Imaging in Severe Light Pollution: Part 2 – Modified DSLR and H-alpha

My slow conversion into a modified camera user shooting narrowband images accelerated last night. Under a very bright, and very full moon I was able to capture some of the best data I ever have. In Part 1 of Imaging in Severe Light Pollution, I talked about four things to help your images in the worst of pollution.

1. Light Pollution Filter
2. Astrotortilla
3. Dithering
4. Settings in DSS

Patience was also in there, but that was more of a reminder that this hobby can’t be hurried, and its even truer in polluted skies. Though Part 1 was written with an unmodified camera in mind, these four things still apply when using a modified DSLR. So what other two things will I hit on in Part 2 that can help get your images to the next level?

1. A Modified Camera
2. An H-alpha and/or other narrowband filters such as OIII or SII

I needed to increase the amount of data I acquired and reduce the amount of time it took to obtain it. I was growing more and more frustrated over the Winter trying to obtain enough data in the short windows I have to create complete images. While I’m happy with the resulting image I got of the Rosette nebula, that single object consumed most of my imaging for the Winter.

I ripped apart my camera, removed the LP1 (anti-alias/moire) filter, and replaced the LP2 (H-alpha cutoff) filter with a Baader BCF-1 filter. There’s is some more information in Part 1 and Part 2 of my camera modification posts. I could have done a naked sensor mod and put in a clip-in IR cut filter for a little cheaper, but I went with the in-camera solution so I could still use the camera for daytime shooting with autofocus intact and I like the idea of a permanent IR/UV filter rather than dealing with additional filters.

I did some quick and dirty testing the night before, and the night of the modification. Here is the Soul nebula. On the left is 48 minutes of data before the modification, and on the right is 48 minutes of data after the modification. Both consist of stacked 120 second exposures at ISO1600 and were “processed” similarly. They aren’t pretty, but were purely for testing purposes.

In this testing scenario, the difference is undeniable. It looks like I have tackled another part of getting my images to the next level while increasing my imaging efficiency. But there was still one other thing I wanted to tackle. Something to give my images pop. Most of the best images you see out there are H-alpha + RGB. Not having a mono camera, I hadn’t really looked at narrowband imaging until I saw some incredible results over on AstroBackyard. Trevor at Astrobackyard has been an incredible help to me and many others in this hobby. In typical fashion, I grabbed the least expensive 12nm H-Alpha filter I could find – The Astromania 2″ screw-in version. I only buy 2″ screw-in filters over clip-ins so if and when I purchase an astro-specific cooled CMOS camera, I don’t have to purchase another round of filters.

I didn’t really see the point of Ha images until I understood what they were. When you look at the black and white images, it appears as just a luminance layer, but it is far from that. It is the specific targeting of H-alpha signal, which resides in the red channel. Most of those beautiful Winter objects in and around Orion, and the Heart, Soul and Wizard in the Northern Winter sky are predominantly composed of Hydrogen gas. The Summer sky presents you with the Eagle, Lagoon, and other nebulae. Some of those Winter targets that dipped too low in February and March also start popping back up as the night wears on.  When you block out other signals and just pull in the Hydrogen, the detail that presents itself is on a completely different level.

I was able to test this filter out for the first time during a very bright, and very full moon. Keep in mind that I am also less than 2 miles outside of the northwest border of Chicago. The image on the left is only 45 minutes of the Wizard nebula. It was low in the sky, just after sunset and ducked behind some trees, limiting the time I could spend on it. On the right is 1-hour and 36-minutes of a section of the Heart nebula. I had a bigger window between the trees on this. The exposures didn’t diminish as the full moon kept rising. Both images are a series of 3-minute exposures at ISO1600. They are very crudely processed incomplete images for examples of testing. Unfortunately, I think I’ll be waiting until next Fall to get a complete Wizard image as it’s quickly disappearing on me.

OIII and SII filters capture different wavelengths and can be used to create some really beautiful images using the Hubble palette and others. In this process you map H-alpha, OIII, and SII to a color channel in its respective image, and then combine in post processing. Starzona has a good guide on assigning palettes for narrow-band imaging found here.

After one night, during a full moon when I wouldn’t have even considered imaging prior to modifications, I am convinced and hooked. At the same time, I am glad I didn’t jump right away. I learned what I needed to about finding targets, focusing and exposing on a slightly simpler setup. That was complicated enough to start. If you’re at the limits of what you can do in light polluted skies and you think there should be more, make sure you doing what’s in Part 1, and then start considering modifications and narrowband imaging. There is definitely value in narrowband imaging through a DSLR while you save your pennies for a true astro-camera.

Clear Skies, Bleary Eyes – KA