"Well that's all very well" I hear you shout "but how do you actually take these flippin' images? Eh?" And the answer, my friends, is simple: On a wing and a prayer. I point my kit at the sky, set the camera for some random exposure length (sort of) and let it go. Hopefully I'll end up with usable subs that I can stack in DSS, process in CS5 and end up with a decent image. Or I may not.
Now I'm assuming in this post that you're using a DSLR camera, and not a CCD. Probably a reasonable assumption, as if you're using a CCD you're probably not going to be reading this blog!
With terrestrial photography, we can set the camera to auto and it will focus, work out the shutter speed, aperture, iso setting if you want it to, white balance etc. etc. It does all the work for you. Not so with astrophotography. There are no astronomical light meters that will tell you how long your exposure length should be (not as far as I'm aware). There may be mathematical equations you can use, but I'm not that technically advanced! I decide how I'm going to take an image of a particular target using the limited knowledge I've managed to gain in the short time I've been doing this stuff, plus knowledge of how well - or more to the point, how badly - my kit performs. Plus no small amount of guesswork and finger crossing :)
Hopefully you've now got a camera and scope (or just a camera and lens - that's equally fine), a motorised mount of some description to enable you to track the stars, and perhaps a polar scope. You'll probably need some additional kit, which isn't essential, but will make your astro life a whole lot easier:
Camera control software
This will connect your camera to your computer, and will enable you to set the exposure length and how many subs you want to take etc. so that the camera can slave away mindlessly in the cold while you're nice and warm indoors. Some cameras come with this software (like Canons). If you have a Nikon, it's an "optional extra" that isn't cheap. Also bear in mind that some older Nikons will only allow up to 30 second subs using the camera control software - bulb mode requires the IR remote control. There is kit available to overcome this, so have a nose around the internet. Generally, Canons are considered better than Nikons for astro stuff.
A mains adaptor for your camera
There is nothing worse than being mid-way through a session when the camera battery dies, and you don't have a spare charged one. Your camera will eat up batteries like nothing you've ever seen whilst doing this stuff: a mains adaptor will overcome this, and in my opinion is worth its weight in gold.
This will stop your telescope/camera lens from misting up, which can happen remarkably quickly during the autumn/winter. A dew band, which is just wrapped around the scope/lens, will raise the temperature of the kit by a degree or so, which is sufficient to keep the thing clear. Or you could keep running in and out with the hair-dryer :) I use Dew-Not, but there are others available. Google "Dew control"
Mains adaptors for all your battery powered kit
You'll find that many of the things you buy, like the dew control, your mount motors and of course your camera, are battery powered. This is so the kit can be used "in the field". Personally, I hate anything battery powered, and as I never go "in the field", all my kit is powered off the mains. When I buy a bit of battery powered kit I run down the local electrical shop (I have a particularly good one here) with the spec, and ask them to sort out the necessary plugs, adaptors and whatever to enable me to operate off the mains. The image below demonstrates how much imagination my local shop had to use to enable me to use the Dew-Nots off the mains, which come with - wait for it - phono plugs!
In my ignorance, I thought phono plugs were only used for audio, but hey, what do I know?! There are probably easier ways to connect a phono to a mains adaptor, but it works and I'm happy :)
You should have the settings right on your camera before you trundle outside - fiddling with the camera controls in the dark can be a pain. As regards the settings to use, that's one for you really, and will largely depend on the quality of your camera. For your initial images, I would suggest you set the iso quite high, so that you at least get something that you can process. Modern cameras go a lot higher than my ancient relic, and are a lot less noisy. I'd suggest at least iso 1600 and see how it goes. If this is too noisy, bring it down a bit - it's a trade off. Turn off your display, as this eats battery power (if you insist on using a battery), and is a little too bright out there in the dark. White balance isn't too important, as you can adjust that in the processing - I set mine to daylight. Obviously the camera should be set to full manual, auto-focus off, and shutter speed set to bulb. If the camera has Long Exposure Noise Reduction, turn it off as it will double the time taken for each exposure - you'll be taking darks anyway, which does the same job (we'll come to that). Image format: RAW. Just RAW - not RAW + jpg. This records as much information as possible.
So, you're equipped and you're out there, eager to get cracking. You've decided on your target, you're POLAR ALIGNED, and ready to go. Did I mention polar alignment? Yes? Oh OK. So now what happens? Well, the first thing I generally do is look up and find that the clouds have rolled in. This generally results in "Oh blast and bother!" (or words to that effect) whilst I drag all my kit back indoors. I'd suggest that whilst you're setting up you keep one eye on the heavens, as conditions can change remarkably rapidly - at least they can here in the UK. The second thing I do is grab a cup of coffee, but that's optional.
If your focus isn't sharp, you'll be disappointed with the final result, and if you do several hours of subs they'll all be wasted. If you're using a scope, consider a Bahtinov Mask, which you can either make (plenty of resources on the internet relating to this) or buy on ebay. When using a Bahtinov, focus on the brightest star you can see (don't wait until you're pointing at your target) - you'll find it a lot easier to focus on a bright star. If you're using a camera and lens, you can't rely on the infinity setting - generally the focus will be out. You can make a small Bahtinov (I made a tiny one for my 52mm filter size zooms) or you can take multiple test exposures, zooming right in each time and making very tiny adjustments to the focus until you get the stars as small and tight as possible - it gets quicker with practice. And when I say tiny adjustments, I mean really tiny - you barely have to move it to make a difference to the focus. Once you've achieved focus, make sure you lock it down tight - you don't want it shifting mid-way through a session. Not so easy with a lens - you may have to resort to a bit of blu-tac over the focus ring to stop it shifting if there's no locking mechanism. Be aware though that blu-tac does leave a residue, so don't do this if you want your lenses to stay pristine :)
Find Your Target
You'll need to have some knowledge of the sky if you're going to find stuff up there, even if you're lucky enough to have a goto mount. I'm not going to teach you that - you'll have to teach yourself! Stellarium is as good a package as anything else, and the price is right! And it will tell you what's in the sky above your head right now at your location.
Don't try anything too adventurous in the early stages, you'll probably be disappointed. M42 in the winter is a cracker, and Cygnus during the summer and autumn (this assumes you're in the northern hemisphere!). You should really have already decided on your target before you ventured outside, so let's assume you have. If you have a goto, I'm going to also assume that you know how to use it. If you don't, and you're struggling (it can be tough sometimes looking for something that can't be seen) I've done a video on YouTube that might help.
Taking your lights
OK - you're on your target. Take a quick test shot to make sure you can see what you're expecting to see and that your focus is OK. Then set your control software to rattle off a few subs of whatever sub length you've decided upon. If you're using a scope and a tracking mount (with motors), but not guiding, the exposure length you can get without pear shaped stars will depend on how good your polar alignment is (very much so), the focal length of your scope, the accuracy of your motors and where in the sky your target is (things appear to move quicker on the celestial equator). When I was using a 200p (eight inch reflector) @ 1000mm, I could get 30 seconds comfortably, 60 on a good day. Using just a lens, say 200mm, a few minutes is possible. For now, take a one minute sub and see how the stars look. If there's evidence of star trails (pear shaped stars), go for 30 seconds. If they look good at one minute, try 90 seconds. You get the idea. You'd be surprised how much data you can pick up with one minute subs. For the image below I took 84 one minute subs, giving a total of 1 hour 24 minutes. I took additional subs for the core, but we won't go into that here :)
|M42 in Orion. 84 one minute subs|
Done that? Taken your lights (or subs, whatever you want to call them)? Good!
After your subs come the darks. The darks are taken with exactly the same settings as the lights, but with the scope (or lens) cap on. That's why they're called darks, because they are - dark. The camera also has to be as near as dammit at the same temperature as it was for the lights, so immediately after your lights, whack the cap on and set the camera to take at least 30 darks of the same exposure length.That's the darks done, which, by the way, take out all the dead pixels you will have in your image, and will take out any amp glow if you're unfortunate enough to have the same camera as me :)
Very important are flats. I've just put up a post on my facebook page about how to take flats, so I'll let you have a look at that. :) Take 30.
These subtract all the electronic noise and stuff that is a characteristic of your camera. Easily done - just put the cap on and set the fastest shutter speed (1/8000 for my Nikon). Everything else the same as your lights, although bias are not temperature dependent, so can be re-used for several sessions (providing all the other settings are the same across the sessions). Again, take 30.
There you go - sorted. Throw all this lot into DSS, and out comes your image - I hope.
This was a long one guys (but nowhere near as long as that DSS video up there!). Hope it helps :)
As always, may your skies be forever cloudless :)