- FLI Five Position color Filter Wheel
- Uses 50.8 mm Filters
FLI CFW-1-5 Five Position Color Filter Wheel for 50.8mm Filters
At less than 7/8 of an inch thick, FLI's CFW-1 color filter wheel is designed to closely and rigidly couple to any CCD camera. The homing device of the CFW is highly accurate and repeatable. The recessed opening of the color filter wheel securely clamps to an IMG, MaxCam, or "AD" adapter mounting flange. This setup adds a minimum distance to the light path and avoids the vignetting issues common with other designs.
The T-mount system was introduced by Tamron in 1957 as a universal lens mount for 35 mm 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 35 mm 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 35 mm 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 42 mm diameter and having 0.75 mm 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-inch in diameter and have 33.866 threads per inch. The inside diameter is typically 1.615-inch.
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-inch. A 1-inch square CCD has a diagonal dimension of 1.41-inch. 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, yet 75-percent 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-inch adapter tube, we've determined that vignetting isn't an issue with f/8 and slower scopes. For faster scopes we have 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-inch adapter tube can vignette the CCD if it is excessively long.
FLI Product Number: CFW-1-5
|Camera/Eyepiece Side Connection||2.930" Pocket|
|Filter OD Size in Inches||50.8" Round (2")|
|Filter Wheel Type||Filters Optional|
|Scope Side Connection||Female 2"—24 TPI|
|Warranty||1 Year Warranty|
INCLUDED ITEMSFLI CFW-1-5 Color Filter Wheel for 50 mm Filters Includes:
- USB Interface Module
- Operating Software
- Detachable 15 Foot Cable