Herschel Wedge Shootout (E)

Lacerta vs. Intes

[Go to Dutch version]

For many years I used a Herschel wedge from Intes to observe and image the sun in white light. Optically the Intes is superb, mechanically however it is far from top notch. So it was about time for a change. At ATT in Essen/Germany I bought the new 1.25” Lacerta Herschel wedge. Because the new Lacerta wedge is better built, safer and yet affordable, I decided to go for it. I chose the 1.25” version including 1.25” nosepiece and eyepiece holder (€ 163). It looked like a good idea to me to compare the New Lacerta Herschel wedge with my old one from Intes.

overzicht

Lacerta wedge (left), Intes wedge (right)

Though the ‘ancient’ Intes wedge is optically fine, it has its drawbacks. As is known, 5% of the sunlight is used by a Herschel wedge. The redundant 95% of the focused sunlight leaves the prism through a mirror at the bottom. You should be be very careful how to point the wedge, to avoid eye injury to folks around. When pointing the wedge straight down however, the focused sunlight might set your shoes on fire!

For the remaining 5% a number of filters has to be applied to dim the light to ambient levels, for example a ND3 grey filter for imaging or ND4 for visual observations, finished with a Baader Solar Continuum filter or a variable polarizing filter. So when using the Intes-Herschel wedge you should know exactly what you are doing!

The new Lacerta wedge is much more secured. There is no dangerous sunlight coming out. The backside contains a finned heat exchanger which catches the focused sunlight. Moreover it is provided with a built in ND3 gray filter.

Besides, the Lacerta wedge features a significant, so called the 56.6° Brewster-angle that provides 100% polarization of the light; this means that the Lacerta Herschel wedge offers continuous dimming with an additional (optional) polarizing filter. If the polarizing filter is screwed into the bottom side of the eyepiece, the overall brightness can simply be adjusted by rotating the eyepiece in the holder.

Practice

I tested both wedges, visually as well as photographically, May 26th, 2016.

Visually

The used telescope was a Televue 76 refractor, with a 7mm Tele Vue Nagler eyepiece (69x). I used grey filters (at the Intes), polarizing filters and the Baader Solar Continuum filter.

Intes

No need to discuss the Intes wedge. It works fine, as long as you take the necessary precautions against escaping sunlight and use the right filters in the right place and in the right sequence. Visually, the Baader Solar Continuum filter delivers an image rich in contrast, with granulation easily seen, as well as sunspot details like structures within the penumbrae.

Lacerta

About visual observations through this wedge only good news. Contrast is fine. With the Baader Solar Continuum filter as well as the polarizing filter numerous details are visible: granulation, faculae and sunspot details at 69x. Adjusting the image brightness by rotating the eyepiece works very comfortably. The wedge housing does not get perceptibly warm, not even the heat exchange part. As far as safety and comfort go, the Lacerta is a massive improvement on the old Intes wedge.

Imaging

For imaging I used the above mentioned Televue 76 refractor and a Coronado Cemax 2x barlow. Filters I did use:

  • a Mega-9 706nm narrow bandpass filter (TiO-line, near infra-red);
  • and the above mentioned Baader Solar Continuum filter (540 nm, green light).

De used camera was a DMK 41AU02.AS ccd-camera.

Intes

The intes wedge has proven its optical quality long since. I took many sun pictures with it, in which granulation is seen all over the solar surface. Faculae were reproduced clearly as well. Especially the 706 nm TiO line filter provides beautiful images because of the photosphere being rich in contrast in the TiO (titanium dioxide) line and the near infrared being less vulnerable from seeing effects.

Lacerta

The lacerta wedge demands much back focus. At first the camera did not even reach focus. Fortunately I could solve this problem by changing the camera nosepiece adapter by a short one which I found in my astro junk box.

But after reaching focus some naughty image issues showed up. Using the Baader Solar Continuum filter ghost images turned up; contrast was spoiled and at sunspots were clearly doubled.

The 706 nm TiO line filter indeed produced a razor sharp and contrast rich image, but a serious  vignetting as well, shifting to almost black at the image edges in prime focus.

Second attempt

At first I presumed the 100% polarizing grade of the Lacerta Brewster wedge might have something to do with it. Normal polarizing- or greyfilters did not cause trouble, but as soon as an interference filter with mirror coatings was applied to, such as the Baader Solar Continuum filter or the 706 nm TiO line filter, things got out of hand.

At a new attempt a few days later I saw a blurred bottom edge; the focal plane was apparently tilted.

I decided to check if tilting the wedge by setting the adjusting screws at the side could help. And it did! After some trial and error the image was sharp all over. After that the imaging results, both with the Baader continuum filter and the 706 nm line filter were fine as well; both ghosting and vignetting were gone!

Setting screws for tilting the wedge

Setting screws for tilting the wedge

Unlike the old Intes wedge, that isn’t adjustable at all and always appears to be spot on, the Lacerta obviously needs secure adjustment of the setting screws. No problem – once you know it.

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Intes wedge, primary focus, TiO line filter

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Lacerta wedge, primary focus, TiO line filter, before collimation, strong vignetting

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Intes wedge, 2x barlow, TiO line filter

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Lacerta wedge, 2x barlow, TiO line filter, before collimation

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Intes wedge, 2x barlow, Baader continuum filter

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Lacerta wedge, 2x barlow, Baader continuum filter, before collimation, ghosting

Lacerta wedge, 2x barlow, Baader Solar continuum filter, after collimation

Lacerta wedge, 2x barlow, Baader Solar continuum filter, after collimation

Lacerta wedge, 2x barlow, TiO line filter, after collimation

Lacerta wedge, 2x barlow, TiO line filter, after collimation

Conclusion

Whether you want to observe the sun in white light,  want to use a Herschel wedge for public purposes, or want to go for imaging the sun with advanced filters, this new Lacerta wedge is an excellent and safe choice with – attached to a good scope – good contast and a wealth of details. Keep in mind it is essential to collimate the Lacerta carefully with help of the setting screws, to prevent imaging problems like ghosting or vignetting.

For public purposes the Intes wedge is not safe, although optically it easily keeps up with the Lacerta.

And never those burning shoes again!

 

Pieter Welters

 

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