| The Paracorr (Parabola Corrector) Universal is an auxiliary coma-correcting lens system designed strictly for 35mm and large-format CCD imaging with fast focal ratio reflectors and TeleVue Imaging System components. It fits into the 2" focusers of all carefully-collimated f/3.5 to f/5 Newtonian reflectors to improve deep space photography by drastically reducing the coma and curvature of field that would otherwise be distracting in the final photographic print. The Paracorr Universal increases the photographic focal length and focal ratio by a modest 1.15x.
This fully multicoated 6.5 oz. four-lens system increases the diffraction-limited field of an f/4.5 system by 36x in area and reduces the field edge coma by 75%-95%. The Paracorr essentially makes an f/4 scope perform like an f/8 in terms of field edge image quality, while still keeping the shorter exposure times and wider field of the faster f/4 focal ratio (actually f/4.6 with the 1.15x focal length/focal ratio gain factored in). Stars at the edge of the field change from comet-shaped flares to points of light, so that stars look virtually the same anywhere in the negative. Faint stars at the edge of the field do not fuzz out to invisibility due to coma and field curvature, but show as tightly focused points of light as much as six times smaller than their size without the Paracorr.
The 2" long 2" barrel Paracorr Universal has threads that will let you use TeleVue Imaging Series accessories to connect it to your 35mm SLR or large format CDD camera. The wide 2" barrel of the Paracorr (which has the largest lens aperture physically possible) is designed to minimize vignetting of a 35mm negative or large 35mm-format CCD chip.
The Paracorr Universal can be used for 35mm photography simply by adding a T-ring and #13TRA Imaging System T-ring adapter to the Paracorr. The position of the film plane at the back of the camera body will provide the correct 55mm spacing (+/-4mm) needed between the Paracorr lens surface and the image plane for optimum coma-correcting performance.
To use the Paracorr for CCD imaging, you will need to know the distance from the CCD imaging chip to the front plate of the CCD camera. Subtract that distance from 55mm to come up with the required additional spacing needed between the Paracorr and the CCD camera. Add the appropriate Imaging System spacers and a #13STL or #13TRA adapter to the Paracorr to achieve the required 55mm spacing and you are ready to image, even with large 35mm-format CCD cameras such as the SBIG STL Research Series.
With CCD chips smaller than 35mm-format in size, the field edge improvement provided by the Paracorr Universal will not be as visible as it is with a larger 35mm-format chip. The Paracorr Universal may not provide enough of a visible improvement to justify its cost if used solely for CCD imaging with a small chip camera.
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