Starting out with One Shot Color; AP Corner September 2022

by Don Selle

m42-early.jpg

When you are starting out in astrophotography (AP) and selecting the components of your imaging rig there are a lot of decisions you need to make. In my opinion your first imaging rig should be adequate for the task, straight forward and easy to use. Rather than acquiring top of the line equipment that well known imagers are using, you should aim to select equipment that is easy to use which will make completing your first images as straight forward as possible. 

For many beginning astroimagers, there can be a steep curve with new technologies that need to be mastered. As a result, for many, the first images completed are the most important. Early success is the surest prevention for the frustration and burn which leads many beginning astroimagers to throw in the towel. Your choice of camera can help make that early success possible.

Back in the day, the standard advice to newbies was that they start out with a good mount, and an 80 mm (or shorter) focal length refractor, still good advice today. Back then, the choice of cameras on a beginner’s budget was very limited. In the Early 2000s, monochrome CCD cameras were the thing, as CMOS camera technology was still in its infancy, and was considered not to have the sensitivity required for AP. 

This all changed around 2005 with the advent and popularity of CMOS based DSLRs. The CMOS technology improved enough to make One Shot Color (OSC from now on) imaging possible. The problem though was that these consumer cameras were not very sensitive for darker red colors, which is where bright nebulae shine, and a new astro-business of “astro modifying” DSLRs was born.

In 2006, when I started in AP, I first purchased a mono CCD camera, filter wheel and LRGB filters. After using it for a while with little to show for the time spent, I purchased a brand new astro-modified Canon Rebel 350XT, and very quickly began to produce images I was proud to show off.  

The image of M42 above is the first good image I captured with the Canon XT350 and is presented as I first processed it in January 2007. I had imaged the M42 with a monochrome CCD camera the year before, and this first OSC image blew the CCD image away. The same data was used when in 2017 I reprocessed the image below. I have included it to give you an idea of what can be done with a very basic OSC camera. With experience you can get a OSC camera to perform at very high level.

m42-later.jpg

Today’s camera technology has improved tremendously since 2007. The huge popularity of CMOS based digital cameras has pushed the development of highly sensitive low noise camera sensors – just what is needed for AP. These sensors can be gotten either in a recent model DSLR (which will need to be astro-modified) or in dedicated astronomy cameras, both OSC and monochrome. The fact that these sensors are being widely used has helped to bring down the price of cameras for AP while considerably improving the quality of the resulting images.

Before we get too far into deciding what camera might be better for a new imager, it would be appropriate to describe how both monochrome and OSC Cameras work. This will make it much easier to understand the reasoning behind my suggestion that new astroimagers start out using a OSC camera.

mono-graph.pngWhat is a Monchrome Camera and How Does it Work?

The bare sensor in a monochrome camera collects all wavelengths of visible and some near infrared light. Typically, it will have its highest sensitivity to green light at around 500nm wavelength which is about where the human eye is most sensitive. 

baader.pngThis is the sensitivity curve of the CMOS sensor for a modern monochrome astroimaging camera. Without a filter in front of the sensor, it collects all colors of light. 

If you are going to assemble a color image, at a minimum you will need to capture subframes with red, green and blue filters. Unlike a OSC camera, each of these color “channels” is captured at the full resolution of the sensor in your camera.

If you want to use a mono camera to its best advantage, you will also capture sub-frames with a luminance filter. This is called LRGB imaging.

In fact, you will want to spend more time capturing the luminance data than the you do on the color channels, because humans perceive most of the detail in an image from the luminance channel.  This is the big advantage of a monochrome camera.

If you plan to add narrow band data, such as hydrogen alpha to your image you can do so by using a single line narrow band filter. Like the color channel, narrow band data is captured at the full resolution of your camera. Exposure times for narrow band will be significantly longer than for the color channels due to the very narrow band of colors that the filter allows to reach the sensor.

In order to assemble and image, you will need to prepare a master frame for each of the four LRGB channels and any narrow band data you would like to include. This will require you to:

  • Take flat frames for each of the channels you plan to use to assemble your image. For LRGB imaging, that means 4 separate sets of flats, one for each filter used.
  • Create a master frame for each channel.
  • Align each of the channels 
  • Blend them appropriately
  • Manually do the initial color balance, as the sky conditions may change between your imaging with different filters.

While there is certainly software to help you complete all of these tasks to get to an image that is “pre-processed” and ready to be further developed into that striking image you are hoping to be able to produce.

So the advantages of using a monochrome camera which are:

  • All channels can be captured at the full resolution of your camera
  • Capturing a longer Luminance channel has the potential to provide greater detail in your images
  • Post processing a Luminance channel – contrast enhancement, noise reduction and sharpening – are more easily done in the black and white of the Luminance channel

Come at the cost of some additional complexity and work needed to assemble your image.

What is a OSC Camera and How Does it Work?

osc.pngA OSC camera is exactly what it sounds like. With each exposure, the camera captures sufficient data to render a color image.  The pixels in the sensor are covered with miniature red, green and blue lenses in what is called a Bayer Array. Each sub-frame which is captured therefore contains all of the color data, albeit at less than the full resolution of the imager.

The Bayer array pattern at the left is a fairly common RGGB pattern. If you look at the Bayer array you will notice that for each red and blue pixel there are two green pixels. This means that the red and blue channels are shot a 25% of the camera’s full resolution, while the green channel is at 50% resolution. Software, either in the camera if it is a DSLR or in processing software must use an interpolation algorithm to render the color image at the full resolution of the camera. As a result, when you image with a OSC camera, combining the RGB channels is done automatically for you, since all three channels are present in a single sub-exposure. 

Combining narrow band data into a OSC image is also possible. As modified DSLRs began to be used more widely for AP several pioneering imagers were successful imaging through single line narrow band filters and blending that data with their RGB images. They also started to convert the single line data into false color images using the Hubble standard color pallet. 

Because the narrow band data is a single color (Ha is a deep red) the narrow band data is collected as less than the full resolution of the camera sensor (25% for Ha), This data must be interpolated to reach full resolution if it is to be blended into an RGB image.

Once again, some very smart people saw a business opportunity. Dual and Tri band filters are now commercially available and are designed to be used with a OSC camera. This allows collection of Ha (red) data at the same time as OIII data (blue and green)! Less imaging time is required, with the trade off being less resolution in the raw data.

There are a couple of other differences that make imaging with a OSC camera a little easier than shooting with a monochrome camera. First of all, you need to shoot only one set of flats which saves time in the field and in processing. This compares to four sets of flats required for a monochrome camera shooting LRGB. (Of course if you use narrow band filters with your OSC camera, flats for each filter used will be needed. More on this below).

Additionally, because all the color channels are exposed at the same time, using a basic algorithm to set your initial color balance. If you are using a modified DSLR and shoot your flats using a photographic white balance card (a good practice), you can use your best flat frame to set the in camera custom white balance the details of which are stored in the file header of your camera raw file. Several image processing programs like Deep Sky Stacker can read this data and use it to perform the same color balance in software as you would have in camera. I have found this color balancing technique to be quite good.

Finally, there is something about a OSC camera that makes it more intuitive to use. Perhaps it is because we are coming to AP with daylight photography experience, perhaps it is just a little less intimidating than learning to master a monochrome astro-camera. Whatever it is, it means a little less stress on beginning astrophotographers.

OSC Camera is Best for Most New Astroimagers

While there are advantages in using a monochrome camera for AP many astrophotographers now start imaging with a OSC camera. In my opinion, if you are new to AP, OSC is the way to go. Whether you are a seasoned daylight photographer, or relatively new to photography, you will achieve better images sooner than you would if you start with a monochrome camera. Once you master OSC imaging, if you so desire, you can always take the next step up to monochrome imaging (there’s always an upgrade!).

There are several reasons for my recommending beginning astrophotographers start with a OSC. 

  • You will be more likely to get enough data to assemble a decent image in a single imaging session with a OSC vs a monochrome camera and filters
  • For most people, using a OSC camera is more intuitive and easier to understand and use.
  • OSC cameras avoid the extra expense of filter wheels and imaging filters.
  • After your sub-exposures have been pre-processed, aka calibrated stacked and integrated, you can use the same image processing software you use for you daylight photos to pretty up your astro images.
  • Additionally, you can get much of the benefit of shooting LRGB with a monochrome camera if you are willing to learn how to create a “synthetic” luminance channel, and post process it similarly to how you would for a true LRGB image.  That was one of the improvements I added to the reprocessed M42 image above
  • As I often say “there’s always an upgrade” OSC cameras can be used for narrow band imaging with single line filters (i.e. Ha) or with the dual or tri band line filters. 

I can’t emphasize the first reason enough. Because you are capturing full color sub-frames with a OSC camera, you are more likely to capture enough data in a single imaging session so that you can assemble an image. Even an hour or two of good sub-frames will give you enough to work with.

When using a monochrome camera, you will concentrate much of your imaging time to capturing the luminance data. When I started with a monochrome CCD, I would shoot all of my luminance first, then shoot the red, green and blue filters. Because conditions change, I have lots of incomplete data sets from a mono CCD camera that have the luminance data but not all the data for the RGB color channels.

Many of the current image capture software packages have the capability to automate the sequence of taking the separate LRGB channel data so that in an hour of imaging some data from all channels is captured. This however, adds a layer of complexity over and above using a OSC camera. Starting out with a camera that is intuitive to use, and which makes assembling and processing an image simpler is also an advantage for the beginning astrophotographer. 

Astro-modified DSLR or OSC Astro Camera?

Starting your AP odyssey with a OSC camera begs the question, “should I start with a modified DSLR or with a OSC astro camera?” I’m agnostic on this question, as I currently own one of each. So rather than suggest one or the other, I will list several attributes of each that may help you make the best decision for situation.

First, let me give you a quick pick up on what an astro modified DSLR is. As I mentioned above, a consumer DSLR uses a filter attached to the camera sensor which blocks both UV and IR light. It may also compensate for the drop off in color sensitivity of the imaging chip. Typically, this filter reduces the amount of deep red light reaching the sensor. As a result, the camera’s native sensitivity to Ha light which dominates most bright nebula is very low. Not a good thing for astroimaging.  

To astro modify a DSLR, the stock filter is removed from the camera sensor and a new filter is added that significantly increases the camera’s sensitivity to the deep red end of the visual spectrum where bright nebulae glow.  Take a look at the image on this website to see this firsthand.  http://imaginginfinity.com/dslrmods.html  Full disclosure – I have had Hap Griffin modify two DSLRs for me. As I upgraded my daylight camera, I had Hap modify the old one.

Please be aware that there are other similar modifications that can be made to a DSLR, one of which is a full spectrum mod used for shooting infrared images. If you get a full spectrum mod, you will need to use a UV/IR blocking filter when doing AP.

This is great for astro-imaging, but not so good for daylight photography, which will have a very red cast unless you apply a custom color balance. Here is a link to help you understand in more detail how to custom color balance a modified DSLR. Personally, I have found I prefer using an un-modified camera for daylight photos, so the modified camera is dedicated to astronomy. https://www.astropix.com/html/astrophotography/customwb.html

If you decide to use a DSLR you may want to browse the website too. 

So, what are the attributes of a modified DSLR?

  • Used modified DSLR’s are readily available for sale at a very low cost. Even if you modify a new camera, you can typically get a larger imaging chip (hence a bigger FOV) than an astro OSC camera.
  • You can use the camera for daylight photos provided you use the camera’s custom color balance feature. Since lighting conditions change throughout the day, you will probably need to do the color balance often.
  • Unlike a OSC astro camera, it is uncooled. This means that the temperature of the camera will fluctuate, and it will accumulate thermal noise much faster than the dedicated astro camera. Using library dark frames is not possible, meaning that some of your imaging time will be taken up capturing darks.
  • You can use the in camera custom white balance feature to set the initial color balance of your image providing your pre-processing has this feature. This is usually quite effective.
  • Wide field astro imaging with camera lenses is very convenient.
  • You can use the camera with a lens on a camera tracker without any additional equipment.
  • You can also use the camera to shoot nightscape photos which show the large glowing clouds of hydrogen in the Milky Way.

The big differences of a dedicated OSC astro camera are directly due to the fact that it is designed specifically for AP.

  • It will be cooled for reduction of thermal noise. This will make a difference in the noise level you need to deal with during image processing. While it is possible to purchase uncooled cameras at a big cost savings, I would steer you away from these, unless you plan to focus on lunar and planetary imaging.
  • You can create a library of dark frames for different exposure times and camera temperatures. This when done at home will make your imaging time more efficient.
  • Using a OSC astro camera has many similarities to using a monochrome camera, both from the camera set up to the use of imaging software
  • A OSC astro camera can be used with camera lenses provided that you purchase an appropriate lens adapter. Auto focusing and setting an f stop is generally not possible though, like it is with a DSLR, unless you use a special adapter which includes electronics ( hey there’s always an upgrade!).
  • A OSC astro camera can also be used on a camera tracker, though it requires purchasing mounting rings designed specifically for the camera (unless your lens has a mounting foot).

Finally, the choice of which camera is yours. There is nothing like the feeling you get when a new camera and it performs flawlessly, and your final image turns out better than you expected. So, whether you decide to start out with a OSC camera or take the plunge right into monochrome LRGB imaging, I wish you all the best and hope you are successful.

Feel free to let me know how you are progressing!

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