It is possible to create great espresso without expensive accessories, but there are some accessories that might qualify as luxuries, but that can be a huge help. A grouphead thermometer is one of those items.
For those who want to know the temperature of the espresso they are brewing there are a few different approaches. One, of course, is to guess. This is not a crazy method especially if you learn the subtleties of your own machine inside and out. This is, in fact, my favorite option, because for me the key question is would the taste of my espresso be better at this temperature, a higher temperature, or a lower temperature. As long as I can answer that question, and then I can figure out how to make my machine create a shot that follows my prescription, it does not actually matter if the temperature is 197 or 205 or more precisely it does matter what the exact temperature is, but it does not matter if I know what that number is.
The flaw this form of thought is that if I want to pull a shot and then talk about it with a friend or on a blog and then someone else wants to recreate it, it is not much help if I proclaim that the ideal temperature is just a little hotter than I started out and that on my machine this is how I controlled the temperature, because it would be impossible for someone else to reproduce the shot. Even if they had the same machine it might not be set up exactly like mine, so knowing exactly what I did would not mean they could reproduce it.
One option is to install a PID in the machine that controls the temperature, but that still does not tell you what the temperature at the coffee bed is because it reads the temperature at the boiler and ambient conditions can change the offset between the setting at the boiler and at the group.
On the other hand Eric Svendson invented a clever thermometer that can be insterted in the grouphead of some machines to read the temperature not far from where the water enters the puck. This is a fairly accurate prediction and allows very good control of temperature. The device only works on some machines (sadly while several of my friends can use these, my machines cannot, for example) but it includes machines like the E-61 semicommercial machines and lower priced Rancilio Silvia.
The thermometer is available from Roaste sponsor Chris coffee or directly from Eric.