A Dog, A Dragonfly, An Experiment, and Variables

Lets start with the dog; a Disney dog in this case. Specifically Pluto. If it can't be a planet, I guess it can be a dog. One of my observing goals has been to “see” all of the planets in our solar system. The last one left would be, of course, Pluto. I know, I know; Pluto isn't a planet any more. But it was when I started observing, so Pluto it is. I can tell you up front, it's spectacularly unimpressive.

Pluto, somewhere in there

Above is an image showing Pluto. It's that tiny dot of light inside the white box. See what I mean? Spectacularly unimpressive. The hunt to find it took quite a long time. But it was fun. Been there, done that...Moving on....

The remaining images in this entry are experiments. The next image of NGC457, also know as the Dragonfly Cluster (or the Owl Cluster, or the E. T. Cluster or the Kachina Doll Cluster, among others; take your pick), was an attempt to overcome problems with light pollution by taking a large number of exposures. The expectation was that it would help to increase contrast, but wouldn't be a substitute for a dark sky site. Pretty much, that seems to be the case. It also became an unintentional segue into working on variable stars and problems with at least one of those. More on that later.

NGC457

NGC457 is an open cluster in the constellation of Cassiopeia. It's about 7900 light years away and quite nice, I think. The two brightest stars are often imagined as eyes and the rest of the stars (about 150 or so in the cluster) as “something”; E. T., a dragonfly, an Owl, or whatever. Technically, the image is quite a challenge; most of the stars in the cluster are quite dim, magnitude 12 to 15. The two bright stars are magnitudes 5 and and 7. Why is this a challenge? (Thank you for asking!) Well, the difference of one magnitude is a difference of 2.51 in terms of brightness. That is to say, if there is a difference of 1 magnitude between two stars, one of them is 2.51 times brighter than the other. So, if the dimmest star I captured is, say 12^th magnitude, the difference between the brightest star (5^th magnitude) and dimmest start (12^th magnitude) is 7 magnitudes. That means the brightest star is 2.51x2.51x2.51x2.51x2.51x2.51x2.51= 627 times brighter than the dimmest star. That's a lot! That difference is referred to as dynamic range. If you consider that the sky is actually the darkest element of the image, the dynamic range is actually much greater than just the difference in magnitudes of the stars. To capture the dimmer stars, longer exposure times are required, which causes the brighter stars to be overexposed or saturated. When that happens with CCD cameras, the overexposed pixels “bleed over” into adjacent pixels. Think of filling a glass with water, which gets overfilled and spills over. The name given to this is “blooming”, and cameras like mine have circuitry called anti-blooming circuitry to help with this. As it turns out, I needed to retouch the image to reduce the blooming that occurred around the bright stars. The original image shows the blooming.

The next experiment involves a different camera, one that's called a “one shot color” camera. It's the one I use for lunar imaging, and is essentially a color webcam. This one has the provision for internal stacking (adding successive frames to make one still frame) to make a long exposure. The target this time was another bright open cluster, M11, also know as the Wild Duck Cluster.

M11

One of the problems with using this camera is the inability to guide the telescope automatically. All telescopes that track the sky do so with some time of mechanical gearing. Because gears can't be made perfect, they cause tracking errors that show up in the image as stars that are not round, but elongated in one or two directions. To correct for that as much as possible, some type of guiding is used. The easiest type is another camera that sends the correcting signals automatically. That's the method I usually use. So, images taken with this camera will always have these guiding errors. That's usually not too bad, because the exposures are usually very short, so little movement is involved. However, it will show up in as little as 10 seconds with the image scale (or apparent magnification) used in this image. What I was able to do was get 10 images, 9 of which were usable, each 10 seconds long to make the single image you see above. Bottom line, I might be able to use this method for bright star clusters and get a color image quicker, but not with as much fine detail, as using the “bigger” camera.

Finally, one of the goals of the Starlight Observatory is to be able to accurately record the brightness of the class of stars called variable stars. As the name says, these stars change their brightness over time, and observations of that variation helps to determine what the cause of the variation is. One way to measure how bright a star is would be to measure the number of electrons in the pixels that are lit up by the variable star, and then do the same thing for a couple of other stars in the same image that have known and documented magnitudes. The process would work like this: 1) take an image showing the variable (V) star, and the two other known stars, called the check (K) and comparison (c) star. 2) the camera software converts the number of electrons into a number between 0 and 65535. 3) Then, subtract the “software”, ie, 0 to 65535 value, of the Check star from the Comparison star. 4) Subtract the “software” value of the Comparison star from the Variable star. Mathematically, K-C and V-C. To find the value of V, I just need to add the known value of C to the value V-C and I've got the value for V. K-C serves as a check to be sure all is ok. Example: Known values for K and C are K=13.956, C=12.743, K-C= 1.213. Observed Value for V-C (ie, the value derived from my image) = 1.791. The derived value for K-C = 1.052. For my data, I get the value, or magnitude of V as 1.791+12.743= 14.534. How close is that to the “real” value? One measure is the K-C value. If it matched exactly, I could be pretty sure. However, there is the difference, 1.213 vs 1.052 so confidence is not perfect. There are explanations for the difference (variation in the cloud cover, camera movement, inexact flatfielding, which is really the most probable cause). But for now, just being sure I'm in the right ball field is good enough. Well, the thing about most variable stars is that they vary over fairly long time periods; weeks, months, or years. There are, however, a few that vary over much shorter time spans, like a few days. Algol is one such star. So, on the night of October 16^th, after checking the Sky and Telescope Algol predictor app, I head out to the observatory to record Alogl's variations. From my point of view, Algol (ra's Al-ghul, in Arabic) lived up to it's namesake as the Demon (ghoul) Star. Either the S & T app was incorrect, or my camera just couldn't record the variation, but I got 2 hours of images similar to the one below.

Algol, The Demon Star, Appropriate for October

The brightest star is Algol. And by my calculations, it never changed brightness significantly. It should have decreased in brightness by over a magnitude ( more than 2.5 times). It could have been my camera; the shortest exposure it can take is 0.1 seconds. Even with that short of an exposure, Algol is still out of the linear response area of the camera (a bad thing for recording variable stars) and very near saturation of the image (meaning overexposed). It was this result that lead me to try using the webcam for star exposures. It can make a much shorter exposure to prevent saturation. Problem is that I would need to do more rigorous testing of the camera to determine it's linear region. I'm not sure I'm ready for that.

Well, that catches me up on all the images for today. As we go into the winter months, we go from “globular season” to “open cluster season” to “galaxy season” and back to “globular season next year. Interesting, and some not so interesting nebulae thrown in for good measure along the way. We'll see what the sky holds for next time.