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The picture with band filters stetta, specifically the H-Alfa, has become very popular. The H-Alpha is the ideal tool to be able to observe the structures of emission nebulae because it provides a high contrast and you can take pictures even in the presence of the Moon or light pollution.
The H-Alpha filter Astrodon has become the preferred choice by Astrophotographers for the following important reasons: p>
The image above shows the remnant of a supernova in the Vela and was obtained with a sensor on KAI11000 Takahashi FSQ106 and H-Alpha filter Astrodon. Image is of John Gleason. All rights reserved. P>
The value FWHM (width of the bandwidth at half maximum of the transmission curve) is 5nm. In the construction of dichroic filters, narrow band, you are reached often a compromise between a value of FWHM very low (sharp bend) and a transmission peak as high as possible. For this the filter Custom Scientific H-alpha 3.5nm has a peak transmission of 72% while the Astronomik 13NM has a transmission peak near the 95%. The Astrodon 5nm, despite having so a sharp turn, currently reaches a peak transmission of 90-92%, which is quite remarkable. For example a Astronomik 6nm not get on average 84%.
The bandwidth of dichroic filters narrow band moves to lower wavelengths with optical short focal ratio. A filter H-alpha very narrow band designed for a perspective f / 10 may not provide much signal when used with a view f / 3.3 given that the bandwidth of the filter may have shifted under the length of emission dell'H- Alfa of 656.3nm. Astrodon 5nm filter has a bandwidth wide enough to support change optical configuration as described above without transmission loss.
Recently, the Custom Scientific has offered the market a filter H-Alfa 4.5nm that overcomes the problems of the previous model . The figure below shows two shots of the Crescent Nebula (NGC6888) obtained with a filter Astrodon H-Alpha (right) and with a filter Custom Scientific H-Alfa 4.5nm (left). The images were taken the same night in sequence, with the same instruments and consist of 6 exhibitions 5 minutes each then processed in the same manner with Maxim DL. You may notice the greatest contrast provided by the filter Astrodon right. The rectangle is drawn in the same region to use the Maxim DL: The table shows the highest value of the mean and standard deviation of the signal, resulting in greater contrast filter Astrodon. P>
| H-FILTER ALFA strong> td>|| MEDIA ADU strong> td>|| SD strong> td> tr>|| Astrodon 6nm td>|| 16279 td>|| 5919 td> tr>|| CUSTOM SCIENTIFIC 4.5nm td>|| 13114 td>|| 5265 td> tr> tbody> < / table> div>|
Many astrophotography using the filters H-Alpha, SII and OIII to obtain images similar to the Hubble. The photo shows an example of what can be achieved by shooting a region of the nebula North America with a Takahashi FSQ106N and a CCD sensor KAF3200.
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