In astrophotography, the optical performance of a telescope directly determines whether the stars you capture look sharp, detailed, and true-to-color. But for astrophotography beginners, charts and technical terms can feel like unreadable code.

But don’t worry. This guide breaks down the key optical performance metrics of a telescope into plain language. By the end, you’ll be able to read them like a pro: Spot Diagram, MTF Curve, Longitudinal Aberration, Lateral Chromatic Aberration, and Relative Illumination.

Spot Diagram: Are Star Points Sharp?

The Spot Diagram is like an eye test for your telescope. It shows how a single point of light (like a distant star) spreads out on your camera sensor after passing through the telescope.

• The smaller and tighter the spot, the sharper the image.
• The more it spreads, the blurrier your stars will look.

Typically, you will see six spot diagrams for different sensor sizes: center of the sensor, 1/1.2 inch, 1 inch, 4/3 inch, APS-C, and full frame (see Figure 1).

Figure 1: Camera's sensor sizes

 

So, how to read this spot diagram of HOPE D60 (Figure 2) and check how sharp the stars can be?

Figure 2: The Spot Diagram of ToupTek Astro HOPE D60 telescope

 

To analyze it, you need to know about the Airy Disk.

When the light from an ideal point source, such as a distant star, passes through a telescope's circular objective lens, it does not converge into a perfect geometric point. Due to the wave nature of light and the effects of diffraction, the light forms a diffraction pattern on the focal plane. This pattern consists of a central bright spot, surrounded by concentric rings of alternating light and dark. This central bright spot is known as the Airy disk (shown in Figure 3).

Figure 3: Airy Disk

 

The Airy disk represents both the theoretical resolution limit of an optical system and the key benchmark for its design and evaluation. Ideally, a spot diagram should approach the size of the Airy disk. As shown in the 10-micron spot diagram for the HOPE D60 astronomical telescope (APS-C format), the black circle denotes the Airy disk's size. See Figure 4.

Figure 4: HOPE D60's Spot Diagram of APS-C format

 

For all sensor formats, the spot diagrams of the HOPE D60 show that all light rays are tightly concentrated within the Airy disk. This superior concentration translates directly into round, sharp star points in your astrophotos, delivering performance that approaches the theoretical limit. Even when capturing vast nebulae and star clusters across a full-frame sensor, the stars at the very edge of the frame remain crisp, ensuring both detailed and aesthetically pleasing images.

 

 

MTF Curve: The Report Card for Sharpness and Contrast

The MTF (Modulation Transfer Function) Curve measures how well a telescope preserves contrast and fine detail. You can think of it as a performance score sheet of a telescope for sharpness and contrast

Here are tips on how to read the MTF curve for astrophotography beginners:

1. Axes: Horizontal is the distance from the image center. Vertical is the MTF value (1 is perfect).
2. Frequency: 10.00 cyc/mm is the low frequency, representing the overall contrast. 30.00cyc/m is the high frequency, representing the fine details. When the low-frequency curve (10.00 cyc/mm) is closer to 1, it means the telescope has excellent contrast, giving the image a strong, clear presence. When the high-frequency curve (30.00cyc/mm) is closer to 1, it indicates high resolution, the ability to render fine details with exceptional sharpness.
3. Lines: Dashed lines (Tangential, T) and solid lines (Sagittal, S). The closer they are, the better the telescope controls astigmatism, avoiding ghosting or doubling.

Figure 5: MTF vs. Field of HOPE D60

The MTF curve of HOPE D60 stays smooth, high, and close to the diffraction limit across the entire field. This ensures consistent clarity and detail, from center to edge.

 

Longitudinal Aberration Curve: Can All Colors Focus Together?

Longitudinal aberration occurs when different wavelengths (colors) of light fail to converge at the same point along the optical axis.

As illustrated in Figure 6, red light, which exhibits the lowest refraction, focuses farthest from the lens. Blue light, with the highest refraction, converges closest to the lens. Green light, with a refractive index between the two, focuses at an intermediate point. The focal points for the various colors are thus spread out along the optical axis. This positional discrepancy of focal points for different colors is known as longitudinal chromatic aberration.

If placing a screen at positions 1,2, and 3 respectively, the resulting blur spot (circle of confusion) will show distinct color fringing. At position 1, the spot will have a red outer halo with a blue core and green in between. At position 2, the pattern shifts to a blue outer halo, a green core, and red in between. At position 3, it appears with a blue halo and a red core, with green in between. This chromatic aberration prevents an on-axis point from being focused into a clean white spot, instead rendering it as a colored blur.

Figure 6: Longitudinal Aberration

 

Figure 7 is the longitudinal aberration curve of the HOPE D60 astronomical telescope.

• Axes: Horizontal = Focal shift (smaller is better); Vertical = Pupil coordinate
• Interpretation: Each line is a different wavelength. Flatter, more tightly grouped lines signify better chromatic aberration correction.

Figure 7: Longitudinal aberration curve of HOPE D60 astronomical telescope

From 436nm to 707nm (full visible spectrum), the axial color shift of HOPE D60 is less than 0.05mm. It shows an exceptional chromatic control of HOPE D60.

Lateral Chromatic Aberration Curve: Will the Edges Show Color Fringes

Lateral (transverse) chromatic aberration shows up as colored fringes (purple, green, etc.) around stars at the edges of the frame (shown in Figure 8 below). It’s more noticeable than axial aberration and can ruin wide-field images. 

Figure 8: Lateral Chromatic Aberration

 

Figure 9 is the lateral chromatic aberration curve of the HOPE D60 astronomical telescope. You can use this graph to easily predict if a telescope will show colored edges (like purple or green fringes) in the corners of your photo. Here's what you're looking at:

• Bottom (X-Axis): Shows the amount of color fringing. Smaller numbers are better.
• Left (Y-Axis): Shows the location in the image, from the center (0mm) to the edge of a full-frame sensor (~22mm).
• Colored Lines: Each represents a different color of light.

How to interpret it:

• Flat Line: Meaning the lens controls fringing equally well in the center and corners.
• Lines Close to Zero: Meaning the fringing is very slight overall.
• Lines Close Together: Meaning different colors focus at nearly the same spot, reducing visible color separation.

Figure 9: Lateral chromatic aberration curve of HOPE D60 astronomical telescope 

Using HOPE D60, all wavelengths from 436-707nm stay well within the Airy Disk, even at the edges. There are no distracting purple halos, even on full-frame sensors.

 

Relative Illumination: Is There Edge Vignette?

Relative Illumination measures how evenly light spreads from the center to the edge. The smaller the value, the darker the corners are. This is what we call vignetting” (shown in Figure 10). The formula is:

Relative Illumination = (Edge Brightness ÷ Center Brightness) × 100%

 

Figure 10: Vignetting

 

Figure 11 shows the relative illumination of the HOPE D60, which measures how evenly bright the image is from center to edge. The graph plots the image height (from 0mm at the center to 22mm at the full-frame edge) against the relative brightness. A perfect result would be a flat line at 100%. The HOPE D60's curve is exceptionally high and flat, declining only gently at the very edges. What this means is that your astrophotos will have minimal vignetting (corner darkening), preserving uniform brightness and detail across the entire field of view.

Figure 11: Relative illumination of HOPE D60

From the above relative illumination curve, we can see that the relative illumination of HOPE D60 stays above 90% across the full frame. This means that the brightness is uniform and no heavy vignetting. Even faint nebulae remain visible at corners without correction.

 

Notes for Interpreting Astronomical Telescope Optical Diagram Reports

1. No perfect optical system exists

All optical systems are the result of trade-offs and compromises between various aberrations. There is no telescope that is perfect in all metrics. Therefore, evaluating a telescope's optical performance requires comprehensive judgment - do not rely on a single chart.

2. Define your imaging needs

• Planetary Imaging: Focus on the spot diagram at the center field (whether it is sufficiently small and symmetric) and the high-frequency section of the MTF curve.
• Deep Sky Astrophotography: Need to pay attention to the performance across the entire field. Spot diagrams should not show severe coma or astigmatism at the edges, and the entire family of MTF curves should remain high.
• Visual Observers: Can tolerate a slightly higher degree of edge aberrations (like coma) compared to astrophotography, but center sharpness remains critical.

3. Pay Attention to Charts' Scales

When comparing optical performance charts from different telescope brands, the most critical aspect is the scale of the charts. When charts are displayed at different scales, the arrangement of spots or curves will appear differently. Therefore, for accurate comparison, ensure the charts are scaled uniformly.

 

Behind every breathtaking image of the night sky lies exceptional optical performance. Key metrics like the spot diagram, MTF curve, longitudinal and lateral chromatic aberration, and relative illumination serve as a telescope's definitive "optical report card." They translate its capabilities into quantifiable data, allowing us to move beyond intuition and objectively understand its true potential.

The HOPE D60 astronomical telescope from ToupTek Astro excels across all these critical dimensions, delivering a balanced and impressive performance. It produces pinpoint-sharp stars across the entire field of view, with exceptional clarity from the center to the very edges. Its precise control over chromatic aberration and minimal vignetting are hallmarks of its advanced design.

Such robust and superior optical quality not only allows beginners to capture the depths of space with ease but also provides seasoned astrophotographers with the reliable foundation needed to pursue extreme detail and uniform image quality.

If you are searching for a comprehensive, trustworthy, and capable astronomical telescope, the HOPE D60 is undoubtedly a clear window to the cosmos that you won't want to miss.