What good is a $100,000 ultra-precision inverted microscope if it can’t hold focus? That’s the problem facing researchers involved in an increasing number of genetic experiments.

Fig. 1. Mercury’s 3500 encoder supplies closed-loop feedback for Mad City’s C-Focus microscope auto-focus correction system.
Sophisticated molecular probes make it possible to view and track the activity of discrete molecules within living cells. The resulting image sequences reveal the true nature of intracellular processes – critical information for drug discovery, disease prevention, biochemistry research, and a host of other life science applications.

To succeed, however, this research requires an inverted microscope to maintain its focus at very high precision to capture digital time-lapse photographs of the fluorescent markers.

In these experiments, the objective lens must remain within 100 nanometers of the focal plane to prevent the image from drifting out of focus during time-lapse photography. At this level of precision, thermal effects present major challenges. Even an ambient temperature shift of one degree Centigrade can cause the focus to drift by 200 nanometers.

Mercury 3500 Encoder Delivers High Resolution

Fig. 2. Mercury’s encoder provides the small size and performance required for a precision inverted microscope.
Mad City Labs, Madison, Wisconsin, has created a high-resolution, closed-loop motion solution to compensate for thermal expansion and other mechanical tolerances. Named the C-Focus auto-focus correction system, it was designed for microscopes capable of resolving 500 micron features, confocal imaging systems, and other systems with auto-focus mechanisms.

Providing the necessary closed-loop feedback is the Mercury(TM)3500 programmable encoder from MicroE Systems. This encoder has programmable linear resolution in integer steps from 5 microns to 5 nanometers. This miniature motion feedback encoder provides ±20-nanometer resolution for the C-Focus system.

Just as important as its high resolution was the small size of the Mercury encoder sensor. At 8.4mm tall, the Mercury 3500 was the only encoder small enough to fit within the cramped confines of the inverted microscope. According to Jim Mackay, Research and Development Engineer for Mad City Labs, “There’s just no space in an inverted microscope. Everything is space-critical.”

“One of the biggest challenges in designing the automated focusing element was finding an encoder that would meet the requirements of the application,” said Mackay. “For example, the encoder needed very high resolution in order to maintain focus on a virus cell of 50 to 100 nanometers in length. At the same time, the sensor needed to be very small to fit within the cramped confines of an inverted optical microscope. We looked at a number of encoders with similar performance, but their sensors were at least 15mm tall. That’s twice the height of the Mercury 3500 encoder sensor. Mounting these larger sensors would have been out of the question as the focal length of the typical inverted microscope is just 7mm. Only MicroE Systems had the solution we needed.” The low-profile encoder sensor makes it possible to retrofit the C-Focus system to virtually any inverted optical microscope.

Mad City Labs based the C-Focus design on its popular Nano-F100 piezo electric stage and Nano-Drive(TM) 85. The system uses the Mercury encoder sensor mounted on a very low CTE Invar bracket fixed to the body of the microscope. Parallel to this is the encoder’s low CTE glass scale, which rides up and down on the aluminum housing to which the objective lens is mounted.

Encoder Integration Enhances Design

To operate the system, the researcher would focus the microscope on an area if interest and hit the “Start” button. The CFocus system with its Mercury 3500 encoder would then provide automatic correction for thermal expansion and contraction.

“The Mercury encoder is easy to integrate because it is a kit encoder with the scale and sensor as separate, individually mounted components,” Mackay added. “The Mercury encoder was also the easiest to set up. It provides the broadest alignment tolerances, a ±2 degree sweet spot in the Theta Z-axis which is three times better than competing technologies. Plus, programmable interpolation integer steps, programmable output frequency, and automatic gain and phase correction circuitry make it very easy to optimize system performance.”

This article was contributed by Will Manning of the GSI Group., Madison, WI. For more information, Click Here .

Motion Control Technology Magazine

This article first appeared in the April, 2008 issue of Motion Control Technology Magazine.

Read more articles from this issue here.

Read more articles from the archives here.