First Attempts Planets

I've decided to try my hand at planetary imaging. As a long-time visual observer with my 22-year-old Skywatcher 127mm MAK, which is still going strong, I've recently upgraded my setup. I've added a Skywatcher AZ-GTI mount and a ZWO ASI 678MC USB 3 camera. While planetary imaging presents a whole new set of challenges, my setup is working well, and the tracking is solid. However, focusing on planets, especially when they're low in the sky, has proven to be the most difficult task. 

General Rule for Planetary Imaging

For planetary imaging, you want to achieve an optimal sampling rate (resolution) to capture the maximum detail, which is often referred to as the "sampling rule". The general rule is:

Optimal Sampling Rate for Planetary Imaging:

1. Formula:
   Focal Ratio=5×Pixel Size (in microns)\text{Focal Ratio} = 5 \times \text{Pixel Size (in microns)}Focal Ratio=5×Pixel Size (in microns)

2. For My Camera ASI678MC:
   Optimal Focal Ratio=5×2.0μm=f/10\text{Optimal Focal Ratio} = 5 \times 2.0 \mu m = f/10Optimal Focal Ratio=5×2.0μm=f/10

3. My Current Setup:

   • My Skywatcher 127mm has a focal length of 1500mm and a focal ratio of f/11.81. This is already very close to the ideal f/10 for my camera, meaning you're almost at the optimal focal ratio.

Using a Barlow Lens

A Barlow lens increases the effective focal length of my telescope, which increases magnification. But you want to be careful not to "oversample," which can lead to softer images.

When to use a Barlow:

• With a 2x Barlow lens:

  • My effective focal length becomes 3000mm, which gives you a focal ratio of f/23.62.

  • This is oversampling for myASI678MC, and while it might work for very bright planets like Jupiter and Saturn, the image may become too soft due to oversampling.

Recommendation:

• My current setup without a Barlow lens for most planetary imaging as I'm already close to the optimal sampling with f/11.81. 

• I could use  a 1.5x Barlow lens giving you a focal ratio closer to f/17.7, which is still within a reasonable range for my camera.

Conclusion:

• Current setup (no Barlow): Ideal for planets, close to optimal focal ratio.

• 1.5x Barlow: Can be used to slightly increase magnification without oversampling too much.

• 2x Barlow: Might result in oversampling and loss of detail for smaller planets.