COMPARATIVE TEST OF BAADER AND SCHULER UV FILTERS

There are two main UV filters today on the amateur astronomy market. The classical Schuler Astro-imaging filter that was first built for photometric applications. Since 2004 and the raise of good UV Venus images, Baader Planetarium has introduced the "Venus filter". I own the Schuler since 2003. But over the last years there have been many images made with the Baader that gave me the feeling that the details on Saturn and Jupiter were not completely identical to those obtained with the Schuler. I then bought the Baader in 2006, and before the arrival of complete successful images, here are a some preliminary tests of both filters.

1° COMPARISON OF THEORICAL TRANSMISSIONS

Comparison of the transmission curves in an extended U band. The Venus filter is a 355 nm passband while the Schuler is a 365 nm that corresponds to the Johnson photometric U band.

The Baader's transparency looks better than the Schuler, because it pics at almost 80 % instead of 70 %, and also because its transmission is higher in the band 300-350 nm.

Some idependant measurements of both filters have confirmed the transmission of the Baader (Arnaud Van Kranenburg) but noticeably corrected the Schuler's (Philippe Rousselle), which looks to transmit only 60 % at best with a pic closer to 370 nm. The last thing to note is that this last filter has a higher transmission in the band 370-400 nm.

The interest was then to :

- Verify that the better transparency of the Baader is really sensitive "on the sky"
- Test the possible difference of details on images as the Baader blocks violet light with more efficiency (390-400 nm)
- Evaluate the importance of each infrared leakage.

2° EVALUATING THE IR LEAK : TESTING WITH A REMOTE CONTROL

The existence of a non negligible infrared transmission is probably the most damaging problem for an UV filer, as even a moderate leak could be seen on the image because of the higher sensitivity of the CCD to very long wavelenghts. The old UV glasses like the Schött UG11 were perfectly transparent to infrared and were then to be used with an IR-blocking glass, which unfortunately bloked also a part of the UV transmission. Modern filters such as Schuler and Baader have built-in IR coatings and can be used alone.

However, some elements have raised some doubts about the efficiency of the Baader's IR coating, as a few amateurs get some "ghost images" while imaging Venus. Indeed, Baader indicates the presence of a residual IR transmission near 800 nm but how low is it really ?
http://www.baader-planetarium.de/zubehoer/okularseitiges_zubeh/filterkurve_u.gif

The remote control of house equipments (TV, Hi-Fi...) work with an IR LED. This is a good test because the emission of IR light is important (one can even detect it visually in complete darkness !). The image made with Astronomik IR 807 gives an idea of how bright it is. The LED is still visible with the Baader but almost fully turned off with the Schuler that proves here its high efficiency. Images taken with a Sknyx 2-0M.

The test however is not representative of real sky conditions. One must not critisize excessively the Baader ; its IR leak is really low, much more than some of my other blue filters ; but the problem can comes from the fact that it could be relatively important in front of the weak UV transmission...

3° EVALUATING THE IR LEAK : REAL TESTS ON THE MOON

Here are some lunar shots taken with the filters, the Skynyx 2-0M and a 7" newtonian at prime focus. Each time, a raw image is taken first with the UV filter alone ; then, without any change of the camera settings, a red light filter, the RG 630, is introduced to block ultraviolet but not the infrared. Therefore the IR leak can be imaged directly.

The visualisation of the images are first done with IRIS at a level of 4095 (corresponding to 12 bits), second at 500 to increase the image's brightness.

Two raw frames with the UV filters alone.

With the RG 630. No signal is detected with the Schuler but the Baader does transmit an image : this is the moon as seen thanks to the IR leak. However it looks mild.

Adjusting the visualisation at 500, the IR leak of the Schuler is now detected, but it's much darker than the Baader's, which allows us to barely observe the principal lunar features...

This test does confirm the higher efficiency of the Schuler, but doesn't invalidate the use of the Baader as the presence of IR looks negligible.

4° EVALUATING THE IR LEAK : REAL TESTS ON SATURN

Same comparison method, at prime focus of a 10" cassegrain telescope.

The raw frames are strictly normal, and reveal unambiguously the caracteristics of Saturn in UV : bright rings, dark globe.

The RG 630 detects a very weak image with the Baader, of course nothing with the Schuler.

Visu 500 : Saturn's shape is observable with the Baader but only a vague pattern hardly emerges from the noise background.

5° EVALUATING THE IR LEAK : REAL TESTS ON VENUS

Test conducted under bad conditions but highy interesting...

While the raw frames are blurred, it's obvious that there is a problem with the Baader's image as an abnormal outgrowth...

Images of the IR leaks at same brightness level. Venus is well visible with the Baader, but almost not with the Schuler..

Visu 500 : the IR leak of the Baader is so bright that it's burnt, while the Schuler is just detected....

A Baader image quickly processed. It clearly shows the presence of an important IR leak, found further south because of atmospheric refraction. Here the problem is really not acceptable and is prejudicial to the image's quality. Venus was at an altitude of 20°, common for this kind of observation, although imaging it higher in the sky should solve the problem.

The Baader filter looks therefore problematic for object that are bright in near infrared ; Venus is an example but Mars could be concerned as well, and maybe the Sun ?

6° COMPARING UV LIGHT TRANSMISSION

As first said, one interest of the Baader was its theorical higher UV transparency. During the tests however, it became completely clear that it was the Schuler that gave the brighter image, and by far.

Measures done during the moon test with identical exposure (0,07 seconde)
- Required gain with the Baader to get a correctly exposed image : 10,2
- Required gain with the Schuler : 7,9

This represents an increase of 29 % with the Baader.

Measures done during the Saturn test :

- Schuler exposure time : 0,6 second at prime focus, rings just under overexposure
- Baader exposure time : 0,8 second

The increase is now 33 %, coherent with the moon test.

In term of performance, the Schuler looks really better because it has not only a much better IR coating, but also a 30 % higher UV light quantity. This is considerable when one knows how difficult it can be to image the planets in ultraviolet ! This could be explained by the fact that the Schuler's transmission heads a bit more toward the red end of the spectrum, in violet light where the quantum efficiency is raising fast (near 400 nm).

7° DIFFERENCES OF DETAILS ?

Looking for details differences between the two filters will be the next step. It comes only from empirical observations of some subtile albedo differences on Saturn and Jupiter between my UV Schuler images and some good UV Baader shots I have seen. If real this would be explained by the differences in the band transmissions.

 

RETURN TO THE FILTER PAGE