Notes
The first time I viewed a galaxy in a telescope I was taken with how solitary it looked. Only later did I learn that many galaxies are part of galactic groups, sometimes so dense as to be referred to as “clusters.” This observing program requires the observer / photographer to view groups and clusters from four different galaxy collections: Abell Galaxy Clusters, Hickson Compact Galaxy Groups, 50 Galaxy Trios, and an additional unnamed collection of 50 galaxies. The observer must observe or photograph 30 galaxies from each of the four collections.
Equipment
I have pursued this program with the remote photographic option, using remote photo services from Sierrastars (SSON) and Slooh. (There are only a handful of SSON photos in this program. The SSON service ceased operation soon after I began the program.) These services employ a number of telescopes, and I have used several of them for this project. Below each photo I note the scope with an abbreviation that follows this convention:
SSON – Warrumbungle in New South Wales, Austrailia = W
- Scope: PlaneWave Instruments 20″ CDK20 (Corrected Dall-Kirkham) at f/6.8
- Camera: SBIG STL6303E
- Latitude: 331:16:30.6 South
- Longitude: 149:11:40.3 East
SSON – Gemini in Sonoita, Arizona, USA = G
- Scope: PlaneWave Instruments 17″ CDK17 (Corrected Dall-Kirkham) at f/6.8
- Camera: Apogee CG42 camera
- Latitude: 31° 39′ 56.08″ North
- Longitude: 110° 36′ 06.42″ West
SLOOH – Santiago, Chile = SC 2
- Scope: PlaneWave Instruments 17″ CDK17 (Corrected Dall-Kirkham) at f/6.8
- Camera: Finger Lakes Instrument Proline PL16803 Monochrome CCD Camera
- Latitude: S33° 16′ 8.4″ S33.269
- Longitude: W070° 32′ 2.4″ W070.534
SLOOH – Canary Islands = C 3
- Scope: Celestron 11″ Rowe-Ackermann Schmidt Astrograph (RASA) at f/2.2
- Camera: Celestron Nightscape 8300 One Shot Color (OSC)
- Latitude: N28° 17′ 58.92″ N28.29970
- Longitude: W016° 30′ 29.736″ W016.50826
The SSON photos use a luminance filter and unless otherwise noted, are of 90 seconds duration. All of the SSON photos are monochrome. Slooh telescopes and cameras operate on fixed filter / exposure formulas based on the type of target in question. Sometimes they take luminance-only shots and at other times they take separate L,R,G, and B photos. I used a variety of integration techniques for the photos in this program. In general, if the photo has color in it, it is RGB or LRGB. If it is monochrome, it is a luminance shot. In some cases I found it best to turn a color photo into a mono photo in order to bring out the contrast between dark space and the objects in the field of view. In all cases, Slooh returns Luminance shots with 50 second exposures and R, G, and B shots with 20 second exposures.
Plate Solving
Part of the beauty of this endeavor (and, I suppose, the purpose of the observing program itself) is to view many galaxies in one field of view. As one begins working with the photos, one sees many, many galaxies. Using a technique called “plate solving,” I was able to identify many of the galaxies in each photo. The solvers I used were the image solver and image annotation scripts in Pixinsight as well as an online resource called Nova Astrometry. For more on plate solving, see here. Below is an example of a solved and annotated photo of NGC 4005 from Pixinsight (click to enlarge):
The red annotations are galaxies from the NGC catalog and the green annotations are from the PGC catalog. On occasion, I added stars from the Tycho Star Catalog to the solving setup; they are in yellow on those photos. I decided that for the Abell targets, I would annotate the original photo showing one bright galaxy (or in some cases, a bright star from the Tycho Star Catalog), and also post the same image, only plate solved. With a bit of hunting and close examination, you can find the galaxy I noted in the left photo on the plate solved image on the right to see how they match up. I hope the use of the solved photo gives evidence of how many galaxies are in such clusters.
The Index
The program also includes the task of creating an index of the objects viewed or photographed. To create the index document, I listed all of the galaxies I annotated in a given photo. So for NGC 4005, I grouped NGC 4018, NGC 3997, NGC 3987, and NGC 3993. See below (click to enlarge):
For each of the four catalog collections noted above, I have a numbered list of my 30 photos. For instance, in the Additional Galaxy Groups category, the photo of NGC 4005 is number 14 on the list. In the index document, I added the notation “AG 14” to all of the galaxies I identified in the NGC 4005 photo. To find them, go to entry 14 in the Additional Groups list. The notation for the other catalogs are as follows: Abell is denoted “AC,” Hickson Catalog is denoted “HC,” and the Galaxy Trios collection is denoted “GT.” I intend to sort by catalog number the entire list of galaxies in the 120 photos used in this program to create a useful finder. I expect to have quite a long list of galaxies when the program is complete. (The actual number turned out to be 381.) And I only included the brightest among the group or cluster. As the plate solved photos show, there are thousands of galaxies in these 120 photos. Just for fun, I re-ran the Pixinsight annotation script on Abell 1185 looking only for galaxies in the PCG catalog. The log file indicates that there are 325 PGC galaxies in that single frame. Obviously I did not include these in the index!
Quality of the photos
The peer reviewer of this program submission noted quality differences in the photos and asked if I could reflect on the performance of the scopes used in this program, and on the photos they produced. Among the 123 photos included in this program, only a few were from the SSON service — denoted W and D in the photo captions. The bulk of the photos were produced by the Canary 3 scope in the Canary Islands (denoted C 3) and the Chile 2 scope in Santiago, Chile (denoted SC 2). For the sake of simplicity I will limit my comments to a comparison of C 3 and SC 2. Though the reviewer did not cite particular photos, I went through the collection and tried to identify some for which a comparison might prove useful. Here are two Hickson photos from the two scopes (click to enlarge):
| Photo from the Chile 2 scope | Photo from the Canary 3 scope |
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The first thing that strikes me about about the two photos has to do with the magnitudes of the galaxies in question. On the Chile 2 scope, the galactic magnitudes are NGC 5208 = 14.2, NGC 5209 = 14.6, and NGC 5212 = 15.9. On the Canary 3 scope the galactic magnitudes are NGC 6119 = 15.4, NGC 6120 = 14.3, and NGC 61222 = 15.5. The magnitudes of the galaxies in the two photos are comparable. But while Chile 2 rendered the galaxies in some detail, Canary 3 only recorded small smudges of light.
The second thing that strikes me has to do with the size of the stars in each photo. Generally speaking, the stars in the Chile 2 photo appear larger than those in the Canary 3 photo.
What might account for these differences? Here are some hunches, in no particular order:
Weather conditions. Even great scopes struggle with poor seeing, cloud cover, wind, moonlight, and so forth. Therefore, side-by-side comparisons, minus a record of weather conditions in situ, may be somewhat unfair to the scope that produced an image of lesser quality. (I have asked Slooh staff if they save such data. I have not heard back but I suspect they do not.) I did think, for just a moment, about getting photos of the same objects from the two scopes at the same time. That won’t work: they are half a world away from each other, and thus do not have identical weather conditions.
Processing realities. I suppose one could write a Pixinsight script that applied processing in exactly the same way to every photo. (That would be complicated, and I am not such a programmer!) Absent such an “equalizer,” I used whatever Pixinsight tools I knew about to improve the appearance of the individual photos. It may be that one of them benefitted from such techniques more than another.
The scopes themselves. The Chile 2 scope is a 17″ Corrected Dall-Kirkham model that takes photos at F/6.8. The Canary 3 scope is a Celestron 11″ Rowe-Ackerman Schmidt Astrograph that takes photos at F/2.2. This means that the Chile 2 scope shoots photos that have a much narrower field of view. In other words, they are “closer in.” In terms of aperture, the Chile 2 scope is a much bigger “light bucket” than the Canary 3 scope. It is bound to capture more information. When the technical level of the two scopes is compared, the differences emerge even more vividly. I checked some retail prices and learned that the Chile 2 scope costs around $24,000 USD while the Canary 3 scope runs for about $3,500 USD. Same with the cameras. The FLI model on the Chile 2 scope runs a hefty $12, 800 while the Celestron camera on the Canary 3 scope can be purchased for about $1,700. Fact is, the Canary scope and camera are what we might call “consumer electronics,” while the Chile 2 scope is a dedicated, sophisticated “scientific instrument.” No knock on Celestron. I own their stuff!
One might then ask, Why the heck did you choose to take so many photos from Canary 3? That’s easy enough. In Slooh, it is easier to book time on Canary 3. Near the end of the program, Chile 2 got hit with three waves of trouble: the weather deteriorated, a storm knocked out the observatory for a time, and fixing the damage was delayed by the ravages of Covid-19 in Chile. So I got as many shots as I could from Canary 3.
Perhaps these considerations account, to some small extent, for the differences in the quality of the photos used in this program.
Experiments
I undertook two experiments toward the end of the program: Depth of Field and Distance between Galaxies.
1. Depth of Field. One of the things that strikes me about all of these photos is their two-dimensionality. The field of view has height and width, but it is often difficult to discern depth. This is especially interesting given the incredible expanse of space and the age of the light that the photos capture. I wanted to figure out a means by which to say something about this on particularly interesting photos.
Imagine this for a moment: You see a photo that shows stars and galaxies. They may look to be comparable in size, but they are not. A single galaxy has billions of stars. The stars we see appear bigger because they are much closer to us, in our galaxy, the Milky Way. I decided to see if, for a given photo, I could look up how far away a star was from our solar system, and how far away a galaxy was from our solar system.
I invented a term called a “Star Unit.” A Star Unit is simply the distance from our solar system to a star. Caveat: the universe is in motion and everything changes over time. For the sake of simplicity, I will use Sky Safari, to find the distances to stars and galaxies from our solar system. Second caveat: toward the end of this project, I discovered that some of Sky Safari’s distances are different than those listed by others, such a NASA. Since the work was largely done, I will stick with the Sky Safari numbers.
If one could travel to a star in a photo, I wondered how may more star units would it take to travel from that star to a galaxy that is pictured near the star in the photo? Easy enough to calculate — divide the distance to the galaxy by the distance to the star. I call that quotient the “Star Unit Ratio.” In the photo notations I have entered this information as follows: [Star Name / Galaxy Name / Star Unit Ratio = N].
In the end, I discovered that while this did provide some cosmic context, it was not always easy to accomplish. Most of the stars in these photos are from the Tycho catalogs, for which distance info seem to be hard to come by. Fortunately, some of the stars in the photos are more common “named stars,” for which data is more readily available. And in some cases, I was discovered some stars had multiple catalog references, so I could get around the Tycho problem.
I count this experiment a modest success. It is amazing to consider the depth of space represented in the photos. The Star Unit Ratios are huge.
2. Distances between Galaxies. No less impressive is the distance between the galaxies themselves. Though the photos may not show depth of field they do represent the distances between galaxies, at least visually. This is not a data point that is usually noted in charts or planetarium programs. It is common to get info about how far an object is from us, but less common to get data on how far objects are from one another. I wondered if I could figure out those distances using common astro software, online resources, and some basic trigonometry.
After a lot of fumbling around, I found a website that features an online calculator that measures distances between stars (and presumably galaxies) if one had the following information for each object: Right Ascension, Declination, distance from our solar system, and visual magnitude. All of the data is available in Sky Safari.
My plan was to use the online calculator to get the distances between galaxies for a dozen or so photos drawn from the Hickson and Additional Galaxies pages, and to add a note about the distances to the photo notes. To test the online calculator results against another source, I decided to use the “measure from” feature in Sky Safari. I was very disappointed in the results of these tests, and called a halt to the experiment. The calculator’s distance and the Sky Safari distance were very significantly off. Here are a three examples:
| Additional Galaxy Photo 4 | Messier 65 to Messier 66 | Online Calculator = 18.3 Mly | Sky Safari Measure From = 4.6 Mly |
| Additional Galaxy Photo 5 | Messier 84 to Messier 86 | Online Calculator = 28.6 Mly | Sky Safari Measure From = 9.7 Mly |
| Additional Galaxy Photo 17 | Messier 106 to NGC 4248 | Online Calculator = 45.4 Mly | Sky Safari Measure From = 10.7 Mly |
Poor results indeed. As a final check, I tried using these tools on stars instead of galaxies. Here are the results:
| Hickson Photo 15 | 4 Aqr to 5 Aqr | Online Calculator = 452 ly | Sky Safari Measure From = 452 ly |
I cannot account on the difference in accuracy between inter-galaxy distances and inter-star distances. My only hunch is that that online calculator is not intended for use on galaxies. I assumed it did because it was based on largely immutable data, namely RA and Dec. In any case, I declare the distance between galaxies experiment a failure. I learned a lot in the process except, that is, how to measure distances between galaxies. Perhaps someone with more expertise can correct any errors in logic or math.