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This is a comparison of the Lagoon Nebula shot with a stock Canon 60D and a stock Canon 60Da. Hold your mouse cursor over the image to see the comparison. The stock 60D records mostly the Oxygen III wavelengths in the nebula and only a little of the red hydrogen-alpha wavelengths because the 60D's long-wavelength filter filters most of the hydrogen-alpha out. The 60Da records much more of the red hydrogen-alpha in the nebula because its long-wavelength filter is specially modified by Canon to pass more of these wavelengths than a stock camera. Both of these images are a stack of two 8-minute exposures at ISO 800 taken with the same telescope at f/8.

For serious long-exposure deep-sky astrophotography, most of the latest generations of low-noise DSLR cameras from Canon and Nikon are excellent for objects such as star clusters, blue reflection nebulae, green planetary nebulae, and galaxies, which have continuum spectrums (they are essentially collections of stars).

The problem with stock cameras is that they almost all have a long-wavelength filter that makes them poor at recording the red hydrogen-alpha light in emission nebulae. It is not totally impossible to shoot emission nebula with a stock camera, but it requires much longer total exposure, and is much more difficult.

This long-wavelength filter is used by camera manufacturers to make the camera's response to color closer to the human eye's response. The human eye is not very sensitive to very deep red colors. It is not sensitive to infra red at all. A digital sensor, on the other hand, is sensitive to deep-red and infra red light. So manufacturers use a filter to remove most of these wavelengths.

For really bright nebulous objects such as M8, the Lagoon Nebula, a stock camera can work well because this nebula contains a lot of Oxygen III wavelength emission. This is mostly what we see when we observe a nebula visually. But large emission nebulae are dominated by red hydrogen-alpha light, which mostly gets filtered out in a normal stock DSLR camera. It can be very difficult to record faint hydrogen-alpha emission nebulae with a stock camera.

Luckily, for those astrophotographers who love emission nebulae and who are serious about getting the best images possible of them, there are several solutions to this problem.

One is the Canon 60Da which is specifically designed for astrophotography with a modified filter that passes 3 times more of the hydrogen-alpha wavelength than a normal stock 60D, making it very good for emission nebula.

The other solution is to remove or replace the stock long-wavelength filter that is built into the camera. This will make the camera sensitive to 90 to 100 percent of the hydrogen-alpha wavelengths.

Several third party vendors offer this service specifically for astrophotography, such as Gary Honis and Hap Griffin in the United States, KW Telescope in Canada, and Andy Ellis in the United Kingdom. Other companies such as Maxmax and LifePixel in the United States also offer modified cameras for daytime infrared work, but with filters that also work for astrophotography.

An interesting hybrid modification by Central DS in South Korea adds a specially designed cooling mechanism to the DSLR camera in addition to replacing the manufacturer's long-wavelength filter with one that is good for red emission nebulae. This greatly reduces the thermal signal and thermal noise. The design is unique in that it places the cooled sensor inside of an airtight chamber to deal with the problems of moisture condensation.

If you are very handy with working with delicate electronics, you can also try modifying your camera yourself. It is, however, very easy to completely trash a camera if this is not done correctly. Directions for modifications of various cameras can be found on Gary Honis' web site.

Unfortunately, modifying a camera invalidates the manufacturers warranty, and can cause problems with the daylight color balance and auto-focus. These problems can be addressed however. Daylight color balance can be corrected in a modified camera by the use of a filter equivalent to the original long-wavelength filter and a custom white balance. Focus issues can be solved by using a replacement filter of the same thickness as the one replaced.

Long-Wavelength Filter Removal vs Replacement

There are two basic modifications that can be performed on the camera manufacturer's original long-wavelength filter. It can be removed completely, or it can be replaced with another filter that passes the red hydrogen-alpha wavelengths of emission nebulae.

There are several things that need to be considered in deciding on which modification to make.

If the original long-wavelength filter is removed completely, the camera's autofocus system will not provide accurate focus. The visual focus in the viewfinder will not be correct either.

A camera with a removed filter can be used for both astrophotography and daytime infra-red photography. Accurate focus can be achieved by using Live View for focusing visually.

Color balance for normal daytime photography will not be accurate either if the filter is completely removed. Correct color balance can be restored by using a filter in front to the camera lens that has the same filter characteristics as the one that was removed. A custom color balance can also be set in the camera, and this may be good enough for informal snapshot photography without the use of a special filter, but some red colors may not be accurate.

If the camera manufacturer's original filter is replaced with one of the same thickness and refractive index, then autofocus and visual focus will be correct.

Replacement Filters for Modified Cameras

There are two main choices for a replacement filter for astrophotography - a UV - IR filter, and a clear filter.

These replacement filters are the same thickness and refractive index as the original filter so the accurate autofocus is maintained in the camera. Manual focusing through the camera by eye is also accurate. The replacement filter goes in the same place as the original filter, just in front of the imaging sensor.

• UV - IR - This filter blocks short-wavelength ultra-violet (UV) light, as well as long-wavelength infra-red (IR) light, but passes most of the light at 656.3 nanometers, the wavelength of hydrogen-alpha light.

  UV and IR light should be blocked because these wavelengths do not focus at the same place as visible light in refractive (lens) optical systems. Glass optical elements that make up refractors, compound telescopes and camera lenses, focus light based on the wavelength and the refractive index of the glass. Therefore, not all wavelengths come to focus at the same location when light is focused through a lens.

  This is why lens and telescope designers go to so much trouble making complicated optical systems, such as triplet refractor objectives made out of exotic glasses - to try to get all of the visual wavelengths to focus at the same location. If they are successful, a white star will have no spurious color around it when it is in focus. If they do not succeed, and with less well corrected refractive optics, you will see a blue or violet halo around a star when it is in focus. The problem is even worse for digital sensors because they are sensitive to wavelengths past both ends of the visible spectrum which we cannot see, and color is not as well corrected at these wavelengths in refractive optical systems.

  On the other hand, mirror-only optical systems, such as Newtonians, do not suffer from this problem because they focus all wavelengths at the same focal plane because they reflect light instead of refracting it.

  Color balance for normal daytime photography is also affected with a UV-IR filter modification, but a custom white balance and special "hot-mirror" filter can be used to correct it.

  A UV-IR replacement filter is the best choice for astrophotography if you do not want to do daytime infra-red photography. It will provide accurate autofocus as well as pass the hydrogen-alpha wavelengths critical for red emission nebulae. It is even a good choice if you have a reflective telescope because you might also want to use your camera lenses for wide-field astrophotography.

• Clear Glass - If you want to do astrophotography as well as daytime infra-red photography, the best option is to replace the manufacturer's long-wavelength filter with a clear glass, anti-reflection, multi-coated filter that protects the sensor and allows all wavelengths to pass, including infrared.

  A clear glass replacement filter of the same thickness and refractive index as the original camera manufacturer's filter will provide accurate autofocus.

  If a clear glass filter is used, a UV/IR filter must also be used on refractive (lens) optical systems for astrophotography. Color balance for daytime use is affected by this modification, so an IR filter, "hot mirror" filter and custom white balance is needed for normal daytime photography.

  Daytime infrared photography is possible with a clear glass replacement filter and an additional filter that cuts out the normal visible wavelengths, such as a Hoya 72 filter.

The camera has to be disassembled to replace or remove the manufacturer's filter, so once these modifications are performed, the new replacement filter is fixed and cannot be removed.

In-Camera Filters

In addition to a permanent UV-IR or clear glass replacement filter, you can also use a set of special removable filters which fit inside the camera body, just behind the lens and in front of the reflex mirror.

Several different types filters are available from Astronomik and Hutech, such as daylight balancing, UV/IR filters, light pollution filters, and narrowband filters such as 6nm and 12nm hydrogen-alpha, oxygen III, and hydrogen-beta filters.

The great thing about these in-camera filters is that you can use them with a variety of different telescopes, or camera lenses that might require different filter sizes.

Hutech calls these "front" filters, and Astronomik calls them "clip-in" filters. They both work the same way. They are held in place in the mirror box with a filter holder with flexible edges. They fit into an area with a small lip on it. These filters can be easily inserted and removed.

Some lenses, however, such as the Canon EF-S series, protrude too far into the camera and cannot be used with these filters. Most other lenses can be used.

Front filters are available for most Canon prosumer and consumer models. They will not fit in a camera with a full-frame sensor such as the Canon 5D, 5D Mark II, or 1D series.

Similar filters are also made for some Nikon model DSLR cameras.