Introduction

A Herschel Wedge is an instrument used to observe the Sun in white light. It is used to study sunspots and granulation but it cannot be used to observe prominences or flares. For prominences and flares I normally use a Coronado Solar H-alpha telescope to observe/photograph the Sun with, but I recently brought a Baader 2” Cool Ceramic Herschel wedge to observe other aspects of the Sun not suited to H-alpha observing.

Click to Enlarge PHOTO 1. The Baader Herschel wedge, filters and storage case

The Wedge

The wedge is extremely well engineered and designed for use with refractor telescopes with a 2” eyepiece holder. It is stressed that the wedge cannot be used with Newtonian, Schmidt-Cassegrain, or Maksutov telescopes, since the optical set-up of these scopes use optical or mechanical parts near the focal plane, which will be damaged by the concentrated heat of solar radiation.

The Baader Herschel wedge comes in two versions – one for visual use (V), the other for photographic use (P). Both versions contain a 2” neutral density (ND) filter of optical density 3.0 (OD =3.0), and a 2” solar continuum filter (SC), but the photographic version also contains three more neutral density filters (OD = 0.6, 0.9 and 1.8). In Australia the version V wedge costs about $650 while the P version costs about $750-800 depending on where you buy it. The extra cost of the P version is for the additional ND filters - but photos can be taken through the V version. 

The solar continuum filter and 3.0 neutral density filter are pre-mounted into the body of the wedge just below the eyepiece holder of the wedge.

The wedge may seem more expensive than other brands but these brands do not contain the solar continuum filter that can cost $220+ if brought separately. Each extra ND filter costs around $65 and buying them in the P version saves money.

Specifications

• 2” nosepiece
• 2” eyepiece holder with ClickLock clamp and brass ring
• 45° specially shaped internal prism
• Optical length = 114 mm
• Weight:  530 grams
• Prism body:  Dye-cast magnesium
• Heat absorbing tile:  cool ceramic
• Filters:  2”  Solar continuum, neutral density OD = 3.0  (additional ND filters in P version have OD = 0.6, 0.9, 1.8).

How the wedge works

The wedge is inserted into the 2” eyepiece end of the telescope (in place of a diagonal) and an eyepiece is inserted into the wedge’s eyepiece holder. Solar light enters the body of the wedge and falls onto its 45° inclined prism surface. Approximately 4.6% of the light is reflected through the filters and into the eyepiece. The rest of the solar light (95.6%) exists the rear face of the prism and would form a focal point outside the prism body.

The existing beam passes through a ‘heat cage’ that is sealed with a special ceramic heat-absorbing tile. The tile traps radiant heat without overheating its surroundings. The body of the wedge is closed on all sides to prevent any light loss.  A really good aspect of the translucent heat tile is that is also acts as a projection screen showing an unfocussed image of the Sun. Aiming the telescope at the Sun is made easy by this image (see photo 2).

Click to Enlarge PHOTO 2. The wedge mounted on the reviewers telescope. The bright image of the Sun on the rear of the wedge helps to aim the telescope at the Sun.

At the top of the wedge a 2” clicklock clamp securely clamps any 2” eyepiece or 2”/1 1/4” reduce with only a small rotation of a lever.

Inside the wedge are two 2” filters mounted above the exiting side of the Herschel prism. Closest to the prism is the solar continuum filter with its strongly reflecting surface facing the prism. Above the SC filter is the neutral density filter OD 3.0 serving as energy reducer. This orientation of filters ensures a solar image free of unwanted reflections.  See photo 3.

Instructions provided with the wedge show how to replace the solar continuum filter with another neutral density one if you want to vary the amount of light entering the eyepiece or camera.

Click to Enlarge PHOTO 3. The prism body opened to show the two filters.

The solar continuum filter

A feature of this wedge is the inclusion of a solar continuum (SC) filter. This is a narrowband filter featuring a 10 nm half-bandwidth at the passband of 540 nm. This produces a lime green image. Human eyes are most sensitive to this wavelength and solar granulation is most contrasty. When using the SC filter, you are observing at a narrowband wavelength and not white light (which is a combination of all visible wavelengths).  The SC filter excludes the red and blue wavelengths and centres on the peak visual wavelength where the telescopes optics are sharpest and free of chromatic aberrations.

The Baader wedge is the only one that comes with a SC filter but if you have another brand of Herschel wedge you can buy a Baader Solar Continuum filter to fit it.  The SC filter can be removed if you want a white light image (but leave the ND filter in place or add another one).

Note: Some wedges use a polarizing filter rather than a solar continuum filter (in conjunction with a neutral density filter) to vary the amount of light entering the eyepiece. According to Baader, polarizing filters are suited to visual use – they do not provide sufficient blocking by themselves nor are they able to block IR wavelengths as completely as a ND filter.

Obtaining focus

I have my Baader Herschel wedge mounted on a William Optics 81 mm GTF refractor scope and use a Baader Hyperion 17 mm eyepiece in 2” or 1 1/4” configuration. Both configurations came to focus. You have to experiment with different eyepieces to see which suits your scope (some do not provide enough back focus or in travel). But the Baader instruction sheet provides suggestions to try if having trouble.  If you wish to observe with 1 1/4” eyepieces, use a reducer adaptor  2” to 1 1/4”. The 2” clicklock eyepiece clamp on the wedge can also be removed and replaced with a shorter one to bring the eyepiece closer to the focal plane.

I found the image produced by the Baader wedge to be excellent – nice and sharp without any chromatic aberrations (right to the edge of the field of view). This is helped by having very good optics in the telescope I use. However, because of the narrow band of the SC filter, chromatic aberrations are not really a problem.

The system delivered excellent detail in both the umbra, and penumbra regions of sunspots with granulation visible when seeing allowed. The sky was a deep black with no scattered light or unwanted reflections.

Photographical images

I take single exposure images through the wedge via a Canon 550d  SLR camera at 1/320 sec and ISO 200 using manual focus (with SC and ND 3 filter). The camera screws onto the eyepiece. The raw image is lime green, but during processing I change the colour to a pale yellow to better match the Sun. Photo 4 to 6  show examples of the images formed.  The images show really good sunspot detail and surface granulation.  Faculae are often visible near the solar limb.

Click to Enlarge PHOTO 4 Lime green raw image

Click to Enlarge PHOTO 5 Processed image of whole disc		Click to Enlarge PHOTO 6 Processed image of sunspot group

If the atmosphere of Earth is rough/turbulent you will see a rippling effect around the limb of the Sun. It is best to wait till the atmosphere settles down.

Summary

Pros

• Very well engineered wedge (solid as a rock); superb optically
• Ceramic tile does not get hot (other brands of wedges do)
• Translucent heat tile acts as a projection screen showing an unfocussed image of the Sun – great for aiming the scope at the Sun
• Variety of filters to vary brightness of Sun to suit your needs
• Comes in an aluminium case with foam internal padding.
• Very good instruction manual (you can download before you buy).
• No internal reflections or chromatic aberrations.

Cons

• Expensive
• May have trouble getting image to focus in some scopes and with some eyepieces.
• Requires responsible handling.
• Must be used in conjunction with a good refracting telescope. Not suitable for use with SCT, Newtonian or Maksutov telescopes.

Overall, I highly recommend the Baader 2” Cooler ceramic Herschel wedge as a useful tool to observe and photograph the Sun through esp. at white light or 540 nm wavelength.
It is well suited to refractor telescopes upwards of 80 mm aperture.

I have found the wedge to be optically better than Baader AstroSolar Safety film filters and much superior to glass solar filters. A reminder – DO NOT ever use glass solar filters that fit inside the eyepiece (they will break with solar heat and damage your eye).

Notes

1. The author of this review is Dr John Wilkinson – science educator and author of science/astronomy books. He operates an observatory from Central Victoria in Australia.
2. Disclaimer: In closing, I should mention that I have no affiliation with Baader Planetarium or the Australian supplier - Extravision (Australia), other than as a paying customer. The wedge can be purchased through Baader Planetarium retailers.

Review by Dr John Wilkinson (John W). Discuss this Review on the IceInSpace Forum.