Lacerta Herschel wedge 1.25″ review

Клин Гершеля Lacerta 1.25
Lacerta Herschel wedge 1.25″

The Herschel wedge is a special optical accessory designed for safe observation and capturing of the Sun through a telescope in conjunction with additional neutral density filters. The principle of operation of this device is very simple – sunlight focused by a telescope is partially reflected from a glass wedge, further attenuated using several light filters and then enters the eye or camera, but most of the solar energy passes through the wedge and hits the radiator. That is, an aperture filter is not used – only a Herschel wedge and additional sub-aperture protective filters. This device was first proposed and used around 1830 by astronomer John Herschel (son of William Herschel). Instead of a wedge, a 90° prism or a more complex design can also be used.
What can you see through the Herschel wedge?
Using a Herschel wedge, you can observe and photograph sunspots, flare fields, and granulation. Prominences are not visible – to observe them you need a special chromospheric telescope or a chromospheric attachment.

The Sun observed through the Herschel wedge in white light and with Baader Solar Continuum filter(540 nm)
The Sun observed through the Herschel wedge in white light and with Baader Solar Continuum filter(540 nm)

Of course, when observing the Sun, you should be careful and be extremely attentive, since if you make a mistake, you can instantly damage your vision or equipment. Considering that light falls on the Herschel wedge without preliminary filtering, the Herschel wedge can ONLY be used with lens telescopes, since overheating and damage to the secondary/diagonal mirror and hood are possible in mirror and mirror-lens telescopes. Yes, there is a Triband-SCT mirror-lens telescope with a full-aperture three-band filter for use with a Herschel wedge, but this is the exception to the rule.

Disclaimer
This review is advisory in nature, in it I express my opinion and share my personal experience of using various equipment for observing and photographing the Sun.
Under no circumstances am I responsible for:
– for causing harm to health with a telescope during use (including when observing the Sun)
– for breakdowns and malfunctions of the telescope, eyepieces, astronomical cameras or light filters that occurred while observing the Sun
– for unjustified expectations, “cloudy picture” or “nothing visible”
– for other problems with health or the telescope that arose not through my fault
– for problems with availability, payment, delivery, warranty or return of equipment
– for manufacturing defects or defects in purchased equipment
– for non-compliance with the stated equipment
– for difficulties with assembling or configuring equipment.
Your equipment – your eyes – your decisions and actions – your responsibility.
If you agree, enjoy reading 🙂 If not, you don’t have to read any further.
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There are several options for implementing a Herschel wedge, but in early 2022 I was able to purchase an unusual wedge with a Brewster angle. In such a device, sunlight is reflected at a certain angle (57°), at which the light is completely polarized. This allows you to adjust the brightness of sunlight using a single polarizing filter by simply rotating the eyepiece with the filters screwed on.

The body of the device from Lacerta is completely metal, inside there is a glass wedge. It is possible to adjust the tilt of the wedge, but I don’t recommend getting into the adjustment unless necessary. At the inlet and outlet there is a T2 (M42x0.75) female thread. At the rear there is a massive radiator. Simple and elegant. I use the visual adapter for 1.25-inch eyepieces. A larger version of the wedge is commercially available for use with 2-inch eyepieces.

There is no cover at the bottom, but the manufacturer claims that this improves cooling and does not affect the picture. I want to screw a standard dovetail plate onto the bottom so I can make a mini solar telescope.

Клин Lacerta, вид сбоку
Lacerta wedge, side view
Клин Lacerta, вид снизу
Lacerta wedge, bottom view
Клин Lacerta, вид спереди
Lacerta wedge, front view
Клин Lacerta, вид снизу в рабочем режиме.
Lacerta wedge, bottom view in operating mode

What are the advantages of a Lacerta Herschel wedge over full-aperture film and glass filters?
+ More durable than film filters. Over time, my Baader Astrosolar Photo filter became covered in small dots that reduced the contrast. And the Herschel wedge that I bought was in use for at least 5 years; I simply wiped it from the inside with a clean microfiber cloth.
+ More universal – I have three refractors (Celestron 102 SLT, Sky-Wtcher BK705AZ3, Levenhuk Ra 66ED), I don’t need to buy an aperture filter for each telescope, a Herschel wedge and a set of eyepiece filters (ND3, polarizing, Continuum, etc.) are enough. . As a result, I completely abandoned film and full-aperture filters in favor of wedge filters.
+ Does not introduce distortions into the wave front – the wedge is sub-aperture and is located in a converging beam; such a surface is easier to produce with sufficient accuracy. However, Baader’s film filter also provides good resolution.
+ The ability to observe the Sun in natural color, since ND filters do not distort color rendering.
+ Excellent contrast and high image detail, especially with the Continuum filter.
+ The wedge reflects ultraviolet light – this is important for shooting with a calcium filter.
+ A large reserve of brightness is a plus when observing with a binoviewer.

Flaws
– Price.
A good wedge is noticeably more expensive than a full-aperture filter, but as the aperture increases, the price difference decreases. Although, structurally, the Herschel wedge is very simple and can be made independently, even based on a 90-degree prism.
– It’s hard to find. They don’t sell them in the Russian Federation, order only from Europe, or look on the secondary market.
– Requires care and caution when using. Pay close attention to which filters you install and in what order. Additional protective ND filters are screwed into the eyepiece, so if another observer wants to change the eyepiece to another and unknowingly does not screw the ND filters into the new eyepiece, there is a risk of damage or loss of vision.
-Works only with lens telescopes, and not with all. A 2″ or 2″ > 1.25″ short adapter may be required.
– There is no built-in sun finder, but these are design features. I navigate by the light spot from the inside of the body, or by the shadow from the telescope. Also, for initial aiming, you can insert a light plug for the focuser instead of the eyepiece.

Important nuances of use
1. All filters, eyepiece, Barlow lens and camera are placed only AFTER the Herschel wedge. Do not screw the filters into the front of the wedge, they will instantly fail.
2. For visual observations, an ND3 filter is MANDATORY in conjunction with a polarizing or another ND filter, and, if necessary, with an additional light filter (Baader Solar Contiuum or another color filter in the visible range).
3. When photographing the Sun, it is necessary to select a combination of filters. In addition, I do without a polarizing filter. Based on personal experience: to photograph the Sun through telescopes with a relative aperture of f/6 with a Baader Solar Continuum filter (540 nm), I additionally use an ND3 filter, and for photographing in calcium – ND96-09.
4. I DO NOT USE AND DO NOT RECOMMEND calcium filter for visual observations due to the danger to vision.
5. An H-alpha filter designed for shooting nebulae will not show the chromosphere and prominences, since the bandwidth of such filters is too wide. To observe and photograph the Sun in the H-alpha line, use either a chromosphere telescope (Coronado, Lunt H-alpha) or a Quark H-alpha eyepiece.
6. With a DSLR camera (for example, Canon), you most likely won’t be able to focus, because… the wedge “eats” approximately 61 mm of the optical path. With some telescopes (for example, Celestron 102 SLT, Sky-Watcher BK1206) it is necessary to use a short 2″-1.25″ adapter, or use a 2″-T2 adapter.
7. When using a telescope with a diameter of 120 mm or more, the manufacturer recommends additionally installing an Optolong KG3 light filter (KG3-IFIRUV2). For visual observations with smaller apertures with an IR-cut, a filter is desirable.

In what order do I install the equipment:
a) broadband visual observations (Sun in white)
refracting telescope > Herschel wedge > ND 3.0 filter > polarizing filter > IR-cut filter > eyepiece.
The image brightness is adjusted by rotating the eyepiece body together with the filter block.

b) narrowband visual observations (Sun in the 540 nm continuum):
refracting telescope > Herschel wedge > Baader Solar Continuum filter > ND 3.0 filter > IR-cut filter > polarizing filter > eyepiece
The image brightness is adjusted by rotating the eyepiece body together with the filter block.

Клин Гершеля Lacerta с установленным окуляром Svbony zoom 3-8 mm SV215
Herschel Lacerta wedge with installed Svbony zoom 3-8 mm SV215 eyepiece. Additional filters are screwed onto the eyepiece skirt.

a) astrophotography (continuum Sun, wavelength 540 nm)
refracting telescope > Herschel wedge > Baader Solar Continuum filter > ND 3.0 filter > astro camera (monochrome or color).
The image brightness is adjusted by adjusting the gain and shutter speed (usually gain to zero, shutter speed near the minimum).

Lacerta + Baader Solar Continuum + ND3 + ZWO 183MC
Lacerta + Baader Solar Continuum + ND3 + ZWO 183MC
Baader Solar Continuum + ND3 + ZWO 183MC
The Sun, September 20, 2023, 13:38
Sun in continuum, September 20, 2023, 13:38

b) astrophotography (calcium Sun, wavelength 393.3 nm)
refracting telescope > Herschel wedge > Antlia CaK 3nm 393.3nm filter > ND96-09 filter > astro camera (monochrome).
The image brightness is adjusted by adjusting the gain and shutter speed (usually gain to zero, shutter speed near the minimum).

Lacerta + Antlia CaK 3nm 393.3nm+ ND96-09 + QHY5III178m
Lacerta + Antlia CaK 3nm 393.3nm+ ND96-09 + QHY5III178m
The Sun, April 15, 2023, 10:37
Calcium Sun, April 15, 2023, 10:37

The process of my using the Herschel wedge for visual observations:
1. I take an eyepiece and screw filters into it: ND3 (required), polarizing (single, required), Baader Solar Continuum (optional), IR-cut (required). I insert the resulting structure into the wedge and tighten the fixing screws on the visual adapter.
2. I point the telescope at the Sun, guided by the shadow or spot in the inner part of the wedge.
3. I look into the eyepiece from afar (from a distance of 20-30 centimeters), loosen the screws on the visual adapter, rotate the eyepiece along with the filters, ensuring that the brightness of the light exiting the eyepiece is reduced to a minimum.
4. I bring my eye closer to the eyepiece, rotate the eyepiece with filters, and select a comfortable image brightness.
5. I tighten the fixing screws on the visual adapter and begin observations.

Eyepieces
As for eyepieces, I actively use Svbony Zoom 3-8 mm. With a telescope focal length of 400 mm (Levenhuk Ra 66ED), you can fit the entire solar disk in the field of view, and the wide range of focal lengths of the eyepiece allows you to quickly select the optimal magnification and view details in small spots.

Filters
I recommend using filters that have threads on both sides (female inlet, male outlet). Unfortunately, many new ND filters are thin and only have a thread at the outlet. To observe and photograph the Sun in the continuum (540 nm, green region of the spectrum), I use Baader Solar Continuum (necessarily in conjunction with ND3.0/polarizing/IR-cut filters). You can also take a closer look at the Player One Photosphere analogue. The continuum filter significantly improves detail by cutting off the red and blue regions of the spectrum, including residual chromatism in refractors and atmospheric dispersion.

Perhaps the yellow or orange color of the Sun is more pleasing to the eye for some. In this case, instead of a continuum, you can additionally install a color filter (for example, No. 21 or No. 12) to the main block of light filters (ND3/polarizing/IR-cut).

The order in which the filters are installed relative to each other is not particularly important. The main thing is that all the filters are located AT THE OUTLET of the Herschel wedge, and that they can be screwed into each other. It is PROHIBITED to install filters between the telescope and the wedge (that is, in the front part of the wedge) to avoid overheating of the filters and damage to your vision.

Светофильтр НПЗ СЗС был установлен в передней части клина Гершеля. После присоединения к 66 мм телескопу поломка поломка светофильтра наступила через пару секунд. Не делайте так!
The NPZ SZS filter was installed in front of the Herschel wedge. After attaching it to a 66 mm telescope, the filter broke within a couple of seconds. Don’t do this!

To shoot in calcium, I use the Antlia CaK 3nm 393.3nm filter in conjunction with the ND96-09 ND filter. Antlia CaK is an interesting filter that allows you to capture details of both the photosphere and the chromosphere. Alas, it will not be possible to photograph calcium prominences with it. It is PROHIBITED to use for visual observations.

Astronomical cameras
For shooting I use two cameras: a monochrome QHY5III178m (shooting in Continuum, CaK) and a color ZWO 183MC (Continuum).

Summary
Lacerta Herschel wedge 1.25″ is an excellent wedge, I use it constantly, productively and with pleasure. Simple design, low maintenance. Over several years of shooting, I have received a lot of images of the Sun. If you can order such a wedge, or see it on the secondary market – definitely take it.

Similar products
Of the similar products, I also managed to briefly test a similar device from HERCULES. It has a more sophisticated design with an adjustable aperture, and instead of a wedge it uses a 90-degree prism, and there is also a solar “finder” at the output. SUDDENLY, the brightness can also be adjusted by rotating the polarizing filter (single) in the eyepiece. An ND3 filter is required, of course. At the same time, the device from HERCULES is MUCH more compact than from Lacerta, and the bend in the optical axis is 90 degrees. The picture is excellent, no complaints, so I strongly recommend it. You can also pay attention to similar devices from Lunt, Baader, but their cost is higher.

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