Preparing for astro season: making your own autoguider - part 2

Ï guess it's a setup?

Last time I talked about the basic building of your own autoguider. Not that hard, right?

But now that we have it all assembled as shown in the picture here, we still need to give it a little TLC to make it work.

Here are the next steps!

First, we need to make sure the sensor is at (or a little bit closer than) the flange focal distance of the lens it is paired with. We can do this by following these steps:

1. Put the lens on something sturdy (or sturdy-ish as you can see in my setup) and aim it at something far far away (something terrestrial helps during the daytime).

2. Focus the lens to infinity.

3. Change the aperture of the lens to something at which the lens is sharp: my old lens only got sharp if I stopped it down to f/11!

If your lens is like mine and the diaphragm is only closed if a lever is moved, find a way to fixate it. With my OM lens (and I'm pretty sure this will work for other OM lenses), I accomplished this by wedging a 1x1 Lego tile in the gap where that aperture lever moves.

4. Plug in the webcam and start up a webcam viewer. I'm using the very very basic WebcamViewer by Bustatech.com (Windows only) which runs stand alone.

5. Slowly push the webcam into the filmcontainer. Check the feed from the cam to see if it's getting sharper.

If it's getting sharper: push it a little more. If it's getting blurrier: stop! You're past the flange focal point. During the first half (up to the flange focal point), the image should get sharper. During the second half it should get blurrier again.

Your image should now look like this:

Image from my setup after "calibration"
(note that this farm is about 1.2 kilometers away)

6. (optional) Marvel at how much atmospheric disturbance you're now able to see.

7. Focus the lens from infinity back and forth again to see if you can get a sharp image. If you get a sharp image somewhere between the final two focus markings on the lens (mine are infinity and 8 m), you're good to go. If you need to focus farther back, you might want to pull the webcam back a little.

8. Fixate the webcam in place with ducktape OR wait for part 3 and make a pretty cam housing!

9. ???

10. PROFIT!


Personally, I like it when the webcam is a little past the flange focal point (in other words: too close to the lens) as this will make the lens focus past infinity. For normal photography this could be an issue as this will make the minimum focus distance larger and compromise the image quality. But we are using this for optical motion tracking: a lens guaranteed to focus to infinity (and thus giving us nice sharp stars to track) is a much more important.

A fun thing we can now do is to determine the crop factor of the webcam by measuring the 35mm focal length equivalent! I've taken pictures at known focal lengths and used them to calculate the focal length equivalent of the webcam package. As we know that the webcam lens focal length (200mm), we can get the crop factor. Take a look at this comparison:

That's a crop factor of 7! I've used the length of the red roof (yup, count those pixels) to calculate the focal length, which is turned out to be a pretty decent measure. Not too shabby eh?

Because of our large crop factor, the motion of the night sky will also be magnified which should lead to better tracking performance as the motion will be easier to detect.

If you are really on a tight budget you could consider yourself done. Also if you intend to use just the one lens, you're also done. But as I've said it once already, why not make a pretty and sturdy enclosure for the cam?

It's not hard and it won't cost much, I promise!

Coming soon in part 3!