Surprises can be the second best part of imaging.  Of course, the best part is learning something new.  The path to this image involved both.   I started on this project just before I headed out on my S. American adventure and I was delighted to see such good data coming from the scope.  First, my efforts to fine tune the focusing system clearly made a noticeable improvement--seen later in the stack of images.  Under the best conditions, I saw multiple frames with FWHM at 1.3" and below.  The best was just a touch above 1"--not surprisingly in the red channel.  Second, the signal strength was really good.  That led to some initial optimism that I could snag a complete image in very little time.  As it turned out, it was too hard to screw around with choosing a new target while I was on the road so I just gathered a boat-load of data with nearly 100 subs for each channel--and that taught me an obvious lesson.  The more data you have, the easier it is to tighten the FWHM threshold to maximize stacked image quality and still wind up with a reasonable stack depth.  I also wanted to add Ha data to the result to show hydrogen emissions so I gathered almost 100 NB Ha frames, which produced a yield of over 50 useable NB frames.

I almost never do very much research on the objects that I image until after I've produced the image.  In this case, I knew that this galaxy was pretty cool, but it was a surprise to find out how much I didn't know about it.  It is among the most studied objects in the sky!  At an apparent magnitude of 6.84 and a size of 25.7' x 20' it is a popular target for both imagers and visual observers alike.  In fact, this was one of the objects that we looked at through the 1-m scope last month and the central dust lane was very clearly visible.  It lies at a distance of about 13 Mly (+/-3 Mly) so it falls in the category of being in the local "neighborhood" to the Milky Way.  The center of the galaxy contains an active supermassive black hole with a mass estimated to be around 55 million suns.  It emits relativistic jets that collide with interstellar material to produce a strong source of both X-rays (very short wavelength) and radio emissions (very long wavelength).  One of the inner jets can be seen in this image above and to the left of the core.   The X-ray jets have been measured at a length of thousands of ly long and the radio jets are over a million ly long.  Centaurus A is one of the brightest radio sources in the sky and if you include both radio and X-ray data, it subtends almost 8 degrees!  Centaurus A’s is thought to be the survivor of a collision between two galaxies that began more than 100 million years ago.  The resulting giant elliptical galaxy contains a star nursery along the central dust ring.  The sprinkling of blue stars along the edge of the lane are young super-giant stars.  If you look carefully, there's an edge in the halo that defines the shape of the galaxy.  This image also shows the distribution of "brown-ish" dust within the inner halo.  This is indeed a VERY interesting object.

As I often do, I took a very quick processing run at this data after I stacked it just to see what it looked like and I lucked out.  I immediately produced a pretty good looking image.  However, as I examined it more carefully, I could see a number of problems that need to be fixed, so I started over.  Well heck, no matter what I did, not only could I not make it better; it got worse!  Version 2 became version 3 and that eventually became version 4 before I wrestled it under control with version 5.  I left bruised and dope-slapped.  So here are a few lessons (that I sometimes have to re-learn):

1) Gradients, no matter how small can kill an image.  Carefully remove gradients right up front.  This is especially true if you are adding a weak Ha signal.

2) Be very careful about TVG Denoise--even if you are just trying to control color noise.  Even with local support and carefully set parameters, it MAY introduce unexpected color halos on stars in linear data--where you least expect it!  I eventually gave up on it in the linear data for this object.

3) Color balance is NEVER obvious and this object is particularly difficult.  Just do a search here on AB and look at all the different results.  There are very few images that show identical colors and that applies to professionally acquired data as well.  I tried every trick in my book and I still had a hard time achieving a result that looked correctly balanced without a bit of "manual intervention."  Remember this rule:  Proper color calibration will show a range of colors in the background stars.  A range of colors in the stars is no guarantee the the balance is absolutely correct but if they all look blue, yellow, or white, something is off!

3) Adding low signal level Ha data can be a tricky endeavor.  I use the Martin Pugh and Vincent Periz method of computing the "pure" Ha signal and adding it to the image.  With this method, it is very important to get the initial color balance right for the LRGB image before adding the pure Ha signal.  The other lesson is that when the signal is very low, it's vital to figure out how to mask out the background in the Ha data so that it doesn't add noise in the red channel, which upsets the background color balance.  It took a lot of hours to figure out out to process the pure Ha channel and to create a pure Ha mask to add Ha to the result without screwing up everything else!  I'm pretty happy with this result but it was the result of a lot of work to get it right.  At least I learned a lot of new PixelMath!

4) Deconvolution (if you choose to apply it) is best applied to the linear RGB result rather than to the individual linear channels before combination.  Applying it to the RGB result allows looking for color artifacts (even when only sharpening the Lum channel) and quickly correcting them when they occur.  Sharpening the raw channels puts off inspecting the results to a point where you have to start all over again if something goes wrong.

5) Carefully inspect all the stars before finishing.  No matter how good the data may be, there's always one (or maybe two) wayward stars that show some abnormal morphology or color artifact that came from...well, who knows where!

6) Masks are essential; however, be extremely careful with them.  They can and (sometimes) will introduce unexpected artifacts--often when you least expect it.   Catching a masking defect at the end of the processing run is likely to send you back to the starting gate.

As usual, I've reached the point where I need to set this image free.  I've provided three versions.  The first shows plate-solved coordinates, the second shows a zoomed view at the 1x1 raw 3.75 um sampling rate and the last (and I hope final) version is an interpolated image to minimize the size of the image for a wider FOV.

Hopefully I didn't screw it up too badly but even if I did (or didn't), I'm happy to hear C&C!


John