Bortle sky and LEDs in Adelaide

[va1erian]

Hi everyone,

I am getting into astrophotography (more exactly, EAA) with my camera ZWO ASI 183MC. My targets are nebulas and asterisms. I discovered that my images become white before nebulas and dim stars pop up. Thus, I realised that the white background was due to the light pollution. Light pollution is fought with the optical filters. The question is, what optical filters ?

Light pollution is usually made by the sodium and mercury lamps which emit as yellow and blue lines in their spectra. Multi-narrowband filters like Optolong L-eNhance are used. But recently, the LEDs started to be installed as street lights. They have continuous spectrum with a heavy blue component. The "notch" filters do not work anymore.

One needs to cut out the "blue" part of spectrum or even the whole visible part, if one wants to image the objects of a continuous emission spectrum, such as stars and diffusion nebulas.

The question is: what kind of lamps are predominant in Adelaide (but I presume the situation is typical for all capital cities in Australia).

I looked up the light pollution map of Adelaide, and I found out I lived in the zone Bortle 5, bordering with the zone Bortle 6. No information about the spectrum.

[Rerouter (Ryan)]

Can't say for your particular region, but the heavy blue component implies a blue excited phosphor type, so they would be fairly wide and flat spectrum,

In this case you would be chasing narrow band filtering or combined narrowband filters to cut out as much spectrum as possible to boost the SNR higher.

[Nikolas (Nik)]

The L-Extreme filter is a better option with blueish light

[va1erian]

The sodium lamps will give yellow and orange glow. The mercury lamps will give the bluish colour with the lines below as is described here, https://en.wikipedia.org/wiki/Mercury-vapor_lamp

404.7 nm violet
435.8 nm blue
546.1 nm green
578.2 nm yellow-orange
650 nm red

Come to think of it, in my suburb, I do not observe any yellow or orange glow. The sodium lamps are found in the industrial areas. Here, it is residential area. The street lights look like the mercury lamps.

Thus, I could have the filters that pass yellow and orange, but block the blue and violet. On the face of it, the Optolong L-Pro should be OK.

I do not think that I should worry about the continuous blue emission of LEDs yet. I have not seen them around.

The other night, I combined my ZWO dual-band filter (15 nm Ha, 35 nm OIII) with the orange filter No. 21. This left me with an affectively H-alpha filter. I noticed that the light pollution disappeared. However, the amount of light coming was low, I had to bump up the exposure and gain of the camera. And I had to do binning, x2 or even x3.

This means that a lot of contamination is coming in the green light around 500 nm. It could be the sodium line, as is shown in the work of the UHC filter exposed to sodium lamp, http://www.astrosurf.com/buil/filter..._pollution.png