Atik 2HS Imaging, Tips and Tricks

The camera I'm currently using for most of my lunar and planetary work is the Atik 2HS. This is a 640x480, monochrome, highly modified TouCam Pro with forced air cooling.  It is capable of both the normal range of short exposures as well as long exposure integration. It uses the Sony ICX-424AL monochrome CCD with a USB interface for image download and short exposure control. There is also a parallel interface for long exposure control. Camera weight is minimal at just under 1 pound. That figure must include the cable because the camera head is very light. Power is supplied directly through the USB interface cable.

After several months of use I have found the camera to be very reliable and easy to use. During that time I've also discovered a few things that are important to know if you're going to get the most out of the camera. Keep in mind that most of what follows should also apply to anyone with a standard TouCam Pro camera and my be applicable to other web cam types as well.

The first item involves frame rate. When shooting the moon and especially the planets you need to gather a lot of frames in a short amount of time. In short exposure mode the camera will deliver up to 30 frames per second. However, because the back circuitry is based around the USB 1 interface the camera will automatically apply higher degrees of image compression to keep the date throughput constant as the frame rate increases. This is great for avoiding dropped frames but bad for image quality. Here are two sets of comparison images that tell the story:

This first test is a vertical slice from a full frame image and shows very clearly how the image deteriorates as frame rate goes up. Both resolution and contrast are effected. Also, at higher frame rates compression artifacts not visible in these images will become a problem when wavelet or deconvolution sharpening is applied.

In these enlarged frames the breakdown in the resolution of fine detail is very clear. Shooting at anything other then 5 FPS is going to have a noticeable impact in the recording of fine detail. An interesting aside from this effect is that it's often easier to focus the camera at 5 FPS then it is at 30 simply because the image is so much sharper. The image can be so soft at 30 FPS that the point of best focus can be very had to judge.

The next item concerns gain settings when operating in fast frame rate mode (>1/2 sec.). At gain settings beyond 30% the camera seems to be integrating double frames to keep up with the increased gain request. This can be easily seen as a persistence of the image of moving objects that's very obvious once you've seen it. The problem is, when you're working at the typical image scales used for lunar and planetary photography the details in your image are constantly shifting in position due to seeing effects, often doing so very rapidly. When the frame doubling kicks in at gain settings over 30% this will result in a blurring or even a double image of your fine details. For a long time I've puzzled over how one set of AVI's can result in such great images and yet the next set is almost unusable. Now I think I know why!  Keep the gain below 30% and you'll have much cleaner frames and less noise as well.

The last item concerns filters. Like most CCD based cameras, sensitivity to the infrared part of the spectrum is fairly high. Here's the curve for the 2HS:

The peak is very close to the maximum sensitivity of human vision but the sensitivity also extends well into the near infrared. As a point of reference, the wavelength of Ha that so many deep sky imagers are concerned with has a wavelength of 656nm, just to the left of center in the chart above. For deep sky imaging the Ha sensitivity is good but if you're shooting any kind of lens system, the extended tail beyond that can result in bloated star images. Unfortunately, this is also a factor if you're using a focal reducer, barlow or ocular projection with an otherwise all reflecting system. The problem is two-fold with the first issue being that none of these optical elements are usually well corrected for good performance into the infrared. The second effect concerns the basic resolution of the system. The size of the airy disk that any diffraction limited system produces is determined by two major factors, the diameter of the aperture and the wavelength of the light forming the image. The shorter the wavelength the smaller the airy disk becomes. Including all that long wavelength light swells the diffraction spot size and reduces resolution. The solution at the very least, is to incorporate a high quality IR cut filter into your imaging train. Here's a simple daylight test that shows how much of a difference that can make:

The difference between no filter and the IR cut is dramatic to say the least. Also notice that by adding a blue filter the resolution of the image improves even more. This is almost like free aperture so it seems worthwhile to take advantage of. As another example here's an excellent test series shot by Bob Pilz from the Lunar Observing Yahoo Group using a 100mm APO refractor:

The only caveat in all this might concern seeing quality and large apertures. It's been all the rage now to shoot planetary images in the infrared because it reduces the effects of seeing. This does in fact seem to be of some benefit when using a large aperture instrument that's largely seeing limited. However, with a small aperture like my 6" and the smaller refractor in the example above, it's really the wrong thing to do. That's because the resolution of smaller apertures are almost always limited not by seeing, but by diffraction. If you use the big scope thinking on a small scope you'll actually loose definition by needlessly bloating the diffraction disk. This is what works with my 6". If you're using a larger scope, say in the 8" to 12" range then much will depend on your local conditions so I would simply run some tests to find out what gives you the best results.

To recap, for best image quality with the 2HS, TouCam Pro or any other webcam:

Shoot at the lowest gain setting possible, never exceeding 30% with the Atik 1HS or 2HS.

Use the lowest frame rate to minimize compression effects.

Filter out the long wavelengths of you're shooting with a smaller aperture instrument.

Additionally, if you're using a standard or modified TouCam Pro you might want to investigate the AVI raw hack. The Atiks have this modification and it greatly improves the quality at 5 FPS. It can also do wonders for color cameras by giving you control over the bayerization color interpolation process. More information on that can be found here:

http://www.astrosurf.com/astrobond/ebrawe.htm

Tony Gondola, 10-05