In Step 4 we are going to estimate the diameter of the comet’s coma.If you have read the instructions that are bundled with Kphot you will see that Kphot includes an estimate of coma size in it’s output file. I do not know why, but this is unreliable so the estimate of the coma size must be made manually (with the help of software).In my case, I stack the comet images using a median stack in Astrometrica to reduce noise and then analyse the stacked image using the “Line Profile” function in Maxim DL. You do not need to use Maxim DL; any software that allows you to plot the number of pixels in a line drawn through the centre of the comet’s nucleus will do the trick.So first we stack the images in Astrometrica using the “Stack Images” function. When you do this make sure to stack using the motion of the comet, by selecting 70P in the “Object” box of the Astrometrica pop-up called “Coordinates, Tracking and Stacking” as shown below:I have attached below a screenshot showing the result of stacking in my laptop. You will notice that the stars are now “trailed” because the stacking has centred on the comet.Save the stacked image and then open it in Maxim DL (or your equivalent software). Here’s a screenshot of the Line Profile function in Maxim DL, with the measurement of profile through the middle of the coma:The estimate of coma size is not exact. Roger Dymock provides advice on the BAA website referred to in Stage 1. In this case I estimate the size of the coma to be about 20 pixels by looking at how the number of pixels varies compared to the general sky background noise. You may get a slightly different number.Because the image scale of my system is 1.35 arc seconds per pixel, this is about 27 arc seconds. The COBS website requires the measurement to be in arc minutes to one decimal place so this is about 0.5 arc minutes, possibly 0.4.That’s the end of Step 4.