Telescope Mount Tracking - The Problems

The above table clearly illustrates the mechanical problems in building our hypothetical perfect mount. A number of factors conspire to insert gremlins into our tracking, broadly these can be summerised as follows:

• Poor Polar Alignment If the RA Axis of the mount is not perfectly aligned with the rotation of the earth, declination drift and image rotation will result.
• Wrong Speed If, overall, the mount is running too fast or too slow it will "loose time" or "gain time" on the sky - ie the RA axis is not rotating at exactly one revolution per day.
• Periodic Error It is impossible to make a worm gear perfectly. They are always slightly wrong. Over the course of one cycle of the worm gear (usually about 10 minutes) the mount will first run a bit fast, and then a bit slow, making a sine wave on your tracking graph. In the long term, with the correct tracking speed, these errors should average out. The magnitude of these errors is usually between +-10arc seconds and +-60 arc second on the average amateur mount.
• Mechanical smoothness If the motion of the mount is not smooth, any jerkiness will result in a tracking error.
• Long term slop As the mount tracks the sky, it can settle into a slightly different position as the centre of gravity of the system moves. Such changes are more of a problem for VERY long exposures.
• Atmospheric Refraction The light from stars is bent by the atmosphere. This effect changes with altitude - hence the stars move across the sky at a rate proportional to their altitude. This effect is quite small.

Telescope Mount Tracking - The solutions

Looking at that lot, its surprising that anything can be imaged! Because astronomers have been dealing with these problems for many years, they have developed a number of ways of solving tracking errors. Very broadly, these methods can be summerised as follows:

• Periodic Error Correction The electronics of the drive system can be trained about the "shape" of the periodic error by watching the motion of a star during the course of a worm period. This training is used by the electronics to correct the drive speed to counteract the periodic error
• Custom tracking rates The tracking rate can be electronically changed to go faster or slower so it can be "fine tuned" to match the sky. Some drive systems will track with "King Rate" which takes into account atmospheric refraction.
• Guiding/Autoguiding My means of an off axis guider, or a separate guidescope, the position of a star is monitored during exposures and the mount electronics are fed tiny corrections to keep the star in the same place. This corrects both DEC and RA errors. This can be done in three ways. By hand. By using a dedicated guider unit, or, by using a normal camera (eg webcam), a computer, some autoguiding software and an interface to your scope.
• Adaptive Optics/ Ultra fast Guiding Units like SBIG's AO7 unit are a type of guider that make very fast corrections to the tracking by using a moving mirror in the image train, and not by adjusting the mount.

This not what you are looking for? Please have a read of some of my other articles:

• The state of astronomy in the mind of the modern public
• The Scale of the Universe
• A rough guide to astronomy
• Telescope Buying Advice - some advice to a friend.
• Astronomical Pet Hates
• Webcam Modifications Summary
• Photons - How many astronomers does it take to make a cup of tea?

Step Three - Polar Alignment

Good tracking needs good polar alignment. Some people will tell you otherwise. I disagree. An equatorial mount is happiest when properly polar aligned. You can autoguide out poor polar alignment, but you are going to get on better without that extra complication.

With good polar alignment you will less declination tracking problems. There is a golden rule of tracking: RA Drift is a drive problem, DEC Drift is an alignment problem. Later on in your quest you'll find that RA guiding is much easier than DEC guiding.

However, don't go too mad. With slop in many mounts good polar alignment in one part of the sky will not hold in another part of the sky. If your DEC drift is less than 2 arc seconds per minute, you are doing pretty well.

There is another link between polar alignment and tracking. As soon as a mount is not correctly polar aligned, it becomes, in part, an Alt-Az mount. As we all should know, alt-az mounts cause image rotation. So, if you guide in both DEC to account for poor polar alignment, small amounts of image rotation will creep in.

The Internet contains a wealth of information on polar alignment. Google around. For Example. Drift Alignment is the best way to refine your polar alignment.

Step Four - Tracking Rate

Now we turn back to our graph. My worm period is about 600s. As you can see, there is a long term downhill drift - the mount is going too slowly!

By using Custom Tracking rates in Autostar, or variable motor controllers it is possible to tweak the drive speed of your mount. If your mount runs of batteries, make sure they are fresh, as speed can change as the batteries flatten.

Before moving on you should aim to get no long term RA drift with perfect polar alignment. Well, a few arc seconds over the course of a worm period aren't going to kill you.

In a related point, switch off ANY anti backlash system the mount has. This is interfere with small guiding corrections.

Step Five - Periodic Error Correction

If your mount has PEC, read the manual and turn it on! Some argue that autoguiding and PEC should not be used together. They might have a point, but you'll have to experiment on your mount.

As a small aside on Meade LXD55 mounts, a lot of users are experiencing problems with PEC - Autostar PEC is introducing a lot of long term drift and screwing up the tracking speed. I am one of these users, and I have not found a solution yet, so I do not use PEC.

Step Six - Guiding and autoguiding

Put the telescope on rails and make it travel west at sufficient speed to counteract the earth's rotation. NB need a big garden. No, no, stop that. Thats silly.

Let us review the whole point of guiding or autoguiding your telescope. The goal of your mount is to keep the scope pointing at the same point in the sky as it rotates around us. However, due to mechanical and electrical inadequacies, it cannot do this. Therefore we use guiding or autoguiding to make small adjustments to the mount to correct these errors.

In the good old days, this was done by watching a star in a reticule eyepiece and manually making the adjustments. Although very purist, this is very hard work. Many exposures have been ruined by the astronomer adding extra vibration to the mount as they fell asleep! Manual guiding is therefore not used much these days. We have the technology to autoguide.

Autoguiding simply means using some clever bits of technology to detect tracking errors and send corrective adjustments to the mount. This will hopefully remove dec drift and RA Periodic error problems.

Which bits of technology? Putting aside the ultra-fast guiding capabilities of adaptive optics systems and restricting ourselves to technology that makes corrections via the mount, we are left with needing two critical components. We need a detector, and a star image.

The detector, unsurprisingly, monitors the light from star, detects the errors, calculates the correction required, and sends it to the mount. Broadly speaking, there are two ways of doing this.

• A dedicated guider camera. This can be in a number of forms. It can be a separate unit, like a mini CCD camera. Or, it could be a secondary CCD chip inside the main camera (popular with SBIG). It could also be part of the main imaging CCD Chip as practiced by Starlight Xpress. These devices generally plug directly into the mount (usually in the port marked "autoguider").
• A webcam or ordinary CCD camera. Any spare ccd camera or webcam can be pressed into service as a guider camera. I use Toucam webcams for this - and the rest of this article will involve webcam guiding. These devices work in tandem with a computer.

The star image is obtained in one of two ways:

• By using a second telescope mounted on the same mount as the imaging scope - this is known as a guidescope.
• By using the light from the main telescope and focusing it onto the detector, either with an off axis guider, or by placing the detector in the same focal plane as the imaging sensor,

Webcam Autoguiding can be setup by using an off axis guider or a guidescope. The principle drawback of an off axis guider is locating a suitable guidestar. The main limitations of a guidescope are usually mechanical - flexure and alignment. I use a guidescope. This image shows some slight flexing between the guidescope and the imaging scope. Usually manifests itself as a drift in the imaging camera in some other direction that up/down or sideways.

My full imaging and guiding setup can be seen here

Your guidescope does not have to be an expensive apochromatic masterpiece. A humble refractor will do. My 80mm/400mm F5 Skywatcher refractor cost about £100. You can always make a short refractor longer with barlows. Barlows are much lighter than telescope tube inches.

A word or two about suitable cameras. A standard Toucam with a CCD sensor and an appropriate adapter will work fine. The disadvantage is the limited number of guidestars available due to the short exposures in a standard Tocuam. Much better is an SC1 long exposure modified Toucam. An exposure of perhaps one second will introduce many more suitable guidestars. The best webcam for this work is one that has had the colour CCD sensor replaced by a same sized black and white sensor. This increases the available guidestars and the resolution by roughly a factor of three. Once of the best things I ever did to improve my guiding was putting the black and white ICX098BL ccd chip into my guide camera. SC3 modified webcams can also be used - although they have slightly less resolution due to the larger pixel size of the ICX424 CCD sensor. It is also useful to cool the CCD sensor, perhaps with a fan. This helps give a better signal to noise ratio, and makes the job of the guiding software easier.

Read my summary of the various webcam mods out there.

One other factor to bear in mind is declination. Stars at the celestial equator move across your field of view much faster than those nearer the poles. Therefore the magnitude of the RA corrections for a certain error needs to diminish relative to the declination. This is why most guiding software features some kind of "aggressiveness" setting.

I use an SC1 modified toucam webcam with a black and white ICX098 CCD chip and a separate guidescope to accomplish autoguiding. The steps are as follows:

• Mount guidescope on your telescope mount pointing in roughly the same direction as the imaging scope. Either piggybacked on the main telescope, or mounted on the counterweight bar. This second approach can save you some counterweights! The mounting must allow some adjustment to help you locate a guidestar. The field of view of a webcam is very small, and the process of finding a guidestar must be made as painless as possible. I devised my own computer controlled guidescope mount to help here. Having the guidescope pointing three or four degrees away from the main scope is not a problem.
• Mount the guide camera webcam on the guidescope. Either at prime focus or by using a barlow lens to increase the effective focal length of the system. The focal length of the guiding system and the pixel size of the CCD sensor combine to give you an image scale (resolution) in arc seconds per pixel. The better the resolution of the guiding system, the more effective it will be. However, as focal length increases, the field of view decreases, and the challenge of finding a decent guidestar increases! Like many things in astronomy, a compromise must be sought. I have guided happily at 400mm focal length and imaged at 1000mm. These days I now guide at roughly 1200mm.
• Find some autoguiding software. The best free offerings are GuideDog autoguiding software and K3ccdtools. I use GuideDog myself. The rest of these notes utilise GuideDog.
• Plug your camera into the computer and make sure you have an image in the software. Using the correct settings in Guidedog is important, but requires experimentation.
• Make sure the camera is rotated so that RA is left and right and DEC is up and down. This point is VERY important. Guidedog lets you flip the axis, but the camera must be square. I hope that the forthcoming new version of GuideDog will have a feature to take into account camera alignment.
• Interface your computer to the telescope mount. With Meade Autostar scopes this may involve an RS232 cable from the handbox to your computer's COM port. Other mounts will need a relaybox interface. You may need to Google for your mount to find the exact details.
• If your camera is a long exposure modified toucam/webcam, then you will also need to plug in the parallel cable to the computer so it can control the long exposure system.
• The setup I use with my Meade LXD55 mount for autoguiding is roughly as above. The webcam is mounted on the 80mm/400mm guidescope at F15, its usb cable is plugged into the computer. The parallel cable from the guidecamera is also plugged into the computer. The computer is connected to autostar with an RS232 cable. The autostar is connected to the mount with its own cable. Please try not to trip over this lot in the dark.
• Now check the balance of the scope as mentioned above in the mechanical section.
• The next stage is to point your imaging system at something you wish to record. First get the object roughly on the CCD chip.
• Now frame the shot and focus the imaging system. At the same time look at the output from the guide camera. Put the guidecamera at the slowest shutter speed (for non modified webcams, for modded cameras use 2 sec exposures) and max gain. When framing most DSOs in a typical imaging system, there is often a certain amount of leeway - move the scope around a bit incase there is a guidestar handy. As soon as a star is visible in the guidecamera preview stop moving!
• If this does not give you a guide star, you will have to adjust the pointing of the guidescope independent of the main scope until you do find one. Either use an eyepiece to centre a suitable guidestar in the guidescope, or, as I do, just move it about randomly until you find one. This sounds easy - believe you me, it can be a very frustrating process. Sometimes, in the act of adjusting the guidescope, you inadvertantly move the main scope a bit, loose the target, and have to start all over again. Time invested in perfecting your guidescope mounting and adjusting system is time well spent.
• The brighter the guidestar you find, the better the guiding will be. So its worth looking for a good one before starting the imaging and getting that cup of coffee/bottle of beer/cigarette.
• Once you are happy with your guidestar, lock Guidedog onto the star. It should remain fairly stationary, drifting slowly in DEC and RA with the errors in your mount's tracking. Get GuideDog connected to your scope, set the aggressiveness at 100 and press guide. Watch the star.
• In guidedog there are two tick boxes for reversing the EW and NS. If Guidedog starts sending adjustments to your mount that cause it to whizz off in one direction, you need to change these tickboxes.
• Watch the guiding for at least a couple of minutes. If you feel its undercorrecting or over correcting, then tweak the aggressiveness.
• The guiding interval is another factor to twiddle with. Longer exposures give you easier access to those elusive guidestars. On the other hand, if the guiding interval is too long, it will not be quick enough to correct your problems. You guessed it: Time to experiment again.
• Make sure your volume is turned up - if GuideDog loses the lock on the star (clouds etc) it will sound a deeply annoying alarm from the computer sound system. You can change this to something slightly less nerve jangling if you want.
• The best thing to do is measure your tracking with the guiding and experiment with different settings to see what it does.

Declination AutoGuiding

For exposures of more than a few minutes, you need to guide in DEC to counteract drift due to poor polar alignment.

It is important to note that DEC Drfit is generally only in one direction. In theory it is not required for the mount to make corrections in the other direction! However, most autoguiding software does not take this into account.

Why is this a problem? Well, with RA guiding, the adjustments take effect instantly. If you change the speed of the RA motor, the effect is immediate because the sky is always moving. Which dec corrections, you must contend with backlash. The guiding software might make many small DEC adjustments to no effect because it is only "taking up" the backlash.

Then, suddenly, it will jerk in DEC as the gears mesh! One thing we can do is make sure the DEC worm gear is "wound up" or "loaded" in the direction of drift before starting to guide. After a while you will learn to mentally visualise whats going on in the telescope's gearbox. Almost as much fun as driving a car without syncromesh (something everyone should do at least once).

As mentioned above, DEC guiding also causes image rotation.

It is best to avoid DEC guiding at first. Start with only RA autoguiding, and progress to dec guiding when you have mastered the RA axis.

Autoguiding and Seeing

As we all know from planetary imaging, the stars wobble in the atmosphere. Adjust your autoguiding so that it does not try to follow this wobble: Chasing the seeing. One trick is to slightly defocus the guidescope so the effects of seeing are less apparent. Using longer guiding exposures also reduces the effect of bad seeing.

As you increase the focal length of the guiding system, seeing becomes more and more of a problem. That would be one of those compromises again.

Webcam RAW mode and Autoguiding

I always use RAW Mode when autoguiding. To be honest I am not sure how much of a difference it makes, but I still use it!

How much focal length do I need to autoguide with?

This a topic about which a great deal of rubbish is discussed. Some folks will tell you that a guiding system must have at least twice the image scale (focal length) of the imaging system. That is, quite frankly, bollocks.

It is true that increasing the image scale improves the tracking... although going too far results in your "chasing the seeing". However, my view is as follows: If you have a poorly tracking mount, then ANY guiding will help. Perhaps you have a 1000mm telescope with +-30 arc seconds periodic error. In a batch of 20second sub exposures you find yourself discarding three quarters of them. Now, even if you try autoguiding with a 200mm SLR lens (image scale on a standard Toucam: about 6 arc seconds per pixel) you will get an improvement: Perhaps you will increase the number of good frames. Its worth trying anything. It might not give you perfect tracking, but it might make things a little bit better. Improvements in astronomical kit must come step by step.

Another arguement against the Long Focal Length Brigade is the advent of modern autoguiding software. This is able to calculate the centroid of a star to sub pixel accuracy - and is far more sensitive than guiding by hand.

Autoguiding and Flexure

Once you move into the realms of minutes long exposures, the main gremlin of autoguiding catches up with you sooner or later. Flexure.

If you are using one camera to image and one camera to autoguide, you might suffer flexure problems. Mounts and telescopes are mechanical devices. As such, parts of them tend to bend slightly. As the mount tracks the sky, the degree of flexure between the parts causes some parts of the scope to move very slightly with respect to others. The autoguiding system will try its best to keep the guidestar locked in one position. In theory, this keeps the imaging camera target in one place on the imaging camera. However, if the guidescope should move very slightly with respect to the imaging camera, the autoguiding system will correct the guidescope, keeping the guidestar centred. The result of this is the imaging target moves slighly on the imaging camera.

If you images still have very slight star trailing, usually in a direction diagonal to the RA/DEC, but the autoguiding system reports no errors, then you are most likely suffering flexure in the scope components.

Examples of things that flex include:

• Camera with a 1.25" nosepiece held in the focuser with one set screw.
• The focuser drawtube moves inside the focuser.
• The mirror settles in its mirror cell.
• The guidecamera flexs in its attachment to the guidescope.
• The guidescope flexures inside the tube rings.
• The guidescope mount flexs with respect to the main telescope (normally the main culprit).
• There are many others!

It is your job to make the all these componants are rigid to avoid flexure effecting your autoguiding.

Other Links

My computer controlled guidescope mount
Some early attempts to autoguide
My full imaging and guiding setup
Webcam autoguided images with the Artemis CCD Camera
Jan Timmermans k3ccdtools autoguiding
Some handy notes.
PEC Analyser Software
Some of my other astronomical articles

Astronomy software for autoguiding

There are various bit of astronomy software out there for autoguiding.
K3ccdtools
Guidedog autoguiding software
IRIS

Some Other Articles

• Lathes and mini-lathes and astronomy
• Moonlight test of Astronomik CLS filter
• The state of astronomy in the mind of the modern public
• The Scale of the Universe
• A rough guide to astronomy
• Telescope Buying Advice - some advice to a friend.
• Webcam Modifications Summary
• Photons - How many astronomers does it take to make a cup of tea?

Setting up GuideDog

Within Guidedog, most of the settings are fairly self explainatory. Below are some screenshots of the setting I am using.