A tale of two Crescents

Thursday night, the 17^th, was another fairly clear night. So, out to the observatory I went. It was a night that I didn't have high expectations because of the 1^st crescent: the moon. For someone trying to image deep sky objects, the light from the moon causes the contrast to decrease significantly. Also, for us earthbound inhabitants, the sky is never really black. While it may seem inky black, it is only a very dark gray at best. That would be because of the lights here on earth that disperse in the atmosphere and light it up. It's called skyglow. OK, there are a few other contributors to skyglow, but man made light is the primary one. When I take an image, it's done by taking a series of images, usually with one each of 4 filters, for a certain length of time and then “stacking” them together. There is a stack for the red filter, blue filter, green filter, and a luminosity filter, which is basically clear, but it blocks infrared light. The 4 stacks are then combined to achieve the color image I present here on this blog. However, if I were looking through the eyepiece, all the images here would appear gray. See the entry for M20. So, if the moon lightens the background gray to the same shade as the nebula I want to see, the nebula effectively disappears. The longer light gathering time of a camera helps with pulling the nebula out of the background, but the fuller the moon, the brighter the background and the harder time I have pulling out the dim objects like a nebula.

Well, that brings us to the 2^nd crescent; NGC6888 or the Crescent Nebula. 

NGC6888

 

 

8” LX200GPS F10, QSI 683

LRGB 33,3,3,3

Imaged in Nebulosity 3

Post processed in Nebulosity 3 and Paint Shop Pro 7

(lots of 3's, huh?)

This image is a stack of 9 minutes each of red, green, and blue, and a stack of 33 minutes luminosity. (Most of the detail is to be found in the luminosity channel.) NGC6888 is formed by the fast stellar wind from the Wolf-Rayet star WR 136 (HD 192163) colliding with and energizing the slower moving wind ejected by the star when it became a red giant around 250,000 to 400,000 years ago. Thank you Wikipedia. At least, that's what I understand. The remaining star is call a Wolf-Rayet after Charles Wolf and Georges Rayet who discovered them in 1867. NGC6888 is about 5000 ly away in the constellation of Cygnus, the Swan (aka The Northern Cross). The reason for so many other stars in the image is because the Milky Way galaxy runs right through Cygnus. So all of those little dots of light are stars in our own galaxy.

Flying with the Eagles

Georgia has had a nice weather event lately; a passing cold front. A REAL cold front. The temperature has dropped about 20 degrees and the skies have cleared as much as I have seen them clear this summer. I have finally gotten a few good nights under the stars. No moon, Milky Way visible. Nice.

For the most part, the equipment has been working OK. I have imaged a few more things, most Messier objects. I got M17, M18, M69, M70, and M16. 

M18 is a open cluster. 

M18

It just looks like a bunch of stars to most folks. What's the big deal. Well, the deal has to do with the fact that all the stars are gravitationaly bound to each other and thus travel through space as a unit. Think of a school of fish in the ocean. I've read something that I currently can't remember about open clusters; something about their position is galactic evolution. I need to find that again, rather than giving out information that I'm not sure about. 

M69

M70

 M69 and M70 are both globular clusters. Same general idea as the open cluster, but obviously much more compact. They usually have many more stars, are usually much older, and generally orbit in the galactic halo, which means above and below the plane of the galaxy. At least that's what I remember about them. M69 and M70 are each only one photo, each a 1 minute exposure, and for the technically minded, only dark subtracted. They are interesting to image, since my eventual goal is to measure the light variance in variable stars. Imaging stars has it's own set of problems, compared to imaging nebulae and galaxies. Obviously, I hope, imaging variable stars will be more akin to imaging star clusters than nebulae.

M17 I've already processed.It's below.

 M16 I just finished, and is the longest exposure I've taken to date. 

M16

M16 is interesting for several reasons. First, it's an open cluster with an associated nebula. That makes the imaging harder, because the dynamic range is larger. What's dynamic range? It's the difference between the brightest object and dimmest object. Think of taking a picture of a flashlight with the sun in the same picture and trying to show the light coming from both. In this case, the stars to the upper right are fairly bright, but the nebula is really dim. There are several ways to solve this; mine was to set the exposure so that the stars did not reach saturation and then try to pull out the nebula in post processing. Since I consider myself a beginner at this, I was pleased with the result. Second, M16 contains the “Pillars of Creation.” See link. They are visible in my image, but the Hubble images in the link are the impressive ones.

Well, if we get some more clear nights, I'll see what else I can get. M31 is possible, but will be time consuming. It will have to be a mosaic; it won't fit on the sensor in one shot. It will be a multi-night imaging session and will probably be better later when it's higher in the sky. For now, I guess I had better go fix the leaky gutter. Yay.

PS. It just occurred to me to mention the "other" eagle; Aquila, the constellation. It contains the bright star Altair, one of the summer triangle stars. Aquila is not all that far from M16, relatively speaking, of course. They are both visible right now.

Two almost clear nights in two weeks!

And I took at advantage of the second one as well, at least for a couple of hours. It seems like this year has been the wettest or cloudiest one in recent memory. There has still been the persistent high cloudiness even on the “clear” nights, which makes the stars harder to see. I did some imaging anyway, this time of M17 and M18. These are located close to Sagittarius and close to each other; might as well take advantage of making a minimal telescope move. So far, I've been able to process only M17, which is also called the Swan nebula, Omega nebula or Horseshoe nebula. I'll let you decide if you can see it; I can't.

 

 

This image presented a different kind of challenge in the post processing. If you know about taking astrophotos, you know about “hot pixels” and thermal noise. If you are not familiar with the ideas, the easiest image I think I can make for you would be something like this: Imagine taking you camera and putting the lens cover on so that no light can reach the sensor (film, if you are thinking in terms of a film camera. However, this phenomena occurs only with digital cameras, so...). The expectation would be that the “image” would be completely black. It isn't, however. Close inspection shows white specks, like salt sprinkled on a piece of black paper. In actually, the image, if “stretched” (meaning putting the black and white points close to each other,more or less) would look like a snowy tv picture with white dots on it. The snow is the thermal noise and the white dots are the hot pixels. A hot pixel means that the pixel puts out too much voltage (meaning whiter) when hit be a photon. As more of an example, let's suppose that normally a photon that hits a pixel puts out 1 volt (it doesn't, this is just an example). If the pixel has 0 (zero) volts, it is completely black. If the pixel is hit by enough photons to allow it to reach it's maximum voltage, it would be 65,565 volts (or there abouts), and would be completely white. In between, we would see it as a shade of gray on a monitor. There is normally a direct relationship between the number of photons that hit the pixel and the voltage; if 20 photons hit, the voltage is 20 volts. If 1000 photons hit the pixel, the voltage is 1000 volts. With a hot pixel, it might be hit by 1000 photons, but instead of 1000 volts, it puts out closer to 65,000 volts. So, my problem was too many hot pixels in the photo.

So, what do they look like?

 

The red, green blue pixels circled are what they look like. Why red, green, and blue? My camera is a black and white (or monochrome) camera. To get color, I have to take series of photos with red, green and blue filters in front of the sensor, then, as part of the post processing, combine them to make a color image.

There are a few things I can do to help reduce them, like cooling the sensor more. I currently operate it at -5 degrees C, but I think I can get it much cooler. That's on my todo list. But what to do about the photo already taken? Well, in the post processing phase of working on the photos, (post, in this case referring to after the photo has been taken), there is a technique to help reduce the effect of noise by using a median filter. Think of if like this: the noise shows up as a white, or light colored dot of the monitor. If it were a drop of white paint on a black piece of paper, we could diminish the effect if we could smear the drop around the paper. The greater the area we can smear it over, the less noticeable it is. If the photo is 10 megapixels, and 100,000 are “hot”, that's a lot of smearing to do. There is another way to accomplish mostly the same thing. If I resize the image from 10 megapixels to 5 megapixels, the resizing algorithm has to throw away 5 megapixels. How does it chose which ones to throw away? I don't know the ins and outs of the algorithm, but part of it works like a median filter; it basically looks at all the pixels around a single pixel and “throws away” any ones that are vastly different from that pixel. That's how it helps eliminate the hot pixels. How well did it work? It eliminated about 90% or more of them. I think by using a cooler sensor and using this “trick” I should be able to make a major increase in quality of the photos, at least as for as the noise problem goes.

A long dry spell

Ironically, a long dry spell in astronomy is usually, but certainly not always, caused by a long wet spell meteorologicaly. If not wet, cloudy at least. Such has been the case for the past, almost 3 months at Starlight Observatory. However, Friday night, September 4^th, it wasn't AS cloudy. A few stars were visible, but there was a definite layer of haze or high cloudiness. Nevertheless, it was the best we've had for a long time. So.... Observatory opened, camera connected, security light out (more or less... it kept coming back on. Maybe the batteries in the laser are getting weak. What laser you ask? Why, the one that I point at the photoelectric cell that turns the light on and off. If I'm able to hit the cell with the laser light, the light goes off. It sounds easy, but try hitting a 1 inch diameter target from 40 feet with a light beam on a creaky old camera tripod; sometimes that is where most of the night is spent!).

This was also an opportunity to try something new; in this case, I was going to spend most of the time recording photons on the Luminosity channel, with a minor amount of time on the Red, Green, and Blue channels. But, alas, the air was not steady. We apparently had a thunderstorm to our northeast; I could see the horizon lit up by the lightening flashes. It would seem that the outflow from the storm was causing the unsteady air. As a means of “tracking”, I use a 2^nd camera to monitor the position of a star. I then feed that information to an “autoguiding” program, which, in turn, sends correcting signals to the telescope mount to keep the star centered in it's tracking “box”. The tracking program displays the errors in tracking, which, on Friday night, were in the 2 to 3 arc-second range. The actual movement of the star was probably 5 to 6 arc-seconds, so I felt pretty good about getting down in the 2 to 3 range. This is where having a spare $35,000 or so would come in handy; I could afford a mount that could track better, say to within 1 arc-second. I don't see that happening any time soon, though.

Anyway, time to shoot. The target was M20, also know as the Trifid Nebula. This is a primarily emission nebula in the constellation of Sagittarius. I managed to get 3 three minute exposures on the Luminosity channel, and 1 three minute exposure in each of the Red, Green, and Blue channels. These four channels would be combined later, in the computer to form one color photo. “Well, what does it look like?” you ask. See for yourself.

M20, September 4, 2015

8” LX200GPS F10, QSI 683

Shot thru high cloudiness

Imaged in Nebulosity 3

Post processed in Nebulosity 3 and Paint Shop Pro 7

This was an interesting, to me, experiment. I wanted to see how well I could image through the high clouds or haze or whatever it was. It's far from a ”perfect” picture; I'm not using the Hubble Telescope and don't have government funding for the computers and software. But, for an amateur with my experience level, I think it's OK. I can, and so can you, see the cloudiness show up as what looks like a hazy background color instead of a really black background. Tracking wasn't perfect, but not terrible, considering. (The stars look basically round, which is an indicator of good tracking.) The colors are at least in the ballpark of what they should be, but there is an artificial look to the nebula, caused by the processing trying to remove the effect of the cloudiness. There were other processing artifacts that I had to remove manually.

The last thing I thought I would try to demonstrate is what the nebula would have looked like if seen through and eyepiece, ie, you were looking through the telescope instead of me taking an image. The following is an attempt to show you what you would see.

M20 as seen though an eyepiece at the telescope

There are a few things to note. First, and foremost, it's hard to see. There is a reason astronomers call these things “faint fuzzies”. The word nebula comes from Latin for cloud. Whisp of a cloud seems appropriate here. Second, the sky is not black. That's because of 1) the cloudiness, 2) light pollution (meaning the lights of all the surrounding cities shinning up, into the clouds and into the 3) water vapor (in the air, of course). All 3 of these things scatter the light, making a dim object dimmer, as well as removing the contrast between the object and its background. Polar bear in a snow storm type of thing.

Well, that' it for this entry. Maybe late September and October will bring better weather for astronomy. Thanks for staying awake this long...