Astro’ Workflow

So if you’re interested in having a go at Astrophotography, or have a passing interest in some of the technical processes, this post is for you. If not and you just like to see pics of things in the sky, LOOK AWAY NOW and I’ll catch you in another post!!!

Hello! Anyone still here? Right, I’m going to try and go through my workflow as clearly as I can without getting too technical and I’ll explain some of the principles behind it. Some of these techniques, particularly in processing, are subject to change. Don’t forget I’m still a Newbie and I keep discovering togs with different techniques to try out and adopt if they work better.

The Shoot

The shoot itself takes a while to set up, although since getting the AZ Mount, it’s a bit quicker. I find I need to have a target in mind; I’ve had a couple of sessions where I haven’t and I’ve wasted a whole lot of time aimlessly shooting different targets without really focusing properly on one. I will also decide what kind of shot I’m after – wide sky, maybe with a 50mm or wider lens or fully zoomed in at 200mm maybe to highlight just one object.

Be Prepared!

When I was using the Nano Tracker (and I will use it again from time to time), I would set up the tripod and tracker indoors – get the declination (up/down angle) as near as I could to 51.2degrees. I was able to do this with an ‘inclinometer’ phone app. 51.2 is the latitude of where I live, so the mount needs to be at that angle to point to Polaris (the North Star).

The importance of polar alignment is because, the stars appear to revolve around Polaris, which is in the same position in the sky all the time. So the earth’s axis points to Polaris, meaning the mount also must in order to accurately counteract Earth’s rotation. But for now, this is only a rouhg alignment – the real one is done once the kit is set up in situ’.

Astro has made me much better at having charged camera batteries and blank SD cards at the ready the whole time. Also, batteries (including spares) and/or USB power banks in order to power various accessories. The Nano Tracker used 3xAA batteries and the intervalometer takes 2xAAA. My lens dew heater is powered by USB and at the moment I have a couple of power units that will feed it for one or two hours respectively.

I’ll also dial in basic camera settings for the shoot. If I’ve been using the camera for a ‘normal’ shoot, there’ll be some changes to make. It needs to be single shot, lens switched to manual focus, shoot in RAW, slow end of the shutter speed dial, and to begin with ISO, turned right up!

Then it’s checking where in the sky my target is, or will be. Again, there are phone apps that help with this Stellarium or Sky Tonight/Star walk are really good apps for this.

I’ve also learned, after a few frustrating sessions, to plug in the intervalomenter before going out in the dark. You try finding a little headphone sized socket whe you can’t see what you’re doing!

Polar Alignment and Focus

So with all of that done, if the clouds haven’t rolled in, we can set up outside. It’s now time to accurately Polar align. The Nano has a very limited aid for alignment – it’s just a tiny hole to look thorugh. Now I can’t use this because I wear glasses, which is why I found a way of using my telescope’s red dot finder. This worked pretty well.

Next challenge is focusing the camera – and this isn’t easy until you get a system that works. I start by using it handheld, aim it at something bright. Maybe a planet like Jupiter, Saturn or Venus – even The Moon, and turn the focus ring until it is as sharp as you can get it,. At this stage, I use the viewfinder.

Then it’s time to carefully mount the camera, switch on the mount then without touching the focus ring, aim your camera towards the target. Now turn on LiveView so you can view on the screen. If the target dim and is in a dark part of the sky (you know what I mean!!) point back at your bright object. Get the object in the middle of the screen, then zoom in using (usually) the + or – buttons. The target will appear as a shimmering blob. Carefully tweak the focus ring until that blob is as small and sharp as possible. You may even see some other dimmer stars start to twinkle on the screen. To make sure you keep that focus fixed, lock the focus ring in place with a piece of tape – masking tape is fine. Don’t use Duct/Gaffa Tape!!

Exposure Settings

Now take a test shot – this is why we have max ISO for now. Maybe a 2sec or 5sec exposure. Use the intervalometer or remote so that you don’t shake the camera. Use this process also to frame your target. You’ll be able to see from the test shots how you’re doing. The shots should be grey (not black) and you should be able to see many more stars than with the naked eye.

Check the result, if its reasonable well exposed, with a good few stars visible, zoom in to see that the stars are round and sharp. If not you may need to tweak the focus a bit more, or if stars are elongated, adjust the alignment of the mount. Keep doing tests till your happy with framing.

Now it’s time to see how long an exposure you can get away with with the camera pointed to the target. If you have a decent heavier duty mount, you should be able to get at least a minute before star trails emerge. With the Nano I could only really get 15secs with a long lens, a bit more with a wider lens. It’s trial and error, but once you know the limits, you don’t need so much experimentation. For this the ISO needs to be back down to 800 or 1600 and the shutter speed set to Bulb. The intervalometer now controls the shutter speed. Again, the overall image should be greyish (it might have a slight brown or red tinge if there’s a bit of cloud.

Start shooting, let the intervalometer do the work, go inside and put the kettle on! For most objects, I try to get 2 hours or more of images (known as Light Frames). 3 would be better, 4 better still! But it’s all down to how long objects remain in view (before they go behind trees etc) and how long you can stay awake for!

Now that I am using my AZ Mount, setup is much easier. I dial in date, time etc, I use the handset to point at a couple of bright stars and lock them into the system, then the GoTo system basically knows where it is, what it’s pointing at and as a consequence, the location of every object in its database! So then I select the target, the mount slews round to it, I can adjust slightly to re-frame with the cursor keys and start shooting.

I generally check how things are going every half an hour or so. Make sure everything’s still running, batteries are OK and I usually stop the shoot, check that the last couple of images are sharp (zoom in on the screen), slightly tweak focus if necessary, maybe reframe a little, then restart.

Calibration Frames

So, providing the cloud hasn’t closed in, with a couple of hours of exposures it might be tempting to clear up and go to bed. Unfortunately the shoot isn’t over – calibration frames are needed to assist with reducing ‘noise’ in the images during the stacking process.

Signal to Noise (S/N) Ratio

Signal is what we want – it’s what is actually there in the sky; Noise is the result of several factors and is generated in the camera itself – it’s due to high ISO, long exposures and the cameras sensor heating up. All things that are an integral part of Astro-shoots! Just increasing signal would also increase noise so it’s necessary to reduce the noise to as low a level as possible before starting to increase the signal.

But here’s the thing – signal is in a consistent pattern; in the same position from one image (frame) to the next. General noise is RANDOM, so will not be the same in across various frames. There might though be some unwanted internal artefacts such as dead or hot pixels, that will be consistent.

Calibration frames allow the stacking software to identify what is signal, what is random noise and other unwanted artefacts. These are the calibration frames that allow the stacker to do this (in addition to the images – Light Frames). Camera settings must be kept the same, apart from changes to shutter speed that are needed:

1. Dark Frames – shots taken in the same conditions (temperature etc) with the same settings, but with the lens cover on. These should be taken straight after the light frames so that conditions are as near as possible the same
2. Flat Frames – these (and the next categories) can be taken indoors! Flat frames are of a brightish, even lightsource and correctly exposed. I point the camera up to the ceiling, put the lights on and cover the lens with a white handkerchief or a folded tissue. Adjust the shutter speed until the exposure dial is correct
3. Dark Flat Frames – the same settings as Flats, but with the lens cap on
4. Bias/Offset Frames – still with the lens cover on, but with the fastest possible shutter speed (1/4000th on my Canon 77d)

You need a good few of these pics (30-50 of each) so that the Stacker can average them out. Put very simply, if there are any consistent white pixels in dark shot, or dark pixels in a flat, then they are unwanted. As it is random, any noise can be averaged out. So the stacker will create a Master of each calibration type to apply to the stack. Are you with me so far??

Now there disagreement over whether you need all of these. Some say that DArk FRames are notr very effective with a DSLR camera (more useful with a specialist Astro camera that has its temperature regulated.

Others say that BIas Frames at a fast shutter speed have no relevance over images shot a slow shutter speeds. I’m not sure where I stand really. I definitely take Flats and Dark Flats – if I have a particularly long total exposure I’ll take Bias and Darks too.

Sorting and Organising

The different types of frame need to be identified and sorted and also any of your test shots really need to be ignored or even deleted.

One way is to just check the info on the camera to see different shutter speeds tand make a note of image numbers for the different types

The other way (which I use) is to start the Import process in Lightroom (but not necessarily import them). You can see by the thumbnails which are Lights and Flats, but not easily distinguish between different types of Darks

Both these ways have their drawbacks, but work OK if your are careful. It might not be the best thing to be doing if you’re feeling tired and ready for bed!

Bear in mind that these are RAW files (so very big) and that teh Stacking program will need access to them (so probably best not to just let it read from your camera or its SD card. I like to copy these files to separate sub-folders inside a project folder (my system starts with the date in the folder name).

Go to bed now! The next post will look at Stacking and Processing!