The Colours of the Stars

As I mentioned in my previous post about The Eight Moons of Saturn , one of my favourite Summer projects is imaging double stars using a Digital SLR camera.

According to Alan Dyer [1], "...digital single-lens reflex (DSLR) cameras have several key features that make them particularly desirable for nighttime photography. First and most important, their large sensors offer much lower noise and cleaner images than do compact point-and-shoot digital cameras, especially at ISO 400 and higher". Dyer's investigation is targeted at long-exposure astrophotography, but the previous sentence made me believe that DSLRs might do a better job at imaging double stars than conventional webcams, particularly around colours. There is one problem. A large sensor implies a large Field of View (FOV), which is the linear dimension of the portion of the sky captured by the camera, but the most popular double stars are usually less then 1 arc-minute wide, with more than half separated by less then 15 arc-seconds [2]:

The FOV of a DSLR camera at prime focus (attached directly to the focuser of the telescope: in this configuration the telescope itself is the lens of the camera) is determined by the following formulae:

FOV (horizontal) =
206265 x Sensor size in horizontal direction in mm/ focal length in mm (1)

FOV (vertical) =
206265 x Sensor size in vertical direction in mm / focal length in mm (2)

For example, the sensor size of my Canon XSi (a very popular DSLR) is 22.2mm x 14.8mm. The focal length of my 10" Newtonian is 1200mm. If I used my DSLR at prime focus, the camera sensor would be able to image a portion of the sky 3815 x 2544 arcseconds wide, more than 100 times wider then the typical separation of the components of a double star. The conclusion is that the large sensor size of DSLRs make double star appears very small, especially when the camera is used at prime focus of telescopes of relatively short focal length.

One way of boosting the effective focal length of an optical system is to use a technique called Eyepiece Projection. In Eyepiece Projection the camera lens is removed, but the eyepiece is left in place. A special adapter is required to connect the camera to the focuser and to hold an eyepiece at the same time, as shown in this image:

My adapter is a 1.25" Variable Universal Camera Adapter sold by Orion. In Eyepiece Projection the effective focal length is given by:

Effective FL = Telescope FL ×Amplification Factor (3)

Where the amplification factor of the telescope-eyepiece-camera system is given by the following formula:

Amplification Factor = S / Eyepiece FL - 1 (4)

S is the distance from the eyepiece to the CCD chip and Eyepiece FL is the focal length of the eyepiece.
For example, if I use my Orion adapter with a 10mm eyepiece, then S = 95mm and according to (4) the amplification factor will be 8.5x. That means that the effective focal length of my setup when I use my 10" Newtonian f/4.7 will be 10,200mm. The FOV will be reduced in both directions by the amplification factor. So the prime focus FOV is reduced (in thise case) by 8.5 times and for my Canon that turns out to be: 449 x 299 arcseconds. Still a little too big, but a Barlow 2X or 3X should solve the problem.

Here’s an image of Albireo obtained using the Eyepiece Projection method with a Canon XSi, a 10mm Plossl eyepiece, a Barlow 2X and a 8in Newtonian f/4.9. :

Notice that the design of the Orion adapter is such that the eyepiece does not slide inside the focuser; the eyepiece sort of “hovers” on top of the focuser. That increases the amplification provided by the Barlow lens, boosting it to 3.2X. The effective focal length of the optical system is then 1200mm x 3.2 x 8.5 = 32,640mm and the focal ratio f = 32,640mm/254mm = 128!! The image above (obtained with my 8" Newtonian f/4.9) is not cropped and the field of view is about 168” x 112” (Albireo’s separation is 34”).

Here's a list of double stars that I imaged over the last two year using the technique outlined above:

• Alpha Geminorum - Castor
• Alpha Herculis - Rasalgethi
• Beta Cygnii - Albireo
• Delta Cephei - Alrediph
• Epsilon Lyrae - "Double-Double"
• Eta Cassiopeiae - Achird
• Gamma Andromedae - Almach
• Gamma Delphini
• Gamma Leonis - Algieba
• Xi Bootes

In some cases (Almach and Achird) I experimented with higher magnifications, in others (Double-Double) I had to stitch together two frames. I usually collect a number of frames and then align and stack them in Registax. The number of frames vary, between 20 and 60 typically. The exposure time is between 0.5 and 3 seconds, depending on the brightness of the target as well as the angular separation and the magnitude difference of the two components. ISO is usually set at 800, although I experimented at 400 and 1600 on a couple of occasions. During processing I saturate the colours a bit, but that's pretty much it. The main enemy is seeing as always when imaging at high magnifications. What I noticed (so far) is that resolving angular separations smaller than 2" is very challenging: that's what the seeing allows most of the time from my backyard in Edmonton, AB.

The same technique can be employed on small objects like the outer planets. Here's an image of Uranus, for example.

The goal of this year is to get better at processing stacked images, particularly around reducing "flaring" caused by mediocre seeing. At the moment I got to the point where I can use eyepiece projection with a DSLR consistently which is the starting point to get better.

The new season has started. More gems to come!

Cheers!

References

[1] Alan Dyer, Cameras in head-to-head showdown, SkyNews: 14-16, 37

[2] http://www.astroleague.org/al/obsclubs/dblstar/dblstar2.html

The Eight Moons of Saturn

After dabbling with Deep Sky astrophotography during the Spring season, much shorter nights due to perpetual twilight suggest it's time to slew to different targets and find new challenges. I am pursuing two projects at the moment:

1. High-resolution imaging of double stars with a DSRL;

2. Imaging of planets'moons using a MallinCam HYPER Plus videocamera.

More on 1. in a future post. Here I am going to focus on 2.

These days the only planet high enough in the sky at a decent hour is Saturn. The ringed planet is getting closer and closer to the Western horizon as days go by, but it still provides excellent opportunities for the avid astrophotographer. Outstanding images of the transit of Titan's shadow taken by Denis Fell (http://spacealberta.com/saturn/sat2009/saturn2009.htm#titan3) are a great example.

Imaging the moons of Saturn is another interesting challenge. Titan, at mag.8.4 is relatively easy even with an unmodified webcam, provided that your scope has enough aperture. Here's an image I took in 2007 using a Philips Vesta 675 and an 8" Newtonian f/4.9 through a 2X Barlow:

However, anyone who had the chance of peeking into a medium-sized aperture telescope (6" or more) will remember the other smaller and dimmer moons accompanying Titan in its dance around the planet. The casual observer is usually able to pick up three more: Rhea (mag.9.8), Tethys (10.3) and Dione (mag.10.5), depending on the severity of light pollution. There are four more that require more expertise and darker skies to be observed: Iapetus (mag.10.1-11.9), Enceladus (mag.11.8), Mimas (mag.13) and Hyperion (mag.14.5). Iapetus is an interesting one because it is much brighter at Western elongation than at Eastern elongation. That's due to the peculiar coloration of the moon's surface: dark as coal on one hemisphere, bright as snow on the other. At the moment Iapetus is close to Eastern elongation and it looks quite dim (mag.11.2).

The challenge is not only to see the dimmer moons, but also to see as many moons as possible in one single observation. To see many moons at once, timing is essential since each moon has a different orbital period. Chances are that there is always one moon that is either transiting in front of the planet or being occulted by the planet itself and so impossible to detect. Then there is light pollution. I usually image from my mag.4 sky (in winter at the Zenith) from my backyard in Edmonton. Anything dimmer than mag.11 proves to be visually very challenging (at least for me) even when looking through my 10" Newtonian f/4.7.

I own a MallinCam HYPER plus (http://mallincam.tripod.com/) videocamera, which was designed for Deep Sky real time imaging and so extremely sensitive. I thought that perhaps I could "enhance" real time views of the Saturnian system using the MallinCam and capture several moons at once. The camera allows for integration times up to 56sec (with 7, 14 and 28 being the other choices) during which frames are collected and added together depending on the exposure setting. Initially I thought that imaging the dimmer moons would not be a problem at all with the MallinCam given that the central star (mag.15) of the Ring Nebula (Messier 57) is visible from my backyard. Then I realized that the longer the integration time, the brighter and bloated Saturn appears. So if one of the moons is too close to the planet it will wash out in the glare. That imposes an additional constraint if we want to observe as many moons as possible at the same time: moons have to be far enough from the disk of the planet and the plane of the rings to be detectable by the camera. Finally there is the altitude factor: the lower the planet is over the horizon, the worst seeing becomes and the larger is the airmass extinction coefficient (that dims objects low on the horizon further).

Using the excellent planetarium software Cartes du Ciel (http://www.stargazing.net/astropc/), I zeroed in on the Saturnian system and fast forwarded in time looking for the time when as many moons as possible would be available for imaging. May 24th at around 0:48 proved to be a good candidate for an attempt so I setup my 10", inserted a 2x Barlow and plugged in the MallinCam. The angular distance between Hyperion and Iapetus was too big and I ended up imaging Iapetus separated from the rest. The most challenging moon proved to be Mimas. Not only at mag.13 is quite dim, but at the time of the observation it was also very close to the rings. Mimas was actually emerging from the glare of the planet, so the longer the wait the better, but at the same time Rhea was diving deep into the bright disk of Saturn, so it turned out to be a trade off. I waited as much as I could to image Rhea hoping that by then Mimas would finally be in plain sight. As I said before, to image a dim moon like Mimas I had to increase the integration time to 28sec which extended the outer boundary of Saturn to the point where I am not entirely sure I was able to pick up Mimas on the laptop screen. By then Saturn was hovering quite low on the Western horizon and that made Mimas'detection even more difficult. I think I caught a glimpse of the Moon just off the Western tip of the rings during those rare fleeting moments of acceptable seeing. Keep in mind that when I say "caught a glimpse" I mean by looking at real time images displayed by the Mallincam on my laptop screen. The software I used to display and record the frames streamed by the camera to the laptop is the excellent VirtualDub (http://www.virtualdub.org/).

The final image is posted here

It is a composite of four images: one for Saturn alone, one for the brighter moons, one for Hyperion and Mimas and one for Iapetus. Each image was obtained by recording a 2000 frame video file in VirtualDub and perform align/stacking in Registax 5.

Thank you for watching!