Thursday, July 7, 2016


A confluence of possibilities came together last weekend that allowed me to get out for a few hours of dark sky observing. Even though our Summer Monsoon season has started, we got a respite on a dark-moon weekend (rare if you ask astronomers), and Melinda was doing well enough to leave for part of a night. Even my observing buddy Roger was tempted to join me for company!

Seems like it has been ages since I'd been to Kitt Peak to observe and other than assisting with an astro-photo class this Spring, it has! Since our astronomy club attendance at the twice-a-year Star-B-Que was dwindling to near zero, I stopped organizing them, so no excuse to go up this Spring at all. My goal was a primitive site at a clearing near mile post 9 - a little level spot off the highway where road materials are sometimes stored. It is far enough off the road that headlights of cars going up or down have no effect on any observing, with room for a few cars and a few observers. The image at left shows a nice panorama of the site from a nearby rise. It is actually darker than the peak of the mountain - hills to the east block the view (and light pollution) from the lights of Tucson, and similarly a little rise to the north does the same for Phoenix. But the view to the south is great - right to the horizon. The panorama shows the view from NE to just west of South. The striking mountaintop of Baboquivari is visible and marks nearly due south for us...

I've talked about Baboquivari several times here. It is a spectacular landmark, about 800 feet higher than Kitt Peak, and about 15 miles to the south. No paved road anywhere near it, unfortunately, though in my more athletic days I've hiked it a number of times on the "Forbes Route". With this trip to check out various lenses on the newish Canon 6D camera, the image at left was taken with a lens from a Pentax 6X7 medium format camera - a 165mm lens (easy to use with an inexpensive adaptor). I set up a couple tracking platforms to run a couple cameras, and Roger set up his excellent little 8" refractor for some visual observing as a side highlight. Before I knew it, darkness was upon us and I got one more exposure of Baboquivari - the stars' short arcs over the mountain demonstrating that due south was actually a little to the west of the unique mountain profile. Just over the peak is a fuzzy star cluster in a rich part of the Milky Way in Norma - NGC 6067. This is a 30 second exposure with the kit lens set to 105mm focal length (F/5, ISO 6400). The slight, barely visible light glow around the peak is likely due to the little town of Sasabe on the Mexican border.

So before I knew it, it was time to get the cameras exposing. Looking to the east, there was a striking view of the Summer Milky Way rising over the hill forming our eastern horizon. This shot is with the 6D and Nikon 16mm fisheye, so has a 180 degree view corner-to-corner. It is tough to realize you are seeing the Milky Way from Cepheus to Scorpius in one view! The glow sneaking over the hill at left is some of the light dome of Tucson... I chose to keep the exposure short at 30 seconds. Any longer and stars would trail. Of course, I could track the stars, but then the hill would trail! A short exposure, w/high ISO minimizes any trailing.

My first target was the glorious Milky Way, just short of transiting in the South. I've heard rumors that the 24-105 kit lens was a pretty good performer and not terribly slow at F/4, so tried that first. Mounting the 6D and hefty lens on the Polarie, I stacked 6 exposures of 3 minutes each at F/5 and ISO 3200. I thought the results were pretty spectacular. Of course, the orange Mars to the upper right and Saturn at upper left dominate the image, the dark nebulae and a numerous reflection nebula and Milky Way make it a memorable first image. Shot at 50mm focal length, the weight of lens and camera was near the Polarie limit. I really wanted to catch all of Scorpius, so for the next set of images, also used the kit lens set to 35mm, and repointed slightly to get the image at right. It certainly picked up more of the Milky Way and all of Scorpius, but also some of the airglow hugging the southern horizon. Still, the image quality and vignetting demonstrate that it might be a decent lens for general use with the large format sensor of the 6D. Also demonstrated here is that the 6D is low-noise enough that I've no longer got to take dark frames while observing. Normally with my older cameras, I would use "long exposure noise reduction" where it would take a 3 minute exposure, then subtract a 3 minute dark to minimize hot pixels. Generally considered bad form for astrophotographers, as it doubles your exposure, or reduces your duty-cycle to 50%, I look forward to not needing to do that any more!

While those images were coming in, I had installed the venerable Canon XSi and 70-200 zoom on an Astrotrac mount. The target - currently the brightest comet in the sky - Comet C/2013 X1 PANSTARRS. I had noticed when looking it up on "Heavens-Above", that it was well-situated, transiting shortly after it got dark, but only about 10 degrees above the horizon in Norma. But I like a good challenge, so went for it! Even with short exposures with the XSi and 200mm focal length, it was easy to sweep up, though I don't think it was near its predicted brightness of 6.5. Perhaps 7.5 would have been a closer guess, though perhaps its low elevation reduced the brightness. Check out the images at left... Both use the same 5 exposures of 2 minutes each (taking a total of 20 minutes with the in-camera darks of the XSi). The difference between them is that at left, the images are stacked on the stars, and at right they are stacked on the comet nucleus. On the left image, the comet moved fast enough to leave a trail, so stacking on the comet nucleus is preferable. If you squint, I think you can see a hint of a tail that goes to the left of the comet. This is cropped considerably from the full 200mm frame...

While first playing with the bigger sensor back in Illinois, I did a comparison of the sensor sizes with a macro lens and suitable subjects - maple seeds and dandelions! But perhaps a star field would serve as a better example of what a sensor twice as large could do... One of my favorite fields is of the Pipe Nebula in Ophiuchus. With the 70-200 zoom set to 200mm and pointed to the field, a series of exposures were taken with both the XSi (APS sensor) and 6D (full-frame sensor), just changing the bodies out between shots. I'll let the images speak for themselves - at left with the XSi and at right, the full-frame 6D. These are the full-frames, so the larger sensor at right translates into a field of view at least twice as large. Between the faster speed, lower noise, and larger sensor, I'm looking forward to getting more from the Canon 6D!

As the evening drew to a close, I finished out with a fisheye view of the transiting Milky Way. Shown here is a 5-frame stack of 2 minute exposures with the 16mm. Tracking on the stars the trees are trailed, as are distant lights of towns on the Tohono O'odham reservation. A memorable view on a memorable evening.

By the way, it was nice having Roger and his scope there too - we got some great planetary views, as well as the comet and a variety of other sights. The goal was to be on the road back to Tucson about Midnight - well, we didn't quite make that, closer to 1220, but I walked into the house before 2am and got to sleep into the 4th of July holiday a little. Of course, once you get a night like that, you can't wait for the next one, and with our rainy season here, the next chance might not come for a while!

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