CN FORUMS ARCHIVE Telescope Reviews: Autoguiding

Let's get the nuts-and-bolts definition of autoguiding out there, so that everyone is clear on what's going on.

You want to take a photograph of the stars with camera 1 and lens 1.

First, you need an equatorial mount, which has one axis that points at Polaris (or, more precisely, at the North Celestial Pole). An alt-az mount, because it must track in both axes to keep an object centered, will induce field rotation in your photograph. So, you need an EQ mount, which only needs to track in one axis, and it needs to be pointed precisely at the NCP, which is for another discussion (but keep the words "drift alignment" bouncing around in your head).

So, in order to make sure that your object stays centered, you realize that if you had lens 2 (attached to lens 1 in some way) and something (or someone) looking through lens 2 at a bright (enough) star that's *close* to your object, you could keep that star centered (in lens 2), and that would mean that your object would stay centered (in lens 1). You could do this yourself, by looking through a reticle eyepiece, and adjusting the mount whenever the star drifted. This is a totally viable option, and also the most boring task I have ever set upon. Enter camera 2, which we'll call the autoguider.

The autoguider's whole job is to sit there, looking through lens 2 at the bright (enough) star, and whenever it detects movement, it sends adjustment commands to the mount to recenter the star. Perfection, b33r, pr$fit, right?

Well, not quite so fast. How do we know that the autoguider's doing a "good enough" job? What are the criteria upon which we will judge this "autoguider" of which you speak? Well, let's put together a crib sheet.

First, let's all agree that a "failure product" of a photograph is "stars which are trailed instead of being points". A "success product" is a photograph with "stars which are points".

So, in order to produce success:
1) The autoguider must be able to find a bright (enough) star that's close (enough) to the object being photographed (or else the star might be moving at a different speed than the object, and badness occurs).
2) The autoguider must see the star begin to drift *before* camera 1 sees it.
3) The mount must be able to respond quickly enough to an autoguider's request for correction that the star is recentered before camera 1 sees it.
4) It would be nice (although not strictly necessary) for the autoguider to have some sort of alarm in case it has some sort of problem (like losing the guide star).

Note that 1) is directly related to the sensitivity of camera 2 itself; 2) is directly related to the focal length of lens 2 (or, rather, the magnification of camera 1/lens 1 compared to the magnification of camera 2/lens 2, with the second pair needing to be necessarily larger than the first); 3) is directly related to the accuracy of the mount; and 4) is directly related to the autoguiding software.

All of these components need to work together properly in order to produce a "success product". If any one of the criteria fails, it is unlikely that a "success product" will result.

So, you need to guide with a camera that is sensitive enough to find a star nearby.
The camera needs to be guiding at a higher magnification than the magnification at which you are imaging.
The mount needs to be accurate enough (and only require guiding that is smooth enough) that the autoguider can keep up.
Finally, the autoguider software should alert you if the guide star is lost, so that you can take action.

So, "a webcam" through "a finder" on "any old mount" *could* work, as long as it satisfied all the criteria. But the better the mount, the more mated the guide lens is to the imaging lens (and the cameras to their lenses and each other), and the more sensitive, accurate, and user friendly the autoguider you use, the better success you are going to have (or at least the better chance of repeatable success).

I hope this helps.

Jimbo

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Order of the Unblinking Eye

NJP 300D SSAG 8"f/5 (Rig)
Guidescope? What guidescope?

I used to shoot Nikon DSLR.
Before that, I shot film.