A Method of Determining the Bortle Class at my Observation Site

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rifleman175
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A Method of Determining the Bortle Class at my Observation Site

Post by rifleman175 »

Having fallen into the rabbit hole of astrophotography for 2 years, I have plenty of imaging nights under Singapore's obnoxiously light-polluted skies. As such, I have been mostly limited to doing narrowband imaging with monochrome astronomy cameras to minimise the adverse impact of these manmade photons on those precious photons from light years away.

It is common knowledge in the amateur astronomy community that Singapore's light pollution falls somewhere into Bortle class 8-9, with some even claiming that it is actually Bortle 9+. Obviously the exact light pollution level will vary with one's location on the island, with the downtown area and Changi airport being brighter than more "rural" areas like Lim Chu Kang. However, I would like to know with some certainty the light pollution intensity in my observation/imaging site close to the Sengkang MRT.

As such, I decided on adopting the following approach:

1) Determine the light pollution rate from the bias, flat, dark and light frames taken under a clear sky with the Moon below the horizon. The light pollution rate (expressed in electron/s) is then matched it to the theoretical sky background electron rate for a given focal ratio, pixel size, quantum efficiency and filter bandwidth from Sharpcap's Sky Background Calculator at http://tools.sharpcap.co.uk/. The Bortle scale and sky magnitude is then displayed in the Sharpcap website.

2) A visual confirmation is done to determine the dimmest star visible on a moonless clear night at similar altitude to the light frame used in step (1). This will then cross-check whether the results from (1) is accurate.

I encourage everyone to give an assessment of your own observation site as well, so that this information may be helpful to others in this amazing hobby as well.
rifleman175
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Re: A Method of Determining the Bortle Class at my Observation Site

Post by rifleman175 »

I am using a ZWO ASI2600MM Pro camera for most of my astrophotography works, so the test will be done with it.

The formula for the light pollution rate is given as such:

light pollution rate = (median ADU of 1 light frame - median of master bias frame) * electrons per ADU / the exposure length of the light frame

In the above formula, one needs to select a light frame the light frame taken on a clear and moonless night at altitude of about 55 degrees with the specific telescope, camera and filter combination. In order to accurately measure the sky background brightness, the electrons per ADU of the camera must be first determined accurately. This can be done either by looking up the value published by the camera manufacturer at the corresponding gain values, or by using the Basic CCD Parameters script in Pixinsight to calculate it. In the later method, one needs to take 2 bias frames, 2 flat frames and 2 dark frames taken at the same temperature. Typically I take the frames as part of the calibration frames in my pre-processing workflow, so the temperature is standardised to -5 degrees Celcius. The frames are loaded into the script, with the camera properties modified to fit your camera parameters. Clicking on "Report" would generate the measurement results as follows:

Image

For my ASI2600MM pro camera, the electrons per ADU can be read out in the "Gain" row as 0.252 electrons/ADU.

The median ADU values of the light frame and master bias frames may be read with the Statistics process in PixInsight (or other image editing programmes). For my case, the median ADU of the 300s light frame and master bias frame is 658 and 501.667 respectively.

Plugging in the values into the light pollution rate formula yields:

light pollution rate = (median ADU of 1 light frame - median of master bias frame) * electrons per ADU / the exposure length of the light frame = (658 - 501.667) * 0.252 / 300 = 0.131 electron/s

The next step would be to go the the Sharpcap website at http://tools.sharpcap.co.uk/, then key in the relevant telescope, camera and filter parameters into the website. The quantum efficiency of the camera is read off the QE chart provided by ZWO at the Ha emission wavelength at 58.5%. Note that the light frame I used is taken with an Antlia 3nm Ha filter, so I input 3 nm into the bandwidth tab.

Image

With the parameters keyed in, I adjusted the bortle class number until the the reported sky electron rate matches to the light pollution rate calculated above. In this example, the 2 figures match when the Bortle class is 7.0.

In summary, the light pollution at Sengkang is around Bortle 7.0 at altitude of 55 degrees looking South from the above analysis. This seems to be much better than what I would expect from Singapore!
rifleman175
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Re: A Method of Determining the Bortle Class at my Observation Site

Post by rifleman175 »

Based on the results from image analysis, it would seem that the limiting naked eye magnitude at my observation site in Sengkang should theoretically be 4.8. However, I regret to say that I am unable to see stars so dim even after having reasonably dark adapted. Under the best conditions, I was able to observe most of the stars making up the constellation Scorpius and the teapot asterism in neighbouring Sagitarrius, which reaches about 4 magnitude at 45 degrees above the southern horizon. On a few clear nights with exceptional seeing, I have managed to spot a few dimmer stars in the shield of the hunter Orion, which puts the limiting magnitude to about 4.4. This puts the light pollution estimate to be about Bortle 7.8 near zenith, and to Bortle 8.0 at 45 degrees altitude.

Compared to the value of Bortle 7.0 measured with the camera, I would lean towards the camera reading as it is more objective than the human eye in perceiving brightness, and is not affected by the degree of dark adaptation in a relatively bright lit urban environment. Having said that, the value obtained from the image analysis tends to underestimate the light pollution as I used a 3nm ultra narrowband filter, which selectively filters out most of the offending light pollution from non-LED street lights. The Bortle class of 7.0 is a representation of the light pollution as viewed in the Ha region, which is still substantial considering that it is a narrowband filter.

I am sure that in a truly dark environ without the interference of artificial lighting, the eye should be able to spot magnitude 4.5 down to an altitude of 45 degrees in a clear and calm night. As such, I estimate the light pollution as follows on a moonless clear night:

45 degrees altitude to zenith: Bortle 8 to 7.5
Below 45 degrees: 8.5 to 8
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