Finger Lakes Instrumentation

 

T-mounts, Aperture, and Vignetting

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The T-mount system was introduced by Tamron in 1957 as a universal lens mount for 35mm cameras. The idea was that each lens would have a standard threaded rear flange. A camera specific adapter would then be attached at the point of sale, as needed. This greatly reduced costs since only the T-mount adapters needed to be stocked, rather than different versions of every lens.

With T-mounts readily available for most camera bodies, it was natural for astronomy buffs to settle on it as the standard adapter for 35mm astrophotography. Without this inexpensive and easy to use adapter, astrophotography might have gotten off to a far slower start than it did. On the downside, the T-mount was designed to solve a very specific optical coupling problem in 35mm cameras. Progress in telescope design and CCD array size is revealing the T-mount as a potential bottleneck in the light path.

The thread used on T-mounts, referred to as the T-thread, is a metric thread 42mm diameter and having 0.75mm pitch. It is correctly written as "M42-.75". If you’re more comfortable with inches and TPI (threads per inch), the thread would be 1.654" in diameter and have 33.866 threads per inch. The inside diameter is typically 1.615".

That aperture seems generous, until you realize that any screw in tube or adapter has to have enough wall thickness to prevent cracking and subsequent failure. Thus, the actual clear aperture for a T-thread system is about 1.45".

A 1" square CCD has a diagonal dimension of 1.41". When combined with the steep light cone from a fast telescope, vignetting is unavoidable. The more T-threaded accessories are installed on the front of the camera, the worse the problem becomes.  

It should be noted that a certain amount of judgment must come into play here. Imagine that you could place your eye at the very corner of the CCD. Looking out toward the primary mirror, you would see the edge of the T-thread cutting off the edge of the mirror. BUT- 75% or more of the mirror would still be visible. Thus, the vignetting we’re talking about isn’t a sharp shadow, but a gradual intensity falloff. In many applications it won’t be significant.

For the FLI IMG series cameras using the large SITe array, our color filter wheel, and a 2" adapter tube, we’ve determined that vignetting isn’t an issue with f/8 and slower scopes. For faster scopes we’ve introduced a redesigned nosepiece with a larger clear aperture. This nosepiece uses the industry standard 2"-24 coupling thread, as found on the rear port of Meade, Celestron, and many other scopes.

As more accessories are placed in front of the camera, there is an additional concern. Even a 2" adapter tube can vignette the CCD if it’s excessively long. Exact dimensions depend on the specific camera, accessories and optical system, but we’ve provided a ray trace drawing in our pdf area that covers the more common configurations.