The Planetaries of the Summer Triangle

Planetary Nebulae are the remnants of Sun-like stars. When our Sun is going to run out (literally!) of hydrogen gas in its core in approximately 5 billion years it will first increase in size up to 100 times its current diameter and then shed its outer layers while shrinking to the size of a planet like Earth and "retire" as a white dwarf star. The inner planets Mercury and Venus will be destroyed in the process. Mars will certainly survive whereas the destiny of Earth is uncertain. Some computer models indicate that Earth should make it, but the proximity to the giant dying Sun will obliterate any form of life still present at that time.

The material ejected by the dying star gives birth to a compact structure called Planetary Nebula. The name is misleading: these nebulae have nothing to do with planets. 18th century astronomers coined the term "planetary nebula" because of the similarity in appearance of these objects to giant planets (albeit much fainter!) when viewed through small telescopes.

Despite the grim Grand Finale, Planetary Nebulae constitute one of the most beautiful type of objects populating the night sky. The elaborated and colourful structure of Planetary Nebulae has been popularized in several famous images taken by the Hubble Space Telescope.

Three of the most famous Planetary Nebulae are located in the region of the Summer Triangle, the portion of the sky within the imaginary lines connecting Vega (α Lyrae), Deneb (α Cygnii) and Altair (α Aquilae). They are the Ring Nebula (Messier 57), the Blinking Nebula (NGC6826) and the Dumbbell Nebula (Messier 27):

I decided to image the three planetaries using my MallinCam HYPER Plus. Summer solstice has just gone by and Edmonton is immersed in perpetual twilight. Whatever is left of the night is very short and not exactly very dark. From my backyard on a moonless night, "darkness" these days lasts between 1am and 3am. The advantage of the MallinCam is that it allows for very short imaging sessions thanks to its superior sensitivity. My Canon DSLR is superior when it comes to overall image quality (after processing), but it requires data collection sessions many minutes long, very good polar alignment, autoguiding and calibration frames (darks and flats). No way I can image three targets in only two hours and so late during the night (I do have to go to work in the morning, after all...). With the MallinCam each planetary nebula was imaged by recording a 1000 frame video (AVI format) which, at a 30 frame per second rate, lasted about 33 seconds! So total imaging time: about 1 minute and 40 seconds. Not bad! The vast majority of the time was spent star hopping to the targets since my Losmandy G-11 is not equipped with a go-to system. The biggest challenge with the MallinCam is to find the right combination of settings to get the most out of your telescope. The focal ratio is definitely the most important parameter, but so is the brightness of your sky. At any rate, the best way to find the optimal settings for your specific situation is to try.

I started with the Ring Nebula (Messier 57). Using a 25mm Plossl eyepiece in my 10" f/4.7 Newtonian, I zeroed in on β Lyrae which, at magnitude 3.4, is visible naked eye in my 4.2 magnitude skies. Using Cartes du Ciel and superimposing the field of view (FOV) of the 25mm eyepiece, I could see that if I placed β Lyrae close to the border of the FOV, M57 should be visible at the same time:

So no hop in this case. After centering on the nebula and carefully replacing the eyepiece with the camera, I began to play with the settings. Focusing wasn't an issue simply because I had already determined in the past where the best focus is on a bright star like Vega and putting a mark on the draw tube of the focuser. By the way, I use a Pinnacle Dazzle framegrabber to stream video frames from the MallinCam to my laptop. To display the frames I use
VirtualDub. Anyway, the first thing I did was to set SENSE to 128x at 2.1sec integration (I didn't touch the switches on the side of the camera). That was barely enough to make out the contour of the nebula, so I bumped the integration to 7sec by acting on the corresponding switch on the side of the camera. At that point the Ring became obvious, although quite dim against the bright background sky that looked blue on my laptop because of twilight (it was past midnight!). Then I set AGC (the gain) to MAN (manual) and bumped it up one notch. The camera is very sensitive and if the gain is set too high, the live image displayed on the laptop would start flashing. So baby steps...As soon as I moved the gain up on notch, I noticed that the image would get a green cast for about 7sec, then disappear for 14sec and eventually display a stable image. This behaviour has been reported and discussed a number of times on the MallinCam Yahoo!Group. You just have to let the camera adjust to the increased gain, that's all. Once the image was stabilized, I moved up the gain another notch and so on until I would get the best possible image in terms of noise, colour and detail. In my case I moved the gain two notches to the left of the midline. At that point the background sky looked really bright, so I set GAMMA to 1.0 and that solved most of the problem. As I mentioned earlier, a 33 sec AVI was captured using VirtualDub before moving to the next subject, the Blinking Nebula (NGC6826).

To get to the Blinking I reverted back to the 25mm eyepiece, slewed to δ Cygnii (visible naked eye) and then hop from there. I noticed that δ Cygnii and the Blinking have almost the same Right Ascension so hopping was easy: I just kept slewing in one direction and checked once in a while how far I was to make sure I would not go past the target:

Again, replacing the eyepiece with the camera, quickly achieving focus and using the same set of settings for the Ring, the Blinking (which blinks only visually) showed up on the computer screen. The main difference between the Ring and the Blinking is the relative size: the Blinking is so small! After collecting a 1000 frames video file, I put back the 25mm eyepiece and slewed to γ Sagittae (3.5 magnitude) to bag the last target of the night: the Dumbbell. The hop was also quite easy in this case, since the Dumbbell and γ Sagittae are very close in Right Ascension:

The first thing I noticed when I put the camera back and started collecting images on the laptop screen was how much dimmer the Dumbbell was as compared to the the Ring or the Blinking. The integrated magnitude is lower (7.5 against 8.8 for the Blinking and 9 for the Ring), but the Dumbbell is much bigger so its magnitude per arcsec square is higher. In other words, on average an arcsecond square of the Dumbbell is dimmer than an arcsecond square of the other two planetaries. To make up for the lower surface brightness I had to increase the integration of the MallinCam to 14 seconds and lower the gain a couple of notches.

Here are the images:
Ring Nebula (Messier 57)
Blinking Nebula (NGC 6826)
Dumbbell Nebula (Messier 27)

Remember: these images are the result of only 33sec data collection sessions! Each of them was obtained by aligning and stacking all the frames in their respective AVI files using Registax 5 and then processing the final images in Photoshop.

It is intersting to notice that the surface magnitude of the three nebulae is quite different:

Name	Dimensions (arcsecxarcsec)	Integrated Magnitude	Surface Magnitude
Ring	84"x60"	9	18.25
Blinking	27"x24"	8.8	15.83
Dumbbell	480"x336"	7.5	20.52

The images above confirm that the Blinking has indeed the highest surface brightness: it appears almost overexposed. The Dumbbell is definitely the dimmest: 14sec integration were not enough to get the same brightness level of the Ring. An interesting question to ask would be: why is the surface brightness of these three planetaries so different? Has that anything to do with their age? Initially I thought that perhaps young, smaller planetaries would be brighter than older, more diffuse ones. The implicit assumption I was making was that the overall brightness of the nebula remains somewhat constant over its lifetime. Well that does not seem to be the case. A quick research showed that the evolution of a planetary nebula is a very complicated business in which a multitude of factors contibute to determine chemical and physical characteristics of this class of objects. So let's leave the answer to the professional astronomer and ask a more mundane (but interesting nevertheless) question: what would the angular size of these three nebulae be if they were placed at exactly the same distance from us? Ignoring for a moment the uncertainty in their actual distances, it turns out that the Blinking is located at 2,000 ly from Earth, the Ring 2,300 and the Dumbbell 1,360. Since the angular dimension is inversely proportional to the distance, if we placed the Ring and the Dumbbell at the same distance of the Blinking (2,000 ly) then the Ring would look a bit bigger (15%) and the Dumbbell 32% smaller. Here's a comparison of what they look like in the sky and what they would look like if they were all 2,000 ly away from us.

Cheers!

The Lyra Challenge: From Delta to Zeta

The constellation of the Lyra is a treasure trove for double stars aficionados. α, β, δ, ε, ζ, η, to name some of the best, are real showcase pairs bright enough to be visible through a small telescope or binoculars. Like double star visual observers, astroimagers can have a great time in Lyra. To prove my point, here's an image of δ Lyrae I took recently.

In a recent post I described a technique based on eyepiece projection to obtain high magnification, narrow field of view images of double stars. In this post I am going to talk about wide(r) field of view double star imaging. The image of δ Lyrae mentioned before is an example. The field of view spans roughly 30'x55'. It was taken with a stock Canon XSi at prime focus of a 10" Newtonian f/4.7 (1200mm focal length). This type of imaging is complementary to eyepiece-projection technique I indicated in my other post, which provides images less than 1 arcminute across.

While the eyepiece projection technique showcases the double star in isolation to underline the colour and brightness difference of the two components in relation to their angular separation, the technique I am going to talk in this post portaits the pair in the context of the region of sky which is surrounded by. For the benefit of the reader, I'd like to point out that narrow and wide are terms relative to my equipment. For example, a shorter focal length telescope is capable of wider field of views than mine. On the other hand, I am not planning to buy another telescope so I am trying to get the most of what I have which, in turn, drives the type of projects and challenges I embark on.

Speaking of challenges, wide(r) field double star imaging lends itself to an interesting one: capture two unrelated double stars on the same photograph showing the components of both pairs as clearly separated stars. Even if my camera at prime focus of my telescope provides a fairly generous field of view, it is not nearly enough to fit two of the most popular double stars in the same frame. I guess it is possible to find two very dim pairs that might fit into a single frame, but they would lack the wow! factor...

Three things have to happen to capture two double star on the same photograph using my setup:

1. The two doubles should be fairly close to each other in the sky;
   
2. A series of images must be stitched together to cover the region from one pair to the other;
3. The focal length of the telescope must be long enough to provide enough magnification to separate the components of the pairs.

Obviously the closer the two doubles, the smaller the number of frames required. On the other hand, the longer the focal length the smaller the field of view and the greater the number of frames to cover the sky that spans from one double to the other.

One comment about the telescope: using a 10" aperture imposes a constraint on the minimum focal length, particularly on a $400 tube which limits the size of the field of view captured by the camera; however, it also makes imaging a little bit easier because the exposure time for each individual frame can be kept pretty short: a 10" mirror is a respectable light bucket!

Now, since I am currently interested in the doubles in Lyra, I thought that δ and ζ would provide a nice opportunity for the type of challenge outlined above. δ and ζ form the top side of the parallelogram of Lyra, right below Vega:

In order to minimize the risk of ruin my little project I started with a bit of planning to determine the position, orientation, overlap and number of frames required to cover the sky from δ to ζ. To do that I opened my favourite planetarium software, Cartes du Ciel ver.3.0 (http://www.ap-i.net/skychart/en/download) and set the Finder Rectangle (click Setup on the menu Bar, select Display and then click on the Finder Rectangle tab) to the field of view of my Canon:

By right-clicking on the star chart and selecting New Finder Circle, a rectangle is displayed to show the field of view of my Canon Xsi:

Notice that the sides of the rectangle are parallel to the Equatorial Grid (more of that in a moment). If we want to create a mosaic of frames to cover the region of sky that goes from δ to ζ, we need to make sure that two adjacent frames overlap to some extent. Here's an example with six frames overlapping by 50% with each other:
Enough planning! Now it's time to start imaging. The first thing to do is to position the telescope so that the camera can cover the first field of view in the sequence of frames from δ to ζ. To do that I typically start by inserting a 25mm Plossl eyepiece (which provides a field of view similar to the Canon, although circular) and adjust the position of the scope until it's more on less on target for the first frame. Then I replace the eyepiece with the camera and turn on Live View. I focus the best I can on the brightest stars I can see and then adjust the rotation of the camera in such a way that the sides of the sensor are parallel to the Equatorial Grid. To do that I position one of the brightest stars displayed by the Live View close to one of the edges of the field of view and move the scope along one of the coordinates using the hand paddle. I keep rotating the camera until slewing the scope in one direction makes the star follow the edge of the field of view exactly. Now that the camera is properly oriented, I slew the scope until the exact field of view associated with the first frame in our mosaic is covered. I snap a couple of shots to make sure we are in the correct position, that focus is close to perfect and to determine the best exposure/ISO setting. In the case of this project, I set the exposure to 15sec and I took 60 subframes for each field of view required to complete the mosaic.

But why rotating the camera to make the sensor's sides parallel to the Equatorial Grid? That's because moving from one field of view to the next is a tricky operation. If the sides of the sensor are parallel to the Equatorial Grid, then it is sufficient to slew the scope in only one direction (Declination, say) instead of two (Declination and R.A.). In the case of this project, once 60 frames of the first field of view were collected, I just had to slew the scope in Declination until about half of the frame had disappeared at the bottom and another new half had entered the field of view from the top.

Interestingly enough, a 15 sec exposure from my backyard is already dominated by light pollution. Individual exposures do not appear to be overwhelmed by light pollution, but when looking at the histogram we can see that there is a sizable gap between the peak and the left hand side:

That means that read-noise is not very important and that 60 subframes 15 sec each are basically equivalent to a single 900 sec (15 min) frame (http://www.samirkharusi.net/sub-exposures.html) in terms of Signal-To-Noise (SNR) ratio.

The software provided by Canon allowed me to set the number of exposures (60), their length (15 sec) and to capture all the frames automatically without me pressing the shutter 60 times...obviously the camera must be connected to the computer where the software is installed, but that can be achieved by using the USB cable provided by the manufacturer.

The imaging session unfolded like this: start the first sequence of 60 subframes, come back after 15 min, slew the scope in Declination until the field of view would match the one associated with the second set of subframes, start the sequence, come back after 15 min and so on until all the frames of the mosaic were completed.

I began processing the data the day after . I knew that mosaics can be quite difficult to put together and I never did one before. Certainly it's not been an easy task. I had to retrace my steps several times until I got it more or less right. The main challenge was to ensure that there were no gradients from one frame of the mosaic to the next. Here's an example of two frames of the mosaic aligned together. As you can see the seam between the two sticks out like a sore thumb!

An article by Robert Gendler (http://www.robgendlerastropics.com/Article3.html) clarified how to make the seams between images impercetible. Non-linear stretching is the key: playing around with curves in Photoshop solved the problem. That said, it was not exactly an easy thing to do. It required me quite some trial and error before being satisfied with the result. Once problem with the seams had been resolved, a couple of passes using the excellent tool GradientXTerminator by Russel Croman removed any residual gradients in the entire mosaic. Standard application of levels and curves helped bring out the fainter stars. Using the technique outlined by Jerry Lodriguss on his website (http://www.astropix.com/HTML/J_DIGIT/STARCOLR.HTM) helped with the colours of the stars.

The final image is posted here

To summarize:
Image

60 x 15sec subframes x 6 fields mosaic

Camera

Unmodified Canon XSi at ISO1600 at prime focus

Baader Coma Corrector

Processing

Each set of 60 subframes aligned and stacked using DeepSkyStacker

Mosaic composed in Photoshop (Levels, Curves)

GradientXTerminator for gradient removal

FocusMagic for sharpening


Now something about the two doubles. δ Lyrae is an optical double, meaning that the two components, δ1 (a 5.6mag, B2 blue giant 1080 light years away) and δ2 (a 4.2mag, M4 red giant 900 light years away) are not physically bound by gravity. However, the two star might be outlying members of an open cluster called Stephenson 1. The two stars are intrinsically very bright: if placed at a distance of 10 parsecs they would shine with magnitude -2 to -3, which is more or less the apparent magnitude of Jupiter (for comparison, the Sun would look like a 4.8mag star at a distance of 10 parsecs). ζ Lyrae is a physical double and interesting information about this double can be found at: http://www.astro.illinois.edu/~jkaler/sow/zetalyr.html

So what's next? Well I would like to apply this technique and capture the portion of the sky that from ζ reaches Vega and then ε (the famous Double-Double). It would certainly be nice to capture the entire Lyra constellation in one giant mosaic!

Cheers!