Author: Ravi Kailas (firstname.lastname@example.org)
This is a follow-up to the basics post (which might be of interest if you are just starting out on this pursuit and looking to experiment with the gamut of subjects). In this post I will elaborate on my work flow to photographing the faint deep sky subjects (Nebulae, Milky Way regions, Galaxies, Constellations and Star Clusters – sometimes all components of a single image).
Please note: The darkest, most transparent (minus haze) skies are a prerequisite for this kind of photography. Try to get as far away from city lights (and other forms of atmospheric pollution), if possible in the mountains and/or desert, and a moon phase that does not interfere with dark skies, when you plan to shoot. In higher latitudes, it would make sense to plan the shoots when nights are long, since the Sun’s spill-over light illuminates the skies for a while, even after sunset and sunrise.
Basic premises for deep-sky photography techniques
With pictures we can see deeper into the night sky (and with colour, since the human eyes cannot see colours well in the dark) than by observation alone (including, in many cases, using advanced telescopes). Since we are photographing faint objects, the obvious solution to capturing this faint fuzz (no fuss intended), is to expose longer* (given that you are using optimal ISO and aperture settings). However, it turns out that regardless of the length of the exposure, combining multiple images greatly enhances the final results (with a great deal of post-processing, discussed in detail, later in the post). Here’s why:
There is a phenomenon called signal to noise ratio (technically, but elegantly, explained in the link). The basic idea is that combining multiple images of deep sky subjects enhances your signal (the good stuff from the actual subject) and lowers, in proportion, random noise (the bad stuff, a result of technical limitations, primarily random high ISO noise). How this works is that since the location of noise is random and signal always appears in the same part of the image, when combining these images, software can then eliminate the bits that do not occur in the same location in the images, hence reducing the effects on noise in the picture, while increasing the effective signal by ‘adding’ the constant parts from multiple images. Since the sky conditions (transparency for example) is dynamic, this process will also enable adding to the data from periods of exposure when sky conditions were optimal (just a higher probability of this when you have camera pointed at the sky at different times).
*It is possible that for some subjects (like the Orion Nebula and Andromeda Galaxy), you might actually need to image the brighter core and surrounding, fainter nebulae/star-fields, with different exposure settings and merge the two sets of images in post processing.
While there is a large variety of equipment (including specialised/modified cameras, sensitive to specific wavelengths of light that bring out target features/colours better) that one could use for deep-sky photography, I am listing here, specifically, only what I use/have used in this pursuit.
Camera and Lens Combos
Canon 6D coupled with:
300mm f2.8 II
100-400mm f4.5-5.6 I
Olympus OMD-EM5 MKII coupled with:
M. Zuiko 12-40mm f2.8 Pro
M. Zuiko 40-150mm f2.8 Pro
At the moment, while the rather bulky Canon set-up gave me some superb data to work with, I now only use the relatively compact, yet competent Olympus system for Astrophotography (for the ease of stabilising/versatility of this set-up).
AstroTrac TT320X-AG (rated for use for upto 5 minutes with a focal length of 200mm, without star trails).
AstroTrac TW3100 (Wedge for fine adjustments during polar alignment)
Manfrotto 055 ProB (Tripod Legs)
Manfrotto MHXPRO-BHQ2 (Ball Head)
Nebulosity 4.0 (specialised for deep sky subjects) for Mac OS
Lightroom CC&6.0 for Mac OS
Vello remote shutter release for Canon DSLRs (with 3 pin)
In-the Field (and just before)
(get most of this done before heading out into the field. Please note that some of this can be pre-programmed and, as such, one need not change the settings, as per below, everytime)
- Change exposure setting to Manual. Pre-set the exposure length, aperture and ISO settings (can be fine-tuned in the field, after test images).
- Set the ISO manually (I typically do not exceed 1600 or 3200 – but yours will depend on the limitations of your camera’s sensor and associated high ISO noise)
- Set your lens to Manual Focus
- Set the ‘White Balance’ manually, to maintain uniformity within multiple images. You can always fine-tune this appearance in post processing.
- Switch off ‘High ISO Noise Reduction’ (this is best done in post-processing (by stacking, as discussed earlier) since the camera software’s version of reduction can actually delete useful data from the images)
- Record Images as RAW Files
- You can either keep the ‘Long Exposure Noise Reduction’ ON (which will then mean that the camera will take approximately as long to write the data on card as the length of the exposure) or OFF (can be done in post processing. I personally keep it on, but could be useful to keep it off if you anticipate only a short window to gather actual data – for example impending bad weather). This process will eliminate the noise (typically spots appearing in the same location on the images), due to sensor heat during long-exposures.
- Set-up a delayed shutter release for the first exposure (especially if you release the shutter via the camera vs a remote release, to reduce vibration). If you are using a DSLR, it would be good to use the Mirror Lock-up option as well, which will further help produce blur free images.
- Set-up your interval timer to automate the multiple-image capture process. You can set the number of exposures (about 20 is recommended for best results from stacking) and the interval between exposures (significant only if you use an external timer, since this interval should take into account actual exposure length per image plus associated data write time, before the camera is ready for subsequent exposure).
- Make sure your batteries are fully charged and you have ample space in your data card, before beginning the imaging sequence
- Remove the camera/lens strap, which could otherwise cause interference during the multiple, long exposure process
With the above settings in place, the camera-lens combo is set-up for deep sky photography, barring minor fine-tuning after test images in the field.
Setting up the tripod, tracker and more
(Ideal, if you can get through this process with some daylight left – saves grappling in the dark)
- Fix the AstroTrac TT320X tracker to the Tripod Head, by screwing on the AstroWedge (the tracker is attached to the wedge). Change the vertical axis of the wedge to reflect the approximate latitude you are shooting from
- Take the tripod (with the attachments) outdoors and place it in a location with an unobstructed view to sections of the sky that you indent to photograph (would be useful if you can envisage the position of your subject over-time and see if would would have unobstructed photo opportunities for the duration of your imaging effort).
- Point the set-up (the point where the Ball Head attaches to the tracker) towards the Earth’s magnetic poles (North if you are in the northern hemisphere and South in the southern Hemisphere) – the approximate axis of rotation of the Earth. Try to be as accurate as possible – it would help to use a compass.
- Using the bubble level on the wedge to adjust the legs of the tripod to simulate flat ground (necessary for accurate polar alignment)
- Fine-tune axis orientation to pole with the aid of the fine adjustment knobs in the wedge and with the aid of the compass
- Attach the Ball-head onto the tracker and the camera-lens combo onto the Ball-head.
- Use the supplied Polar Scope (ensure it is properly collimated) and follow the instructions to fine tune polar alignment (necessary for long-exposures when using focal lengths beyond 28mm, full frame equivalent. For wide-field step 5 would suffice). The AstroTrac TT320X is rated for use upto 5 minutes of continuous exposure for a maximum focal length of 200mm full frame equivalent, for images without trailing. I have only ever used the set-up for a maximum of 2 minute exposures, but at at a focal length of 300mm full frame equivalent and achieved trail free images).
- Attach the supplied battery-pack to the tracker and test the tracker, by pressing the play button. Be very careful after step 7 to ensure that you do not move the set-up by mistake (even as much as nudge it with your foot), or you would have to fine tune the polar alignment over again.
- Using the live-view option (default in mirrorless) in your camera, point the camera towards the section of the sky that you are interested in photographing and focus. Cameras these days allow the option to magnify in live-view, which will greatly aid in fine focus.
- Once you achieve focus, set the tracker-on and take a test image to see if your composition/exposure/white balance/tracking accuracy are ok.
- If satisfied with all in Step 10, you can now start the tracked, multiple exposure sequence
Having gathered the data, which is essentially the RAW, tracked images, there is still considerable post-processing work to be done. Assuming that these images are already processed for Long-Exposure Noise Correction (like I do, in-camera, in the field), here is the process I follow:
- Import into Lightroom and correct for Vignetting (inevitably quite strong given that we use large apertures for image capture) and Lens Aberrations (automated). Export processed files as TIFF 16 bits, no compression (to retain all the data from the original image)
- Import the files into Nebulosity 4.0 and work on the following steps*:
- Align Images: A process by which tracking errors are corrected. Any minute differences in the location of objects between images can be corrected with this process.
- Stack Images: Once aligned, the images can be combined to enhance signal and reduce proportionate noise (as discussed earlier in the article)
- More procession on the stacked image to bring out finer details: The stacked image can now be worked on with specialised software features to bring out the finer details and colour as desired for the final output. Some of this post processing can also be done on advanced photo software (I use Lightroom 6.0 for some of it).
- When satisfied, export file in format as desired for sharing/display (I choose jpeg)
*This excellent review video for Nebulosity gives a step-by-step guide for the the above processes (however please use some discretion on which steps are relevant to your specific images).