What is the best way to visualise the size of objects in astronomy images? How does one comprehend the size of objects when trying to photograph objects which may be more than ten degrees wide (in the case of the Large Magellanic Cloud) or just a few arc-seconds (in the case of many objects, like many planetary nebulae for example)?
Will they be worth shooting? Will they extend outside the FOV or will they just be tiny blobs in the middle?
I base my rough estimates on the Moon, which is known to vary in size from 29.3 to 34.1 arcminutes. As far as I am concerned, that is just over half a degree or maybe 32'.
So, curious as to which size of objects might be most suitable for imaging in my new 120mm Sywatcher Esprit triplet refractor, I took the following uncropped photo of the waning gibbous Moon:
Knowing that the Moon diameter is about half a degree, I was able to estimate that the size of the images in my f/7 refractor telescope will be about 1.5° x 1.0°.
Using a 2X Barlow would reduce the field by a half, or about 45' x 30'.
A 4X Barlow would further reduce the field to about 22' x 15'.
By way of comparison, earlier in the year, my old LX-90 Schmidt Cassegraine f/10 telescope, with a 0.62 reducer, took this uncropped image of the Moon:
From this I deduced that my Meade LX-90 operating at f/6.2 would produce images approximately 0.75° x 0.5°.
Finally, some time ago I took the following image with the LX-90 at f/10 with no reducer:
The deduced size of images produced by the LX-90 at f/10 was therefore roughly 33' x 22'.
Knowing the field of view is very useful information when researching various objects to image next time you go out in the field.
All images were taken using my Canon 60D camera.
Images © R.Powell