My Astrophotography Journey: First Steps in Autoguiding

One of my main goals in starting out in astrophotography is to avoid the need to spend all night outside in the cold watching the telescope. The requirement of long exposures and multiple images where you can’t actually see what’s happening is not that appealing to me. I have been looking into methods for automating the acquisition of images without actually sitting in the dark by the telescope. This series of posts follows me in my journey towards an automated astrophotography setup that can be used at home or elsewhere without breaking my budget.

In this post I cover auto guiding with the ASIair and a separate guide scope and camera.

In my previous posts I have discussed how I have set up my camera and lens on a Sky Watcher Star Adventurer GTi Goto star tracker. Using this basic set up I have been successful in reliably taking 30 - 60 second exposures with a 400mm focal length lens. Even 120 seconds are achievable under good conditions, but more often than not I have to reject around 50% of exposures due to elongated stars or the beginning of star trailing. At longer exposures there is significant trailing due to the effects of periodic error within the mount.

The question then is, why would you need to take longer expoures when you could just stack multiple 60 sec images to get longer total exposures. Stacking multiple short exposures helps reducing the noise associated with the image, but extending the exposure time actually increases the signal, often significantly increasing the amount of detail that can be seen on dim objects. 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 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. Modern equatorial telescope mounts are quite capable of compensating for the Earth’s rotation —that is after all what they were designed for, however, deep-sky astrophotography is a very demanding application for any entry-level to intermediate equatorial mount. As you increase your telescope’s focal length, the tracking accuracy becomes even more critical because we are now looking at smaller and smaller areas of the night sky. This zoom effect can potentially highlight even the smallest amount of error in your telescope mount.

Depending on which mount you’re using, subtle errors in the accuracy of your polar alignment can begin to show in even short exposure images. 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 long exposure images. The telescope mount itself may also have shortcomings due to wear in gears or low-cost materials used in its construction. Even if your polar alignment is spot-on, your payload is perfectly balanced, and your mount is behaving itself, it is often necessary to have additional assistance to track your deep sky object smoothly enough for long exposure astrophotography.

This is where autoguiding can help.

Autoguiding is accomplished by using equipment that can monitor how the mount is performing and send small correction pulses to the mount to correct any identified error. This pulse guiding of an equatorial mount can significantly improve its tracking accuracy.

A Quick Maths Lesson

A quick internet search suggests that my Star Adventurer GTi mount has an unguided tracking periodic error of about 40 - 80 arc seconds with a period of about 7 minutes. 7 minutes is equivalent to 420 seconds, so if we assume a mean value of 60 arc seconds periodic error, and that it is evenly spread over the 7 minute period, that would give us approximately 0.15 arc seconds error per second of exposure, i.e. a 10 second exposure would show a tracking error of 1.5 arc seconds, 20 seconds would have an error of 3 arc seconds, 60 seconds would have an error of 9 arc seconds, and 300 seconds an error of 45 arc seconds etc.

To put this figure into context for a moment, at a distance of 100m, one arc second represents a distance of just under half a millimetre. I don't know about you, but that astonishes me for a relatively cheap and lightweight mechanical gear driven star tracker.

Using my Nikon Z7ii mirrorless camera, with its 45MP sensor having a pixel size of 4.34um, you can calculate the plate scale as 206265 x pixel size (mm) / focal length (mm). This gives 202265 x 0.00434mm/400mm or 2.24 arc seconds per pixel using my 400mm lens

If we take the tracking error and divide by our calculated plate scale we can calculate the tracking error in terms of actual pixels. For a 60 second exposure the error is 9 arc seconds / 2.24 arc seconds per pixel, or just over 4 pixels, i.e. for a 60 second exposure we should expect a single pixel point star to be spread out over 4 pixels. For 300 seconds this would equate to an error of approximately 20pixels.

By guiding, however, you should be able to achieve a tracking error of better than 2 arc seconds. This would mean that our single pixel point star should have an error of less than 1 pixel no matter how long the exposure was made for.

Auto Guiding Setup

My current astrophotography imaging setup comprises a Sky Watcher Star Adventurer GTi with a Nikon Z7ii and Nikkor Z100-400mm zoom lens. These are connected to and controlled by an ASIair Plus mini computer so it made sense to choose a guide camera and guide scope that would be compatible with the ASIair. Also given the weight of my equipment and the payload capacity of the Star Adventurer GTi it was important that the guidance system was kept reasonable light. This is particularly important as I was looking to replace the lens by a proper refractor telescope.

If you’re using a separate 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. For my 400mm lens this would mean approximately 130mm for the guide scope. A quick Internet search identified the ZWO 30mm f4 guide scope (120mm focal length) and ZWO ASI120MM Mini guide camera as being a good choice, especially as they were available bundled together for a good price.

***ZWO Mini Guider & ASI 120MM-Mini Bundle***

The ASIair Plus has a built in version of the PHD2 autoguiding program that communicates with the guide camera and telescope mount just like PHD2 does on a standalone computer. Setting up guiding is very easy using the ASIair App.

Fitting the Guide Stop and Camera

Fitting the guide scope and guide camera to a standard telescope isn’t normally a problem as there are dovetail assemblies that are designed for this purpose. With a DSLR or Mirrorless camera and photographic lens though this can be more problematic. I tried fitting to the front of the vixen dovetail, but if the guide scope was mounted above the rail it clashed with the camera lens, and if fitted below the dovetail it would clash with the mount. I wasn’t keen to try the option for fitting the scope to the hot shoe of the camera due to its weight potentially straining the hot shoe. Also much of the feedback suggested that the hot shoe mounts were prone to movement during slewing. I also tried following Peter Zelinka’s approach by fitting an arca swiss clamp to the guide scope and trying to mount it to the side of my camera l-bracket. This gave the best option but it introduced weight off axis causing strain on the lens collar locking mechanism and unbalancing the whole setup.

In the end I decided to mount both the guide scope and camera and the ASIair Plus on the dovetail to try and balance each other out. I bolted a spare long arca swiss rail on the vixen dovetail at 90º to the axis of the lens. This allowed me to mount the two items either side of the camera and by adjusting the distance from the centre I could balance them both out. This is shown in the pictures below.

Now that the equipment was set up it was time to mount it all on the Star Adventurer GTi and boot it up. The ASIair recognised the ZWO ASI120MM Mini straight away and all I had to do was enter the guide scope focal length of 120mm.

The first task is to make sure the guide scope and camera are focused correctly. It’s best to start this in daylight and focus on a distant object. To make things easier you can set up the ASIair to use the guide camera as the main camera, just make sure the correct guide scope focal length is set as the main scope. You will need to set the camera gain to its lowest value and reduce the exposure time to a small setting to make sure you get an image on the screen. The guide scope is focused by loosening the red locking ring and rotating the front of the telescope. I found I had to unscrew the objective quite significantly to get the image in focus.

Once you are close you can fine tune the guide scope focus once stars become visible. I did this by adjusting the focus until the stars were as small as possible but you could also use a Bahtinov mask if you have one small enough to fit the scope. Once happy with the focusing you can tighten the locking collar to keep the focus set. As long as you don’t move the camera you shouldn’t need to keep repeating this focusing stage.

The main objective of your autoguiding system is to lock onto a star in the guide telescope’s field of view. The guide camera continuously captures short exposures through the guide scope and checks the position of the star to see whether it has changed as each image is acquired. If the system detects that the star is drifting it sends signals to the mount to correct for the drift, maintaining a lock on your target by making subtle corrections to the tracking motors. This keeps the guide star position controlled and therefore automatically ensures the main telescope/lens is tracking correctly.

To start the autoguiding on the ASIair you simply click on the guiding graph on the main screen. If the graph isn’t shown press on the Guide icon and then click the graph. The guiding screen is shown as below.

Once distinct stars are shown on the looping display click on the guide icon (circle with crosshairs) to start the calibration process.

***ASIair Guiding calibration display***

Once the calibration process starts the ASIair will intentionally move the mount to check how far the guide star moves, calibrating the distance the star moves to the mount commands. The screen will show increasing West steps and increasing distance. Once the star has been moved a sufficient distance the ASIair will start to move back, increasing East steps and counting down the distance. The number of steps required to move the stair a set distance (roughly 24 units) should be checked. Typically it is recommended that around 12 steps should be sufficient. If it is much less than 12 the calibration may be too coarse. Conversely if its much more that 12 it will take too long and may fail.

Using the default setup for the ASIair my calibration was taking 26 steps, significantly longer that the recommended value. To change this you need to exit the guiding screen and return to the guide camera settings menu and scroll down to the Calibration Step setting. The default value is 2000ms giving 26 steps on my GTi. I increased this value to 3000ms and repeated the guide calibration. This time the calibration was completed in 14 steps, much closer to the recommended value. I left the Max DEC Duration and Max RA Duration to their default values.

***Adjust the Calibration Step setting if the guide calibration is taking too many steps.***

Once the calibration has completed the ASIair automatically stars guiding.

For my first attempt I must have been quite lucky. Guiding started and rapidly settled down to provide roughly 0.94 arc seconds total error, significantly better than I expected. The whole process was completed in a matter of a couple of minutes. The guiding screen and guide graph shown blow indicate how accurately the system was tracking. To say that I was pleased with my progress would be a massive understatement.

Guiding progress with my Star Adventurer GTi showing 0.79 arc seconds RA error and 0.50 arc seconds Dec error for a total guiding error of 0.94 arc seconds. Note I left the RA aggression and DEC Aggression values at their defaults.

To test the effectiveness of the autoguiding I returned to the Preview mode in the ASIair and varied the exposure time to see how long I could image for without seeing any star trails. If you remember I could achieve around 30 - 60 seconds unguided - anything more and I was losing around 50% of my images due to star elongation or trailing. With guiding the exposure time could be increased significantly without any evidence of trailing.

The image below shows a 1000 second (16 minute) exposure which is the maximum exposure that can be set with the ASIair and my camera.

***Single 1000 second exposure (zoomed) showing no evidence of star trailing (zoom in on the centre stars to check - those near the edges are distorted due to slight lens coma)***

The goal of autoguiding 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. To achieve staisfactory exposures of over 16 minutes with a simple, cheap, lightweight mount like the Star Adventurer GTi is quite impressive.

Final Thoughts

Autoguiding is something that you won’t even think about once you’ve got it working properly. The ASIair Plus makes autoguiding a breeze. Within 2 minutes of connecting the guide camera to the ASIair I was able to achieve guide errors of better than 1 arc second and exposures of 1000 seconds per image with consistently sharp stars and no evidence of trailing. This is considerably longer than the 30 - 60 seconds I could achieve unguided.