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.

guide scope

Autoguiding improves the tracking accuracy of your telescope mount for astrophotography.

Overview and Quick Setup

To start autoguiding, all you need is a guide camera, a small guide scope, and software to run the camera and utilize the autoguiding port on your astrophotography mount. You can put together a reliable autoguiding package for about $250 USD (see the example kit below).

A simple camera and guide scope package like this will allow you to harness the power of autoguiding for primary imaging telescopes with a focal length of up to 1000mm. Image capture software tools like N.I.N.A, PHD2 Guiding, and the ASIAIR all have autoguiding modes to utilize this feature. 

To use autoguiding, you must use a computerized telescope mount with an autoguiding port and/or USB port to connect it to the guiding software. Modern equatorial telescope mounts like the Sky-Watcher EQ6-R Pro and strain eave drive mounts like the ZWO AM5 support autoguiding. 

You will also find autoguiding ports on portable star trackers like the Star Adventurer GTi. If you are looking to add a small guide scope package to your existing setup, you can build a system using the components of your choice or invest in a ready-to-go guide scope package. 

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 a small 50mm guide scope package way back in 2011, and this is still a popular starter system for many backyard imagers. 

guide camera

The ZWO ASI120MM Mini guide camera is affordable and works exceptionally well. 

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 that more compact and affordable solutions are available than ever.

Below, I’ll offer you some affordable autoguiding solutions that I have used to guide several 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 two main software tools astrophotographers use for autoguiding are PHD2 Guiding (which integrates with several camera control software) and the ASIAIR smartphone app. Both are easy to use and allow you to dither your images between each frame. 

Autoguiding a telescope mount

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 your telescope’s focal length, 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.

The Veil Nebula

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 that communicates with your guide camera to the mount. You can also autoguide using the pulse-guiding method that directly connects your PC to the telescope mount.

It is said that pulse guiding (with an ASCOM-compliant) equatorial mount improves guiding accuracy. This can be measured using the tools 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.

telescope mount

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 Earth’s rotation —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 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.

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.

astrophotography telescope

My Sky-Watcher HEQ5 Pro mount with a refractor guide scope mounted to the primary imaging telescope.

The telescope mount itself may have shortcomings due to worn-out gears or low-cost materials used in its construction. If the issues are severe enough, even autoguiding won’t help.

For this reason, it’s always best to invest in a quality telescope mount that has been proven reliable for deep-sky astrophotography. 

telescope mounts

Thankfully, basic autoguiding is enough to correct most of the issues associated with modern equatorial mounts. 

What I’m Using

I am currently using various autoguiding systems, including my  Sky-Watcher EQ6-R Pro, CQ-350, and ZWO AM5. I have used a number of different guide scopes, 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 it 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 allows you to shoot longer, and 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
  • Starfield 50mm Guide Scope
  • Starfield 60mm Guide Scope
  • William Optics Z72 APO Doublet

I primarily use a William Optics 50mm Guide Scope because it is 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’s focal length is 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. 


William Optics Guide Scope

The camera I use most often with the 50mm guide scope is the ZWO ASI 290mm mini. This is a highly sensitive monochrome guide camera and compatible with the ZWO ASIair Wi-Fi 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 continuously captures short exposures through the guide scope, usually between 1 and 3 seconds in length.

The guide camera and your telescope mount communicate to maintain a lock on your target by making subtle corrections to the tracking. This is accomplished using great free software developed by Stark Labs called PHD2 Guiding. 

PHD2 Guiding

Our computer can communicate with the telescope mount using autoguiding software such as PHD2 Guiding. 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 is very easy to use once everything is set up properly.

This software can also do other useful astrophotography tasks, such as drift alignment, which is helpful for those who cannot use Polaris for Polar Alignment.

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.

PHD guiding graph

Please consider visiting the Learn Astrophotography section of this site to explore my techniques in real-life backyard situations.

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 ” off-axis ” starlight and does not enter it.

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 correct the telescope mount for long-exposure astrophotography.

Lumicon Easy Guider

Using an OAG for autoguiding on an iOptron SkyGuider Pro.

No matter which autoguiding method you use, the goal is simple: to capture long-exposure images with round, sharp stars. If you’re able to collect images over three minutes in length through your telescope, your autoguiding system has done its job. 

Final Thoughts

Autoguiding is something that you won’t even think about once you’ve got it working properly. For those having issues 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. 

The Tadpoles Nebula