Imaging Messier Marathon: Imaging Focal Length

Last time I mentioned planning for an imaging Messier marathon (IMM), where the goal was to produce images that recreate the impression a visual observer would have looking through a 6" or 8" telescope. This time I'll look at the choice of a telescope for this task.

Focal Length:

Under ideal circumstances imagers try to choose an imagin focal length that best matches the target object. In the IMM that's not really possible without spending a lot of time on swapping gear around. We need to use one telescope. Since this is quick and dirty imagaing with little concern about "going deep," any reasonable focal ratio from f/5 to f/10 will do, with faster scopes being preferred.

More important in my judgement is the ability to frame the larger of the Ms in single shots. If smaller objects end up looking small, that's fine, for that simulates the eyepiece view.

Since I'm planning this for myself, my calculations will be based the relatively common KAF-8300 sensor (18x13.5mm) that is in my CCD camera. DSLR imagers with APS-C sensors will have somewhat larger fields of view.

You can follow along with what I'm doing if you have a good planetarium program. I recommend Sky Tools 3, a package that is very good for planning. Also helpful is this list of Messier objects sorted by size.

Let's get started by examining some of the Messier objects.

The Ms range from the enormous M31 (about 3°x1°) to the tiny M57 (105"x78"). 400-500mm is about right for the entirety of M31, while M57 probably looks best at 2000mm or longer. This is what M57 looks like at 400 and 700mm FL:

M57 @ 400 mm FL,  5.4um pixel size, enlarged 4X

M57 @ 700mm, 5.4 um pixel size, enlarged 4X

The first image is very pixelated, while the second is smoother. Surrounding stars are better seen in the 700mm image. (We don't care about the central star, since that isn't available to visual observers except using  very large telescopes.)

The second largest Messier object is M45 (M44 is almost as big). Here's what it looks like at 700mm:

M45 @ 700mm, 5.4 um pixels. Red box is FOV in KAF-8300

It fits very nicely. At 800 or 900mm we could still get most of the cluster in, but it would seem a little cramped. (Here we don't care about the nebulosity because it's essentially invisible to the eye of a beginner. We just want to capture the stars with enough dark space around them to make it look like a cluster.) 

We could continue, but there's a balance to be set, and it's a very subjective balance at that. Longer focal lengths favor the more abundant smaller objects, while longer focal lengths favor relatively few objects. Short er focal lengths give us a chance to capture several Ms in single frames, which helps us do the IMM. Longer focal lengths will eat up more time with getting targets centered in the camera's field of view.

 From the above I would suggest any focal length between 600 and 1000mm should serve well. 

The ultimate choice depends on the telescopes you have available to you. In my case only one scope fits: My TV102, which has a FL of 700mm with its flattener attached.

Next time I'll look at which Ms can be combined on single frames given a 700mm FL and KAF-8300 sensor.