The Planetary Imaging Barlow Calculator works out the focal ratio your Camera wants for planetary imaging, and the Barlow that gets your Telescope there. You enter the Telescope’s aperture and focal length and the Camera’s pixel size, and it gives you the answer for mediocre, OK and excellent Seeing side by side. It never asks which planet you are aiming at, because the rule depends only on your pixels and your air. The same answer holds for Jupiter, Saturn and Mars.
How to use it
- Pick your Telescope from the catalogue, or type its aperture and focal length in mm as it is, with no Barlow fitted. The page opens on 203 mm and 2032 mm, so you see an answer before you type anything.
- Then pick your Camera, or type its pixel size in µm. The pixel size is the only thing about the Camera that goes into the answer. Resolution and sensor size play no part.
- At the top right of the table you’ll find the native focal ratio, which is simply the focal length divided by the aperture.
- Choose a column: mediocre seeing, OK seeing or excellent seeing. On most nights OK seeing is the place to start, and you should move toward excellent only when the air is steady.
- Read the recommended Barlow. This is the amplification, in ×, that takes your native focal ratio to the target. When the amplification would be 1.05× or less it reads None, which means your Telescope already meets the target or comes within about 5% of it.
- Next comes the closest common Barlow, the nearest of 1.5×, 2×, 2.5×, 3×, 4× and 5×, along with the focal ratio it gives you.
- Last, read the target focal ratio, then the effective focal length in mm and the pixel scale in ″/px at that target. Keep in mind that these describe the target itself, not the common Barlow.
How it works
A planetary Camera wants a focal ratio matched to its pixel size. The tool uses a rule of thumb that sets the Target Focal Ratio at the pixel size in µm times 3 in mediocre Seeing, 5 in OK Seeing and 7 in excellent Seeing. If the focal ratio is too short, you lose fine detail. If it is too long, the image turns dim and soft, and you need longer exposures that freeze less of the Seeing. Treat it as a rule of thumb rather than a limit.
To get the Barlow, the tool divides the Target Focal Ratio by your Telescope’s native focal ratio and shows the result to one decimal. It never suggests a reducer, so when the native focal ratio already meets the target, the answer is None. Bear in mind that a standard Barlow amplifies more the farther the sensor sits behind it. Spacers or an ADC raise the amplification, and a 2× can end up at 2.5× or more. Aim for the recommended value and adjust the spacing until you reach it. A telecentric amplifier is different, since it keeps the same amplification at any distance.
The arithmetic is exact, but judging the Seeing is up to you. The tool doesn’t measure the night, and it knows nothing about the planet, the filter or the exposure.
Formula
Barlow = (p × k) ÷ N, with N = F ÷ D
p is the Camera’s pixel size in µm, and k is 3, 5 or 7 for mediocre, OK or excellent Seeing, with no unit. Read as a focal ratio, p × k is the Target Focal Ratio, f/(p × k). N is the Telescope’s native focal ratio, its focal length F divided by its aperture D, both in mm. The Barlow comes out in ×.
Worked example
Say you have a Telescope with a 150 mm aperture and a 945 mm focal length, a Camera with 2.9 µm pixels, and OK Seeing. N = 945 mm ÷ 150 mm = f/6.3. The Target Focal Ratio is 2.9 µm × 5 = f/14.5. Barlow = 14.5 ÷ 6.3 = 2.30, which the tool shows as 2.3×. The closest common Barlow is 2.5×, giving f/15.8, because it sits 0.2 from 2.30 while 2× sits 0.3 away. At the target, the effective focal length is 14.5 × 150 mm = 2175 mm and the pixel scale is 0.275 ″/px.
Common questions
What focal ratio should I use for planetary imaging?
Multiply your Camera’s pixel size in µm by 3 in mediocre Seeing, 5 in OK Seeing or 7 in excellent Seeing. With 2.9 µm pixels, that gives f/8.7, f/14.5 and f/20.3. On most nights, start from the OK figure.
What Barlow do I need for my Camera?
Divide the Target Focal Ratio by your Telescope’s native focal ratio. Take a 203 mm aperture with a 2032 mm focal length and a Camera with 2.9 µm pixels. In OK Seeing that asks for 1.4×, and the closest common Barlow is 1.5×, which gives f/15.0.
Does the answer change for Jupiter, Saturn or Mars?
No. The tool doesn’t ask about the planet at all, because the rule looks at your pixels and the Seeing, not at what you are pointing at.
Why does the recommended Barlow say None?
It means the native focal ratio already meets the Target Focal Ratio or comes within about 5% of it. Since the tool doesn’t recommend reducers, a native focal ratio well above the target also reads None. For example, a 203 mm, 2032 mm Telescope with 2.9 µm pixels reads None in mediocre Seeing, where the target is f/8.7 and the native ratio is f/10.0.
How accurate is this Barlow calculator?
The arithmetic is exact. The Target Focal Ratio, though, is a rule of thumb rather than a limit, and which Seeing column you pick is your own judgement of the night. Remember too that a real Barlow amplifies by the spacing you give it.
Does it need an account, and is what I type saved?
You don’t need an account. Everything is worked out in your browser, and nothing you type is sent to us or kept. The only things remembered are your language and your light or dark Theme Mode.
Related tools
In the store
Equipment this tool helps you choose, at David Astro: