Messier 24 - The Small Sagittarius Star Cloud - in Widefield LRGB
Date: July 26, 2021
Cosgrove’s Cosmos Catalog ➤#0078
Messier 24 - The Small Sagittarius Star Cloud. That is a lot of stars! (click image for full resolution via Astrobin.com)
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About the Target
Messier 24, also known as IC 4715 and “The Small Sagittarius Star Cloud,” is located appropriately in the constellation Sagittarius. It appears as a large and dense population of stars with a few areas of dark dust. As we look towards Sagittarius, we are looking towards the highly populated core of our own galaxy. Unfortunately, we don’t see the core as many molecular gases, and dark dust clouds obscure our view.
However, in the case of Messier 24, what we are actually seeing is a hole in the obscuring gas and dust, which provides us a glimpse of the dense stars of the Sagittarius-Carina arms of the Milky Way. The visible starfield is quite large - roughly 1.5 degrees across and is located about 10,000 light-years away. The volume of space filled by M24 extends a further 10,000-16,000 light-years. It is also quite dense - in binoculars alone, about 1,000 stars can be seen. In the image captured here, you can see an incredible density of stars of all types. The dark rifts and globules are areas of high dust concentration that have their own designation in the Barnard catalog.
The Annotated Image
The Location in the Sky
About the Project
The month of July in Rochester has been horrendous for Astrophotography. We have only had two nights where subs could be gathered, and these were marred by thin clouds and smoke plumes from the forest fires from out west. I have already published the image of Messier 16 - The Eagle Nebula - that came from these two sessions. Messier 24 was shot at the same time, only using the Askar FRA400 Platform.
The Askar FRA400 platform is designed to be portable, and as such, has a lightweight mount and smallish telescope. The Astrograph has a focal length of 400mm, which combined with the ASI1600MM-Pro camera, provides an angular field of view of 2.3 x 1.9 degrees. Compared with the other two scope platforms, this is about 2X wider - which makes it a good choice for something as large as M24.
A Very Low Object and a Very Wide Field of View
M24 sits pretty low in my skies as we look to the south. It never gets much higher than 25 degrees. When I am shooting it, the scope has to look edgewise through a lot more atmosphere than when I am looking at the zenith. Not an optimal situation normally for astrophotography but doubly bad when you are concerned about high humidity weather, thin clouds, and smoke plumes made even worse by the oblique angle through the atmosphere.
With the wide-field camera, I am noticing another interesting problem. I can’t start my exposure until the target has cleared the tree line - and I have to stop the exposures before the target sets into the tree lines. With a wide field of view, I have to start even later as it takes a while for the camera field of view to clear the trees because it is so wide. Same problem on the other end - a wider view hits the trees before a narrower view does. So I lose a frame or two at both ends! When your access time is restricted, losing frames is losing time!
The Data Was Better Than I Thought it Might Be
I had not gotten to this data because I was working on launching my website. Now that that is done, I finally took a look at the data for this project. Yes - there were some bad frames, but the good ones looked better than I expected. But -just like with the last project - I needed to optimize the data I had.
First - I used the GradentScaleNormalize script in Pixinsight to normalize the frame gradients to a reference frame. This would allow me to keep more frames in still get some value from the compromised frames. Then I decided to do selective rejection on the frames that show tree artifacts.
For example, below is a single sub with trees entering the field of view.
I then took the DynamicPaintBrush tool and filled the dark areas in with the back (code values of 0.0).
Now, when I do ImageIntegration, the black areas will be rejected by the low criteria but the data from the rest of the frame will be there and useful!
Trees starting to enter the camera field of view - with a wide-field system, this occurs earlier than with a narrower field of view.
Tree shadows masked off with code 0.0, ready for rejection.
Will Elon Musk Kill Astrophotography?
While we are talking about rejection - let me talk about Satellite trails. People are always asking me if Elon Musk is ruining astrophotography with all of his satellites. To be honest they don’t impact me much. I already have trails from satellites, airplanes, asteroids, and cosmic rays in my images. Elon’s Satilittles are not much worse than that. The logic of image integration is such that outlier pixels like those are identified and removed in the normal course of operations. Here is an example of trail pixels rejected from this image:
This is the rejection map from the ImageIntegration script - the the trails are neatly removed.
On the other hand - what does hurt astrophotography is Global Warming. Global Warming causes the extreme weather we are now seeing. It’s likely why we were so cloudy this month here in Rochester. It’s also likely the cause of the forest fires out west. I would expect the percentage of clouded nights to go up as Global Warming kicks in more and more. That’s what we need to worry about and take action on.
Undersampled Data
The resulting image came out surprisingly well given the low integration times and the poor skies. In general, I don’t do wide-field images, and rarely do I go after targets that are low in the sky. In this case, I was doing both - but I was also pointing towards the core of the galaxy.
When I first looked at the master images, my reaction was very similar to David Bowman’s when he first looked into the obelisk in orbit around Jupiter: “My God, it Full of Stars…..”
The ASI1600MM-Pro undersamples this scope slightly, so the small stars are a bit squarish, but that does not show up so much at normal viewing scales. Since it is undersampled, I would expect that not much would be gained by using deconvolution on this image. I know this is based on theory - but I can be pig-headed, so I tried it anyway - and I can confirm that the theory holds. All I really got out of the exercise was some wasted time! ;-)
Now I could have drizzled the data. Drizzling is a method first developed for the Hubble Space Telescope to maximize image resolution. The net effect of the way I would use it would be to effectively double the resolution of the image. At that point, it would no longer be as undersampled, and deconvolution could do more good for us - in theory. I’ve decided to put that experiment off until this winter - when we are clouded out!
Also - there are so many stars in this image - and the little ones are on the square side - that it was a little hard to deal with gradient reductions and noise reduction in the normal fashion. They tend to distort the small background stars, as you might expect. But at the proper viewing scales, I would contend that this is not really noticeable.
During a season where there is little productive astrophotography going on, it is very nice to have a new image to share with you!
I hope you like it!
Capture Information
Light Frames
The number of frames is after bad or questionable frames were culled.
52 x 90 seconds, bin 1x1 @ -15C, unity gain, ZWO Gen II L Filter
30 x 90 seconds, bin 1x1 @ -15C, Unity gain, ZWO Gen II R Filter
15 x 90 seconds, bin 1x1 @ -15C, unity gain, ZWO Gen II G Filter
29 x 120 seconds, bin 1x1 @ -15C, unity gain, ZWO Gen II B Filter
Total of 3.2 hours
Cal Frames
30 Darks at 90 seconds, bin 1x1, -10C, gain 0
30 Dark Flats at Flat exposure times, bin 1x1, -15C, gain 0
Flats done separately for each evening to account for camera rotator variances:
15 L Flats
15 R Flats
15 G Flats
15 B Flats
Capture Hardware
Scope: Askar FRA400 73MM F/5/5 Quntuplet Astrograph
Guide Scope: Sharpstar 66EDPHII
Camera: ZWO ASI1600mm-pro with ZWO Filter wheel with ZWO LRGB filter set,
and Astronomiks 6nm Narrowband filter set
Guide Camera: ZWO ASI290Mini
Focus Motor: Pegasus ZWO EAF 5V
Mount: Ioptron CEM 26
Polar Alignment: Ipolar camera
Software
Capture Software: PHD2 Guider, Sequence Generator Pro controller
Image Processing: Pixinsight, Photoshop - assisted by Coffee, extensive processing indecision and second-guessing, editor regret and much swearing….. Given the problems on this image, more than the usual whining….
The portable scope platform is supposed to be, well, portable. That means light and compact. In determining how to pack this platform for travel, I realized that the finder scope mounting rings made no sense in this application and I changed them out with something both more rigid and compact - the William Optics 50mm base-slide ring set.