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:
- The two doubles should be fairly close to each other in the sky;
- A series of images must be stitched together to cover the region from one pair to the other;
- 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.
60 x 15sec subframes x 6 fields mosaic
Unmodified Canon XSi at ISO1600 at prime focus
Baader Coma Corrector
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!