the astroscopic labs - article: eyepiece projection photography

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article: eyepiece projection photography

The eyepiece projection is used for two main reasons:

Enlarging the prime focus picture for small objects
Coupling a fixed objective camera to a telescope

As the name of this method is implying the picture of the primary focus is sent through an eyepiece. So we need some mechanical structure to hold an eyepiece and beeing able to connect the two ends to the telescope's focuser and to a camera. Here are two examples:

assembled and ready to use...

disassembled to show the parts...

Both systems use the t-thread on both ends. The t-thread is M42 x 0.75 thread. Here you can screw directly to the t-adapter of a camera or a t-thread focuser. There are adapters from t-thread to a 1.25" nose piece (the barrel you will insert to a focuser) to connect it to a normal 1.25" focuser. Other adapters change from 2" to t-thread so the usage in a 2 " focuser is no problem as well. The lower system has already an adapter screwed in place to hold the 1.25" nose piece very often used with CCD cameras.

The eyepiece is screwed into the right part of the systems just like into an ordinary focuser. The first model (the all-black one) shows a shifting mechanism between eyepiece and camera. The second model relies on the usage of more or less adapter rings to change the distance between eyepiece and camera. This distance together with the focal length of the eyepiece is giving the effective amplification factor. The ampflification factor is comparable to a Barlow Lens. These lenses come with their ampflification factor printed on the outside like 1.5x, 2x or 5x as a practical maximum.

Enlarging the prime focus picture for small objects

For small objects like the planets very often the primary focal length of the telescope is not giving enough resolution and the object is imaged too small.

While using a Balow Lens for enlarging is restricted to about a maximum of 5x the eyepiece projection method can go far beyond that without too much aberrations. The magnification comes with the price of a slower f-ratio of the whole system.

The effective amplification, total focal length and total f-ratio is depending on the eyepiece and the distance to the camera:

The Amplification Equations:

	The amplification factor also called magnification factor. For a Barlow lens it's simply printed on the barrel as i.e. 2x or 3x and must not be calculated.
	The distance from the eyepiece to the film plane or CCD chip. For the eyepiece you can start measuring from that point where the nose piece barrel (1.25" or 2") is connected to the lens housing of the eyepiece. That is the edge touching the focuser when the eyepiece is inserted into the focuser.
	The focal length of the eyepiece, i.e. 20 mm
	Effective focal length for the whole system including primary of the telescope.
	The focal length of telescope itself.
	The effective focal ratio of the whole system, i.e. f/20
	The diameter of the primary objective or mirror of the telescope, i.e. 200 mm or 8 inch
Obviously the bigger S and the smaller the focal length of the eyepiece the bigger the amplification will be. That's fine because eyepieces with very long focal lengths are rarely fitting into the mechanical structure of the projector anyway. Same is true for most of the wide angle eyepieces.

Coupling a fixed objective camera to a telescope

The problem in that case is that the camera will always image through its own built in objective. The situation is comparable to the usage of the telescope with the eye. Hence the above formula for eyepiece projection with a camera without it's own lens cannot be applied. The direct image of the primary objective of the telescope can only be used by a camera without own objective and hence must be turned into a real image by an eyepiece.

Like the eye is focused to infinity looking through the eyepiece the digital or film camera must be focused to infinity too. The whole focusing process during imaging should hence be done with the telescope focuser itself. Pay attention to the fact that most cameras are not really at infinity when they are set to their infinity mark.

To calculate the overall magnification the prime magnifictaion of telescope and eyepiece is multiplied by the camera's objective magnification.

To estimate the cameras own magnification factor simply use this rule:

A picture in a camera is said to be not amplified ( or A=1 ) when the objective focal length is same as the diagonal of the film or the CCD chip. For an ordinary 35 mm film camera the diagonal is about 43 mm. That is why the 45 to 50 mm objectives are regarded to be 1x zoom, hence no zoom at all. A 100 mm tele objective is regarded to be about 2x zoom.

A digital camera example: the Minolta Dimage 7i digital camera has a CCD chip diagonal of about 17 mm (2/3 inch CCD chip). This chip has an active diagonal of about 14 mm. The real focal length of it's zoom objective is ranging from 7.2 to 50.8 mm. Minolta claims this to be like 28 mm to 200 mm for a 35 mm film camera and because people are used to it that's what they have written onto the Dimage's zoom objective.

But because the CCDs diagonal is only about one third the size of a 35 mm film diagonal this should be better written as 21 mm to 150 mm in reality. In respect to it's CCD chip the Dimage should produce 1x at it's 65 mm setting and a maximum of 3x at its 200 mm setting.

To calculate the telescope and eyepiece magnification simply devide the telecope's focal length by the eyepiece's focal length.

An example:

Telescope focal length = 1200 mm Eyepiece focal length = 25 mm Camera Magnification = 3x

Total magnification is 1200/25 * 3x = 144x

A homegrown eyepiece adapter for the Dimage 7i

1) Focuser with T-Thread (M42x0.75)
2) Quick Connector for T-Thread, rotatable
3) Eyepiece holder for T-Threads, holding a 25 mm Plossl covered by the extension tube
4) 50 mm plumbers ring for water pipes
5) Extension tube for T-Thread, the length is so that the eyepiece is almost touching the first camera lens
6) Adapter to change T-Thread to the 49 mm filter screw of the camera
7) Plate to give more stability and not to overload the zoom objective of the camera

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