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These were purchased from Kitt Peak, as orginally specified by J Mould. They have flat topped profiles and are intended for use with CCD detectors (unlike earlier filter systems). Figure 2 shows details of these filters. Three sets exist; one for the INT, one for the JKT and a spare set at Herstmonceux. The original specifications are given below:
All filters to be 2" x 2", 3mm thick, blocked to 1.1 micron, better than 75% peak transmission (except B 70%), MgF coated, no pinholes, optical quality glass.
Filter 7200Å = short-pass hard-coated on 3mm CG590, blocked to 1.1 micron, 75% or better peak transmission, MgF coated on glass side.
Filter 9000Å = Short-pass hard-coated on 3mm RG-M9, blocked 1.1 micron, 75% or better peak transmission, MgF coated on glass side.
Filter 6000Å = Short-pass hard-coated on 3mm GG 495 blocked to 1.1 micron, 75% or better peak transmission, MgF coated on glass side. (Sent back did not come up to specification, 1/6/81)
Filter 4900Å = Short-pass, hard-coated on the glass substrate indicated below, blocked to 1.1 micron, 65% or better peak transmission, MgF2 Coated on glass side. Glass substrate to be cemented combination of Schott GG 385 and either Hoya GM-500 or Hoya C-500 (whichever will adequately suppress the red leak). Total thickness to be 3mm as with the other items.
SIZE: All Filters to be 2 x 2 inches and standard 3mm thickness.
NOTE: All filters to be on optical glass quality and with no pinholes.