The output voltage of an astable multivibrator is not a perfect square wave; the voltage when viewed from an oscilloscope tends to resemble a curve. I'm told that this is because the voltage across the capacitors cannot change immediately and therefore the R1 capacitor current somehow disrupts the voltage waveform at the transistor collector. Apparently this can be fixed by isolating the capacitor from the collector using a diode:
Now C1 charges through its own resistor, R2. But I don't understand how the R1 current in the first picture disrupts the collector voltage. When Q1 turns off it's off and therefore above ground. How does the capacitor alter this? And when the diodes are included, how is the situation different? To me it seems that the collector is simply around 0.7 volts lower than the capacitors R2 plate.