According to Alan Dyer , "...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 :
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!
 Alan Dyer, Cameras in head-to-head showdown, SkyNews: 14-16, 37