Condenser Alignment.
Posted: Mon Mar 04, 2024 6:28 pm
To most microscopists, this means centering the condenser under the objective. What if the condenser itself isn't aligned?
A condenser is an optical device that focuses light along an axis but also usually includes a mechanical device that reduces the intensity of the light by vignetting the periphery of the beam, usually an iris diaphragm. For certain illumination techniques such as phase contrast, interference contrast or dark field patches to name the most common, a further beam modifying device is inserted under the condenser. Each component that modifies the beam needs to be in axial alignment.
Now some of these devices, such as a phase contrast diaphragms are adjusted to be in alignment with the objective and while they will work under such a condition, they may still not be in alignment along the condenser's optical axis. The light still gets where it needs to go in order to perform it's needed function but follows a zig-zag pathway, thus carrying a degree of distortion and or aberration to the eyepieces. This can be anywhere from dramatic to subtle but once proper alignment is attained, there is usually a noticeable improvement in image quality and never a deterioration. Mis-alignment is much less of a problem in BF but usually worth attending to when the condenser is called upon to focus specialized illumination beams. It is one of the principle culprits when someone is having trouble with DF patches and is particularly nasty at flumoxing attempts to align a 40X patch.
Condenser housings are all similar in function but of variable construction. They aren't normally part of an adjustment routine but I have long ago included it in my routine of microscope set up.
To adjust it, a condenser must have a way of adjusting the condenser lens pack relative to the condenser iris diaphragm and thus perfectly align the condenser iris with the field diaphragm. Sometimes the field diaphragm is independently adjustable relative to the condenser iris and sometimes not but ideally both need to be on the same axis as the condenser lens pack, which we have to assume is internally axially well aligned. If the condenser is a sealed unit you just have to take it as it is and assume it was aligned in the factory. Almost all of this type will be BF condensers, so outside of attempts to install DF patches, they should work o.k. This little article is related to condensers that are mostly used for enhanced contrast techniques and need to be critically aligned for best performance. Many used ones have been taken apart and reassembled without due attention to alignment.
So I have a couple of condensers at hand here, a Bausch & Lomb 1.25 abbe aplanatic from a Balplan and a 1.25 abbe phase contrast condenser , also from a Balplan. I guess this is Balplan week. I also re-aquainted myself with just how good those Balplan planachros are. They were the first long barrel objectives at roughly 60mm parfocal , coming out in the mid-70's and blew everything away at the time and for many years after. The 40X .65 in particular will image punctae on diatoms where most .65 objectives can barely image the striae. It functions more like a planfluor and seems to function at a higher N.A. than .65. ....and they made a 50X .80 planfluor as well!
I used that objective to test a newly aligned condenser in BF but also it's phase analog objective for the phase condenser. So, the alignment was successful. Excellent imaging for both.
Here is the simplest way to align one.
The BF condenser I have in hand has 3 set screws around it's perimeter. Loosening the set screws allows the condenser lens section to be removed from the diaphragm housing and adjusting them differentially to align the condenser to the condenser iris . The phase condenser has three cap screws on top that fasten the top plate including the lens section on to the phase diaphragm section and the iris section. It can be aligned by orbiting the optical section in the xy plane against the condenser iris or visa versa, if you see it that way. To adjust the phase condenser, you do it with the open port and once aligned each phase diaphragm is adjusted to it's corresponding objective, through the aligned condenser.
Bring in a 10X objective then, and focus it on some slide.
The first thing is to just barely loosen the screws that fasten the condenser into it's housing, then install the condenser and establish Köhler with focused field iris leaves and center the contracted leaves of the field diaphragm. Then using either screwdrivers or allen keys on the side or your fingers on top for condensers that have top screws,, establish that you can move the condenser lens pack manually while looking through the eyepieces. You will see the field diaphragm move while you manipulate the slightly loose condenser lens section. Then center the field diaphragm again at it's completely closed position and lower the condenser about a cm. You will see the edges of the field diaphragm blur considerably. Then close the condenser iris. By focusing the condenser a little up or down, there will be a point where both the field diaphragm and condenser diaphragm leaves are both enough in focus to see each closing on the other. You want to get them completely concentric. Usually, by centering the field diaphragm and then by manipulating the condenser lens pack manually you will get one overlaying the other perfectly. It will take a few recenterings of the field diaphragm because every time you move the condenser lens pack you will mis--align the field diaphragm optically. Once you have the two irises opening and closing centered on each other, you have to carefully tighten the condenser lens pack down without disturbing it's centering. For those condensers with top fastening screws, an L-shaped scewdriver helps or gently removing it to tighten it down. Might take a few tries.
With a phase condenser you can then align each phase diaphragm to it's objective. You can test your alignmrnt by looking through the 10X phase objective with the open port of the condenser, focused on a slide , then closing, centering and focusing the field iris until it is the smallest. Then close the condenser iris completely, which should just dim the bright center spot . Then switch to the 10X phase diaphragm. There should be no light. As you open the field diaphragm then to fully, you should have a large black center spot surrounded by a complete dull but clearly light halo. Opening and closing the condenser iris should cause the spot to increase and decrease in the center of a well lit, even field. If it is seriously skewed, either your phase alignment is off or the condenser alignment is. Check your phase alignment first.
A condenser is an optical device that focuses light along an axis but also usually includes a mechanical device that reduces the intensity of the light by vignetting the periphery of the beam, usually an iris diaphragm. For certain illumination techniques such as phase contrast, interference contrast or dark field patches to name the most common, a further beam modifying device is inserted under the condenser. Each component that modifies the beam needs to be in axial alignment.
Now some of these devices, such as a phase contrast diaphragms are adjusted to be in alignment with the objective and while they will work under such a condition, they may still not be in alignment along the condenser's optical axis. The light still gets where it needs to go in order to perform it's needed function but follows a zig-zag pathway, thus carrying a degree of distortion and or aberration to the eyepieces. This can be anywhere from dramatic to subtle but once proper alignment is attained, there is usually a noticeable improvement in image quality and never a deterioration. Mis-alignment is much less of a problem in BF but usually worth attending to when the condenser is called upon to focus specialized illumination beams. It is one of the principle culprits when someone is having trouble with DF patches and is particularly nasty at flumoxing attempts to align a 40X patch.
Condenser housings are all similar in function but of variable construction. They aren't normally part of an adjustment routine but I have long ago included it in my routine of microscope set up.
To adjust it, a condenser must have a way of adjusting the condenser lens pack relative to the condenser iris diaphragm and thus perfectly align the condenser iris with the field diaphragm. Sometimes the field diaphragm is independently adjustable relative to the condenser iris and sometimes not but ideally both need to be on the same axis as the condenser lens pack, which we have to assume is internally axially well aligned. If the condenser is a sealed unit you just have to take it as it is and assume it was aligned in the factory. Almost all of this type will be BF condensers, so outside of attempts to install DF patches, they should work o.k. This little article is related to condensers that are mostly used for enhanced contrast techniques and need to be critically aligned for best performance. Many used ones have been taken apart and reassembled without due attention to alignment.
So I have a couple of condensers at hand here, a Bausch & Lomb 1.25 abbe aplanatic from a Balplan and a 1.25 abbe phase contrast condenser , also from a Balplan. I guess this is Balplan week. I also re-aquainted myself with just how good those Balplan planachros are. They were the first long barrel objectives at roughly 60mm parfocal , coming out in the mid-70's and blew everything away at the time and for many years after. The 40X .65 in particular will image punctae on diatoms where most .65 objectives can barely image the striae. It functions more like a planfluor and seems to function at a higher N.A. than .65. ....and they made a 50X .80 planfluor as well!
I used that objective to test a newly aligned condenser in BF but also it's phase analog objective for the phase condenser. So, the alignment was successful. Excellent imaging for both.
Here is the simplest way to align one.
The BF condenser I have in hand has 3 set screws around it's perimeter. Loosening the set screws allows the condenser lens section to be removed from the diaphragm housing and adjusting them differentially to align the condenser to the condenser iris . The phase condenser has three cap screws on top that fasten the top plate including the lens section on to the phase diaphragm section and the iris section. It can be aligned by orbiting the optical section in the xy plane against the condenser iris or visa versa, if you see it that way. To adjust the phase condenser, you do it with the open port and once aligned each phase diaphragm is adjusted to it's corresponding objective, through the aligned condenser.
Bring in a 10X objective then, and focus it on some slide.
The first thing is to just barely loosen the screws that fasten the condenser into it's housing, then install the condenser and establish Köhler with focused field iris leaves and center the contracted leaves of the field diaphragm. Then using either screwdrivers or allen keys on the side or your fingers on top for condensers that have top screws,, establish that you can move the condenser lens pack manually while looking through the eyepieces. You will see the field diaphragm move while you manipulate the slightly loose condenser lens section. Then center the field diaphragm again at it's completely closed position and lower the condenser about a cm. You will see the edges of the field diaphragm blur considerably. Then close the condenser iris. By focusing the condenser a little up or down, there will be a point where both the field diaphragm and condenser diaphragm leaves are both enough in focus to see each closing on the other. You want to get them completely concentric. Usually, by centering the field diaphragm and then by manipulating the condenser lens pack manually you will get one overlaying the other perfectly. It will take a few recenterings of the field diaphragm because every time you move the condenser lens pack you will mis--align the field diaphragm optically. Once you have the two irises opening and closing centered on each other, you have to carefully tighten the condenser lens pack down without disturbing it's centering. For those condensers with top fastening screws, an L-shaped scewdriver helps or gently removing it to tighten it down. Might take a few tries.
With a phase condenser you can then align each phase diaphragm to it's objective. You can test your alignmrnt by looking through the 10X phase objective with the open port of the condenser, focused on a slide , then closing, centering and focusing the field iris until it is the smallest. Then close the condenser iris completely, which should just dim the bright center spot . Then switch to the 10X phase diaphragm. There should be no light. As you open the field diaphragm then to fully, you should have a large black center spot surrounded by a complete dull but clearly light halo. Opening and closing the condenser iris should cause the spot to increase and decrease in the center of a well lit, even field. If it is seriously skewed, either your phase alignment is off or the condenser alignment is. Check your phase alignment first.