Magic wands

Clive’s Corner by Clive Bagshaw

A feature of MicroNews, Clive’s Corner is a place created for the sharing of knowledge, tricks, and tools. The Corner is where you read about clever microscopical hacks - and submit your own. Clive’s Corner is the namesake of SFMS Member Clive Bagshaw, who has spent a lifetime looking into microscopes - including 50 years studying protein reactions. 

In this Corner: This is installment #4 of Clive’s Corner, in which Clive describes how microscopists can put a vacuum cleaner to proper use.

One of the simplest tricks that a savvy microscopist can do is to remove an eyepiece and invert it to make a magnifying glass. This is convenient for looking at the surface of an objective lens to check whether cleaning is warranted (Fig. 1a). If an inverted eyepiece is held above another eyepiece, the combination of lenses works as a phase telescope to examine different focal planes within the microscope tube. Typically, this device is used when aligning a phase condenser to the ring within a phase objective lens. However, even for non-phase optics this is a useful hack. When a condenser is first installed on a microscope, it is important to center it to the optical axis of the objective lenses. This can be done by removing an eyepiece and looking at the nearly closed condenser iris through the microscope tube and objective lens (see Clive’s Corner #1). The edges of the condenser iris need to be centered within the field of view. This process is more precise if a phase telescope, or two eyepieces are used because the iris image at the back focal plane of the objective can be brought into focus. The challenge in the latter case is holding the inverted eyepiece on axis and at just the right distance to bring the objective back aperture and condenser iris into focus. Enter the magic wand.

Figure 1a. Using an inverted 10X eyepiece to examine an objective lens.

Figure 1b. Using two eyepieces, connected by a vacuum cleaner tube, to make a phase telescope.

Figure 1c. “Exploded” view of phase telescope.

For several years we had an old vacuum cleaner stored in our garage – just in case our replacement one failed. But it was only a year or so ago that I noticed the associated extension tubes looked like a useful diameter to make optical devices (after a thorough cleaning of course). Typically these “extension wands” as they are called, are tapered and have an outer diameter of 1 ¼ inches  (32 mm) diameter at the narrow end, which fit into a second wand or the vacuum cleaner hose connection. The internal of the tube tapers from 32 to 26 mm which spans the outer diameter of my compound microscope eyepieces (28 mm). From linear algebra, I estimated where the tube internal diameter would be a snug fit around the eyepiece and cut out a 75 mm length from the wand. Sliding one end over the 10X eyepiece and inserting an inverted 10X eyepiece in the other enabled me to view the condenser iris with ease (Fig. 1b,c). By sliding the inverted eyepiece within the tube, different parts of the optical path could be brought into focus - and revealed some dust which had accumulated on the objective lens and on the prism surface within the binocular head. Not what I was looking for, but it suggested a cleaning was in order.

Figure 2a.

Vacuum cleaner tube for C-mount camera connection.

Figure 2b.

USB camera mounted on a vacuum cleaner tube at the primary image plane

Other parts of the vacuum cleaner wand proved useful. The larger end (outer diameter 38 mm), with a couple of turns of electrical tape around it, was a snug fit into the binocular head flange of my Amscope compound microscope (Fig. 2a). Using an appropriate length of the wand to maintain parfocality (10 cm in my set-up)  with a C-mount adapter glued into the end of the tube, enabled a USB camera to be mounted in place of the ocular head (Fig. 2b). Why would you want to do this? Well, on my inexpensive Amscope trinocular microscope, the light is split 50:50 between the eyepieces and the camera (i.e. each eyepiece receives 25% of the light). For brightfield work this is no problem, but when the light intensity is reduced (such as with darkfield or fluorescence), it can be helpful to direct 100% of the light to the camera or a single eyepiece to get the brightest image. More expensive microscopes have a slider to direct 80% or more to the camera or 100% to the eyepieces, but this option is not available on many student microscopes. So, switching ocular headpieces provides a way to maximize light throughput in any one channel. 

Another adaptation was to glue a 37 mm diameter filter ring onto a length of vacuum cleaner wand, which allowed a wide-field eyepiece to be incorporated. The particular eyepiece I used (EagleEye) was an old accessory from my terrestrial telescope and was designed for attachment of a camera for Digiscoping (Fig. 3a). This eyepiece gives a wider field-of-view (effective field number about 22) than the standard 10x eyepiece provided with the microscope (field number = 18, i.e. a 1.8 mm field-of-view when used with a 10x objective). The EagleEye eyepiece also has a larger exit pupil designed for attachment to a digital single lens reflex (dSLR) camera or camcorder for afocal photography (Fig 3b). Apart from the 100% light path, another advantage of using a separate vertical ocular tube is stability. Mounting a heavy camera on, or in place of, the angled eyepiece of a monocular or binocular microscope, results in a top-heavy combination. Another option, with the right adapters, is to attach a lensless dSLR camera body directly to the microscope, but the camera sensor has to be positioned at the primary image plane. Finding the right adapter(s) is only one consideration – the optics and aberrations involved in the different methods of camera attachment also have a significant effect and this will be covered in a future corner. 

Figure 3a. Wide-field telescope eyepiece mounted on microscope using a vacuum cleaner tube.

Figure 3b. Camcorder attached to wide-field eyepiece for afocal video photography.

Finally, before you decide your vacuum cleaner needs replacing so that you can cannibalize the wands, I should point out you can buy replacement wands for about $7 a piece.