GradientXterminator - Brian Fontenot, Author

The weather hasn’t been especially great for view or astrophotography this past month, which might be a blessing in disguise. I took what I thought were going to be some great images of the Whirlpool Galaxy, Messier 51. I had 4 hours of good 5-minute exposures in pretty clear skies, but I guess there are just limits to what you can do when you’re pointing your scope over the umbral of city light.

M51 was one of my first DSLR astrophotography targets almost a year ago. I previously discussed my early process and equipment, but I can at least share one of those older images here for context and convenience.

You can see how the light from the city creeps up into the image. The closer to the horizon your scope gets, the worse it becomes. But, the sky didn’t look too bad that night I set out to grab 4 hours of exposure.

I believe my first mistake was I started too early. The sun was set, but there was still some of its ambience in the “night” sky even though I could make out my DSO of choice. It wasn’t until I started to stack and process the image that I began to see the issue. It’s pretty telling.

Even though I dutifully captured my flat and darks, neither seemed quite prepared to overcome the result. Hour to hour during the image capture, that light you see in the bottom of the photo above as a reference, would shift lower and lower. So after stacking and even using GradientXTerminator, you could still see “waves” of light in the background of my data.

After multiple attempts to process the image in Photoshop, I decided to restack the raw files, but exclude at least the first hour of imaging to reduce the light pollution I was getting. Here’s what that looked like after some basic image adjustments. Because the image is cropped and depending on your screen, it can be hard to make out the waves in the background. Just note the bottom left corner is bright, then there’s a band of darker sky, then some brightness just above the galaxies, and then a darker corner. Also, there was not a lot of color.

Whirlpool Galaxy, M51, stacked and roughly processed image from exposures taken on 2-20-21.

Undeterred, and not having any new data to stack and process, I tried again. This time around, I applied a color-preserving luminance layer to preserve the star color and more gradually stretched the data with the arcsinh functions and balanced the light levels. Still, I think I oversaturated the image. It may look a little blue, but notice there are some orange stars, too.

*Whirlpool Galaxy, M51, stacked and more carefully processed image from 3 hours of exposures taken on 2-20-21.*

While I think this version looks better, if not prettier. It’s still not as clear and as detailed as I would like and I’m still seeing what I can only describe as a “flat” coloration where I would expect to see something that would more resemble nebulosity/clouds.

But really, I probably just need better skies and more Adobe knowledge. Do check out A.V. Astronomy’s video for some good tips and tools for quick image processing in Adobe. I learned about how to create the luminance layer and a method for removing light pollution from his video.

“Vignetting” remains a problem for my astrophotography and has become an even worse since using my new astrophotography camera, a Canon EOS Ra. The pricey camera was a surprise Christmas gift and it works really well, so much so it feels like I just got my learner’s permit and someone handed me the keys to a Lamborghini. But it’s so much more sensitive to light with a larger sensor that it’s almost unusuable in my light polluted sky and backyard.

Reduced, JPEG version of my original stacked TIF file of the Orion Nebula, M42. You can make out the ring vignetting and uneven light gradient interfering with my processing.

But what is vigentting? My understanding is inexpert, but I would describe it as the background light in my photographs not being exposed evenly across the frame. So, my corners will be really dark, the middle of my image typically has a brighter rings, and then the middle is the brightest part. This disformity or unevenness in the light gradient makes balancing the background light and stretching the image cleanly almost impossible (at least for me…). As you try to darken the background, especially the brightest parts, you lose the image or you have this blob of overexposed noise.

There are a variety of ways to remove or reduce the vignetting. One tool I attempted to use after hearing it recommended by AstroBackyard was GradientXTerminator. You can download the tool to try for free for 45 days, but past that it costs money. I think it’s a great tool and would be really useful going forward, but I had limited success with my current batch of stacked in images. Here’s what the TIF image above looks like when GradientXTerminator is used on a medium/aggressive setting.

The image of the TIF file above run through a round of GradientXterminator. It’s a great tool, but my current batch of images are a little beyond its capabilities or my experience.

There are also more manual methods that can be performed in Adobe Photoshop that create fuzzy screening layers to even out the background noise or rely on using the radial gradient tool. I didn’t have much luck with either.

Really what I should have done is take some flat frames. These frames are used by stacking software like Deep Sky Stacker to balance out the background light to remove the vignetting. These images should consists of an uneven frame of light, not too bright, across the entire censor so the exposure artifacts and anomalies like dust can be substracted from your final image.

I used to think the flats were just unhelpful and not that necessary. Whenever I did take them, I either didn’t notice a difference or they made the final TIF file worse. But that’s largely because I was taking them incorrectly.

Unless you have a light panel or light box to stick onto the front of your telescope, you’ll be advised to use the “T-Shirt Method”. This method entails stretching a cotton white T-shirt over the end of your telescope and then exposing the end to an even light source like either the dawn sky, a tuned-down all-white LCD screen or light panel, or whatever light source you can at hand that’s both not too bright and also uniform. The T-shirt helps diffuse the light for a more even spread. Once you have the shirt attached and a light source, you snap some frames, like 20 or 30, at the same ISO setting and focus as the frames you took of your deep sky image, but generally with a much shorter exposure. As I understand it, non-technically, you want to return a grayish frame, not an all white one and certain DSLR cameras have an AV mode that will automatically adjust the exposure time for you to achieve this result.

With these flat frame, the frames of your image (light frames), and some dark frames (and potentially some bias frames but I’m not there, yet), you should be able to composite a much more balance TIF file from your stacking program.

That’s all theory to me for the moment, however. My first attempt at flam frames saw me using a rubber band to affix a few coffee filters onto the opening of my smaller refractor telescope and then pointing the scope at a 100-watt-equivalent LED bulb in a shop light. To make matters worse, I still relied on my intervaluemeter to take the snaps and it couldn’t take snaps of less than 1 second of exposure. So, my “flats” were blowing out my image to the point of uselessness.

See this bright white block. That was one of my first “flat frames”. It didn’t turn out so well.

The next not too cold and humid clear night I get, I’m going to give the flats another shot. I’ve actually bough some brand new T-shirts and figure I can still use the shop light with a lower wattage bulb and possible a second T-shirt over it. I’ll post about how well it goes eventually. Will it work or will I fail? Wait and find out.

In the meantime, here’s an image of my ongoing misadventures in learning to process these images. I guess I inadvertently activated the hidden Sailor Moon plugin.

Tag: GradientXterminator

Swirling Down the Whirlpool Galaxy

DSLR Astrophotography – Vignetting and Flats