Browsing around the Internet you will find a decent amount of information about replacing the glass filter that sits in front of the sensor in a digital SLR camera to make it more effective for astronomical imaging. Additionally, you'll find a lot of information about how to continue using the same modified camera for regular daytime imaging. Unfortunately, you'll also find a lot of misinformation on both topics, especially the latter.

I don't pretend to be an expert on cameras, light, photography, nor optics. But my experience with imaging over the last several years, specifically with my modified Canon 20D, has given me a pretty good understanding of many of these basic processes and principals.

The core of the misunderstandings seem to revolve around

1. The types of modifications that can be performed and the implications of each
2. Methods for using a modified camera for regular daytime photography and the implications of each method

Let's approach these one at a time:

1. The types of modifications that can be performed and the implications of each
   
   Generally speaking, a digital SLR can be modified in one of three ways:
   
   
   1. The factory IR filter is removed. No replacement filter is inserted in its place.
      — This is a relatively uncommon modification but it can be and has been done. To use the camera for regular, daytime imaging, you will need an IR/UV-cut filter inserted into the light path and you must manually focus your photos.
      
      This modification allows the full, inherent sensitivity of the sensor to be utilized. It allows all incoming light to fall on the sensor; which, of course, means that IR and UV light reach the sensor. There are many implications here. (i) The camera can be used for infrared imaging. A lot of people have created some spectacular landscape photos in infrared. (ii) The camera's autofocus feature will not work. Removal of the glass filter changes the physical light path so that the factory set autofocus will no longer be accurate. (iii) If you are using refractive or catadioptric optics and you want to use the camera as a one-shot color camera, you must use an IR/UV-cut filter at some point in the light path. As they pass through a refractive optical element, each portion of the light spectrum will get bent to a slightly different degree, especially the purple and UV rays. This means that not all of the light will focus at the same point. This results in color fringing (chromatic aberration) and star bloat. These symptoms are less concerning if you want to use the camera for monochrome imaging applications such as Ha or IR (although star bloat can still be an issue). They are not an issue if you are using fully reflective optics.
      
      
   2. The factory IR filter is replaced with clear, anti-reflection glass.
      — This is a relatively common modification that companies like Hutech Astronomical offer. To use the camera for regular, daytime imaging, you will need an IR/UV-cut filter inserted into the light path.
      
      (i) Infrared photography is still an option. (ii) Autofocus function will be restored since the light path has been restored (assuming the replacement glass has the proper refractive index). Issue (iii), above, still applies.
      
      
   3. The factory IR filter is replaced with an IR/UV blocking filter that still allows H-alpha light through.
      —This is the most common type of filter modification performed on digital SLRs. Companies like Hutech Astronomical offer this type of modification service.
      
      (i) IR photography is not an option. (ii) Autofocus functions normally. (iii) A wider range of astronomical telescopes can be used without additional filters because the offending IR and UV wavelengths are blocked (of course, the color fringing associated with achromatic telescopes is still an issue, as with any color imaging situation). Since light in the H-alpha spectrum is now being allowed through, the camera is better suited for certain types of astronomical objects (the camera's sensor will collect more of the emitted light in this portion of the spectrum [656nm]). It also means that regular daytime photography will still require some additional steps to make the images look as though they were taken by an unmodified camera—which leads us to the next discussion (#2, below).
      
      

Where does the Canon 20Da fit in?

The limited production Canon 20Da (discontinued) is similar to a stock 20D that's had modification C (described above) performed on it. The differences between a stock 20Da and a modified 20D are: The stock 20Da allows more H-alpha light through than the stock 20D but LESS than a user-modified 20D. Canon did this so they could still keep the camera useful for regular daytime photography without requiring the user to go through the steps discussed in section 2 (below). The stock 20Da allows live focusing. Until the Canon 1D Mark III, the 20Da was the only Canon dSLR to offer this function. The stock 20Da allows you to zoom in 5x or 10x on the LCD while using the live focus feature. The stock 20Da has "optimized" noise reduction algorithms. To my knowledge, Canon never quantified the noise differences between a stock 20D and a stock 20Da.

2. Methods for using a modified camera for regular daytime photography and the implications of each method
   
   The remainder of this discussion focuses primarily on modification C, described above. When applicable, I've made special notes concerning the other types of modifications.
   
   For daytime imaging, there are two primary methods for getting images to appear as though they were taken by an unmodified camera: (A) placing an additional filter in front of the camera's sensor and (B) using the camera's custom white balance function. Both methods have advantages and drawbacks.
   
   
   1. Placing an additional filter in front of the camera's sensor
      The purpose of such filters (like the VLC and the XNite CC1) is to pass and block different wavelengths of incoming light in the same way that the camera's original, factory filter did when it was in place. This is the easiest way to restore the camera's color balance since you simply put the filter in place and shoot away, in all types of lighting conditions. There are 2 variations: filters that are placed behind the lens (directly in front of the camera's mirror; these are referred to as "drop-in" or "plug-in" filters) or traditional front filters that screw on to the front of your lens.
      
      

      Advantages

      • It's quick and easy, requiring minimal additional interaction with the camera's settings. Set it and forget it.
      • Resulting images are well color balanced.

      
      

      Drawbacks

      • Scenes through the viewfinder appear bluish-green
      • Images may appear darker than they would have without the filter
      • Filters that are placed behind the lens prevent you from using Canons EF-S lenses (the rear of EF-S lenses sit closer to the camera's chip than EF lenses, thereby hitting any filter placed in front of the mirror box of the camera).
      • If you get a front filter, you'll need to have a different size filter for each different size lens (workarounds include using stepper rings [which can cause vignetting] or using one, all-purpose lens [which doesn't always work for everyone nor for every situation]).

      
      
   2. Using the camera's custom white balance function
      The purpose of this method is the same as using a filter; to restore the color balance of your images. But the method is entirely different. You must have a card or other surface that is roughly 18% gray. You then take an image of that surface with no filter and using the camera's automatic settings. You then use your camera's menu to tell it to use the image you just took as the custom white balance image. Finally, you must tell the camera to use the custom white balance setting instead of the auto white balance setting or any of the white balance presets (some cameras may do this automatically after you select a custom white balance image).
      
      

      Advantages

      • When done correctly, this is the most accurate way to restore your camera's color balance.
      • Can be done with minimal or no additional expense.

      
      

      Drawbacks

      • New custom white balance images need to be taken as the source of light or lighting conditions change (more on this below). This takes a little extra time and means you need to carry around a little piece of cardboard or paper.
      • Automatic exposure settings may be negatively impacted (more on this below).

      
      When using this method, two questions typically come up:
      
      (1) If I'm trying to set the white balance why do I use a gray card?
      The goal of using custom white balance settings is to tell the camera what a known surface looks like under the given light. Using a color like red or blue would be difficult because there are so many hues of those colors. White too easily takes on the color of surrounding color sources. Gray is color-neutral and, while it can reflect surrounding colors, it does it less so than white.
      
      (2) When I take a white balance image as instructed, why is the image pink or red?
      The white balance image itself will appear pink, red, or even orange, depending on the light source. That's how the modified camera sees white and gray under the given lighting conditions. The stock filter is designed to block and pass light so that the sensor captures the same wavelengths that our eyes perceive and our brain interprets. The internal white balance settings then further refine the captured image to ensure the chromaticity appears natural to our eyes. By replacing that stock filter, you've just thrown all those presets into disarray. They no longer work as expected because more red light is being permitted than is expected. So, the reds, pinks, and oranges are exactly what you would expect to see through the replacement filter. With a custom white balance setting, you are telling the camera, "Even though this card looks pink to you, it's actually gray, so treat colors like this as gray... not pink."
      
      More on drawback #1 (new custom white balance images): This method does not physically interact with incoming light the way a filter does. It's simply a software/mathematical setting, if you will. This means that you must take new white balance images when the lighting source or conditions change. Since the modification allows a broader spectrum of light to reach the camera's sensor, the camera's built-in white balance features no longer function the way they were intended. This is why you can't just use one white balance image to account for all lighting conditions. No camera, modified or not, compensates perfectly for the multitude of light types and conditions a typical photographer/camera will encounter. Different light sources emit varying wavelengths of light. This is why cameras have white balance presets for sunny daylight, cloudy daylight, incandescent, fluourescent, tungsten, etc. A camera's auto white balance setting lets the camera automatically detect the type of lighting and account for it when exposing the image and applying internal processing. The purpose of a custom white balance setting is to allow you to tell the camera what the ambient light looks like when reflected off an even, color-neutral surface (which is why an 18% gray card is preferred). If you hold a piece of white copier paper under bright outdoor daylight, outdoor shade, and indoor incandescent lighting, the color of the paper will appear different in each of those settings. The paper hasn't actually changed but its appearance has because of the type of lighting.
      
      More on drawback #2 (automatic exposure settings): Now that the camera's sensor is receiving a broader spectrum of light, it's also receiving more light overall. You will find that the camera will often slightly overexpose images while using its automatic exposure settings. The preset functions of the camera weren't set to accommodate the properties of a filter that passes H-alpha in addition to the "normal" visible spectrum. They were set to account for the properties of the stock filter, which blocked H-alpha. You can use the manual settings and adjust exposure length or f/stop one setting lower than the light meter suggests or you can set the camera to automatically take the image at one setting lower.
      
      

Notes for taking an outdoor custom white balance image

Taking a custom white balance isn't difficult but there are a few things to be aware of. Here's an overview: On a clear, sunny day, near noon, position yourself with the sun behind you. Place or hold the gray card in front of the camera ensuring that the card is fully and evenly illuminated by the sun. Make sure the card isn't reflecting any light other than the sun's (keep it away from colored surfaces that might be reflecting colored light onto the card). Do not do this through a window; be outside. Make sure there are no shadows falling on the card. Do not use any filter on the camera lens. Position the card and camera so that the card takes up the entire frame. Set your camera to the full auto mode (let it automatically adjust the exposure length, f/stop, and white balance). Set your camera or lens to manual focus. If the card fills up the entire frame, autofocus likely won't work, which may prevent you from taking the photo. Using manual focus will overcome this. It's not important that the card is in focus. Take the image. Switch your camera to use the custom white balance setting and make sure you select the image you just took as the image on which to base the custom white balance setting. Additional tips and information on white balance: http://www.digitalartsphotography.com/instructions.htm http://www.earthboundlight.com/phototips/white-balance-gray-card.html http://www.photoxels.com/tutorial_white-balance.html

A somewhat unexpected but pleasant side effect of replacing the stock filter is that images are sharper; not tremendously sharper but noticeably upon close inspection. The stock filter performs a process called anti-aliasing. This is basically a very gentle blurring effect that is meant to minimize something called moire. Moire is a side effect of digital imaging sensors (as well as LCD monitors). It appears as a slightly perceivable grid pattern throughout the image. Some people notice it more than others. Implementing a filter that slightly blurs the image reduces the effect of this symptom. So, when the stock filter is removed and replaced with a filter that doesn't have an anti-aliasing effect, the resulting images are inherently sharper.

^ BACK TO TOP