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Projection Imaging

See an example of a projection attachment for a video camera elsewhere on this web site: Video Camera and Attachments
See details of a projection attachment at the end of this article. For Photo of attachment press here.

The topic of eyepiece projection comes up from time to time when a very high magnification is required for objects that are quite bright. Planetary photography especially comes to mind. A few comments about this topic is appropriate because projection imaging is often not well described. For many reasons the results are quite often rather poor. I have done quite a bit of projection imaging with microscopes and a bit with telescopes as well. Only positive projection is described here. Negative projection, such as with a Barlow, is generally limited to low projection powers (typically 2 to 4 times image magnification). It does work well with a good Barlow (negative) lens. My telescope experience has been mostly with projection of planets onto a video camera chip. While it is certainly possible to get good images by projection with an eyepiece, I think they are used mostly because they happen to be available rather than being chosen for their optical effectiveness.

When setting up an optical system for image projection, it helps to think about the differences between visual observation of the telescope's real image and projection of that image. Eyepieces are designed to be magnifying glasses for the eye and not projection lenses. They are designed to let you look at the real image that the telescope objective creates at the eyepiece position (actually at the field stop of the eyepiece). They are basically quality magnifying glasses which are designed to have a large field of view as seen by the eye. This is called the apparent field of view. The eyepiece has special properties that observers seem to covet. They are designed to have a large apparent field of view and a large field stop so that the actual field of view is as large as possible. The actual field of view depends only on the focal length of the telescope and the size of the field stop. They need to be bright and free of reflections. They often have many thick lenses in them and are designed to control flatness of field and off axis distortions as seen by the eye.

As an aside, one should note that the largest field stop possible is a bit smaller than the diameter of the eyepiece tube. This means that a 1 1/4 inch eyepiece usually has a maximum field stop of about 28 mm. The reason for going to a 2" eyepiece is to get a larger field stop and thus see a larger part of the sky with a given focal length telescope. Of course this large field stop also requires large lenses in the eyepiece and the costs rise rapidly. You can estimate the diameter of the field stop by looking into the back side of the eyepiece and hold a millimeter scale against it.

The compound (and complex) lens part of the eyepiece is designed to provide a view of the real image that the telescope objective creates at the field stop. This requires a design that has a large and flat field and projects a virtual image at some convenient distance in front of the eye. The power of the eyepiece is usually considered to be the focal length divided into 250 mm. The sharp edge of the field stop can provide a reference point for focus. But since the eye has considerable accommodation there is sometimes a cross hair or reticule located at the field stop upon which to fix the eye's attention.

The point of describing these factors is that the optical design of the eyepiece requires a large field and a curvature of field that fits that of the telescope. The real image is usually slightly within the focal length of the eyepiece and thus the projected image is a virtual image that the eye can see. (Note the eye cannot see a real image unless it falls on a ground glass or impinges on smoke.) Additionally the eyepiece must cover a large field. That is, it must cover the entire area of the field stop.

Real images that need to be magnified by projection are not large in size. They are generally planets which have tiny but bright real images. One often looks at these with a short focal length eyepiece (a high power eyepiece). This type of eyepiece has a small field stop but that is fine since the image is small.

Her is shown the basic difference between projecting a real image formed by the telescope onto a film (or CCD) surface (top) and viewing the real image with the eye through an eyepiece. (bottom)

 The idea of image projection is to focus the real image (called the object in the projection system), which is formed by the telescope objective into another real image (called the image) which is a significantly enlarged version of the object. In this case, the object lies outside the focal length of the projection lens by a small amount and the projected image is outside the focal length by quite a bit so it is substantially larger than the original. To do this the lens used should be a projection type lens designed for a real object to real image magnification of about the desired amount. (usually 3 to 20 times) Lenses made for this purpose are microscope objectives, enlarger lenses, copy lenses, microfiche lenses, small format lenses, movie lenses. video lenses and the like.

Some of these lenses are designed for specific magnification ratios and for flat but not necessarily wide fields. They are usually designed specifically for high resolution and high contrast. The size of the lens needs to be somewhat larger than the object, that is the real telescope image, so that it intercepts the entire cone of illumination of the object. This is so that there will be no vignetting of the real image. Since the largest real image of a planet, Jupiter or venus, is only 0.4 mm with a 2000 mm telescope, we are looking at very tiny images; even compared to a CCD chip. This is where positive projection works the best. Magnification of 3X to 10X will fill the CCD chip or video camera chip or even give a useable image on 35 mm film. Most books on photography of planets recommend, or hint at, using a lens other than an eyepiece for positive projection. (See references at the end of these comments.)

Ideal lenses for this application are small copy lenses or microfiche lenses. Short focal length photographic enlarger lenses and photomicrographic lenses are also very good. If an eyepiece is used, a simple orthoscopic type is better than a complex and costly wide field type. That is because the complex wide field eyepieces have too many glass surfaces and too much of the design is directed to factors that are important for visual work but not important for projection applications

The desirable lenses mentioned above are usually designed specifically for image magnification ratios of about 10 to 1. They are all of a speed of f4 or faster and thus will easily accept the object light bundle without vignetting. Edmund Scientific is a source for high quality lenses of this type. There are other sources such as surplus optical suppliers. Some photo shops have selections of used lenses that are also suitable. You may have to have an adapter tube or some sort of holder ring made to mount them in the tubes you are using.

Typically the microscope objectives are designed to work at a projection distance of 160 to 250 mm. So a 10X objective will be just about right for many purposes. Such an objective has a focal length of 16 mm. I personally use surplus microfiche lenses which can be found for only $20 or $30 each in focal lengths of 10 to 30 mm. They are very excellent for this purpose. You will find them in surplus optics catalogs.

Basically, the optical arrangement you want to use is to place the projection lens a distance of 150 to 250 mm (6" to 8") from the imaging plane, that is the film or CCD chip and focus the telescope to place the real image in front of the lens at a distance slightly greater than its focal length. Believe me that for a very modest cost for the projection lens you will obtain projection results better that most eyepieces can ever deliver.

Projection Attachment

Here is shown an example of a projection attachment. The attachment is a 2" O.D. tube which is blackened inside. The tube has on the right a standard 48 mm filter adapter ring to hold an IR filter of other filter as necessary. There is a position setting ring which allows consistent setting of the tube in a standard 2" focuser. In this case a JMI focuser. The lens is slipped into the tube inside a retaining ring mounted inside the tube. The lens shown is one of a set of four microfiche lenses of different focal lengths that can be inserted. This provides for a variety of magnifications. The lens is held in place by the set screw shown.

The left end of the tube is terminated in a standard T-thread. To the T-thread in turn any number of camera attachments can be appended. In this case a Canon breach mount is shown. This mount in turn can be bayoneted to the second adapter shown, a Canon to C-type adapter, or to other adapters designed to fit a CCD imager, or a standard Canon camera for that matter. The projection side spacing chosen for the particular lenses used in this projection tube is approximately 6 inches. Generally 6 to 8 inches is appropriate for the projection side of micro lenses or microscope objectives. The position of the adapter in the photo corresponds to the above diagram.

projection 2

I have found that using the Canon bayonet mount as an intermediate adapter has been very convenient. In this fashion all tubes lenses, cameras and imagers can be made parfocal and easily changed. Because of the breach mount, which can be made very tight, there is no slack in the entire structure.

Several books that give good advice on these issues are:
 Astrophotography for the Amateur - M Covington - Cambridge rev. 1991
 Astrophotography an Introduction - HIP Arnold - Sky and Telescope 1995
 Astrophotography II - P Martinez - William Bell 1987

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