What is difference between triplet astrograph with dedicated field flattened and quadruplets astrograph?
The difference between a triplet astrograph with a dedicated field flattener and a quadruplet astrograph mainly comes down to optical design, performance, and ease of use. Here’s a clear breakdown:
🔭 1. Triplet Astrograph + Dedicated Field Flattener
- Design: Has three lenses (triplet) in the main objective.
- Purpose: The triplet corrects chromatic aberration (color fringing) and spherical aberration.
- Field Flattener: An additional optical element is needed to flatten the curved focal plane for astrophotography (most triplet objectives naturally have some field curvature).
- Modular: The flattener is a separate accessory, often matched to the telescope, and adds back focus length and complexity.
- Pros:
- High optical quality and contrast.
- Flexibility — flattener can be swapped for a focal reducer if needed.
- Cons:
- More setup complexity — spacing between flattener and camera must be correct.
- Slightly more potential for misalignment.
🌌 2. Quadruplet Astrograph (Built-in Flattener)
- Design: Has four lenses, usually including a built-in field flattener element.
- Purpose: Designed from the ground up for astrophotography — chromatic and field aberrations are corrected in a single, integrated optical train.
- Built-in Flattener: You don’t need a separate field flattener — it’s already designed into the scope.
- Pros:
- Plug-and-play — simpler imaging setup with no flattener spacing to worry about.
- Flat field across a large image circle (ideal for full-frame or APS-C sensors).
- Cons:
- Less modular — you can’t remove or swap the built-in flattener.
- Often slightly heavier and more expensive due to more glass.
💡 Summary Table
| Feature | Triplet + Field Flattener | Quadruplet Astrograph |
|---|---|---|
| Number of elements | 3 main + separate flattener | 4 (one is usually a flattener) |
| Field flattening | Requires external flattener | Built-in |
| Chromatic correction | Excellent | Excellent |
| Imaging setup complexity | Higher (spacing critical) | Lower (plug and play) |
| Modularity / flexibility | High | Lower |
| Cost | Often slightly cheaper | Usually more expensive |
🧠 Bottom Line:
- If you want flexibility (e.g., to swap flatteners or reducers), or already own a field flattener, a triplet may be the better choice.
- If you’re focused purely on imaging convenience, want a flat field out of the box, and are using a large sensor, a quadruplet is usually better suited.