Sunday, January 15, 2012

M33: Two nights at Dino

Here's M33, the Triangulum galaxy:

(There's probably an issue with orientation or `flipping' of the image, but since I've stared at it for so long in this orientation, this is becoming `how it looks to me'.)

This image has me thinking about two `themes':

1) The pleasures of imaging from a nice dark site, like Dinosaur Point.

2) The difficulties of getting good data on M33, the Triangulum Galaxy.

I shot these data on two successive Saturday evenings, October 22 and 29, 2011, from an observing site called Dinosaur Point. It's a boat ramp on the San Luis Reservoir. The reservoir is part of California's enormous system of water projects, which control floods, supply water, and supply electricity. One function of the San Luis reservoir is, essentially, as a giant electrical storage battery. Water gets pumped uphill into the reservoir at night, when electric rates are low, and the water is drained downhill (through generators) during the day.

Dinosaur Point has long been a favorite winter dark-sky site for Bay Area observers. It tends to be too windy during the warm months. But in the late fall and winter, if the `tule fog' from the nearby Central Valley hasn't covered it, Dino can be a very dark site. I really enjoyed setting up there and imaging M33; the sky was nice and dark. One night, in the wee hours of the morning, we even saw the adaptive-optics laser beam from Lick Observatory, shooting towards some object in the south.

It's very important to note, though, that observing access to Dino is subject to some very specific conditions. If you're a Bay Area observer who hasn't been there, make quite sure that you've read and understood the `gatekeeper' access protocol! You can also check the TAC list and the TAC Observing Intents page to see if a gatekeeper is going. Don't just go there without checking all of these details first!

I acquired these data with the same rig as the last couple of shots - my Orion ED80 refractor (80mm f/7.5) with the SBIG ST-8300M CCD camera. I shot unbinned luminance data, and 2x2 binned color data through R, G, and B filters. If I recall correctly, I think I have a couple of hours from each filter. That would make for 8 or so hours of total exposure time, give or take.

I think that M33 has some potential to be a frustrating object for beginning astro-imagers. Typically, I think a lot of us undergo a pattern like this: a) We get a CCD camera during the summer, and by autumn we have a basic understanding of how to use it. b) During the fall, we shoot M31, which is so bright that we can get a decent signal-to-noise ratio over most parts of the galaxy, without too much trouble. c) Next, we say to ourselves `Aha, look what's nearby - M33! There's another big bright galaxy just waiting to be shot!' As it turns out, however, M33 has a lower surface brightness than most of M31, and it's tough to build enough SNR to get a good image. Unless you're using an optical system with a very fast focal ratio, M33 is going to take a long time to build a decent dataset.

This dataset really isn't long enough, but I decided to go ahead and try to process it anyway. I probably won't be able to shoot M33 again until summer or fall 2012, so here's what I've got, so far. With a considerable amount of time invested in Pixinsight, I was able to get something semi-presentable.

Processing in Pixinsight:

I started with the usual calibration routine, using light, dark, bias, and flat-field frames, and I extracted the small amount of light-pollution gradient that one gets at Dino. This gave me linear (i.e. unstretched) luminance (L) and color (RGB) images. These images had the usual background-neutralization and color-calibration corrections applied to them. Then it was time to get a little more from the linear images. First, a bit of noise reduction using the Multiscale Median Transform tool. Then I used the new DynamicPSF module to build a model point-spread function for each image, and fed that PSF into a gentle application of regularized Richardson-Lucy deconvolution. This helped to bring out a bit more detail in the central part of the galaxy.

Then it was time to go non-linear with each image. I did this the easy way: For each image, I did an auto-STF (Screen Transfer Function), and applied each of those auto-STFs to instances of the Histogram Transformation tool. This gave me stretched images that had very similar histograms - and that's just what the LRGB combination tool wants.

If I recall correctly, I did a bit of SCNR (Selective Color Noise Reduction) to take out some of the `galaxy green' in the RGB image, before performing the LRGB combination. I increased the saturation a bit when making the LRGB image, and used Pixinsight's magic Chrominance Noise Reduction routine.

With the LRGB image in hand, it was time to perform two parallel lines of attack, which would later be combined:

1) Compress the dynamic range a bit with HDR wavelets, so as to take away some of the `over-bright dominance' (for lack of a better term) of the central part of the galaxy, and then punch up the contrast with Local Histogram Equalization.

2) Try my hand at the mystical `multiscale processing', a la Rogelio. I split a copy of the LRGB image into large-scale and small-scale components, following the general method of Rogelio's and Vicent's multiscale tutorials. I didn't to anything extra to the smallscale image; I just didn't have the mental energy. But I did some Histogram Transformation (and possibly HDRWT, IIRC) to the large-scale image, brightening the midtones and re-setting the black point. Then I combined everything back together with PixelMath:

a) The LRGB image
b) The LRGB image that had been HDRWavelets-ed and LHE-ed
c) The smallscale image
d) 0.25 * the stretched-even-more largescale image.

Following this recombination, I made a Star Mask (with default parameters), and used Morphological Transformation to dim/shrink the small and medium-sized stars. At that point, I said `Stick a fork in this sucker, it's done. Put it on the blog.'

Room for Improvement:

When I look at this image, it seems to me like it's still afflicted with a bit of `galaxy green', but when I applied an additional round of SCNR to it, it didn't seem to change. Some of the stars also wound up looking a bit pink, but at this point, I'm too tired to fight about it.

Next, there are the big, bloaty stars. These are the bane of all my images. My temptation is to blame them on the small aperture of my telescope. An 80mm scope will have a big, fat point-spread function, and if I want tiny stars, I'll need a bigger scope. That's probably true, to some extent, but I'll bet it's not the whole story. I am beginning to suspect that the big, halo-y stars are a consequence of the fairly severe stretching that the image has undergone. M33's dim, and it takes a lot of stretching. This probably brings the outer parts of the PSFs up to an objectionable brightness. With a longer total exposure time, I could probably get the faint parts of M33 to show up without as much stretching. (Of course, this raises the question of whether those outer portions of the PSFs would show up, too... hmm...) I'd love to figure out how to shrink those stars, so that it looks like I used a bigger scope. After a lot of fiddling around with Star Mask and Morphological Transformation, however, I haven't found a way. It remains a dream.

With more integration time, I think I could show more of the faint outer portions of M33. I'd love to get in night after night on this object, and really punch out every part of this galaxy. M33 is full of resolved stars and HII regions like NGC 604. I often think of M31 and M33 as the closest thing we've got the Magellanic Clouds up here in the NoHem, and it would be nice to make the deepest, sharpest images of them that I can.

Naturally, many people have gotten some very nice, very deep images of M33. One of my favorites is this one by Stephane Guisard, because he shot it from the Atacama region of Chile - exactly the `wrong' place to get a good image of M33. Shows you how good places like Paranal are! And of course, there's a nice Hubble image of NGC 604, the most prominent star-forming region in M33. (In my image, the way I've got it oriented, NGC 604 is down and to the right of the galaxy's center, above two prominent, bloated orange field stars.)

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