IX. Frames
The microscope frame is the central module in the modern microscope. The frame has the attachment points for the various modules that make up the microscope system. The frame also has the focusing mechanism. Focusing mechanisms come in several forms, nosepiece or stage and infinite or finite.
Focusing mechanisms can either move the stage or the nosepiece. The A.O. microscopes are the most famous nosepiece focusers. The advantage to focusing the nosepiece is that the stage should be more stable. The claim is made that a nosepiece focusing microscope can't break slides but I haven't found this to be true. The mass of the nosepiece mechanism is enough to break a slide in some circumstances. A spring loaded objective is as good as nosepiece focus.
The disadvantage of nosepiece focus is complexity. The focus mechanism extends over a large part of the microscope. This leads to slop and play in the instrument. Stage focus is used by most manufacturers because it is easier to make, more direct and if properly engineered the stage is very stable.
Inverted microscopes use nosepiece focus for other reasons. Inverteds are frequently used for micro-manipulation. A fixed stage is much easier to use since the micro-manipulators don't have to be re-adjusted every time the focus changes. The other problem with inverteds is stage size. Modern inverted microscopes have very big, heavy stages, nosepiece focus is easier to do.
Most manufacturers focus mechanisms can drive the stage from the top of its travel to the bottom using only the fine focus. This is called infinite fine focus. If the fine focus has a limited range this is called finite focus. The trade off is precision versus ease of use. If you run out of travel with a finite focus you have to refocus with the course focus and try again. The advantage of finite focus is it tends to be finer and more progressive than infinite systems. The best way to use a finite system is to set the fine focus is the middle of its travel and use the course focus to get close then use the fine focus. Many finite systems have range marks on the side of the microscope to help you set the fine focus travel.
Most modern fine focuses are infinite systems. The designs and material choices are so good that it doesn't really matter that they are inifinite. If you do a lot of work at oil make sure that the fine focus is smooth and progressive before you buy.
There are two main types of microscope frames, upright and inverted. The upright is the most popular type while the inverted is used in metallurgy, cell culture and aquatic studies. If the specimen you are using is in a suspension or very large an inverted may be the microscope for you.
Upright microscopes come in three frame sizes student, clinical or bench top and research. The student microscope is the smallest and the least versatile. It is designed for bright field and possibly phase. The design goal for a student microscope is to be inexpensive and tough to break. Usually these microscopes will not have Koehler illumination but use diffusion illumination. They will not have the light output of a Koehler illuminated bench top microscope.
Usually these frames are sold in quantity to someone equipping a student lab. The price breaks to users on quantities of student frame should be large since manufacturers believe that all the user wants to see is the discount and not the actual price. Olympus and Nikon are the two leaders in student frames. The discount to dealers is 40% to 50%. List prices start at $1,100 to $1,800. These microscopes are very good for practioners such as veterinarians, dermatologist and health clinics. If you do not use the microscope intensively and you don't need very high resolution a student frame microscope will be for you. They are very easy to use and tough to break.
The bench top or clinical microscope is sold across the widest range of users. Every manufacturer sells their bench top into all the main market areas. These microscopes have Koehler illumination and are able to do all the major techniques and photography. If photography is the most important purpose of the microscope then a research frame would be better but if photography will only be a part time use a bench top microscope will work well. Bench tops can also use multi-viewing, most research frames can't.
Every major manufacturer's bench top microscope is good. You pay your money and take you chances but you won't be disappointed. This isn't some cop out it is a wonderful fact of life, all major manufacturers make excellent bench tops. Since this is the main commercial battle ground competition has bred some excellent microscopes. Prices start at $3,200 and go to around $18,000.
Research frames are the big deal in microscopes, the big enchilada, the big...well you get the idea, big is the operative word. And big they are, 40 kg. and more of metal, glass and electronics. A modern research frame is as much electronics as stand. Research microscopes are designed to do any technique or combination of techniques at the highest level of performance, have excellent stability and then document the specimen in any way imaginable. What this leads to is a big instrument with a lost of camera ports on it. Typically a modern research microscope will be able to have two 35mm cameras, one large format (4X5 or 8X10) camera and a tv. attached to it. The user will be able to select the camera they want to use by pushing a button or lever. The camera system will include full automation,
film id. systems, manual over ride and perhaps automatic focusing. The observer has superwide field eyepieces and an array of motor driven controls.
Not every research microscope has all these features but the research microscope is moving towards more electronics and more complexity. What these microscopes are good at is documentation. If you do a lot of photography this could be the microscope for you. Just remember to bring money. Prices start at $26,000 and go to $125,000. If you are in bio-med and your work involves tissue culture, aquatic research or anything were gravity affects the specimen you need an inverted microscope. The objectives in an inverted microscope point upward. The modern inverted microscope can accept all of the modules in the manufacturers line including phase, Nomarski and Hoffman.
Typically a bio-med inverted frame is equipped with long working distance condensers and objectives since most specimens used on inverted are in thick containers, thicker than a cover slip anyway. These containers usually contain not only the specimen but usually some growth media or other liquid. This leads to all kinds of problems including condensation and the location of the cells in the media. Inverted bio-med frames come in two sizes small and large. The small frame microscopes are used for routine culture checking. They don't have fine focus mechanisms but since they are usually not used with objectives over 20X. They are equipped with phase and brightfield. These microscopes are designed to be used for routine examination of cultures. If you are doing tissue culture work you will need one of
these.
If your work is more demanding and you need an inverted then the research inverted frame is for you. The research inverted has a fine focus, fixed stage, capacity to accept a fluorescence module and a powerful transmitted light module. A research inverted will have all the capacity of an upper level bench top microscope and a few things more.
A research inverted will have excellent capacity for documentation. Some manufacturers have built in camera ports on the microscope. This can be a real plus making documentation a lot easier. The disadvantage of a research inverted over a bench top microscope is when using oil. When you try to use oil with an inverted it runs down over the objective. If you are real careful you can do this but it is messy and a real pain. If your work requires oil a bench top microscope is usually best.
All bio-med inverteds have fixed stages. This makes it easy to install micro-manipulators. Since the stage doesn't move the manipulators do not have to be reset each time you focus. This also means that the specimen doesn't get shaken up from focus.
Inverted for industry are used by metalurgists. An inverted microscope is preferred by many metalurgist because their specimens are large. A metalurgist may look at a huge, whole specimen. The other consideration is that most metalurgical specimens are very carefully surface polished. This means that the front of the specimen is very flat. When placed face downward on an inverted the specimen will be flat. This reduces focusing between fields.
The disadvantage of using an inverted for metallurgy is that you can't see were you are on the specimen. If this is a real problem use an upright microscope and a hand press. The press is used to get a flat specimen mounted on a metal slide. The specimen is placed on some plasticine on the metal slide and the hand press is used to press the specimen flat down into the plasticine. While this works it is not as easy to use as an inverted and it tends to be not as flat.
Metalurgical inverted or metallographs are available in two sizes, routine and research. Routine metalographs are usually based on the manufacturers research inverted. This means that they have fixed stages, good documentation capability and the price is moderate, about $12,000 to $18,000. Research metalographs are a different breed. They are huge monsters with eyepieces, camera ports and viewing screens. Since they need a lot of light 450 watt Zenon arc lamps are common. These microscopes can do just about anything from contrast enhancement techniques to all forms
of documentation.
Not all manufacturers make these microscopes for good reason, the market is small and the costs are high. The ones that are out there are all excellent however. If you need one of these bring money. The entry price will be $80,000 at least and will go up rapidly from there.
What this means to you:
There are different sizes of frames for different purposes. These frames include student, bench top and
research upright microscopes. There are also small and research inverted and the research
metalograph.