I’ve covered the importance of seeing, focus and image scale in an earlier post - High Resolution imaging of the Moon (http://www.thelondonastronomer.com/it-is-rocket-science/2018/4/12/high-resolution-imaging-of-the-moon), so won’t restate the criticality of these factors here. This section covers what I believe are the key points to consider regarding camera settings and capture duration, assuming that you are imaging at the longer focal lengths required for high resolution planetary photography.
Just as the Earth rotates on its axis, so do the other planets in our Solar System, some faster and some much slower. This rotation can have a significant effect on the sharpness of your images as features will smear when stacked due to their movement over the capture period, the longer the duration of your capture video the worse it will be.
This blurring can be mitigated by using software such as WinJupos and AutoStakkert (which I will cover in detail later), but I try to keep my ‘sub-captures’ within the limits detailed below, and the overall capture period (remember we are taking multiple sets of data) in the case of Jupiter to around 30 minutes to reduce blurring around the limbs of the planet.
The actual theoretical maximum capture time you can get away with without blurring will depend on the pixel size of your camera and the focal length of your telescope so timings will vary. I’ve given some suggestions below to act as a starting point but these are only suggestions, it definitely pays to experiment with your own set up. The times I’ve given also take into consideration the fact that we are adding the time taken to capture through each of the 4 filters together, though each suggestion is for an individual filter run. I aim to shoot around 5 sets of IR, R, G, B data, as a general rule the more data sets you have the better so this is not a hard and fast limit. I try to keep my imaging runs within a reasonable time frame to reduce limb blurring, as mentioned earlier, but if you are shooting with a colour camera, or your target rotates slowly you can get away with a lot more, its then about your level of desire to process large amounts of data!
- Mercury – Rotational period 58.6 days. No practical limit to length of captures or number of datasets. Generally you only need IR as you will not be looking to bring out colour but rather attempting to show phases and surface detail, you will however need lots of IR captures for stacking.
- Venus - Rotational period 243 days. No practical limit to length of captures, however if imaging cloud structures in UV you may want to restrict to a few minutes especially as image brightness tends to be quite dim in this wavelength. In other wavelengths Venus is so bright that time limit shouldn’t be a factor anyway. Generally IR will suffice and if you want to reveal cloud structures IR & UV.
- Mars - Rotational period 24.6 hrs. Time will vary with the apparent size of Mars as it presents very differently when it is at opposition compared to when it is further away from us on its journey around the Sun. At opposition I would suggest limiting each capture to a maximum of 90 seconds.
- Jupiter - Rotational period 9.9 hrs. Try to keep individual captures below 60 seconds, preferably around 45 seconds. As Jupiter presents as a bright target in IR, red, green and blue wavelengths you should still be able to capture a large number of frames at a reasonable frame rate within these limits.
- Saturn - Rotational period 10.6 hrs. Saturn has a lower surface brightness than Jupiter when imaging at long focal lengths so exposure times will have to be longer resulting in it taking more time to capture a decent number of frames. I’d suggest limiting capture times to 120 seconds if conditions allow.
- Uranus - Rotational period 17.9 hrs. Unless you are imaging at a very long focal length with a large scope it is unlikely that you will be lucky enough to capture detail within the atmosphere of Uranus so rotation limits are pretty academic. Also the planet will present as a small dim target so you will be imaging at high gain. In this case the more frames the better.
- Neptune – Rotational period 19.1 hrs. See advice for Uranus.
Capture Area, Exposure, Gain & Gamma
I can’t give any hard and fast rules here as the optimal settings will depend on your individual camera and scope, however I can give a few general pointers from my own experience.
I aim to capture around 3000 frames through each filter, within the time constraints detailed above, so I try to take this into account when setting exposure and gain. You will want to be shooting at around 60 frames per second for a target like Jupiter, though this may have to drop lower for targets like Saturn (~ 30 fps) or the outer planets (~10 fps).
- Capture Area - The capture area selected will have a big impact on frame rate, fortunately (or unfortunately depending on your point of view) a planet will present a small image on your camera chip so you can get away with only using a small proportion of it, usually 640 x 480 or 800 x 600 pixels.
- Exposure – The most important thing here is not to over expose any part of your image. As I want to minimise the amount of fiddling around between each filter capture (I already have to re-focus between filters) I assess exposure using the filter than produces the brightest image of the target. For Jupiter this tends to be the green filter, but you can determine this by using the histogram function of your image acquisition software (SharpCap, FireCapture etc.). Generally I shoot Jupiter with the histogram at around 50% through the brightest filter and check that image brightness isn’t an issue through the other filters (for dimmer targets you probably won’t even come close to 50% of your histogram). If your framerate drops below your desired value you will have to adjust gain. A useful feature in SharpCap is ‘Highlight OverExposed’ which causes the over exposed parts of the image to flash on screen, simply reduce your exposure until the flashing stops (with a little bit of leeway added). The issue with this approach is that you will have to adjust colour balance in post processing, the best colour balance will be obtained using these settings for Jupiter, Saturn & Mars:
- Jupiter - The Red, Green and Blue channels should reach ~80-90% on the histogram
- Saturn - Red channel should be at ~70%, Green ~50% and Blue ~25-30%
- Mars - Settings as per Saturn
- Gain – Gain can be compared to the ISO setting on your digital camera, the higher the gain or ISO the more electronic noise there will be in your images. When imaging bright objects you can get away with lowering the gain which will give a cleaner image from a smaller stack. If your target has a dim surface brightness then you probably won’t have any option but to shoot at high gain as increasing exposure time may not be an option with respect to maintaining a decent framerate. When shooting at high gain you will want to shoot as many frames as you can, within the constraints already mentioned, to counteract image noise by stacking.
- Gamma - Within SharpCap I keep the Gamma setting at 50 which is the recommendation for planetary imaging with ZWO cameras, you may want to check any manufacturer specific advice for your own planetary camera . In my experience gamma is only a consideration when shooting targets with extremely high contrast variations such as the ISS.
- File Naming – Most image acquisition software, certainly SharpCap and FireCapture, enable you to automatically name your videos in a format that is recognisable by WinJupos i.e. the date and time of the capture is in the file name. Make sure you enable this as it will make your life much easier later on.
Now you’ve completed your five IR, R, G, B imaging runs and are the proud possessor of twenty .AVI or .SER video files you need to process them, starting with stacking each of the videos. You’ll repeat this process 20 times, once for each video (OK some elements of this workflow are a little tedious).
With twenty individual videos to process it can be very easy to get yourself into a total mess regarding where you have got to in the workflow so the first thing I do is create a sub-folder for each of the IR, Red, Green and Blue datasets. By doing this I can process them independently and go off and to other things without losing my place, bringing them back together later in the workflow.
For stacking I use the free application AutoStakkert! (https://www.autostakkert.com/wp/download/). It really is hard to beat in terms of the quality of the stack it produces, so I’ll briefly take you through the main things you need to know about its use. There are nuances depending on the target which I’ll try to cover when relevant.