So far we’ve looked at astrophotography as a whole, explored essential kit and discussed wide-field astrophotography.
The next natural step would be to venture into the vast expanse of deep space photography.
It might seem unattainable and the spectacular imagery produced from deep space imaging can be daunting.
The images you can produce are literally out of this world and like nothing else you would ever be able to capture on Earth. What deep space photography, also called astroimaging, offers us a personal glimpse into that mysterious realm outside of our sphere of life.
It is difficult to imagine a hobby where you will be peering thousands, millions and tens of millions of years into the past – with a chance of discovering something never discovered before.
Deep Sky Astrophotography How To.
Video by AstroBackyard
But, that is exactly what Astro-imaging allows. Deep space imaging is, at the most basic level, the use of a telescope, mount, and camera to take photographs of deep space objects, such as galaxies, nebulae or star clusters, just to name a few.
The galaxies can either be our very own Milky Way, or other distant galaxies far beyond our own. In short, deep space is considered anything outside of our Solar System. This article will try to delve deeper into the basics of deep space astro imaging and how to get started.
Choose a deep space subject.
Depending on your location on Earth, including which hemisphere and the time of year, you will have some decisions to make.
In general, during the summer at the location described in this article(United States), many hydrogen emission (HII) targets (aka, nebulae) are visible as the edge of our Milky Way crosses into our night sky.
As winter approaches, there are somewhat more galaxies available.
Considering what you enjoy the most, or have the most interest in, can be the biggest driving factor.
Planetarium software, such as Stellarium, will set the date/time based on your computer’s time, and you need only set your location.
Once set, you can go forward (or backward) in time to see what will be rising that night and when it sets again on the opposite horizon.
Using Stellarium, you can then look for deep space objects, select them and read the information – such as the name of the object, size in arc seconds, and magnitude/brightness.
Stellarium Tutorial.
Vidoe by Emily Welch
If the object would reasonably fit into the field of view (FOV) based on the camera/telescope combination and would be high enough above the horizon for several hours, then that would likely be a target.
In general, imaging two objects in one night is a normal procedure, waiting for them to cross the celestial meridian so that the scope will be on the east side of the mount, and track to the west as the object sets.
This helps to prevent the camera and back of the scope from impacting the tripod legs, for example.
Deep Space Photography what you can image and when.
Although differences in months for seasons may vary between countries, here are some targets for the general seasons in the United States of America.
Spring: Galaxies to image include M100, M64, NGC 4565 and M81/M82, just to name a few. Nebulae include M27 and NGC 6820.
Summer: Summer is generally tough as the high temperature will not allow the camera to cool down to a lower CCD temperature.
In addition, for the location discussed in this article, summer normally means mosquitoes, so another consideration for an imaging night. As for targets, there are many nice objects to the image in both galaxies as well as nebula categories, and even clusters (not described in detail here).
Summer is also great for wide-field (DSLR) images of the Milky Way.
Galaxies to image include our own Milky Way, as well as M31 (which overlaps between seasons for this location) and NGC 4565. Nebulae/Clusters include M16, M27, NGC 6995, and M13.
Winter: Generally the best season for imaging for the United States in terms of weather. Normally dry and cool, which implies good weather conditions due to the dry air, plus lower temperature setting (-15C) for the camera CCD due to the lower ambient temperature (also, no mosquitoes!).
Thus, many of the images you can see on popular magazines and on the web are from this season.
Galaxies to image include NGC 7331, Stephan’s Quintet, M33 and NGC 3184.Nebulae include IC 434 and M33 (Horsehead Nebula), NGC 2664, NGC 7000 (North America Nebula), M42 (Orion Nebula) and IC 5070 (Pelican Nebula).
Getting set up for deep space photography.
A deep space setup can be quite involving and time consuming, but in order to shoot the best possible imagery you should make sure that you prepare well in advance.
For imaging using anastroCCD, multiple steps are required to ensure that long exposures of deep space objects are possible. Following physical setup of your equipment, you must undertake several other steps before you begin collecting image data.
First you must polar align your mount. This varies between different kits – with the Takahashi EM-200 mount, the polar scope is built into the mount.
Use the user’s guide, which describes how to do the polar alignment – align the GE mount with Polaris.
Takahashi Telescope EM-200 Mount.
Using your software, connect the camera, guide camera, filter wheel, and telescope mount. When connecting the mount, the Takahashi portion of the ASCOM driver will prompt for scope position.
For the scope position prompt from the ASCOM software, make sure scope/counterweights are vertical, and then select ‘Scope pointed at the pole, CW down’ and the connection will be made. Use the polar scope to find the North Star.
Then, using your camera software, take a very short three-second image and find the North Star in the FOV of that image.
Then, switch to the Focus mode, select that same star, and the focus will update the image at some frequency (normally two seconds or so, depending on the brightness of the star).
Make sure you make a large-enough window so you can move the star and see it move, then move your mount using your software until you have the North Star close to the centre of the main FOV.
Take another test image to see where it is at. Repeat the process until the North Star is very close to the center of your image frame.
Next, pick one of the ‘smaller’ stars near Polaris and near the centre. Finding a centre star is not too important for me (as the scopes have very flat fields), but in cases of outer coma, you will want to use a star near the centre.
Set the exposure on the camera at 1-3 seconds, with Focus mode binned to 2×2 or 4×4 (depends on camera). Poor seeing conditions will require that you use shorter exposures so as to not confuse poor seeing with poor focus.
Use your focus method (motorized or manual) to bring your focus star to as close to 1×1 pixel as possible (many dependencies here, but this is in general). A three- (or more) point alignment is required when using a portable setup.
This allows your mount to have a reference when slewing to a particular target. Using your software (e.g. a ‘Goto’ list that can be created and loaded into Astroart), select Polaris, and Goto it.
It is important that you provide the correct information when youconnect the mount e.g. ‘counterweights down/ scope pointing at pole’). Next, select a star that you know is visible – have the mount Goto that star, then centre the star in the FOV of the camera – and ‘Sync’. Do this for one more star (or common object,e.g. M31 if visible).
At this point outside, you will have already decided what you will image for the night (per previous section). Using the Goto of your mount, now move to your target, which should be close to the FOV for your image frame.
Take some test images to better centre your target. Then, re-sync your mount (in case you need to find this exact same position again, e.g. a glitch – reboot and tell the mount to ‘keep last sync’ and you will not have to redo the three-point alignment).
Now set up your autoguide; this differs depending on setup, but in general, the corrections will differ depending on whether your scope is on the east or west side of the celestial meridian.
Once auto-guide is running properly, you are set for your LHaRGB (or other filters) exposures – so, you are now set up to start imaging!
Capturing a deep space object.
About 12 million light years from Earth lies one of the most beautiful galaxies in the entire night sky.
Bode’s Galaxy, also known as M81 or NGC 3031, can be found in the constellation of Ursa Major (The Great Bear) and shares its region of the sky with another galaxy of completely different shape, known as the Cigar Galaxy, cataloged as M82. What makes Bode’s Galaxy outstanding is its glorious spiral arms.
It is almost face on to us, so we get a very good view of the entire galaxy. The galaxy itself is interesting, as it is thought to contain a supermassive black hole at its centre.
It is, without doubt, a very photogenic object in the night sky. It is therefore a very popular object for amateur astronomers and astrophotographers. If you plan to image this galaxy in all its glory for yourself, there are a few things you’ll need before you begin.
First of all, you’ll need a telescope; between 85mm and 100mm aperture for a refractor, or between 150mmand 200mm aperture for a reflector are good sizes.
Of course, an even larger aperture will make imaging the galaxy and its spiral arms even easier. A tracking mount is also essential and an equatorial mount is preferable with a drive or a GoTo system.
You’ll also need a DSLR camera and an adaptor to fit it to your telescope. A useful device to have here is a remote shutter release, as this helps to minimize vibrations when opening and closing the shutter on the camera.
Depending on the focal length of your telescope, you may need to use a Barlow lens to increase the magnification and to fit the galaxy comfortably into the frame of the picture.
Even though the galaxy is quite bright, you’re still going to need to use long exposures to capture it well. You’ll also need to be able to set your camera to manual settings to do this and adjust the ISO setting to get a good image.
It is a good idea to use a series of shorter-exposure shots and then stacks them together in software, rather than taking one long-exposure image.
This helps to minimize the risk of something going wrong during the exposure and can also help to increase the contrast in the final image.
Take lots of exposures and check for good focus regularly, and adjust where necessary. Once you’re happy with your shots you can process the images in Photoshop or similar software. Want to know more find our Mastering Deep Space Photography post here.
Image processing.
The primary steps for image processing are almost as involved as the setup and capture. The first step for image processing is the calibration phase.
The normal calibration frames are flats, darks, flat darks and bias – in the data library for the images in this feature, ten of each of these calibration frames are used to create a master for each by combining through software.
Details of how to collect these calibration frames and combining them are beyond the scope of this article, but are well documented on the web and other sources.
For the application of the calibration frames, Astroart handles this through the Preprocessing feature, where it enables you to add your ‘light frames’ (the data images), as well as each of the calibration frames.
Following calibration, the stacking of the data is performed. The stacking is accomplished through the Astroart Preprocessing feature. In some rare cases (Ha data), a test can be done with a SigmaAverage result (rather than just Average that is shown) to remove ‘cosmic ray’ strikes on the sensor that are picked up at such a low signal level, as is the case with a Ha filter.
Use hotpixel removal (single and groups) in case there is a hotpixel that the dark frame master did not pick up. Once the stacking is run, save the result as both a FITS file and a PNG (the latter for use in importing into PS).
Generating colours from each of the Red, Green, and Blue (RGB) channels is considered the RGB combine step.
The RGB combine within Astroartis the method Color → Trichromy and allows the combining of same-size images from each of the three channels. There are a few different options provided before the combine, such as coefficient, white balance and colour balance.
Each target is different, so experimenting with the different selections is normally the best process.
Once adjusted, save each channel as a ‘master’ FITS as well as a PNG file. For RGB, also sometimes applied is AA’s Gradient Removal feature – depending on the target and if the target was in an area that had significant LP.
Once theLum and/or Ha are saved as PNG files, they are then imported into Photoshop. First, run levels and then later curves – always paying attention to the histogram so as to not clip data at either end. You can also use GradientXTerminator(Photoshop filter) to help reduce gradients from light pollution.
Once the RGB is saved as a single master PNG, it is imported into Photoshop. Similar to Lum/Ha, run levels first and then later curves (again, paying close attention to the histogram), and possibly the GradientXTerminator and/or Hasta La Vista Green (HLVG) Photoshop filters. Once the majority of processing is done to Lumand/or Ha and RGB, use Photoshop to combine.
Related questions.
Why do people doing CCD imaging often stack, say, five 1-minute exposures instead of taking just one 5-minute exposure?
Modern digital cameras capture faint astronomical objects with much shorter exposures than their film-based counterparts did, but it still takes an exposure of many minutes to produce a good picture.
So-called image stacking is the easiest way to achieve the equivalent of a long exposure, for two reasons.
The first is that many image-processing programs have reduced the stacking process to a few mouse clicks. And the second is that most telescope drives can manage 1- or 2-minute exposures unattended, without the effort and auxiliary equipment needed for guiding a long exposure.
As a bonus, it’s far less frustrating when you lose a minute-long exposure to a gust of wind or an airplane flying through the field than it is to lose many minutes of work.