The LACERTA Herschel prism is built quite different compared to others on the market (INTES, Lunt, Scopium etc…). It makes use of the full polarization possibility at Brewster angle, compared to only partial at the 90-degree versions.
Why Brewster angle?
Since the polarization comes close to 100% at Brewster angle (57°), it enables continuous and even complete dimming with the mandatory ND3 neutral density filter and an additional Polarization filter – in reality a dimming range between ND4.07 and 6.37 – usually even more (equivalent to 240x dimming factor)!
Without Polar filter the Herschel prism operates at ND 4.07 – good for photos. The 90° “traditional” Herschel prisms can do 4:24 to 5:00 (equivalent to 6x dimming factor only).
The visual perception at various neutral densities are shown in the images below. It can be seen that the granulation in traditional Herschel prisms is well observable only with an additional third filter (ND3 and polarizing filters are required!). Herschel prisms with Brewster angle do not need an extra filter, because the dimming range here continues to until ND6.37!
How to use the Herschel prism?
– First the ND3 filter is screwed as directly as possible into the Herschel prism (eyepiece side). If no filter thread exists there, the filter must be screwed into the eyepiece. With Herschel LAC2s, the 2″ ND3 filter glass is removed from the filter frame, and fixed directly into the Rotation-adapter (using its own fixing ring from the filter frame)
– Then the polar filter is screwed into the rotation adapter end. If we have a Herschel prism without rotation system, the Polar filter must be screwed into the eyepiece. The overall brightness then is adjusted by the rotation of the eyepiece.
– Please note, that the optical elements strictly need to have the following sequence: Telescope – Herschel prism – ND3 filter – Polar filter – additional filter (eg IR block or contrast filters of your choice) – eyepiece (or camera)
– Be aware that when switching eyepieces, all the filters must be used again in the correct order. When using a rotation adapter, switching eyepieces is much easier, since both filters are screwed into the rotation adapter: the ND3 sitting in rotation adapter lower part and the Polar filter in rotatable end. In this case, the eyepiece can be changed easily!
Why is the Lacerta Herschel prism open at the bottom?
– The open design enables a much better ventilation than other designs available on the market
– The amount of stray light that enters through the open part is totally negligible compared to the bright sun entering from the telescope
– no light escapes through the open part, so your shoes wont be on fire!
Whats in the package(Herschel LAC2s):
– Lacerta Herschel Prism (core product) in Brewster angle with T-threads on both sides
– Eyepiece holder barrel (shortened and comes with a T-thread for direct connection to CCD/CMOS cameras or DSLR cameras via a T-ring)
– ND=3.0 neutral density filter