New Baader Herschel Wedge Solar Prism

Review by Ted Wolfe

Normally, I image the sun’s chromosphere with H alpha filters, such as the Coronado systems. While I have viewed the next layer down, the photosphere, in “white light” I’d never photographed it.

In the fall of 2009 I began to see descriptions of a new solar wedge from Baader Planetarium, which promised excellent visual and photographic detail of this intriguing region of the sun – along with greatly increased safety. White light observing with a typical Herschel wedge focuses an enormous amount of solar energy which exits the prism. This forms a diffused, but still potentially harmful beam behind the housing, especially after prolonged usage.

Baader had been studying this problem for 15 years. Their new approach involved the use of a “heat cage” (multi-layered perforated steel screen) with a heat absorbing ceramic tile in the back of the housing. Baader believes this combination results in the safest white light wedge on the market.

Additionally, they designed the inner chamber of the prism to be as pitch black as possible. The back of the prism is also multi-coated to reduce light scatter. Baader maintains that this produces a darker sky background for high magnification viewing and imaging without breakdown in the observed features. The new “ND” filters also incorporate special anti-reflection multi-coatings for maximum image contrast.

The new wedge was expected to be available to US dealers in December, 2009. However, delays ensued. Winter became spring, and then summer. Johannes Baader explained the delays were associated with the decision to use a magnesium housing for the new wedge. Magnesium was chosen to reduce weight, but it also dissipates heat faster than aluminum.

I had placed an order for one with OPT, and it finally arrived in mid-September. It came beautifully cased as shown below.

Figure 1.

There are two versions available from Baader – the “V” version which costs $509, and the “P” version which costs $590. V for visual and P for photographic. (By the way, this wedge works only on refractors). The P version has 3 more interchangeable filters for use in changing the amount of light entering the camera. The objective is to get the exposure period down to about 1/1000 second to “freeze” the atmosphere for solar imaging.

I found the directions for using the wedge to be clear, and easy to follow. The safety warnings were prominently displayed on orange pages.

The workmanship on the wedge, connecting adaptor, and filters was simply top of the line. At the time of shipment Baader was unable to include a “Solar Continuum Filter” which is necessary for visual use on achromatic refractors, and recommended for photography on all refractors. In its place they shipped a 2” polarizing filter, which I found essential for good visual observing.

In my case I was unable to reach focus with my 80mm F/6 Stellarvue “LOMO” refractor coupled to my Lumenera skynyx 2.1 camera. The OPT website recommended the use of 2 separate components to reach focus in this situation. You substitute a T-2 1 ¼” Clicklock Eyepiece Clamp with a 2” to 1 ¼” T-2-27 adaptor to reduce the distance between the camera, and objective lens. Since my Lumenera camera has a 1 ¼” tube this looked promising.

These components were available from Alpine Astronomical, which has a large inventory of Baader adaptors, rings, filters etc. I found both OPT and Alpine quite knowledgeable about the Baader product line.

Since I was effectively switching over to a 1 ¼” system I also swapped over my 2” polarizing lens for a 1 ¼” version. Lastly, I found that while nobody had the 2” Solar Continuum in stock Alpine had 1 ¼” versions on hand, and I bought one to fill out my requirements.

 
Figure 2. Shown with 1 ¼” clicklock eyepiece clamp

It worked! I was able to easily reach focus with plenty of room to spare with the items mentioned above.

Baader recommends that you start out initially by using their 1:1000 ND 3.0 filter plus the Solar Continuum Filter for photographic work. I found that my F/6 system could only get down to about 7/1000 second exposures with the ND 3.0. By replacing it with the supplied ND 1.8 + ND 0.6 filters I could take 1/1000 second exposures.

For visual observing I used the ND 3.0 and polarizing filter with my Stellarvue 80mm scope. (The Solar Continuum Filter is not necessary if the refractor is aprochromatic). In the 160 to 180X range I found the view crisp. The system delivered excellent detail in both the umbra, and penumbra regions of sun spots with granulation visible when seeing allowed. The sky was a deep black.

To test the wedge’s heat dissipation features I used low power for a prolonged period. After 45 minutes at 20X, with the sun still centered in the eyepiece, I placed my hand behind the back of the prism housing, close to the ceramic tile. I detected only an extremely small amount of heat.

One real plus from the tile is the dimmed image of the sun, which appears at the back of the diagonal. This allows you to correctly center the sun in the eyepiece from a seated position. See picture below.

Figure 3.

Photographically, I easily captured faculae with the spots when they were rotated into the limb region. In another image below, when a spot grouping was moving down toward the central region of the sun, you can see light bridges crossing the larger spots, and granules in the lower half of the image. (Granules are visible toward the brighter central regions of the sun, since the curving horizon effect in the limb regions masks them.) I took 1500 exposures, each 1/1000 second long, and selected the best 300 on Registax for stacking to acquire the image. A 3X Barlow was used to change the focal ratio of the system to F/18.

Figure 4.

What didn’t I like? If you use the Solar Continuum Filter visually you get an odd color for the sun. It’s a bright lime green, which I didn’t find very pleasing to the eye. I am involved with my astronomy club in Naples, Florida with a lot of public viewings of the sun. It’s confusing for the uninitiated to see the sun in “white light” in one scope, and then in vivid red in another (H alpha). To introduce a lime colored version might just take too much explaining.

However, this does not prevent me from giving the new Baader Herschel Wedge Solar Prism a solid “A” photographically, and visually, especially if you are working with an aprochromatic refractor. Superb workmanship, premium materials, and a safe design all come together very nicely.

Net/net it was worth the wait!