Autoguiding a Telescope for Deep-Sky Astrophotography
Below, I’ll explain how to start leveraging the power of autoguiding for deep-sky astrophotography. Many amateur astrophotographers are hesitant to add any additional complexity to an already steep learning curve, but the truth is, autoguiding your camera and telescope isn’t overly difficult to execute.
I’ve been using a small “guide scope” with an autoguiding camera to take longer exposures through my telescope for several years. With the right equipment and a little trial and error, you’ll wonder how you ever lived without it.
The setup above shows a William Optics Zenithstar 73 refractor with a 50mm guide scope and camera mounted on top. If you are looking to add a small guide scope package to your existing astrophotography setup, you can build a system using the components of your choice, or invest in a guide scope package that’s ready to go.
Keeping weight to a minimum has many advantages and a miniature system like the one shown below is an attractive option for beginners. I started out with an Orion 50mm Mini Guide scope package way back in 2011, and this is still a popular starter system for many backyard imagers.
The ZWO 30mm Guide Scope is ultra-small and adds very little weight to your telescope.
Basic Autoguiding for Astrophotography Made Simple
I’ll admit that autoguiding can seem a bit daunting in the early stages of building your first deep-sky astrophotography kit. The great news is, there are more compact and affordable solutions available than ever before.
Below, I’ll offer you some affordable autoguiding solutions that I have personally used to guide a number of telescope setups. They’re really not that hard to get up and running, and they can make a big difference to the quality of your images.
The Concept of Autoguiding
Whether you’re shooting with a DSLR or a dedicated astronomy camera, capturing longer exposures means that more light (or signal) can be recorded in a single shot.
You’ll often reveal much more signal on a deep-sky object in a 5-minute exposure than you would in 30-seconds. This makes being able to consistently capture long exposure images with sharp, focused details a real benefit for astrophotography.
However, to do this requires extreme accuracy from your equatorial telescope mount as it slowly tracks the apparent movement of the sky. Even the slightest amount of periodic error can ruin a long exposure image of your target.
As you increase the focal length of your telescope, autoguiding becomes more important. This is because we are now “sampling” a smaller (zoomed-in) area of the night sky that can potentially highlight the smallest amount of period error in your telescope mount.
This image of the Veil Nebula was captured on an affordable equatorial mount using autoguiding.
Autoguiding is accomplished by sending small corrections to your telescope mount via an ST-4 cable communicating from your guide camera to the mount. You can also autoguide using the pulse-guiding method that utilizes a direct connection from your PC to the telescope mount.
It is said that pulse guiding (with an ASCOM compliant) equatorial mount results in improved guiding accuracy. This can be measured using the tools found in the PHD2 guiding software, mainly in the total RMS error reading.
Over the years, I have made it very clear on my YouTube channel that I don’t obsess about these values much. However, if you are looking for a general benchmark, Jerry Lodriguss shared a helpful reference in this Cloudy Nights thread:
- Good seeing (2″) averages around 0.3 arcseconds RMS in the guiding
- Average seeing (2-3″) averages around 0.5 arcseconds RMS in the guiding.
- Bad seeing (more than 3″) averages around 1.0 arcseconds RMS in the guiding.
My portable deep-sky astrophotography setup with a 50mm William Optics 50mm guide scope riding on top.
Why it’s Useful for Astrophotography
Modern equatorial telescope mounts are quite capable of compensating for the rotation of the Earth, that is what they were designed for. However, deep-sky astrophotography (especially at longer focal lengths) is a very demanding application for any entry-level to an intermediate equatorial mount.
Depending on which telescope mount you’re using, subtle errors in the accuracy of your polar alignment can begin to surface in images as short as 60-seconds. The slightest bit of off-balance in your imaging payload can put stress on the gears in your mount, which often leads to less than perfect stars in your long exposure image.
The QHY PoleMaster device is great for those looking to easily improve their polar alignment accuracy. Be sure to check out my full review of this electronic polar scope.
Even with your polar alignment spot-on, and your payload perfectly balanced, autoguiding is often necessary to track your object smoothly enough for astrophotography. This is especially true on beginner-level GoTo mounts such as the Sky-Watcher HEQ5 Pro pictured below.
My Sky-Watcher HEQ5 Pro mount with a refractor guide scope mounted to the primary imaging telescope
The mount itself may have shortcomings due to worn-out gears or low-cost materials used in its construction. There is a myriad of potential culprits for periodic error in your mount’s tracking ability. Have a look at the list of upgrades applied to a “hypertuned” Celestron CG-5 mount. It’s a great general list of reasons why your mount may not be tracking accurately.
- Complete inspection of the mount head.
- Complete dismantling of the mount head.
- Inspection and cleaning of each part – removing all metal burrs, shavings, thick grease and foreign objects.
- High-resolution sanding and polishing of moving internal parts resulting in high smoothness of internal metal parts and “glide like” performance from your mount.
- Inspection and adjustment of motors and encoders with gear set screws replaced as necessary.
- Replacement of stock worm bearings with ceramic hybrid bearings.
- Replacement of plastic bearings with PTFE bearings.
- Reassembly using high-grade synthetic grease.
- Resetting of worm gear meshing.
- Adjustment of backlash settings in RA and DEC.
If the issues are severe enough, even autoguiding won’t help. For this reason, its always best to invest in a quality telescope mount that has been proven to be a reliable choice for deep sky astrophotography.
Thankfully, basic autoguiding is enough to correct most of the issues associated with modern equatorial mounts. The use of autoguiding made a dramatic difference to the types of images I was able to capture on my old Celestron CG-5, (Now the Celestron Advanced VX).
What I’m Using
I have used a number of different autoguiding telescopes from the Orion 50mm Mini Guide Scope to a William Optics 72 APO Doublet. If you’re using a guide scope for autoguiding, a good rule of thumb is to use one with a focal length of at least a third of your primary imaging scope.
An off-axis guider solves this problem by utilizing the native focal length of your imaging telescope but can add weight and complexity to your camera system. Both systems have their strengths and weaknesses, but I personally prefer the simplicity of an auxiliary guide scope over an OAG.
In the following video, you’ll see me use autoguiding to collect 3-minute long exposures on the Cocoon Nebula, using a DSLR camera and a 73mm telescope. When using a DSLR camera for deep sky astrophotography, autoguiding not only allows you to shoot longer but dithering (which helps reduce noise in your stacked image) is now possible.
Here is a complete list of guide scopes I have used in the past:
- Orion 50mm Mini Guide Scope
- Starwave 50mm Guide Scope
- Starfield 50mm Guide Scope
- Starwave 60mm Guide Scope
- Starfield 60mm Guide Scope
- William Optics Z72 APO Doublet
These days, I primarily use a William Optics 50mm Guide Scope because it is just so lightweight and easy to mount to various telescopes.
This little telescope features the William Optics Rotolock system, a design feature that securely holds your 1.25″ barrel guide camera. I have found it very convenient to adjust the Rotolock system to move the guide camera in-and-out of the optical tube to find focus.
The guide scope has a focal length of 200mm @ F/4. You’ll need to fit the tube into an appropriate set of tube rings, or your existing finderscope rings on your telescope.
The camera I use most often with the 50mm guide scope is the ZWO ASI 290mm mini. This is not only a highly sensitive monochrome guide camera, but it’s also compatible with the ZWO ASIair wifi camera control device and software.
The ASIAIR Plus has its own autoguiding program in the application that communicates with the telescope mount just like PHD2 does.
What you need to start Autoguiding
The basic equipment needed to accomplish a successful night of astrophotography with autoguiding is a secondary telescope (guide scope) and a guide camera. The guide scope rides atop your primary imaging telescope and is usually much smaller. The autoguiding camera is traditionally lighter than your primary imaging camera, and will often include a small mono CCD or CMOS sensor.
Once properly connected to your computer using the appropriate cable (in my case, a USB 2.0 A-male to B-male cable), the autoguiding camera will broadcast a live-loop image through the guide scope to your computer.
How it works
The main objective of your autoguiding system is to focus and lock onto a star in the guide telescope’s field of view. The camera will continuously capture short exposures through the guide scope, usually anywhere between 1 and 3-seconds in length.
The guide camera and your telescope mount communicate with each other to maintain a lock on your target by making subtle corrections to the tracking. This is accomplished by using a great piece of free software developed by Stark Labs.
By using autoguiding software such as PHD2 Guiding, our computer can communicate with the telescope mount. PHD2 Guiding is the successor to PHD Guiding, which I used for several years before upgrading to PHD2. PHD stands for “Push Here Dummy”, and it very easy to use, once everything is set up properly.
In early 2017, I upgraded my autoguiding system to include a more sensitive camera, and an improved guide scope:
PHD2 Guiding includes a user interface that allows you to enter in your specific connection type and autoguiding camera model. Currently, I use an Altair GPCAM2 (AR01030 Mono) guide camera with a Starfield 50mm guide scope. This combination works extremely well, and I spend very little time dealing with anything related to autoguiding these days.
When autoguiding with the iOptron CEM60 mount, I can utilize the dithering and autoguiding features directly from my laptop computer to the mount. This means that I no longer need to connect the ST4 cable to the guide port from the camera, as the ASCOM interface communicates with the mount directly.
- Attach 60mm Guide Scope to finder shoe of the primary imaging telescope
- Install Altair GPCAM2 camera to the guide scope
- Connect camera to computer via USB 2.0 A-male to B-male cable
- Start PHD2 Guiding and Connect to Camera and Mount
- Slew to your deep-sky target, center, and frame using APT
- Clear mount calibration and start a new live loop (1-second exposures)
- Select Tools > “Auto Select Star”
- Run PHD2 calibration routine on the star chosen by PHD2
- Make sure Dither function on APT is selected under the “Guide” tab
I use a 1-second refresh rate on the Altair GPCAM2 to display an assortment of stars within the field of view. It is important to make sure that the guide scope is properly focused to ensure accurate star tracking. When PHD2 is running, I usually open on the “graph” window to monitor the accuracy of the tracking.
Please consider visiting the Learn Astrophotography section of this site to explore my techniques during real-life situations in the backyard.
What is Off-Axis Guiding?
Off-axis guiding (or OAG), involves using a device that sends starlight to your guide camera from the optical axis of your primary imaging telescope. It does not affect your primary imaging camera, as it uses the starlight that is “off-axis” and not entering your primary imaging camera.
I have used the Lumicon Easy Guider for off-axis autoguiding with my iOptron SkyGuider Pro. The pick-off prism sent useful guide stars to my ZWO ASI290mm mini guide camera to make corrections to the telescope mount for long-exposure astrophotography.
Using an OAG for autoguiding on an iOptron SkyGuider Pro.
No matter which method of autoguiding you use, the goal is simple. To capture long exposure images with round, sharp stars. If you’re able to collect images of over 3-minutes in length through your telescope, your autoguiding system has done its job.
Autoguiding is something that you won’t even think about, once you’ve got it working properly. For those having issues with autoguiding early on, I urge you to ensure that your telescope mount is properly polar aligned and balanced (with no cable snags) before trying to diagnose autoguiding issues.
Also, don’t obsess about the numbers and guiding graphs within PHD2 guiding. If you are chasing numbers, chances are you’re not taking pictures. If you’re taking pin-sharp 5-minute exposures at a focal length of 1000mm or more, chances are your autoguiding system is working just fine.