I already have a 100mm EF-L lens but found myself wanting even more reach. I got it in my head that it would be cool to frame M31 and M33 in the same field. I thought the 100mm would be able to do it but after a valiant attempt (laying in the dirt making adjustments while looking at the camera back) to squeeze them both into the opposite corners of the frame .... Oh, so close but no-go. And about a week later after Rogelio Andreo put up this image of just the conceptual frame I imagined. He did it with a 3x4 mosaic'd panel (and considerably more skill and artistry than I would have!). Incredible and inspiring but my skills are not up to trying a mosaic panel just yet.
So a few days before CalStar last October I ordered a 10-22mm EF-S lens. Yeah, a bit overboard but I wanted the lens for terrestrial imaging as well. I wanted wide ... lets go wide!
But Robert Burns knew how this would turn out.
The best-laid schemes o' mice an' men
Gang aft agley,
An' lea'e us nought but grief an' pain,
For promis'd joy !
I like the lens for terrestrial photos. It is ... fun. It's a good lens, not a great lens, but actually fun to use. But as we know, a good lens, even a fun lens does not a good lens for astro imaging make. I knew there would be aberrations at the edges of the field but there was more than I expected. Oh, focusing. Don't even get me started. What a pain. No camera back focusing here. I had to connect to a computer and do a live view focus on jupiter just to see anything.
And then the vignetting. Lots of it. Flat fields that look like driving through a tunnel. OK, a bit of an exaggeration but still. And no sky flats possible with a field as wide as that. I took much more time trying to get a decent flat frame than I did getting the actual images. Much more. In the end I took my flats off my laptop screen.
After all that ... calibrating with my flats made the images worse. I'm still trying to puzzle that one out. The flats severely over corrected the vignetting. I dunno ... only thing I can think of is that my flats were not representative some how. Something to figure out.
So after wrestling with flatfielding I decided to only calibrate with darks and bias frames and deal with the gradients in PixInsight which has some powerful tools to throw against it. I tried a bunch of background models and finally stumbled into one that was "good enough".
|Cassiopeia nearly centered. That's Deneb and the North American nebula at left. Andromeda upper center. And a zillion stars from the Milky Way. Click image for larger version or here for flickr.|
Of course the tree was out of the field when I started but slowly crept in during the hour I was taking the images (in the end I only used 10 of the 30 2min exposures I took). During star alignment that tree area becomes a blurry mess so I just pasted the tree from the last frame in place.
There is a lot in there. Double cluster right of center. M31 mid top and if you look on a larger version you will find M33 just near the top of the frame to the right of M31 (yes, by design ... I got my wish). I should work on an annotated version some cloudy, rainy night.
So, more lessons learned. Maybe I should have spent the time learning to put mosaics together after all. But there is something kind of neat about this field of view. When I was taking the images I noticed that the field was an approximation of what you see with the unaided eye. Plus or minus. Look North and Up and that is what you would see ... if your eye could integrate photons.
North and Up
Taken at Lake San Antonio, CA October 9, 2010
Canon T2i (stock), Canon EF-S 10-22mm @ 14mm f5
Astrotrac Travel System Mount
10 120sec exposures @ ISO1600
no flatfielding, vignetting and gradients removed in PixInsight
32 bias frames
calibration, registration and post-processed in PixInsight