Here's a breakdown of the pros and cons:
Pros:
* Extreme Reach: 800mm is a *very* long focal length, allowing you to capture detailed images of the Moon, planets (though still small), and some larger deep-sky objects like the Orion Nebula.
* Affordable: For an 800mm lens, the RF 800mm f/11 is relatively inexpensive compared to other telephoto lenses of similar focal length. This makes it an accessible option for astrophotographers on a budget who want to experiment with long focal lengths.
* Lightweight: Its relatively lightweight design (compared to other 800mm lenses) can be a plus for portability and potentially less stress on your tracking mount (though you still need a good one - see below).
* Canon RF Mount Compatibility: Directly compatible with Canon's mirrorless cameras, meaning no adapters needed if you're already in the Canon ecosystem.
Cons (and these are significant):
* Slow Aperture (f/11): This is the biggest limitation. f/11 is *extremely* slow for astrophotography. This means:
* Very Long Exposures: You'll need to use extremely long exposures (potentially minutes) to gather enough light, increasing the challenge of tracking.
* High ISO Required: You'll likely need to use high ISO settings, which will introduce significant noise into your images.
* Dim Objects Will Be Very Difficult to Capture: Faint nebulae, galaxies, and other deep-sky objects will be challenging or impossible to capture well.
* Fixed Aperture: The fixed f/11 aperture gives you no flexibility to adjust the amount of light entering the camera.
* Diffraction: At f/11, you're getting into the territory where diffraction can soften your images, especially on cameras with smaller pixels.
* Potential for Aberrations: While reasonably well-corrected, any lens at this price point is going to have some degree of chromatic aberration (color fringing) and other optical imperfections, which can be magnified at long focal lengths and in astrophotography conditions.
* Requires Accurate Tracking: An 800mm lens requires *very* accurate tracking. Even small errors in your mount's tracking will result in blurred stars and elongated objects. A good equatorial mount is essential.
* Vibration Sensitivity: Any vibration will be amplified at 800mm. You'll need a very stable setup, remote shutter release, and potentially mirror lock-up (if applicable to your camera) to minimize vibration.
* Not Ideal for Widefield: It's obviously not suitable for widefield astrophotography. You'll be limited to smaller, brighter targets.
Tips for Using the RF 800mm f/11 for Astrophotography:
* Focusing: Accurate focus is absolutely critical. Use Live View with maximum magnification on a bright star to carefully achieve pinpoint focus. Consider using a Bahtinov mask or similar focusing aid.
* Tracking Mount: Invest in a good, solid equatorial mount with accurate tracking capabilities. This is non-negotiable at 800mm. Consider autoguiding for even better tracking accuracy.
* Stable Setup: Use a sturdy tripod and ballhead, or even better, a dedicated astrophotography mount.
* Remote Shutter Release: Use a remote shutter release to avoid introducing vibrations when taking photos.
* Experiment with ISO and Exposure Length: Find the optimal balance between ISO and exposure time to minimize noise while gathering enough light. Don't be afraid to take lots of test shots.
* Stacking: Take multiple exposures of the same object and stack them using software like DeepSkyStacker, Astro Pixel Processor, or PixInsight. This will significantly reduce noise and improve image quality.
* Dark Frames, Bias Frames, and Flat Frames: Calibrate your images with dark frames, bias frames, and flat frames to further reduce noise and artifacts.
* Moon and Planets: The RF 800mm f/11 can be useful for capturing detailed images of the Moon and planets, though you'll still need to use techniques like stacking and sharpening to get the best results. Consider using a planetary camera for planetary imaging.
* Bright Deep-Sky Objects: Target brighter deep-sky objects like the Orion Nebula (M42), the Pleiades (M45), or the Andromeda Galaxy (M31). Don't expect miracles with fainter objects.
* Post-Processing: Be prepared to spend time in post-processing software (Photoshop, Lightroom, etc.) to remove noise, correct color balance, and enhance details.
* Consider a Barlow Lens or Teleconverter: These can extend the focal length even further (but will also increase the effective aperture, making it even slower). Use with caution.
In conclusion:
The Canon RF 800mm f/11 is *not* an ideal lens for astrophotography due to its slow aperture. However, with careful technique, a good tracking mount, and a lot of patience, you can capture some interesting images of the Moon, planets, and brighter deep-sky objects. It's a budget-friendly way to dip your toes into long focal length astrophotography, but be aware of its limitations and the extra effort required to get acceptable results. If you're serious about astrophotography, especially of deep-sky objects, you'll likely want to invest in a faster, dedicated astrophotography telescope or lens eventually.