Hi, These are required to correct for IR focus. This is needed for monochrome cameras which have significant IR sensitvity. Without them the images are degraded because the IR light cannot be focused to the same plane (ie the surface of the chip) as the visible light. The images look "soft" without it. If using a colour camera they can be left out as colour cameras tend to have an IR cut filter in front of the chip. Cheers, Bill
In the documentation that accompanies the lenses, the items is referred to as a "B/W Aberration Compensation Filter" (ACF). One of the reasons the Watec 902H series is so sensitive is due to the sensor having a degree of sensitivity down into the IR. As we all know, different wavelengths come to focus at slightly different focal planes and hence on a mono camera, the resultant images look slightly out of focus.
What's not clear is whether the ACF is a filter or a weak lens. I suspect may be a weak lens that helps to bring IR light to the same focal plance as visible light.
Towards the end of last year I purchased a "43mm Optical UV-IR CUT filter for Camera Lenses Canon Nikon Sony Pentax" from eBay - no specific details were provided with the lens to indicate the specific wavelenghts that were blocked / transmitted. I did try it on the front of a Computar 8mm f0.8 lens but found that while bright stars appeared more tightly focussed, this may very well have been due to the fact that the lens was blocking a significant amount of light (I suspect IR light) and hence stars appeared dimmer. Faint stars became invisible.
The ACF does provide a significant advantage (see meteorobserver.proboards.com/thread/93/lens-comparison). I think blocking the IR part of the spectrum leads to too much loss of sensitivity and ultimately I'd rather have good sensitivity with somewhat "soft" images as opposed to low sensitivity with "crisp" images.
Hope this helps. With the Watec 902H and 902H Supreme, the lens mount design prevents the ACF from being fitted. This issue was corrected on the 902H Ultimate. It may be possible to modify the lens mount on the earlier design cameras to allow the ACF. And if anyone has surplus Computar B/W Aberration Compensation Filters for 1/2" lenses I'd be very interested :-)
Thanks William that may save me some time experimenting. I can find no other info on the ACF so I was not sure if was purely a filter or had any focusing characteristics like a lens. I'll keep digging for info, and am looking for ACF's myself. (no luck so far)
In recent months I"ve been in touch with Computar. What's frustrating is the fact that they do not appear to have a part number for this particular item. After much negotiation they did send me some ACFs but alas they were for 1/3" lenses as opposed to 1/2" lenses. It looks like they've stopped manufacture of the 1/2" ACFs as well as the fast 1/2" lenses.