Ever since (even before that) I acquired my AT65EDQ refractor I have been listing Deep Sky Objects that should fit the frame of my 500D. With enough photographic or observing experience, I believe this becomes less relevant but when one is just beginning his Astro Imaging journey the question “What’s a good target for this equipment?” makes a lot of sense.
I played with programs like Google Earth and Stelarium for the longest time and I have always liked they both provide the Field of View that is displayed on the screen. I like having references to compare things, and this information is to me the best way to compare the Apparent Size of Deep Sky Objects.
Now, the question becomes what is the Field of View of my pictures? Well, it depends on two things: one is the Focal Length of the telescope and the other is the actual size of the imaging chip.
The Focal Length is a specification of the telescope and of any Focal Reducer that may be used.
The APS-C sensor in the Canon 500D measures 22.3 x 14.9 mm and has a crop factor of 1.6x. Each pixel on this sensor measures 4.7 um.
The Field of View (in degrees) is given by approximately: Sensor Size * 57 / Focal Length where Sensor Size is either the sensor’s width, length or diagonal. The plots below show the resulting Max (Sensor’s diagonal) Field of View when using a Canon APS-C CMOS sensor at different Focal length.
Alternatively you can also calculate the resolution in Arc-second per Pixel by the formula: Res. = 206.265 * Sensor Size (in um) / Focal Length (in mm).
The Field of View drops very quickly from 10 mm until about 750 mm and considerably slows down after that.
The image below is another way to illustrate this is to compare actual FoVs in function of the Focal Length.
This translates how much more details can be pulled of an image taken with a 2000+ mm Focal Length telescope. As an example, my M31 image almost fits entirely the FoV of my 420 mm Focal Length AT65EDQ. At 1600 mm or more the Galaxy’s core itself wouldn’t even fit in the frame.
Choosing the right target for the equipment: The way I do this is, read, read a lot about the different DSOs out there, I look up their apparent dimensions and see how well they fit the 3 x 2 degrees FoV of my AT65EDQ. So far, I’ve decided on the following targets: M42, M45, NGC2237 Rosette Nebula, M8 + M20 and I will keep adding to this list until I move onto a new scope!
