Autoguiding with a webcam for astrophotography and astronomy page 2

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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 summarised as follows:

Tom's guide to getting the best tracking for your telescope

Let me first say that tracking problems are not going to solve themselves. When I read somebody's imaging reports telling stories of a great image ruined because of telescope tracking problems the first thing I think of is this: Have you tried to fix it? Or did you just go out and image, hoping naively that it would all work?

It is only by making an investment of time to get the best possible tracking out of the mount will you get the best results. People who go out and spend hours trying to image and do nothing but curse their tracking, and try quick bodges under the clear skies are foolish. Darwinian Evolution soon weeds out the foolish. We are fast drifting into my astronomical pet hates.

So, spend some clear nights working on your tracking with no intention of imaging. Nights during the full moon are ideal - you cannot do much imaging and you can see what you are doing.

Most of what I have to say is bloody obvious, but sometimes the obvious needs pointing out.

Step One - Measuring Tracking Error

Learn to measure your tracking. The best way of doing this is to use k3ccdtools and a simple webcam. Point your telescope at a bright star, and use the k3ccdtools drift explorer to make a graph of your tracking. Any measurement should be across two cycles of your worm gear. K3ccdtools will output to a text file that you can graph in Excel. You may well end up with something like this:


Time in seconds is along the X axis, and Error in RA is on the Y Axis. You can plot of DEC error and RA error. Some conclusions can be drawn from this graph about tracking speed and periodic error, but for now, all that needs to be mastered is the method of making the graph.

Standard unmodified Toucam webcams will be able to see down to at least magnitude 6 in the average telescope. Chose a star close to the celestial equator - these move the quickest.

Step Two - Mechanical Fixes

The mount must be mechanically well set up for two reasons. Firstly, it needs to be steady and smooth as possible. Secondly, it needs to be setup to allow the guiding system to correct small errors without causing larger ones!

Learn to adjust the worm gears to get the best tracking without excessive backlash. Make sure the equatorial head is firmly mounted on its pier or tripod. With the scope on the mount, physically wobble it, and try and see where the flexure is happening. Try is tighten or fix these points. If the head goes clunk-clunk back and forth, look at the backlash again.

Take off the motors and turn the worm gears by hand. You should not feel any roughness, stiffness, or sticky points. If you do, things need adjusting.

With some mounts it is advisable to strip the mount, clean off all the gunky far Eastern glue (they call it grease!) and lubricate the mount with a better product. I use white lithium grease. Websites such as Astronomy Boy and will give details on this procedure. Google for your mount and see what people have done to improve it mechanically.

Another important part of mechanical setup is balance. When driving the mount the motors prefer it if things are slightly out of balance on the RA axis. The gears and motors ought to be lifting the load up, not slowly dropping it down. So, if the telescope is on the east side of the mount (going up!) adjust the balance on the RA axis so that the scope end is slightly heavier. When the tube is on the west side of the mount (going down) make the counterweight side a bit heavier.

Keep fiddling with everything until your RA Graph is a smooth as possible with the minimum periodic error.

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. That's 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.

The star image is obtained in one of two ways:

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.

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