There isn’t a perfect one-size-fits-all telescope that can do all kinds of astrophotography.
Some models are better suited to planetary imaging than deep-sky astrophotography, the two main forms of astronomy imaging.
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Planetary imaging – as the name suggests – involves photographing the Solar System’s planets, but we also often include imaging our Moon in this bracket, as the equipment and capture requirements are very similar.
Meanwhile, deep-sky photography encompasses objects such as star clusters, galaxies, nebulae and planetary nebulae.
If you decide on planetary imaging, you’ll likely end up with a very different setup to if you went after deep-sky objects (DSOs), especially considering astrophotography is about more than just the telescope – it’s also about the camera and the mount.
If you’re in the camp who pursued the planets first, you may now like to try your hand at some deep-sky targets.
After all, planetary astrophotography has unique challenges, including battling unpredictable atmospheric conditions and the limited number of targets.
The planets don’t always present themselves well; the best conditions for capturing the superior planets (those further out from the Sun than Earth is) are when they’re high in the sky and free from the pollution that hangs near our horizon.
And then there’s the wait for planets to be at opposition, when they’re at their largest and brightest, which doesn’t always happen every year; Mars, for example, reaches opposition once every 26 months or so.
So, if you’re ready to give imaging objects in the deep sky a go instead, we’ll guide you through the different equipment requirements of both imaging types, what your planetary equipment might be capable of, and choosing a couple of DSOs to turn your telescope to for a first imaging run.
Four targets perfect for deep-sky astrophotography
The Orion Nebula
- Designation: M42
- Magnitude: +4.0
- Apparent diameter: 1°
- Best months to image: December to March
The Andromeda Galaxy
- Designation: M31
- Magnitude: +3.4
- Apparent diameter: 5°
- Best months to image: September to November
Bode’s Galaxy and the Cigar Galaxy
- Designation: M81, M82
- Magnitude: +6.9 and +8.4
- Apparent diameter: 21 and 11 arcminutes
- Best months to image: All year
The Dumbbell Nebula
- Designation: M27
- Magnitude: +7.5
- Apparent diameter: 8 arcminutes
- Best months to image: July to November
Planetary imaging
To begin with, let’s cover the basic requirements of planetary imaging setups versus deep-sky setups.
What makes them so different? Appreciating the differences between planets and deep-sky objects helps explain why the equipment requirements vary: planets are bright, local sources, whereas deep-sky objects are faint and often unable to be seen with the naked eye.
For a planet-grabbing telescope, focal length is key.
We want a long focal length to bring the target as close as possible, meaning we’re generally looking for something that delivers 1,200–2,000mm focal length.
A telescope offering this in a compact design, such as a Maksutov– or Schmidt–Cassegrain (SCT), is a popular choice.
Newtonian reflectors are sometimes chosen, but their focal length is slightly shorter.
Meanwhile, Ritchey–Chrétien (RC) telescopes tend to be avoided by planetary imagers due to large central obstructions, despite their long focal lengths.
A long-focal-length telescope often means a high focal ratio (determined by dividing the focal length by the telescope aperture).
The higher this number, the ‘slower’ the telescope: f/8 and upwards is often a desirable attribute for planetary telescopes, as it steadies the atmosphere and increases the chance of clear views.
The focal length of the telescope can be increased with a Barlow lens: a 2x Barlow lens increases a 1,000mm telescope to 2,000mm, but also doubles its focal ratio. This makes planets appear larger, but also dimmer.
Credit: The Secret Studio
In addition to the telescope, the camera is perhaps what differentiates a planetary setup most from its deep-sky counterpart.
Designated planetary cameras have small sensors to ensure planets fill as much of the frame as possible.
These tend to take video rather than still images, to combat atmospheric or seeing conditions.
A single static exposure will (unless the seeing is excellent) return a blurred photo, while 3–5 minutes’ worth of video will capture thousands of frames, which can then be sifted with planetary stacking software to select the best ones to stack for a brighter, still, final image.
Taking bursts of sub-1-minute videos is a popular approach.
In terms of the mount, many planetary imagers opt for an altazimuth type as they’re easy to set up
– these are also the mounts that are offered as part of a complete planetary setup by manufacturers, with a telescope included.
Not only can we forgo polar alignment, but these mounts are perfect for short-exposure daylight photography too.
Deep-sky imaging
Deep-sky setups are very different. A compact refractor is generally best for nebulae and galaxies.
They’re lightweight and easy to use, yet offer crisp and high-contrast images due to quality glass lenses.
Refractors also offer superior colour management to get the best from these delicate targets.
Aperture is also key, as it boosts the light-gathering ability of the telescope and makes it more efficient.
Shorter focal lengths are generally more desirable: 400mm–700mm will provide the ability to capture a wide range of targets from the Messier and NGC deep-sky catalogues.
A wide aperture and shorter focal length mean a lower focal ratio, or a ‘fast’ telescope: f/5–f/7 means a reasonably fast deep-sky refractor.
Smaller focal ratios don’t spread light out as much, so they deliver brighter images, which is exactly what is needed when capturing faint objects.
In addition to refractors, Ritchey–Chrétiens are excellent deep-sky telescopes as they combine high magnification, wide fields and fast focal ratios in a compact tube.
Ultra-sensitive cameras
Instead of using cameras that take video, deep-sky astrophotographers tend to use designated CCD or CMOS astro cameras: low-noise, cooled devices with large sensors and pixels, purpose-built for long exposures.
Taking long exposures through traditional cameras introduces electronic and thermal noise into an image as the sensor starts to heat up.
However, astro cameras reduce this through built-in cooling fans which keep the camera cool for longer exposure times.
Depending on the sensor and its cooling abilities, these cameras can take exposures of up to (and even over) 20 minutes per frame.
A DSLR camera is also a popular deep-sky choice, as it has the functionality of a deep-sky camera but can be manually operated without a computer; we can easily change its light sensitivity (ISO) settings and exposure settings to suit our target.
While these don’t come with cooling systems in place, we can still comfortably take exposures from 30 seconds and over with them, even into the 3–5 minute range for some of the high-end models.
Image exposure length also depends on the mount. To maximise the duration of our frames, an equatorial (EQ) mount is a must.
These have one axis aligned with the celestial pole, allowing us to track a target across the sky in an arc and keep it central in the image frame.
A mount that’s accurately polar-aligned lets us take exposures several minutes long or, by adding a guiding system, even longer.
Making switches to your kit
So, how can we make the transition from planetary to deep-sky photography?
First, consider the telescope. If you’re using a Newtonian telescope, you’re off to a good start!
Many are reasonably fast – a 1,000mm reflector with 200mm aperture has an f/ ratio of f/5.
Schmidt–Cassegrains are also versatile enough to make the move from planets to DSOs, although they are likely to have a slower f/ ratio.
There are accessories to speed up sluggish scopes and ready them for deep-sky objects.
It’s time to ditch the Barlow lens and buy a focal reducer. These cause the light entering the telescope to converge at a steeper angle, leading to a wider field of view and reducing the focal length.
As the telescope’s aperture can’t change, this reduces the f/ ratio.
Not only do these make the telescope faster for DSOs, but they’ll increase the number of targets you can fit into the field of view.
The next thing to do is to check your planetary camera’s specification: how large is the sensor, and is it able to take still pictures in addition to video?
If so, it could be that it is capable of some basic deep-sky images. If not, then a DSLR camera might be a sensible next purchase.
There’s an excellent second-hand market for DSLRs, meaning you can pick up an entry-level model at a reasonably modest price.
The key features needed are a Live View function for star focusing, a ‘Bulb’ mode that allows us to extend exposure times into minutes, and an adjustable ISO function.
Before making any accessory or camera purchases, check out a field-of-view calculator such as www.astronomy.tools and browse your favourite deep-sky objects.
Check whether they’ll fit into the field of view provided by your particular telescope and camera.
This will help you to determine whether your current equipment (or planned purchases) will return the results you’re expecting.
Using what you've got
If you have an altazimuth mount, the good news is that deep-sky imaging is possible. However, you’ll be limited to sub-1-minute exposures to account for field rotation.
Because altazimuth mounts don’t align with the celestial pole, the field of view of the camera remains orientated on the horizon.
This means over time, our view of the sky slowly rotates - long exposures will therefore show the stars trailing.
You can maximise the possible exposure time by aiming for a deep-sky target close to Polaris and the celestial pole; the closer to the celestial equator your object is, the greater the star trailing will be.
If your telescope is particularly slow and you don’t have a reducer to hand, pick the brightest deep-sky objects to capture as much light as possible within these short frames.
You can also combat the short exposure requirements of an altazimuth mount by bumping up the ISO setting on a DSLR camera, to pack as much light into each fame as possible.
Image your target for as long as possible and then stack these images in a stacking freeware such as DeepSkyStacker or Sequator.
If you already have an equatorial mount, longer exposures are possible – but the focal length of a planetary telescope does complicate things.
In addition to polar alignment, deep-sky imagers use guiding systems, comprising camera and software that corrects tracking errors during long exposures.
The longer the focal length, the more pronounced these tracking errors are, so an off-axis guider will be required to keep stars pin-sharp.
Depending on your kit, some level of deep-sky capture should be possible.
Temperamental weather can sometimes put a dampener on this rewarding hobby, but hopefully, by expanding your astronomy horizons and trying out new things, not only will you make the most of your kit, but you’ll gain a deeper appreciation of our stunning skies.
Planetary vs deep sky: at a glance
Telescope
Planetary: Reflector, Schmidt–Cassegrain, Maksutov
Deep-sky: Refractor, Ritchey–Chrétien, Schmidt–Cassegrain
Key feature
Planetary: Long focal length
Deep-sky: Aperture, fast focal ratio (f/5–f/7)
Camera
Planetary: Planetary video camera
Deep-sky: DSLR, CMOS/CCD cooled astrocam
Sensor size (diagonal), mm
Planetary: 6–12
Deep-sky: 26.7–28.4 (APS-C), 43.2–43.3 (full frame)
Mount
Planetary: Altazimuth
Deep-sky: Equatorial
Accessories
Planetary: Barlow lens, atmospheric dispersion corrector
Deep-sky: Field flattener, guiding system, narrowband and LRGB filters
Image-sequencing software
Planetary: FireCapture
Deep-sky: Sequence Generator Pro
Stacking software
Planetary: AutoStakkert!, RegiStax
Deep-sky: DeepSkyStacker, Sequator
Editing software
Both: Photoshop, PixInsight, GIMP, Affinity Photo, Astro Pixel Processor
Locating
Planetary: Finderscope, Go-To mount
Deep-sky: Go-To mount, plate solving
Polar alignment
Planetary: Not needed
Deep-sky: Yes
Focusing
Planetary: Manual or electronic
Deep-sky: Manual or electronic
Exposure time (per frame)
Planetary: Fractions of a second
Deep-sky: Up to 20 minutes
Integration time (total duration of exposures)
Planetary: Minutes
Deep-sky: Hours
Guiding
Planetary: No
Deep-sky: Yes
Multiple sessions
Planetary: No
Deep-sky: Yes
If you're an astrophotographer, send us your images and they could appear in a future issue of BBC Sky at Night Magazine.
This guide appeared in the January 2025 issue of BBC Sky at Night Magazine.
