NGC 2403 (also known as Caldwell 7) was discovered by William Herschel in 1788. It is a intermediate, flocculent spiral galaxy in the constellation of Camelopardalis with an apparent size of 23.4 arc-minutes and an apparent magnitude of 8.2. Its large size and brightness makes it an attractive imaging target for smaller telescopes. Simbad lists the morphological type as SAcd, which is an early type spiral with a dominant core and a large halo of stars. Other sources list the type as SAB(s)cd. It is an outlying member of the M81 Group at a distance of 8-11 MLy that bears a strong resemblance to M33. NGC 2403 has a large number of huge H II clouds that contain numerous star formation regions. NGC 2404 located in the northern arm is the largest such region, which is larger than the tarantula Nebula located in the Large Magellanic Cloud.

I started this LRGB+Ha imaging project early last November and it was quite a slog to get it done. The conditions in Northern New Mexico this winter have been especially poor this year. Clouds shut things down quite often but wind and the prospect of frost often kept the observatory roof closed even when the skies were clear. And somehow whenever the conditions were good enough to get the roof open, the seeing was often just terrible. It was uncanny how the weather seemed to go down whenever the moon was new and it took three lunar cycles to acquire enough good data to finish this image. I gathered a little over 400, 20-minute subs for a total of 136 hours of exposure time in 5 channels and out of that, just 33 hours of data where useable. For example, I only got 12 useable Ha subs out of the 90 that I took—and I had to relax my threshold to FWHM<=2.4 arc-sec to even get that much! Imaging under these conditions has just been a game of odds and to make it worse, the problems with my guide camera locking up have returned. I’ve disconnected power to the cooling circuit, which greatly improves the reliability of the camera but every so often the camera hangs and that takes my whole system down for the rest of the night (unless I happen to catch it early.)

This image includes 40% of the Ha signal added to the R channel to emphasize the Ha regions. This approach tends to make color calibration a bit more challenging and that was the case here. I struggled to settle on a correct blue/green balance. For the final version, I selected a slightly cooler color temperature that was very slightly "de-greened" to better emphasize the blue arms in keeping with Hubble data and Adam Block's interpretation of the same objet (which is considerably more blue.) Version B is a bit closer to the color balance in Eric Cole's APOD image and it's what came straight out of using the whole galaxy to define the white level. I can't say for sure which one is "right" so I went with the balance that I thought might best represent this type of galaxy taking into account the colors commonly shown in M33. This isn't a photometrically accurate representation so I'll claim a little latitude in choosing how to display the data.

I thought that the Ha image was interesting enough to show it off in rev D.

As usual, C&C is welcome so let me know what you think.

- John