Globular Clusters

Beautiful Evening

Evening, May 6th, a lovely night with no moon and temperature falling from the 70's to the 60's. I decided to try a ZWO 174 monochrome camera for night sky imaging. The camera was initially attached at prime focus on a Stellarvue 130mm refractor seen below waiting for darkness to fall.

The telescope is not permanently mounted, so its polar alignment is only approximate after each initial setup. Consequently, the mount alone will not track accurately enough for long exposure imaging. At present I have no way to make automatically guided images with a guiding camera. Although the mount tracks well enough for visual observing, stars slowly drift in the field of view as time goes on. This limits camera exposure times to values where star trails don't show up. I spent time trying different exposure times to see when star trails became visible. For example, at prime focus, I discovered exposure times less than 15 seconds were required to keep stars nearly round.

I then pointed the telescope at globular cluster M3 in the constellation Canes Venatici and stacked 40 individual 15-second exposures to make a single image equivalent to a 10-minute exposure. The next picture shows the result.

Notice the diagonal streaks above. These are caused by "hot pixels" in the camera chip. Actual stars don't trail in the image because software aligns all star images before adding them together in a stack. Unlike stars, however, hot pixels were not aligned, so the diagonal streaks show how stars would trail during a single equivalent 10-minute exposure if stars were not aligned before stacking. The hot pixels appear because I didn't take a dark frame for this image. A dark frame, or image obtained when no light falls on the camera chip, records the hot pixels and can remove them by subtraction. I manually removed the diagonal streaks in the next image, but a slight glow protruding on the right edge still remains.

As you can see above, the image scale is too small at prime focus. Stars look blocky instead of round. Star images look better with a 2X Barlow lens which produced the following image of globular custer M13 in the constellation Hercules. This is a stack of 40 individual 5-second exposures made with a gain setting of 350. I used a dark frame this time, so no diagonal hot pixels streaks are visible.

I don't know why I limited myself to 40 frames. In the future I will surely try for more. Next, I tried globular cluster M92 in the constellation Hercules. The following image is a stack of 40 individual 7-second exposures for a total exposure time of 280 seconds.

Finally, I had the best luck with globular cluster M5 in the constellation Serpens. There was a download malfunction while capturing images, so only 27 individual 7-second frames were stacked with gain setting at 320.

I moved the telescope to globular cluster M4 in Scorpius, but it was too near the horizon for good results. In the future I'll need larger stacks for longer equivalent exposure times. More magnification would also help. Although the grayscale images here are nowhere near as good as those produced by experienced dark sky imaging experts, I'm amazed at these first time results.