Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of Veeco Instruments (Nasdaq: VECO), which makes equipment used to manufacture solar cells and LED lighting modules, have fallen more than 13% today on several times the normal trading volume.

So what: Citigroup downgraded Veeco from buy to hold on concerns of a saturated market in China. If Chinese equipment orders come to a standstill, as Citi predicts will happen in the first half of 2011, Veeco's largest market would take a severe blow to its revenues and profitability.

Now what: Citi also downgraded Veeco rival Aixtron (Nasdaq: AIXG), which also got slapped with a second downgrade by Kaufman Brothers -- without taking nearly the damage Veeco's shares did. Then again, there is a massive short interest in Veeco but nearly none in Aixtron, which goes a long way toward explaining the two stocks' very different levels of sensitivity to the news. After today's haircut, Veeco's shares are still up 37% year to date yet reasonably priced at just 10 times trailing earnings. Veeco's gravy train will soon come to an abrupt end if Citi's misgivings are correct, but I think those fears are overblown -- and Foolish godfather David Gardner would agree.

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Fool contributor Anders Bylund holds no position in any of the companies discussed here. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool is investors writing for investors.