Saturday, October 10, 2015

Upgrading guide camera. Goodbye, SSAG...Hello, QHY5L-II.

For a while now I have been troubled by the strong horizontal banding in my StarShoot Auto Guider (SSAG). These bands appear when the gain is set to 95% (default value) in PHD2 and turn into a checkerboard pattern at gains near 10%. Here is a PHD2 screen capture of a SSAG dark frame showing the horizontal bands.

SSAG dark frame. 0.05s exposure @ 95% gain.

To compound the problem, the bands shift up and down slightly, so dark frames do not help because the pattern is not consistent. Not all SSAG users report on this issue, but those who do have narrowed it down to either excessive USB cable length or sensitivity of the camera to high frequency noise in the computer USB port. I did my own detective work and found the same issue on all USB ports on two computers and with three different USB cables, so I never found a way to resolve the issue with my current hardware.

If the problem were benign, I would not worry. However I have noticed consistently that the movement of these lines affect the centroid calculation of my guide star when autoguiding. Since my image to guide scale is nearly 3:1, this causes unnecessary corrections at every guide cycle as evidenced between the correlation between horizontal band movement and the guide star profile. In short, I am convinced the centroid follows the bands.

After much thought I have decided to move on to a new autoguider. Today, a brand spanking new QHY5L-II monochrome planetary/guide camera just showed up my home, courtesy of OPT Corp and the money I raised by selling an old telescope. This autoguider comes highly recommended due to its small pixel size and sensitivity. After following the advice of folks on Cloudy Nights to install the high compatibility driver from QHY, the camera worked perfectly with PHD2 (Ver. 2.5). I am relieved to see that horizontal lines are not present in the dark frame:

QHY5L-II dark frame. 0.05s exposure @ 95% gain.

I have not tried the autoguider functionality of QHY5L-II just yet since the purchase of new gear always brings clouds - especially near new moon. Until then, this looks to be a promising upgrade to an essential piece of gear. As for my SSAG, it will serve as a nice camera for outreach events.

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