If you have already started doing some research on getting started in Astrophotography, you have likely heard quite a bit about guiding and its role in achieving sharp stars over long exposures. In this article I am going to talk about what guiding is and what an acceptable guiding error is for your particular set up.
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The Reason For Guiding
Even if you have an equatorial mount that tracks the night sky, you will more than likely still want a guiding set up. The reason for this is because equatorial mounts all have inherent imperfections and a degree of periodic error that doesn’t allow them to track perfectly for indefinite periods of time.
For this reason many astrophotographers use guiding equipment and software which allows them to increase their sub exposure time without developing star trails. This is common in broadband imaging and is even more crucial in narrow band imaging, where sub exposures of 5 minutes or longer are required to draw in signal.
Guiding vs Tracking
Many people confuse tracking and guiding and they are in fact two different things. Any time you use a motorized equatorial mount you are effectively tracking the night sky.
Guiding takes this a step further using either an off axis guider or a secondary guide scope and camera to monitor one more stars in the general region of your target. Phd2 is designed to use the movement of these stars to send periodic corrections to your mount, and to smooth out the mechanical imperfections in its tracking.
Guiding Error
Now that we understand that mounts aren’t perfect lets complicate matters a bit further and talk about the fact that your guiding in phd2 has an error of its own called the root mean square, or RMS error.
The degree of this error will depend on several different factors and keeping it to a reasonable level can make or break your sub exposures and the shapes of your stars.
If you search through enough astro forums you are probably going to find that a commonly accepted RMS number for “good” guiding is 1 arc second per pixel. This seems to be the metric that everyone strives for and chances are that most people will be in fairly good shape if they can average an RMS error of 1” or less.
This being the case it may not necessarily be true for you and depending on your equipment your RMS error may need to be lower, or you may be able to get away with it being much higher.
So what is the biggest factor in determining an acceptable RMS error for my equipment?
Simply put, its image scale. Image scale refers to the amount of sky that covers a single pixel on the sensor of your camera. Guiding errors become much more apparent in set ups with smaller image scales, and less apparent in set ups with large image scales.
Rather than focusing on achieving an RMS of 1” or below, you should focus on achieving an RMS that’s equal to or less than your image scale, at any given point in time.
The formula for calculating your image scale is pretty simple. Its (the pixel size of your camera / the focal length of your telescope) x 206.3.
The ideal image scale is said to be between 1 and 2 arc seconds per pixel, so keep this in mind if you are trying to decide on how to pair a camera with a scope. I personally don’t believe this to be a hard and fast rule and my own set up is technically under sampled.
Lets do the math on this using my telescope and camera which is a Meade 70mm Astrograph and an ASI 1600mm Pro.
The calculation is as follows: (3.76/ 350mm) * 206.3= 2.216 arc seconds per pixel
By most standards that is a very large image scale but not only have I really enjoyed the targets I can image at this focal length, I have had very few issues guiding it successfully and keeping the majority of my sub exposures.
If I were imaging at 800mm instead of 350mm, my image scale would be .97 arc seconds per pixel, and an RMS error of 1 arc seconds per pixel would probably result in me tossing a lot of sub exposures with egg shaped stars.
Looking Beyond your average RMS error
Take a look at one of my most recent guide logs below using a Cem25p.
After tearing down and reassembling the declination axis multiple times, I’ve been able to get this little mount to average in the mid .80” range for RMS error on a pretty consistent basis.
One thing you must keep in mind here is that error is an average and in this case it was over the course of several hours. There are times where my RMS error will drop to the low .6”s
And there are times where it will jump up well over 1” in to the 1.25” range or higher.
However with this particular telescope and camera combination my guiding never gets so bad that it even comes close to my image scale in normal imaging conditions.
What I’m trying to illustrate here is that just because your average RMS error is equal to or less than your image scale, it doesn’t mean that you will get to keep all of your subs if you are toeing the line too much.
If guiding error spikes are periodically pushing your RMS error above your image scale, you may have to toss those sub exposures even if your average at the end of the night is below your image scale.
What Causes RMS error to Increase?
Any number of things can cause your RMS error to increase for a period of time:
- A stretch of bad seeing conditions
- Backlash in your mount
- Wind
- Passing clouds
- Vibrations from things like vehicles passing nearby
- Poor cable management
Its best to leave yourself a margin of error so that when some of these aforementioned challenges inevitably present themselves, it doesn’t destroy your whole imaging session.
Conclusion
- Your image scale is the most important factor in determining an acceptable guiding error for your telescope and camera combination.
- All else the same longer focal lengths will result in a smaller image scale.
- Its best to start astrophotography with shorter focal length optics.
- Don’t toe the line. Leave yourself a margin of error when choosing your image scale so that periods of imperfect guiding don’t ruin your imaging session.
- Be mindful of what mount you’re using and it’s guiding capabilities.
- If you choose to start with a small image scale look at mount options beyond entry level. Think EQ6 R pro class.
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