After several years of persevering with my flimsy home-made mount, I finally picked up a secondhand Astrotrac from eBay and last night was my first opportunity to give it a try. I already have some beefy Manfrotto gear to mount it on (058 Triaut legs and 405 Pro Geared Head), so adjusting polar alignment was not too bad. I think there's room for improvement on that, and I've found out how to get the polar scope focused properly.
I tried a few different targets, first with the 70-200/4L at 200mm, and then, to give it a sterner test, with the 100-400L at 400mm, both on the 7D. I found pointing at the target so much easier with such sturdy gear. All these images were stacked in Deep Sky Stacker and then processed in Photoshop. My processing skills are still quite rudimentary, added to which I think the "flats" I shot are not as flat as they should be, so I might need to revisit that.
First, here is the Andromeda Galaxy at 200mm, made from 14 30s exposures at ISO3200, plus two 60s exposures at ISO1600 (these were just tests of tracking at longer exposure - the 120s exposure I took had slight trailing, so I didn't use it). I lost colour in my DSS processing here. I'll probably come back to it after I've improved my flats. Limiting magnitude in this and most of these shots is about 15.0.
I then turned to the near zenith and shot the North America Nebula region beside bright star Deneb. This was only two 30s exposures at ISO3200, but with the darker zenith sky I've got close to mag 16, and the nebulosity is showing up quite well. DSS counted 6,000 stars in this image.
I then put the 100-400 on and returned to Andromeda. This is just five exposures at f/5.6, ISO3200, four of them at 30s, one at 60s.
I then moved to the Triangulum Galaxy, which I've only ever captured as a faint blob before. This is just four exposures, f/5.6 ISO3200, three at 30s and one at 60s. I'm so pleased to be able to see traces of the spiral arms.
Next the Perseus Double Cluster, just three 30s exposures, other settings as before. Focus had drifted slightly here - the 100-400 seems quite sensitive to temperature changes.
Finally I turned towards the Ring Nebula, M57. I know it's very small, but I thought I'd see how it came out at 400mm. This is seven 30s exposures at f/5.6, six at ISO3200 and one at ISO6400. The limiting magnitude here is about 16.3, the faintest I've yet managed.
Also, when scouring the image to remove hot pixels (DSS doesn't seem to do this even though I've got the box ticked) I discovered a faint fuzzy blob in the extreme bottom-right corner. I checked against Wikisky and identified it to be NGC6700, a 13th magnitude galaxy at a rough distance of 200 million light years, my most distant object imaged. To show it, and also the full-res image of M57, I clipped them out at 100% here.
I've been really pleased with this first go with the Astrotrac - I was only outside for about an hour, though processing today took considerably longer. I'm sure I can do better, by improving the alignment and improving my flats to allow more severe processing, not to mention improving my processing tricks.
I thought people interested in the Astrotrac would like to see some examples (which is why I haven't cropped these images, except to remove edge artefacts, so you can see the field of view), and I'd also be very happy to hear comments and criticisms on these images. I'm keen to learn from some of the more experienced astro-imagers on here.
