CONFRONTO CON IL FILTRO H-ALFA BAADER - Skypoint Srl

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CONFRONTO CON IL FILTRO H-ALFA BAADER 

Recently, Baader Planetarium, a manufacturer and distributor of astronomical equipment in Mammendorf, 

Germany, introduced a Hydrogen-alpha, or H-a, 656 nm filter specifically designed for astrophotography 

using CCD cameras. The filter is stated to have a FWHM (full-width at half-maximum intensity) of 7 nm and 

a transmission of 90% and is made with scratch-resistant coating technology. The interesting aspect of this 

introduction is the list price of $335 for a 50.8 mm diameter filter. This price includes several markups for the 

primary and secondary U.S. distributors. This is considerably less than list price of the 4.5 nm Custom 

Scientific H-a filter at $595 or 6 nm Astrodon at $695. The purpose of this study is to compare the Baader 

filter with the Astrodon 6 nm H-a filter. 

Spectral curves were obtained using Varian Cary 500 high resolution spectrophotometer. 

The Astrodon 6 nm filter reaches its maximum throughput of 92.3 %T at the H-a emission wavelength of 

656.3 nm. The Baader 7 nm filter reaches its maximum throughput of 89.1%T at 657.5 nm. However, it only 

reaches 85.8 %T at the H-a emission wavelength. 

Some manufacturers design their peak transmissions on the high side of the emission wavelength, since the 

passband shifts toward shorter wavelengths with faster optics. Studies underway show that the Astrodon 

filter will maintain its >88%T specification with systems slower than about f/4.5. An angular study has not 

been performed on the Baader filter. However, if this spectrum is representative, its use with f/7-10 systems 

will be degraded due to its 85%T relative to 92%T of the Astrodon. This goes to the Signal (S) part of the 

equation to determine the signal-to-noise, S/N, of the filter. 

Narrower filters are nearly always preferred. The problem with narrower filters is manufacturing 

reproducibility. For example, Custom Scientific 4.5 nm FWHM filter are specified to have maximum %T 

(throughput) >70%. Some filters have 90% and others near 70%. This does not reflect negatively on the 

quality of these filters. It is just a recognition of the fact that narrow filter are more difficult to make 

CONSISTENTLY. We do not have access to different lots of Baader H-a filters, but we can present such 

lot-to-lot data for recent Astrodon 6 nm H-a filters, as shown below:




Three of the most important parameters in narrowband filters are the FWHM, the maximum %T and the out-of-band 

blocking. We have discussed the first two, and most people are somewhat familiar with what they mean. The out-ofband 

blocking is very important. It refers to the degree to which light is rejected OUTSIDE of the passband. Poorer outof-band 

blocking will let more light come through your filter, degrading its contrast. 

Realize that these H-a filtes let light pass in about a 10-12 nm region, whereas the silicon-based CCD detectors used in 

amatuer cameras are sensitive to light from the ultra-violet (UV) to the near-infrared (NIR) up to about 1100 nm. Thus, 

more and more coating layers are needed to eliminate the potential light leakage of the filter outside its passband. If you 

integrate the area under the curve from 300 to 1100 nm compared to just the area under the curve of the narrow 

passband, a small amount of out-of-band leakage adds up quickly. This, in fact, is how you characterize the CONTRAST 

of a filter relative to a chosen detector and light source. 

You cannot see how much light is leaking through the filter outside the passband in %T plots, as shown above. The 

signal would be




The comparion of out-of-band blocking between the Astrodon 6 nm and Baader 7 nm H-a filters is show 

below: 

The NIR (>700 nm) difference is large. Both filters have excellent out-of-band blocking at shorter 

wavelengths of OD 6. However, the Baader filter has significantly poorer out-of-band blocking at NIR 

wavelengths where many CCD detectors still have considerable sensitivity, such as the KAF3200ME, 

KAF6303, back-thinned detectors and the new large KAF16803 andKAF09000. This difference is nearly 3 

orders of magnitude and certainly explains part of the cost difference. 

This would not be acceptable for professional filters, and Astrodon (and Custom Scientific) filters are made to 

professional standards. How does this affect the type of astrophotography mostly done by amatuer 

imagers? It will likely not make much of a difference for bright objects, such as M16, M20, the Veil 

Supernova remnant, etc. As with narrower filters, it will manifest itself in the fainter areas and fine detail, e.g. 

contrast. 

To test NIR leakage, I used an Apogee Alta U16M CCD with a Kodak KAF16803 large format CCD 

operated at -20C in my dimly lit office with a standard flashlight directed at the ceiling. A flashlight was used 

over just a few minutes of time, so changes in lamp intensity should not be a factor. Further, the overhead 

room lights are the coiled fluorescent bulbs, and a continuum from an incandescent bulb with NIR intensity 

was most appropriate. A calibrated light source is unnecessary to demonstrate the difference in S/N and light 

leakage in these tests. The camera was pointed up. A 2" nose piece was secured into the camera holding 

a standard 48 mm Astrodon Schuler photometric (Bessell, Johnson-Cousins) IR filter made with 3 mm thick 

piece of Schott RG9 glass and 1 mm of KG4 glass. This filter is used to compare the IR leakage of the two 

H-a filters, because it blocks the filter's bandpass and all shorter wavelengths. It's scan (Is) is shown below:




MaxImDL was used to collect the 4s scans and CCDStack was used to define identical regions on all scans 

and compute the image statistics. 

LEFT: Apogee U16M with Bessell IR filter in 2" nose piece 

CENTER: H-a filter placed over opening above IR filter 

RIGHT: Stack of AP tubes pointed at dimly lit ceiling




LEFT: Photometric IR filter only MEAN ADU 64,074 (saturated) RIGHT: Astrodon 6 nm H-a with ~6 OD Blocking MEAN ADU 332




Baader 7 nm H-a with ~ 2.5 OD Blocking Mean ADU 10,022 

Thus, even blocking nearly all UV and VISIBLE light with the IR filter, the KAF16803 detector in the CCD 

was saturated with only a flashlight pointed to the ceiling in these tests. The Astrodon 6 nm FWHM H-a filter 

rejected all but 332 ADU, whereas the Baader 7 nm filter allowed 10,022 ADU. Thus, the Baader allows a 

factor of 10022/332 = 30.2 times the amout of NIR light through that the Astrodon filter rejects with this 

camera and broadband incandescent light source. This, of course, is expected from the spectral scans that 

show considerably more NIR light leakage from the Baader filter. [Note - there was a dark horizontal band in 

the Astrodon image above in the original posting. One of the three frams showed this for unknown reasons. 

The above represents a second FITs file take at the same time. The earlier results showed 365 mean ADU 

and this showed 332 mean ADU. Thus, this small change is likely within experimental error and does not 

change any conclusion]. 

So, what does all this mean for imaging? As most of you know or suspect, attractive H-a images can be 

taken with a variety filtes from the wide 13 nm Astronomik, the 9 nm Astrodon-Schuler, the 7 nm Baader, the 

6 nm Astrodon, the 4.5 nm SBIG/Custom Scientific and the special order Custom Scientific 3 nm. There is 

no right answer because it depends upon your budget and your objectives for H-a imaging. Greater 

transmission with the same FWHM increases S/N. Narrower filters with the same transmission increase 

S/N. Thus, with the above optical performance, you will more likely be looking at subtle differences in 

contrast with lower level, or background features. With this in mind, we will determine the S/N of several 

areas of the Eagle Nebula, M16 taken on the same night under idential condtions with the U16M on an 

RCOS 12.5" RC reflector with the Baader 7 nm and Astrodon 6 nm H-a filters. The S/N was determined in 

MaxIm, using the following formula (ADU was multipled by the camera gain of 1.2 to convert to e-): 

S/N = [(signal - background) ] / sqrt [ Signal + background standard deviation^2 ]




The regions shown were taken in areas without stars. The same dark background region was used (not 

shown). The Baader 7nm H-a filter is on the left and the 6 nm Astrodon H-a filter is on the right. A brighter 

region next to the Eagle Nebula is analyzed first. 

Baader S/N = 3.69; Astrodon S/N = 4.10 11% improvement with Astrodon 

A darker region further away is analyzed next: 

Baader S/N = 0.92; Astrodon S/N = 1.11 21% improvement with Astrodon. 

A brighter region with ridge structure inside the nebula resulted in an improvement in S/N with the Astrodon filter of only 

4%. This is consistent with the expectations described above. Both filters produce a pleasing image of the Eagle 

Nebula. The advantages of the narrower filter with slightly higher transmission and much better out-of-band blocking 

show up in fainter areas and are thus more subtle. Again it really depends upon your goals in astrophotography. 

Astrodon will continue to make the best off-the-shelf narrowband filters possible for astrophotography, even if they are 

more expensive. We will not sacrifice blocking, transmission, prevention of halos, or manufacturing consistency for a 

lower narowband price. For some of you, this is overkill. For others who have spent thousands or even tens of thousands 

of dollars or Euros on scopes, mounts and cameras, we offer you filters wihtout compromise. 

In short, as in any other aspect of life, there is "no free lunch".

CONFRONTO CON IL FILTRO H-ALFA BAADER - Skypoint Srl

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