Q. How can I figure out the adapter length required?
A. The Adapter Effective Length
is the net distance the custom adapter will add to the
(If you are designing a SLR or DSLR lens adapter for an astronomical camera, please check
If you need an adapter consuming the minimum possible amount of back focus, just enter
0 for the Adapter Effective Length
to let Build-An-Adapter calculate the shortest possible
adapter that will connect the devices selected. The actual, calculated, length will be displayed
in the length field after the Build button is clicked. The length is displayed in inch or mm
according to the unit selected. The calculated value may or may not actually be zero; the minimum
length is constrained by the specific mechanical features required for this particular adapter.
Always check the 2D drawing displayed on the Build page for a visual confirmation of how the
effective length is measured.
For a practical example how to calculate an adapter length please check this
Noting that a custom adapter designed at minimum length may not be adequate for a
or for the camera side of a field flattener or reducer: to reach focus or achieve a flat image field,
a flattener, reducer, or lens must be located at a precise Optical Distance
in front of the
The recommended Optical Distance
for a reducer/flattener is also known as
the Metal Back Distance
and the value is generally supplied by the device manufacturer.
To determine the Adapter Effective Length
for such optical system, the
Image Train Back Focus
needs to be calculated precisely and subtracted from this
- Adapter Effective Length = Optical Distance - Image Train Back focus
In the simplest example, if we have a flattener requiring an Optical Distance
of 55 mm and we want to connect a camera having 17.5 mm of back-focus to that flattener, the Image Train Back Focus
is simply 17.5 mm and we will need a custom adapter with an Effective Length
of 37.5 mm
(55 minus 17.5) to end up with the correct spacing for the flattener.
This example, although simple, is not very realistic as image trains generally incorporate
several different accessories inserted in front of the camera. Here is a more complete example:
We have a reducer requiring 86 mm of Optical distance
, and we have both a filter wheel
and an off-axis guider inserted in the image train between the reducer and the camera. Such devices clearly
consume back focus so the thickness of ALL these accessories must be taken into account
in calculations. The Image Train Back focus
is then the sum of these components
thicknesses and back-focus, then as before we use the formula above and subtract the total
Image Train Back focus
from our Optical Distance
For the example shown in the drawing we end up with an Image Train Back focus
of 39.5 mm (10 + 12 + 17.5)
and consequently the adapter effective length will need to be 86 mm minus 39.5 mm, or 46.5 mm.
Additionally, if there is any glass filter installed between the reducer/flattener
and the camera (eg. in the filter wheel), the effective length may need a small correction to take
into account light diffraction through the filter glass: each 3 mm of glass thickness INCREASES
the Optical Distance
by about 1 mm and the correction must be ADDED in the formula,
therefore increasing the Adapter Effective Length
by the same amount to compensate for
the glass correction.
In our example image train above, if our filter wheel is equipped with 3 mm thick glass filters, our
corrected Adapter Effective Length
is now 47.5 mm (86 minus 39.5 PLUS 1 mm for the filter correction).
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