How much does H-alpha sensitivity matter for astrophotography?
Asked 6/1/2011
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I’m currently using an unmodified DSLR for astrophotography. I’ve read that standard DSLRs include an internal filter that reduces sensitivity in the deep red part of the spectrum, including hydrogen-alpha (H-alpha). How much difference does that make in practice for astrophotography, especially for nebulae, and is a camera modification worth considering if I want to get more serious?
Originally by Jeff Snider. Source · Licensed CC BY-SA 4.0
Jeff Snider
15y ago
2 Answers
5
Although the CCDs in DSLRs are quite sensitive to the deep red colors (including H-alpha), the camera manufacturers added a filter over that part of the spectrum to make the photos they take appear more like the images we see with our eyes (that aren't so sensitive to those wavelengths).
However, in astrophotography, H-alpha is important. Often, big gas clouds and nebulas are composed largely of hydrogen. When they are lit up by radiation from other stars, they re-emit a large portion of that energy as H-alpha light, because the lowest visible light band for hydrogen is H-alpha. The filter over the CCDs don't target H-alpha light of course, they just reduce the sensitivity of the CCD to that color--at the H-alpha wavelength, only 20-25 percent of the light actually gets through to the CCD.
Some DSLRs, such as the Canon 20Da were designed specifically for astrophotographers, where the filter was changed to let up to 70 percent of this light through. That camera has been discontinued however.
The filters can be modified but this can be tricky and expensive (especially if you brick your sensor). You are still able to photograph these nebulas, just not with the intensity that you would be able to without that filter.
Originally by Carson Myers. Source · Licensed CC BY-SA 4.0
Carson Myers
15y ago
0
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It can make a big difference for certain subjects. Standard DSLRs are quite capable of detecting deep red light, but most include an internal filter that reduces those wavelengths so everyday photos look more natural. Around the H-alpha wavelength, only about 20–25% of that light may reach the sensor.
For astrophotography, that matters most when shooting emission nebulae and other hydrogen-rich gas clouds, because they glow strongly in H-alpha. An unmodified DSLR will still work, but those subjects will appear much weaker and require more exposure time or more aggressive processing to bring out the red nebulosity.
For targets that are not dominated by H-alpha emission, the benefit is much smaller.
So: if your main interest is deep-sky objects like emission nebulae, improved H-alpha sensitivity is very worthwhile. If you mostly shoot other astro subjects, an unmodified DSLR may be fine.
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