Camera on an eyepiece or on a Barlow Lens or... not at all.

To make photos or videos of a celestial object you can either place the camera directly on the telescope or via a Barlow lens or a modular eyepiece. I asked Lumo, the AI-tool from Proton to elaborate on this. As I have Baader Hyperion eyepieces I mentioned them.

Why would you put a camera on a modular Baader Hyperion? Does that work like a Barlow lens?

Why mount a camera on a Hyperion? The Baader Hyperion is a very high-quality eyepiece with a wide field of view and excellent optical quality. By mounting a camera (such as a webcam, a dedicated astrocamera, or even a DSLR/mirrorless with a T-ring) directly onto the eyepiece (often via a "T2 adapter" that screws into the 2-inch or 1.25-inch holder), you can capture the image formed by the eyepiece directly onto the sensor.

Advantage: You utilise the high quality of the Hyperion optics to magnify and correct the image, rather than relying solely on the telescope optics. This can lead to sharper images at high magnifications. Application: This is almost exclusively done for planetary imaging. Because planets are small and bright, you need significant magnification. The combination of telescope + Hyperion + camera works perfectly for this.

Does it work like a Barlow lens? No, it does not work like a Barlow lens, but it has a similar effect on the focal length and magnification, albeit through a different mechanism:

Barlow lens: A Barlow lens is a negative lens placed in front of the telescope's focal point. This extends the effective focal length of the telescope (for example, doubling or tripling it). This increases magnification without needing to use an eyepiece with a shorter focal length. Camera on Eyepiece (Projection): When you mount a camera on an eyepiece, you place the sensor where the eye would normally sit (the focal point of the eyepiece). You do not change the focal length of the telescope. You do not change the focal length of the eyepiece. What you are doing is projecting the image from the eyepiece onto the sensor. The crucial difference: In planetary photography, a Barlow lens (or a PowerMate) is often placed between the telescope and the eyepiece to increase magnification. If you mount only a camera on a Hyperion without additional lenses, you get the magnification corresponding to the telescope's focal length divided by the Hyperion's focal length.

However, there is a nuance: Many astrophotographers use the eyepiece projection method. Here, the camera is not placed exactly at the eyepiece's focal point, but slightly further away, causing the eyepiece to function as a positive lens and project the image onto the sensor. In this configuration, the eyepiece does function as a kind of magnifying lens (similar to a Barlow), but in combination with the distance between eyepiece and sensor.

Conclusion: If you screw the camera directly onto the eyepiece (so the sensor lies at the focal point): It does not work like a Barlow. You get the standard magnification of that eyepiece. If you place the camera at a distance (eyepiece projection): The eyepiece then functions as a magnifying lens and increases the effective focal length, which is comparable to the effect of a Barlow lens, but with the optical properties of the Hyperion itself. Why do people choose this setup? Quality: The Hyperion often has better correction for curvature and chromatic aberration than inexpensive Barlow lens optics. Flexibility: You can use different Hyperions to achieve different magnifications without buying extra Barlows. Cost: It is often cheaper to buy a good eyepiece-camera adapter than a high-quality Barlow lens of the same quality. In short: It is a clever way to achieve high magnifications with high optical quality, but technically it is a different method than using a Barlow lens, unless you specifically apply "eyepiece projection".

What happens when you increase the magnification?

When you increase the magnification (for example, by adding a Barlow lens), the following happens:
• Image becomes larger: You "zoom in" on the detail that the mirror has already captured.
• Empty Magnification: If you magnify too much (above the resolution limit or beyond what atmospheric conditions allow), you do not see any new detail. You only see a larger, blurrier image. Your camera's pixels are displayed larger, but they contain no additional information.
• Brightness decreases: The more you magnify, the less light falls on your sensor or retina per unit area. The image becomes darker.

The utility of higher magnification in planetary photography
Although the resolution does not increase, higher magnification is useful for planetary photography (such as Jupiter or Saturn):
• Sampling: Camera sensors consist of pixels. If the image of a planet is too small on the sensor (undersampled), you miss detail because the pixel size is larger than the detail the telescope can resolve.
• Optimal magnification: By using a Barlow lens, you magnify the image until it exactly matches the resolution of your camera pixels. This is called critical sampling. You get the maximum out of the telescope and camera combination, but you do not extract new resolution from the sky.

In summary
• Resolution: Remains exactly the same (determined by the mirror size).
• Detail visibility: Can improve if you were previously "undersampled" (too little magnification for your camera), but deteriorates if you apply "overmagnification" (too much magnification for the atmosphere or mirror quality).
• Image quality: Often becomes slightly less sharp due to extra optical elements (the Barlow lens) and atmospheric turbulence (seeing), which becomes much more clearly visible at high magnifications.

Conclusion: A Barlow lens helps you make the existing detail from your telescope fit better onto your camera sensor, but it does not create new detail that the mirror could not have already captured.

Important nuances

Wavelength dependency: Resolution is better (smaller θ) at shorter wavelengths (blue light) than at longer wavelengths (red light).
Theoretical limit: This is the theoretical limit for perfect optics. In practice, resolution is often limited by:
o Atmospheric turbulence (seeing): This is usually the biggest limitation. Even a perfect telescope cannot achieve better resolution than the atmosphere allows (often 1-2 arcseconds, sometimes worse).
o Optical quality: Aberrations in the mirror or lenses.
o Collimation: The correct alignment of the optical elements.
Other criteria: The Rayleigh criterion is one of several criteria. The Dawes limit is a similar empirical formula (θ ≈ 116 / D in mm) that is slightly more optimistic, but for practical purposes, both yield comparable results. In short: The formula gives the fundamental physical limit of what a telescope can resolve, but the actual performance depends on atmospheric conditions and the quality of the optics.

The Rayleigh Criterion
The calculation for the Rayleigh criterion:
Using the formula θ ≈ 138 / D (mm):
θ ≈ 138 / 203 ≈ 0.68 arcseconds

Why the difference?
There are two different criteria that are often used:
Criterion Formula (mm) Result for 203mm
Rayleigh 138 / D ~0.68"
Dawes 116 / D ~0.57"
Rayleigh criterion: Based on diffraction patterns (the first minimum of one star coincides with the maximum of the other). This is more conservative.
Dawes limit: Empirically based on observations of double stars. Slightly more optimistic, but often achievable in practice with good optics.

So, based on the foregoing, what is the maximum possible Barlow lens to achieve the Dawes limit with the ASI585MC with 3840x2160 pixels?

Good question! This concerns sampling and the relationship between camera pixels, telescope focal length, and theoretical resolution. Let's calculate this step by step.

Specifications of the ASI585MC
Parameter Value
Pixel size 2.9 µm
Resolution 3840 × 2160 pixels
Sensor size ~11.1 × 6.2 mm
Nyquist Sampling for the Dawes Limit
To adequately sample the Dawes limit (0.57"), you need a minimum of 2 pixels per resolution element according to the Nyquist theorem. In practice, 3–5 pixels are often recommended for planetary photography to ensure optimal stacking and detail retention.
The formula for image scale (arcseconds per pixel):
Image Scale = 206,265 × pixel_size (µm) / focal_length (mm)
Calculation for Different Sampling Levels
For a 203mm mirror with a Dawes limit of 0.57":
Pixels per Resolution Element Image Scale ("/px) Required Focal Length Barlow Factor
2 pixels (Nyquist minimum) 0.285" ~2100 mm ~1.0x
3 pixels (practical optimum) 0.19" ~3150 mm ~1.55x
5 pixels (conservative) 0.114" ~5250 mm ~2.6x
Important Limiting Factors
Atmospheric Seeing: This is the primary limitation. Even if your telescope can resolve 0.57", the atmosphere will often restrict this to:
Good seeing: 1–1.5"
Average seeing: 2–3"
Poor seeing: 3"+
With 2" seeing, higher sampling offers little benefit—you won't see extra detail.
Practical Recommendation: For the ASI585MC with your 2032mm telescope:
Without Barlow: Image scale = 206,265 × 2.9 / 2032 ≈ 0.29"/px
This is approximately 2 pixels per Dawes limit (Nyquist minimum).
Perfect for good seeing conditions.
With 1.5x Barlow: Image scale ≈ 0.19"/px
This is approximately 3 pixels per Dawes limit.
Ideal for stacking and detail retention.
With 2x Barlow: Image scale ≈ 0.15"/px
This is approximately 4 pixels per Dawes limit.
Good, but you enter the "empty magnification" zone if the seeing is poor.
Conclusion
Maximum useful Barlow: 2x
Optimal: 1.5x – 2x Barlow for the best balance between sampling and seeing tolerance.
More than 2x: Likely "empty magnification" unless you are working in exceptionally good seeing conditions (such as at a mountain observatory).
Recommendation: Start with a 1.5x or 2x Barlow (or a Baader PowerMate of a similar factor). Then test with different settings to see which yields the sharpest results under your local seeing conditions.

Would you like me to work out the calculation for a specific Barlow factor in more detail?