Using a light pollution filter to combat sky glow
Astrophotographers use one of two types of cameras: one shot color, and monochrome. I prefer monochrome. It allows me the freedom to capture narrow band images as well as RGB images. When using filters, however, one needs to worry about filter vignetting.
The biggest challenge with deep sky astrophotography I have is light pollution. Where I live, and depending on the night (Friday night football games are very common in my area) the light pollution limits my sub exposures to less than two minutes. Anything exposed over 90 seconds is even questionable. The amount of sky glow that can saturate my image sensor. Because of this, I can benefit from the use of a light pollution filter. The major drawback to using the light pollution filter is the fact that I’m filtering light that has already been filtered by a Red, Green, or Blue color filter. Such ‘double filtering’ means that I will require long exposures. I also run the risk of uneven field illumination due to filter vignetting.
The first post of my DSO with the ASI174MM camera series explained how I plan on using RGB filters and a light pollution filter in my optical train. As you can see from the map, where I live has lots of light pollution. I need a way to combat the light pollution using filters.
Taking RGB plus light pollution filter exposures
My first field test to find filter vignetting actually took place on August 15th. The delay in getting setup under the stars was largely due to life getting in the way.
Enough about me. As the sun set, the temperature slowly crept below 90 degrees F. The temperature didn’t fall much further below 85 degrees F by the time I wrapped up. On a hot August night and under light polluted skies I attempted some Andromeda (M31) images. I slewed to Andromeda around 23:00 local time, and fired off a few sixty second red filter plus light pollution filter exposures. To my surprise, I was able to see quite a bit of detail in that exposure! To my horror, I was also able to see way too much amp glow and sensor noise! The amp glow and sensor noise made it nearly difficult to find any filter vignetting in the sub exposures. I needed to attempt some ‘lab’ experiments to eliminate the amp glow and other sensor noise issues.
I decided to do a proper filter vignetting test
I used my home office as a test environment when acquiring the test frames. My first order of business was to get a good handle on using APT for image acquisition. I quickly learned the basic ins-and-outs of how to take an exposure that was not overly exposed. I pointed my scope at my 24″ monitor and proceeded to create several libraries of 5 flat exposures in various configurations. Each library of flat frames consisted of three focuser positions:
- All the way in
- Half way out
- All the way out
Each of the above three focuser positions were tested with three filter combinations:
- With no filters
- A green filter
- A green filter plus light pollution filter
The flats were saved as .FITS originally by APT. Keeping the files in their native FITS format, I dropped them into Deep Sky Stacker with some random other frame as a “Light” frame. I created the 9 master flat files from the source images. I combined them into a single image showing the results in a matrix. Keep in mind that the exposure times were identical for each test. The more restrictive test involving the green filter and light pollution filter is very dark. Regardless, you can see there is nearly no sign of filter vignetting caused by having the light pollution filter in the nose of the filter wheel.
Light pollution filter vignetting test conclusions
I’ve concluded that it doesn’t cause a much filter vignetting when the light pollution filter is placed in the nose of the filter wheel. This is good news for me, because my original thoughts were that this is precisely where the vignetting would occur. Now I know my filter wheel will not cause significant vignetting when combined with a light pollution filter. I can begin working on other tests to quantify the sensor noise generated by the imaging chip during long exposure photography! The more I test the camera, the more I begin to believe that I can do long exposure deep sky astrophotography with it!