Optec 58mm Square Dichroic Red IR Filter for RGB Imaging
The new 58mm square filter standard from Optec offers a clear diagonal measure of 79mm or 3.1 inches. This size makes full use of the aperture available with the 3-inch line of Optec instruments. The large 35-37mm square CCDs are covered without vignetting with these filters.
These filters have the same quality and transmission specifications as the popular 50mm round LRGB H-alpha and OIII filters. Scratch and dig is held to 60-40, wavefront distortion to less than 1/4 wave, and peak transmissions greater than 90% in the pass band.
Filter thickness is 2mm. The unique construction of these filters with the hard dielectric layer on only one side avoids multiple reflections between the two sides of the filter. These multi-reflections with bright stars can cause halos and ghost images. The side opposite the dielectric layer is coated with a high transmission multi-layer anti-reflection coating which further reduces the multi-reflection problem. Finally, the price of the Optec 58mm square filters is less then the premium 50mm square filters available from others.
A PlaneWave CDK 12.5 inch telescope has a f/8 cone and a wide well corrected field. Cameras with the KAF-09000 CCD have a diagonal image area that is 52mm across. A 50mm square filter would have to be mounted within 5.4 inches of the CCD to avoid vignetting the corners. This would not allow the use of TCF-S3 to be used between the filter wheel and the camera since it's back focus distance by itself is 5.5 inches to the mid point of travel. The use of 58mm square filters provide about 9.5 inches between the filter and CCD which allows the TCF-S 3-inch to be used. Having the filter far from the CCD avoids shadows from dust spots or filter defects from effecting the image. In addition, the ghost images and halos from multiple reflections between the filter and the CCD surface are also reduced.
Optec Product Number: 17270