work with increased exposure times which leads to less accurate guiding.

A guide camera need not be as sophisticated and expensive as your main imaging camera. But ideally, the guide camera should have the following characteristics:

  • High sensitivity to allow for short exposures and smaller-aperture and lighter guide scopes
  • A monochrome sensor because, while color sensors will work, but there's no advantage to using them and they will result in a loss of resolution and sensitivity because of the Bayer matrix in front of the sensor
  • Low noise
  • Minimal "hot pixels" (faulty pixels that are always on) so that guiding software won't mistake them for stars
  • Compact and lightweight to minimize load on the mount and focuser of the guide scope.
  • Poweredby the USB data connection
  • Fast download rates (at least USB2.0)
  • A built-in ST-4 guiding port to connect the guide camera directly back to the equatorial mount for corrections (which are supplied through the camera from the computer and autoguiding software connected to the camera through the USB port). An ST-4 port is not required but can be convenient

Sensor size is also a consideration. Larger sensors give a wider field of view and more potential guide stars, but they are expensive. If the camera has a relatively recently-made sensor, even a small one with a diagonal of 6-7mm, it may likely pick up enough guide stars to do the job. It's rarely necessary to get an autoguiding camera with a considerably larger sensor.

If money is no object, nearly any monochrome astronomy camera can serve as an autoguiding camera. But if you want to save money, you can use an older "obsolete" astronomy camera. Or you can acquire a newer but relatively basic monochrome astronomy camera that you can also use for applications such as lunar or planetary imaging. Or you can get a camera designed especially for autoguiding.

Some Recommended Autoguiding Cameras
Advances in semiconductor technology and manufacturing have made sensitive monochrome astronomy camera extremely affordable, so this is an excellent time to acquire anautoguider camera. As mentioned in the previous section, nearly any modern astronomy camera will serve as an autoguider. But if you want to keep your costs reasonable, the following cameras offer excellent autoguiding performance at a reasonable price:

ZWO ASI120MM CMOS Astronomy Camera. ZWO A popular camera with beginning imagers, this compact and affordable CMOS astronomy camera also makes a great autoguider. With small 3.75-micron pixels, low power consumption, and ST4 port, this camera works well with guide scopes of 50mm aperture or larger. This camera also works as a respectable planetary, lunar, and solar camera. The USB2.0 version costs less than $200, while the USB3.0 version, the ASI120MM-S, is $50 more.

QHYCCD QHY5L-II Monochrome Camera. Like the ZWO 120MM, this camera is affordable and can be used as an autoguider or lunar/planetary camera. The sensor is very similar to the 120MM, and the package is the size of a small 1.25" eyepiece. It also includes an ST4 port and USB2.0 interface. In 2016, QHYCCD released an upgraded QHY5-III series of cameras with USB3.0 interface. The sensitive Linked TEXTQHY5III178 is well suited for autoguiding. These cameras can also be combined with the cigar-sized QHYCCD miniGuideScope mentioned above.

Starlight Express Lodestar X2. Specifically designed as an autoguider, the monochrome Lodestar X2 has high sensitivity and low noise. It's also sensitive enough to image brighter deep sky objects. Because of its sensitive CCD sensor, the X2 is considerably more expensive than other autoguiding cameras.

Orion StarShootAutoguider. Another dedicated autoguiding camera, the StarShoot has been around for many years and also works well as an entry-level astronomy camera. It also comes paired with an 80mm f/5 refractor as a complete autoguiding package.

7. Putting It All Together

Once you've selected your guide scope and guide camera, it's time to put it all together. The details of connecting your camera, mount, and computer and using them to autoguide during imaging are beyond the scope of this article. But in general terms, once the camera is mounted on the guide scope and the whole assembly is mounted on your telescope, you connect the guide camera to your computer with a serial cable, usually with USB connectors, to transfer data and supply power to the camera.The computer has many roles in imaging. It controls the main imaging camera. It may also, if you choose, run a separate application such as SkyX or Stellarium to slew your telescope to your imaging targets.The computer also runs an autoguiding application that monitors the signal from the guide star generated by the guide camera, calculates guiding errors, and supplies a corrective signal for the telescope mount. Some cameras come with proprietary guide software, but many imagers use the open source (and free) PHD or PHD2 application as the guiding software. PHD is short for "Push Here Dummy".

To get the corrective signal back to the mount, there are two options. If the guide camera and mount have ST-4 ports for autoguiding, you can connect the camera and mount together with the appropriate cable, which is often included with the camera. If the camera does not have an ST-4 port, you can supply the corrective signal to the mount from a serial port on the computer to the serial input on the mount's controller.

You can get a more in depth understanding of using PHD autoguiding software, and autoguiding in general, in this excellent 2017 article by Rod Mollise:

http://uncle-rods.blogspot.ca/2017/03/issue-534-getting-your-phd.html

8. Summary

This article has taken you through the options and key considerations for choosing guide scopes and cameras for astrophotography. Compared to other methods of guiding, a guide scope and camera are conceptually straightforward and inherently flexible for beginning (and experienced) astrophotographers, especially when using a main imaging scope with a focal length of less than 1000mm to 1500mm.

The essential features of a guide scope include sufficient aperture and field of view to bring a larger number of guide stars into view, a long focal length to allow for more precise guiding, and an overall weight which does not tax the telescope mount. It's also important to use rings to mount the guide scope on the main scope to minimize differential flexure between the two scopes.

The choice of guide scope also has an influence on the guiding camera. More sensitive cameras, while they may cost more, can relax the aperture and weight requirements of the guide scope. And while nearly any astronomy camera can be used as an autoguider, cameras with monochrome sensors, small pixel size, and high sensitivity work best.

Brian Ventrudo
About the Author

Brian Ventrudo is a writer, scientist, and astronomy educator. He received his first telescope at the age of 5 and completed his first university course in astronomy at the age of 12, eventually receiving a master's degree in the subject. He also holds a Ph.D. in engineering physics from McMaster University. During a twenty-year scientific career, he developed laser systems to detect molecules found in interstellar space and planetary atmospheres, and leveraged his expertise to create laser technology for optical communications networks. Since 2008, Brian has taught astronomy to tens of thousands of stargazers through his websites OneMinuteAstronomer.com and CosmicPursuits.com.