Tuesday, September 16, 2014

A Few Hours Under Dark Skies

Last weekend between hurricanes Norbert and Odile, which is scheduled to hit Wednesday, we had some pleasant weather.  With a nearly last quarter moon, and some new hardware to learn, I called buddy Pat, and invited myself out to his observatory near Benson.  It is always more fun to have a friend under the stars with you, plus he had experience with the auto-guider I was trying to learn to operate.

I'm still learning more about using the refractor obtained a few months ago.  The task for Saturday was to get the auto-guider working with the AP1200 mounting.  I always try to keep things as simple as possible out in the field, and the use of any auto-guider means using a computer - believe it or not, new for me as it requires a power beyond a 12 volt battery...  Melinda and I got a little portable notebook computer a couple years back that hasn't found a permanent application, so was going to try to run new software and hardware on a relatively new mount and scope - what could go wrong?!

Nearly all telescope drives have slop, periodic and alignment errors that limit tracking accuracy.  In the past, macho astronomer that I am, I'd normally guide manually when it was needed.  The image at left shows the little auto-guider I first used with Pat's setup in June.  Even with a scope so much smaller than the one shooting through, small pixels in the compact cameras and sophisticated software work at keeping the tracking nearly perfect for long exposures. 

And let me talk about digital cameras for a
minute.  Virtually all of the images taken in the 800+ posts here are taken with couple Canon DSLRs that I own.  Compared to olden days using film these newer cameras are a godsend!  No waiting for processing to find out the telescope or lens was out of focus, or poorly tracked, or object decentered.  The digital images go immediately in the telescope for stacking or further processing.  There are issues though - consumer cameras work great for most applications, but for long exposures, electronic noise is added to the signal.  Specialized astronomy cameras are cooled to reduce noise, separate dark exposures can be taken to minimize the noise, and many exposures can be averaged to reduce noise compared to the faint signals.  The exposures shown here were taken Saturday at Pat's.  At left is a full-resolution partial frame of Messier 20.  It is a 3.5 minute exposure with the TEC 140.  The red, green and blue speckles are actually "hot pixels" where some of the little sensors have more noise than others.  Fortunately, they mostly repeat really well, so darks can be taken separately, like at left.  This is the exact same piece of the sensor, so the hot pixels repeat in both images.  Check out the green clump of them above center.  The "dark" is typically subtracted to reduce noise in the object's image.

Most modern cameras have built-in tools that can help too.  My 6-year-old XSi, used for these images, has a "long-exposure noise reduction" feature.  The image at left was taken right after the above exposure, with that feature turned on.  After the 3.5 minute exposure was taken, it closes the shutter and takes a 3.5 minute "dark" exposure, which it subtracts from the object image before writing.  The result is an image with fewer of the hot pixels as above.  It has been shown that you are better off to take many "dark" exposures when not under dark skies.  Most consider taking the darks sequential to the object exposures as wasting half of your potential exposure time.  As long as I'm going after reasonably bright objects, I sort of like the convenience of doing this part of the calibrations in camera...

As demonstrated in these exposures, the auto-guider worked great!  After attaching the guide scope to the telescope, hooked it up to the notebook, focused, and calibrated, and it worked great, no issues at all!  I ended up taking 7 exposures with a total of about 25 minutes.  When stacked together with some levels and curves adjustment, the image is shown at left.  I'm thinking it looks pretty good for a consumer unmodified camera.  If I had taken the images with the 20Da (with slightly modified red response), the red part of the nebula due to hydrogen emission would have been more prominent.  The blue part of the nebula is from the hot blue stars near the center reflecting off the dust and gas clouds...

The moon came up shortly after 10pm, so it was an early night.  Just before breaking down the scope, I took a couple frames of the moon a couple degrees off the horizon.  Nothing special, but still fun to see what the nearly 1,000mm focal length scope can do.  Of course, it would have been better high in the sky, but it is a phase of moon I rarely photograph, so the fun was in the taking! 

It was a fun night - Pat helped me with some guider stuff, and I helped him with some collimation issues, so we both benefited by our company.  We'll likely do it again soon...

No comments: