Autoguiding with a webcam for astrophotography and astronomy

Autoguiding guide Page 1
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Step by step autoguiding instructions

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:

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 what's 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 argument 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:

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

Autoguiding guide Page 1
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