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Tuesday, June 17, 2008

Using a Barlow Lens

The new T-ring adapter arrived, so once again I can trod out to the backyard and play around in the dark. I decided this night I would spend a little more time with that Barlow lens, maybe I was distracted last time I attempted this when the T-ring failed.

Increasing the magnification, compounds the effects of any errors you might have. I read that on a forum somewhere. It truly is the case. Focusing with a Barlow lens that effectively doubles the magnification, took 10 to 15 minutes! Really had to work to dial it in. This night was particularly challenging. A near full moon was out and monsoon season has officially started, so humidity was up. Any slight fluctuation in the upper atmosphere was readily apparent in the frames I collected.

The following picture of The Ring Nebula (Messier 57) was taken using a 2x Barlow at 1600 ASA, 175 x 15 second frames and automatic dark frame subtraction.

For the next several rounds of nightly shoots, I've decided to try working with JPEG format directly from the camera. I expected that it should dramatically reduce my processing time and have some impact on noise. The frames of this M57 photo were all captured in JPEG format. The optical effects of the Barlow don't help here in getting a fair comparison, but I found that capturing 150+ frames in JPEG format dramatically improved signal to noise ratio as compared to my other runs of 150+ frames in RAW format. It was really no comparison. I'm anxious to try this again in a normal prime focus configuration. For me, I'll use the Barlow approach sparingly where zooming in on a small object seems reasonable.

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