CONFRONTO CON IL FILTRO H-ALFA BAADER - Skypoint Srl

More documents

Recommendations

Info

SKYPOINT s.r.l. VIA ZORUTTI (S.S.13) 145/11 CAMPOFORMIDO – UDINE – TEL 0432 652609 FAX 0432 663473 www.skypoint.it e-mail: info@skypoint.it 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 <strong>CON</strong>TRAST 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
SKYPOINT s.r.l. VIA ZORUTTI (S.S.13) 145/11 CAMPOFORMIDO – UDINE – TEL 0432 652609 FAX 0432 663473 www.skypoint.it e-mail: info@skypoint.it 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:
Page 1: SKYPOINT s.r.l. VIA ZORUTTI (S.S.13
Page 5 and 6: SKYPOINT s.r.l. VIA ZORUTTI (S.S.13
Page 7: SKYPOINT s.r.l. VIA ZORUTTI (S.S.13

CONFRONTO CON IL FILTRO H-ALFA BAADER - Skypoint Srl

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?