Category Archives: Astronomy

Random musings on viewing the night sky

Refining the Process – APT and Dark Frames

A second clear night during the holiday weekend and another round of imaging with the 102GT. Last night’s imaging run was conducted using Astro Photo Tool (APT). I had purchased this software a couple of years ago and never really used it. In a nutshell, it allows for automatic exposure runs which saves the hassle of using Canon’s EOS utility and manually clicking each time for an exposure. In addition to using APT, I finally turned off the camera’s built in noise reduction and shot dark frames after each run. In essence, dark frames are produced by covering the telescope objective (putting the cap on) and shooting with the exact same ISO and time as light frames (regular exposures). The benefit to this is a massive increase in the total number of exposures during a given run as the camera takes twice as long to do an exposure with noise reduction enabled.

All in all, it was just nice to be able to enjoy the sky while the camera clicked away rather than being tethered to the laptop screen and mouse for every shot. I could even pop inside to make a cup of coffee or two as the night progressed. The targets for this run where M11, M13, and M38. I wanted to try and make it til Jupiter and the Orion Nebula came up high enough to shoot, but by 2:30 am the temperature and fatigue won out over the coffee. My stargazing companion had already started nodding off beforehand as seen in the picture below:

Processing flow is DeepSkyStacker, convert 32bit tiff output to floating point with FITS-Works, crop + levels & curves adjust in Photoshop, noise removal in NoiseWare, and final tweaks and tagging in Paint.NET. StarSpikes plugin was used on M11 and M38 to simulate diffraction spikes.

I suspect that these images represent the limit of what a slow f-ration refractor is capable of. There is definitely a diminishing returns situation with regards to increasing the exposure counts for a given target, and no amount of tweaking curves in Photoshop will bring out detail that isn’t there to begin with. While the $59 102GT really does perform admirably for visual usage, it is not designed for astrophotography. Looks like it’s time to start saving for a better imaging OTA.