How big are Messier objects in pixels?

Now I am getting some more experience with some of the brighter Messier objects, I decided to have a look at the catalogue and see how bright and how large some of the objects are – basically what I can expect to see in my telescope.

All the copies of the Messier catalogue floating around on the internet state the size of the objects in arcminutes (1 arcminute = 1/60th of a degree, just like 1 minute = 1/60th of an hour). I don’t actually know what an arcminute looks like, nor how many of them will fit in my telescope’s field of view – which depends not only on the telescope but also on the size of the camera’s sensor. I decided to put together a quick spreadsheet to do a few vital calculations about the telescope and camera combo, and then to generate a version of the Messier catalogue that shows the size of the objects in pixels. For photographers, this is a much more familiar unit and can help decide which objects to attempt to photograph, and whether or not you should enable drizzling to boost the resolution.

If you’re keen, you can grab the spreadsheet right here. I recommend you read my brief notes below for best results (sorry, I know it’s dull).

Messier objects

Notes on how to use the spreadsheet

Go to the first tab, Constants. In the top grid, labelled Enter these constants, do as it says and fill the numbers in. My 6″ f/5 telescope has a focal length of 750mm and the sensor in my Canon EOS 600D is 22.3×14.9mm and 5184×3456 pixels in size.

Now look at the lower grid. Here are a bunch of statistics calculated from the data you entered. The useful figures here are the size of the pixels in microns (4.3μm in my case, pretty small compared to a dedicated astronomic CCD) and the X and Y field of view in arcminutes and degrees. This is the data we need to compare the size of the Messier objects to the size of my sensor.

Now flip to the second tab, Catalogue. This is a pretty normal version of the Messier catalogue except that as well as showing the width and height of the objects in arcminutes, it also shows it in pixels. Just to make it explicit, there’s also a column that tells you if the object will fit on your sensor or not.

I hope this quick spreadsheet proves useful to someone. Please do let me know if you spot any mistakes etc.

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