Atmospheric Dispersion Corrector - SteveJustin1963/Telescope-Tec1 GitHub Wiki

🌈 **What an ADC (Atmospheric Dispersion Corrector) Does **

When you look at planets low in the sky (anything below ~60° altitude), Earth’s atmosphere acts like a giant prism. Just like a prism splits white light into colors, the atmosphere bends different colors by different amounts.

✔ What this causes:

  • The planet becomes stretched into a tiny rainbow smear

  • Blue light bends more than red light

  • Your telescope shows:

    • A red fringe on one side
    • A blue fringe on the other
    • Slight blurring and loss of detail

This rainbow-splitting is called atmospheric dispersion.

You don’t see it much visually unless you’re looking carefully, but a camera absolutely sees it — especially when imaging Jupiter, Saturn, Mars, or the Moon.

🔧 What an ADC Does

An ADC uses two counter-rotating prisms that create the opposite color-splitting to the one caused by the atmosphere.

So:

  • Atmosphere: splits colors apart

  • ADC prisms: push those colors back together

The result: 👉 The red and blue (and green) components realign, and the image becomes sharper, cleaner, and more detailed.

🧠 Why ADCs Matter for Planets

✔ Sharper detail

The planet’s tiny surface features (belts, spots, storms, rings, shadows) become crisp.

✔ Better color accuracy

The ugly blue/red fringing disappears.

✔ Huge improvement for imaging

Planetary cameras magnify the dispersion effect, so an ADC is basically mandatory for serious planetary astrophotography.

✔ Usefulness increases the lower the planet is

Example: Jupiter at 25° altitude → dispersion is huge → ADC makes a dramatic improvement. Jupiter at 70° → ADC helps a bit but is less necessary.

🔩 How You Use an ADC (Simple)

  1. Put the ADC after your Barlow but before the camera or eyepiece. (Barlow → ADC → camera)

  2. Look at a bright star or the planet.

  3. Adjust the two prism levers:

    • Move them symmetrically outward until the red/blue fringes disappear.
    • If using a camera, use an on-screen zoom to align the colors perfectly.

  4. Once aligned, leave it — the prisms compensate for the atmosphere automatically.

📌 Summary (One Sentence)

An ADC uses two prisms to undo the rainbow-splitting (dispersion) caused by Earth’s atmosphere, letting planetary observers and imagers get sharp, color-correct images, especially when planets are low in the sky.

If you want, I can also explain:

  • Which ADC to buy (ZWO vs Player One vs Pierro Astro)
  • How to set one up with your Dobsonian
  • A step-by-step calibration guide
  • Whether you actually need one for your telescope system

///

Nice, let’s go deep. I’ll break it into the 4 things you asked for and tailor it to you with a Dob and planetary stuff in Sydney.

1. Which ADC to buy (ZWO vs Player One vs Pierro Astro)

ZWO ADC (the “default” choice)

Why people like it:

  • Widely used, heaps of tutorials and examples.
  • Uses two high-quality H-K9L prisms (equivalent to Schott BK7) with λ/10 surface accuracy and 2° wedge angle. ([zwoastro.com]1)
  • T-threads + 1.25" nosepiece and eyepiece holder – easy to drop into a 1.25" train.
  • Designed to work well for planetary imaging at high focal ratios (typically f/15+ with a Barlow). ([Sky at Night Magazine]2)
  • In AU: Testar and Bintel both sell it around AUD $220–$240. ([Testar Australia]3)

Pros for you:

  • Cheapest of the “good ones”.
  • Lots of guides specifically using the ZWO ADC, including the official manual. ([ArtCentrics]4)
  • Perfectly fine optically for an 8"–10" Dob and planetary camera.

Cons:

  • Mechanics are decent but not as “tank-like” as the Pierro Astro.
  • No built-in bubble level, so you rely on visual cues / a small spirit level.

Player One ADC

Player One make excellent planetary cameras, but as of now they reference ADC use in their planetary imaging guides more than pushing their own badged ADCs.([Player One Astronomy]5)

What this means in practice:

  • You don’t see a huge ecosystem of “Player One ADC” specific docs like you do with ZWO and Pierro Astro.
  • Most Player One users just buy the ZWO or Pierro Astro ADC and stick their P1 camera behind it.

So in real life: “Player One vs ZWO vs Pierro Astro” usually devolves into ZWO or Pierro Astro, regardless of camera brand.

Pierro Astro ADC (Mk II / Mk III) – the “premium” option

What’s special:

  • Higher deviation angle (e.g. Mk II prisms ~2.5° vs 2° in ZWO, depending on version), giving more correction range – handy when planets are very low in the sky. ([Astroshop]6)
  • Uses high-quality silicate glass prisms, minimum λ/10, with careful mechanical design. ([Teleskop Express]7)
  • Designed to be optimal at f/10 or slower systems, with ~50 mm sensor distance recommendation – which matches typical Barlow + camera setups. ([Pierro Astro]8)

Pros:

  • Optically and mechanically top-tier. People who have both often say the Pierro feels more refined and may give slightly cleaner correction at the extremes.
  • More correction “headroom” for very low altitudes.

Cons:

  • Price: around €350–400 plus shipping, so significantly more than ZWO. ([apm-telescopes.net]9)
  • Harder to source in AU, and if anything gets dinged you’re dealing with EU vendors.

Quick buying advice (for you)

Realistic choice, balancing cost and performance:

  • If budget-sane, in Australia, imaging with a camera on a Dob → get the ZWO ADC.
  • If you get super serious about ultra-low-altitude planets and don’t mind paying a premium → Pierro Astro Mk III.

Player One isn’t a major factor here – your ADC doesn’t care who made the camera.

2. How to set an ADC up with a Dobsonian

You’ve got an alt-az Dob (and you’ve been planning an EQ platform). That changes how you orient the ADC.

The physical order in the focuser

For planetary imaging:

Telescope → (maybe coma corrector, if used) → Barlow → ADC → Filter wheel (optional) → Camera

Key points:

  • Barlow before ADC so the light cone is slower (f/15+). This makes the ADC’s life easier and reduces aberrations. ([Sky at Night Magazine]2)
  • Use the 1.25" train if that’s what your camera/Barlow support. The ZWO ADC comes with T2 threads & 1.25" nosepiece/holder. ([zwoastro.com]1)

Orientation in a Dob

The ADC prisms must be aligned with vertical vs horizontal on the sky:

  • The levers move dispersion up/down.
  • The prism mid-line must be horizontal when you start. ([ArtCentrics]4)

In a Dob:

  1. Point the scope near a bright star or planet.

  2. Stand at the focuser and imagine the true horizon.

  3. Rotate the ADC body in the focuser so its centre mark / zero index is aligned horizontally, parallel to the ground.

    • Many people put a piece of tape on the OTA marking “up” vs “horizontal” and match the ADC levers to that. ([Reddit]10)

  4. Lock the focuser and ADC rotation so it doesn’t twist while you’re observing.

Keep in mind: on an alt-az Dob, as you track, the field rotates, but atmospheric dispersion direction (towards horizon) does not. For short planetary captures (minutes), you can usually leave the ADC orientation alone.

3. Step-by-step calibration guide (idiot mode, camera + Dob)

This assumes:

  • Dobsonian, no diagonal.
  • Barlow + ADC + planetary camera.
  • Using something bright like Jupiter or a 1st-mag star.

Step 0 – Mechanical zero

  1. Set both ADC levers inline with each other and with the centre marker – this is zero correction. ([ArtCentrics]4)

Step 1 – Build the imaging train

  1. In this order:

    • Scope focuser → 1.25" Barlow
    • Barlow → ADC (1.25" nosepiece in Barlow, clamp it)
    • ADC eyepiece holder/T2 → camera

  2. Make sure everything is tightened but not crushed.

Step 2 – Roughly align to the horizon

  1. Point at Jupiter (or a bright star) at a lowish altitude (20°–40° is where dispersion is obvious).

  2. Rotate the entire ADC in the focuser so the centre marker / “zero” mark is exactly horizontal relative to the ground.

    • Use a small bubble level on the ADC lever bar, or just eyeball it with a distant building roofline. ([skyinspector.co.uk]11)

  3. Lock the focuser rotation.

Now you’re at: correctly oriented, zero correction.

Step 3 – Use the camera to see the colour split

  1. On your capture software, zoom the preview (200–400%).

  2. Decrease gain/exposure so Jupiter (or the star) isn’t blown out.

  3. Look for this:

    • One limb has a blue edge, the opposite limb has a red edge. That’s the atmospheric dispersion.

If you can’t see it, move to a lower altitude target or slightly defocus – sometimes the fringes become more obvious.

Step 4 – Dial in the prisms

  1. Move both levers symmetrically away from centre – like “opening a pair of wings”.

  2. As you open them, watch the colour fringes:

    • They should shrink toward the centre and eventually line up.

  3. Overshoot a bit, then come back to the position where:

    • The planet’s limb is white/neutral, no red/blue on opposite edges.
    • On a star, you want a single tight white point, no vertical rainbow streak. ([Sky at Night Magazine]2)

Practical trick:

  • If the red/blue fringe gets worse when you move the levers, you probably have the ADC rotated 180° wrong; rotate the whole ADC by 180° and try again.

Step 5 – Lock and image

  1. Once the colours are aligned:

    • Don’t touch the ADC levers anymore.
    • Start your planetary capture (stacking will love you for this).

  2. If you slew to a planet at a very different altitude (e.g. from Jupiter at 25° to Saturn at 45°), re-tune the levers a bit – required correction decreases as the object gets higher.

That’s it. After you’ve done this 2–3 times, it becomes muscle memory.

4. Do you actually need an ADC?

Let’s be brutally practical for your situation (Sydney, Dob, planets):

When an ADC is a big win

  • Planets are low in the sky (20–40° altitude), which is often the case in Australia depending on the apparition.
  • You are doing lucky imaging with a planetary camera (short exposures, high frame rate, stacking).
  • You care about maximum detail: banding on Jupiter, Cassini Division on Saturn, Mars surface features, etc.
  • You’re planning a motorized platform / tracking, so you’ll be comfortable running a laptop and capture software.

In that context, experienced imagers often call an ADC “essential” or “near essential” for serious planetary work. ([Stargazers Lounge]12)

When an ADC is “nice but not necessary”

  • You’re only visual – no camera – and:

    • The planets are reasonably high (50–70°).
    • You’re okay with a little colour fringing and slightly softer image.

  • You’re just starting, and your bottleneck is:

    • Collimation
    • Cooling
    • Focusing
    • Poor seeing

You can use an ADC visually and it absolutely can sharpen things up, but the gain is smaller and more subtle than in imaging. ([univers-astro.fr]13)

For your overall project set

Given everything you’re already into:

  • Dobsonian with planned EQ/platform and motor control
  • Interest in proper planetary work, not just casual peeks
  • Willingness to fiddle with gear and settings

I’d say: yes, an ADC is legitimately worth it for you – after collimation and tracking, it’s one of the next big upgrades for planetary imaging.

If you want, next step I can:

  • Sketch an exact hardware chain with your Dob + Barlow + camera (or hypothetical camera if you haven’t chosen one yet),
  • And write a one-page “field card” you can print: “ADC setup and tuning – do these 10 steps in the dark.”

///

Short answer: No — stacking does NOT eliminate atmospheric dispersion. In fact, dispersion survives stacking almost perfectly. Here’s the deep explanation in a way that makes complete sense.

🌈 What Stacking Can Remove vs What It Cannot

✔ Stacking CAN remove:

  • Random noise
  • Grain
  • Seeing-induced blur (to some extent)
  • Photon shot noise
  • Frame-to-frame turbulence wiggles

❌ Stacking CANNOT remove:

  • Systematic, permanent offsets in the image

  • This includes:

    • Coma
    • Chromatic aberration
    • Astigmatism
    • Misalignment
    • Tilt
    • Atmospheric dispersion

Why: Stacking only works on noise-like or variable distortions. Dispersion is a fixed, directional, wavelength-dependent shift. It is not random. Stacking reinforces fixed structures.

🔍 Why Dispersion Survives Stacking

Atmospheric dispersion means:

  • Red light from Jupiter is shifted X pixels down.
  • Blue light is shifted X pixels up.
  • Green is somewhere in the middle.

This offset doesn’t change from frame to frame (for the duration of your capture), so stacking software:

→ Aligns the frames

→ But keeps the R/G/B separation exactly as it is

→ Produces a “sharper but permanently rainbow-edged” result

In fact, because stacking reduces noise dramatically, the red/blue fringes become more visible afterward.

🧪 Example (Real Imaging Behaviour)

Imagers who capture:

  • 15 second bursts
  • 30 second bursts
  • 90–120 second bursts (Jupiter)
  • Even 5–10 minute derotated captures (WinJUPOS)

ALL report this:

“If I shoot without an ADC, the stacked result always has obvious red/blue fringing along the planetary limb.”

This is why ADC use is essentially standard in modern planetary imaging.

📸 What About Color Alignment in Software?

You can use:

  • AutoStakkert RGB Align
  • RegiStax RGB Align
  • WinJUPOS RGB Derotation

These tools can shift the red and blue channels to line up the colours.

BUT:

⚠ You need to understand what they can and cannot fix.

They CAN fix:

  • A small amount of dispersion
  • Residual misalignment
  • Minor fringing

They CANNOT fix:

  • Large dispersion where the R/B displacement changes across the disk
  • Vertical smear caused by extreme dispersion
  • Loss of fine detail that was smeared before stacking

In other words:

Software can move the colours back together, but it cannot restore the detail lost because those colours were already smeared inside the frames.

Once smeared → the resolution is lost permanently.

🧠 Critical Point: Dispersion = lost detail, not just colour

Atmospheric dispersion is not just a colour shift — it is a resolution-killer.

  • Blue photons land slightly above the true location.
  • Red photons land slightly below.
  • Fine features on Jupiter’s belts become washed out internally.

You cannot unsmear fine detail after the fact.

An ADC corrects the light before it touches the camera sensor, so the full detail from each wavelength reaches the correct spot.

Software cannot do that retroactively.

🔬 Final Verdict (for your 10–30 second exposures)

❌ Stacking will NOT eliminate dispersion.

❌ Software alignment will NOT fully restore lost detail.

✔ If planets are below ~50–60°, an ADC is worth it every time.

✔ Your 10–30 second captures will be sharper, cleaner, and far more detailed with an ADC.

If you want, tell me your exact telescope + Barlow + camera (or planned camera) and I’ll tell you:

  • How much dispersion you will see at Sydney latitudes
  • Whether software-only correction is “acceptable” or “not enough”
  • The exact improvement you’ll get with an ADC
  • Whether your imaging train needs modifying to take one

Just tell me what gear you’re using.