AO phase contrast condenser 1240

[mnmyco]

Hey everyone,

I have looked through the manuals and other documentation I found online, but I cannot find the NA of the mentioned condenser. It is an AO phase contrast and dark field condenser. The catalog number is 1240. The lens on the condenser is catalog number 1242. The only thing I can find is that it is for standard working distances. It came off of a AO series 20-A if that helps.

Thanks.

[apochronaut]

Yes. They are ellusive about those N.A.s. They probably felt, that data wasn't necessary because it was all designed to work optimally for phase. Probably , at p.o.p. there would have been some user info.

I found out drom literature published in 1951 that the earlier version from the series 4 is in fact a 1.25 abbe, , and based on the disassembly and perusal of the 1242, I suspect it is the same, just a condenser designed to cover a wider circle.
However, at the time I was not looking specifically for any aspheric curves on the lens surfaces, so I will do that a.s.a.p. It might be a 1.25 abbe aspheric.
The condenser would be a .90 without oil, and AO states that the 1242 condenser should not be oiled for phase use. Phase , like DF works at an N.A somewhat below the capacity of the condenser, below 1.0 N.A.

When it is being used as a BF condenser, it can be oiled for use with the 1.25 N.A. achro and planachros.

[apochronaut]

I had a few minutes to spare, so I collected the following condensers together and pulled them all apart.
Cat.# 1000250 phase condenser for the series 2/4, a Cat.# 1242 phase condenser for the series 10/20, a Cat. # 1087 abbe aspheric condenser for the series 100/120, a Cat.# 1201 phase condenser for the series 100/120 and 400/410 and a Cat.# 1973A made for the series 400/410.

Information from brochures , catalogues and other literature gives the following information on the condensers.
Cat.# 1000250. An abbe 1.25 oil condenser. It was designed to be used dry( N.A. approx. .90) for phase contrast and oiled at N.A. 1.25 for BF. It is a two element condenser.
Cat.# 1242. I, like you, nmmyco have not found any published information regarding the performance parameters of this condenser. By comparing it's design to the other condensers , I have formed an educated guess regarding it's correction level and N.A. It is a three element condenser in two groups.
Cat.# 1087. An abbe aspheric 1.25 oil condenser . This was the standard default BF condenser for the series 100/120 microscopes. It is a two element condenser with the bottom element having a pronounced aspheric curvature on the bottom surface.
Cat. # 1201. An achromatic aplanat .90 dry condenser. This one was used for BF and as the phase condenser for the series 100/120 and 400/420. It was the replacement in the AO infinity corrected system, for the Cat.# 1242.
Cat. 1973A. An achromat aplanat 1.4 oil condenser. I only have blanks of this condenser carrying the 1.4 N.A. top oil lens. The Cat. # 1973 condenser was the D.I.C. BF and fluorescence condenser for the series 400 microscopes. The 1973A, is the same in the housing design but appears to have one critical difference; it has an alignment mark. It was possibly of significance for pol with higher N.A. objectives. Nevertheless, despite the incompleteness of the example I have, I know what should be in there from published literature. It is a 4 element condenser in 3 groups. It's value, in this assessment is in the curvature of the top element, which can be used to determine the N.A. of a condenser, when it is not known.

O.K., so the 1242 is not a 1.25 abbe, as I had vaguely assessed it as many years ago. Perhaps the example I was looking at then, did not have a very definable seam in the lower element. The current example I am looking at is clearly an achromat with a corrected doublet for a lower element . It is hard for me to tell whether it is in fact an aplanat but in comparing it to the 1201, with it's very similar structural design and based on the fact that I know of no other Spencer or AO achromatic condenser that is not an aplanat, I would say that it is most likely an achromat aplanat.

The curvature of the front lens is the determining factor when it comes to N.A. In this little comparison, I have 2- 1.25's, 1- .90, 1- 1.4 and an unknown; the 1242.
There is a considerable difference in the curvature of the rear surface of the top condenser lens between a .90 and a 1.25 and again between a 1.25 and a 1.4. The 1242 falls somewhere between the 1.25 examples and the 1.4, so I suspect that it may be a 1.3.

[apochronaut]

Determining the N.A. of oil condensers when used dry.

Very few condensers are marked with both an immersed and dry N.A., yet increasingly condensers are being used dry, with the small sacrifice in resolution being accepted in comparison to the increase in convenience.
Condensers marked with a dry N.A., can be used to determine the relative N.A. of an immersion condenser when being used as a dry condenser. While , with some experimentation a special test scale could be made up in order to measure the N.A, , in this test I am simply using a linear measurement to determine either a greater or lesser angle of acceptance for each condenser. The tools to do this test took less than 5 min. to assemble.

The set up is simple in this case because all of the condensers being tested will screw into the same condenser housing. If condensers from differing mfg. were to be tested, one would need a simple jig ( a stage block for instance, or condenser focus block) to mount the condensers on, so that the bottom lens of each condenser could be adjusted to the same distance from the scale. Each condenser in turn was placed over a small light table or back lit screen, with a flat ruler graduated in mm on top of it. Any such measuring device with gradations as fine as a mm will do. 32's" or 64's would even be better but the ruler needs to be wide enough that it completely extinguishes the back light, when the condenser is placed on top of it .

By placing the scale over the light source and then the condenser over the scale, the scale can be adjusted so that it completely extinguishes the light coming into the condenser. By sliding the scale in one direction, it is possible to allow the end of the scale to just barely allow light into the edge of the condenser. This is the beginning point of measure and the opposite side of the condensers circle provides a measurement of the linear distance of acceptance of that particular condenser. In order to read the scale, I shone a small flashlight through the filter gap in the base of the condenser. Condensers have an extreme level of field curvature, so I measured to the mid point in the condensers field and multiplied by 2. It is easy to see the beginning point, due to the light coming past the end of the ruler but condensers have an immense curvature of field, so it was difficult to read the scruntched numbers on the opposite side. I therefore read the mid point and multiplied by 2. Using a mid point was easy. In this fashion, I determined that the Cat. # 1201 .90 achromat aplanat condenser could "see" 25mm of the scale. The # 1242 could see 24mm and the # 1087 abbe aspheric could see the same.
Based on this measurement , it seems that both the abbe aspheric 1.25 condenser and the 1242 ( 1.3 achromat?) are working at somewhat less than .90 as dry condensers. But how much less?
At the same time , I tested 2 D.I.Y. condensers that I have made over the past couple of years, using the Cat.# 1970A housing blanks. They have the built in 1.4 N.A. top lens. One has been fitted additionally with an achromat doublet as the receiving lens a condensing and a meniscus lens lens and the other with an apochromat triplet and a meniscus lens. Neither of these have been fully tested as immersion condensers but they have been, as dry condensers. Both show an enhanced N.A. and brilliance when compared to the # 1201 .90 achromat, when used dry but neither technically can have an N.A. over 1.0. or possibly even .95. When tested using the same set up above, the D.I.Y. with the achromat bottom lens measures a 31mm acceptance and the one with the apochromat bottom lens measures 30mm. Since the difference between these and the .90 condenser is considerable and the difference in N.A. can be no more than .10, it seems that the # 1087 and 1242 condensers are working very close to .90, when used as dry condensers, probably .88 or so.

[MichaelBrock]

Apochronaut, everything you write here is educational and worth reading. Thank you very much for your posts!

[mnmyco]

Thanks for the info. I had guessed that it was at least 1.25 NA, but I don't have much experience with AO.

mnmyco