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 Polarisation 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!

When to use the 2" Herschel prism?
Up to 2000mm focal length the 1,25" version is fitting well, as the sun image is not so big yet. Above 2000mm focal length the 2" version is better, but it needs about 130mm backfocus. Due to the additional rotation unit it is somewhat longer than a 2" diagonal.

How to use the Herschel prism?
- First the ND3 filter is screwed as directly as possible into the Herschel prism (yepiece 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!

Here is the thread on the german board, where the design was discussed: www.forum.astronomie.de/phpapps/ubbthreads/ubbthreads.php/topics/1116653

Whats in the package(Herschel LAC2s):
- Lacerta Herschel prism (core product) in Brewster angle, telescope side with M48 thread, the eyepiece with M54 thread,
- M48M48 37mm barrel as telescope side plug-adaptation,
- RotaM54p Rotation Adapter with internal M48 thread to take 2" filter glass
- ND3 filter (2"), incorporated into the fixed RotaM54p rotation adapter,
- RoteT2 Rotation Adapter with internal 2" filter thread, or instead T2 on demand RoteM48

backfocus:

1.25" Version: 61mm from T2 to T2 thread ("S"-Version ca. 89mm)
2" Version: 81mm from M48 to M54 ("S"-Version with rotation adapter ca. 106mm)

weight:

1.25" Version: 308g
2" Version: 685g

We recommend:
- 2" polarizing filter for continuous dimming.
- KG3-IFIRUV2 for additional infrared dampening with telescopes > 120mm aperture


Attention: Herschel prisms are generally relatively bright - an ND3 filter needs to be used mandatory! Continuous dimming is only possible if a polarizer is taken. But as many Astro friends have these two filters already, we offer the "naked" prism also solo as "core product". Please refer to the "recommended products" below!