Friday, October 19, 2012

Tracking the stars

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OK - the next exciting instalment. Sorry this stuff takes me a while but I have a decreasing amount of time I can devote to this. I'm doing my best :)

 If you want to take images of the heavens that are longer than just a few seconds duration, you'll need to track the stars. You may have noticed that the stars move across the sky, which is a bit of a nuisance really when you're trying to take pictures of them. The stars themselves don't move (well, they do, and pretty damn quick, but they're so far away we don't notice). The apparent movement of the stars is due to the Earth's rotation (which you probably already know) and in order to do long exposures, we need to match that rotation so that the stars remain in a constant position on the camera sensor. Easier said than done :)

There are some options available to the budding astrophotographer, but in my limited experience, the EQ (Equatorial) mount is probably the way to go. If you're heavily into DIY, you may want to have a crack at a Barn Door Tracker (I personally would rather stick needles in my eyes). A Barn Door Tracker is for use with your camera only (not with a scope). If this is something that interests you, here's a couple of links to get you started:


Dave Trott 

Equatorial Mount
You're unlikely to be able to make one of these, so you'll have to buy one, which is a bit of a pain, but hey ho :). To give the Wiki definition:

An equatorial mount is a mount for instruments that follows the rotation of the sky (celestial sphere) by having one rotational axis parallel to the Earth's axis of rotation.

There you go - that just about says it all. :) You're probably already aware that the Earth is leaning over, and if it wasn't for this we could all track the stars in one axis using an Alt-Az mount (specifically using the Azimuth axis - left, right). But unfortunately the Earth is leaning over and we have to get around this by aligning our mount with the Earth's axis of rotation - this enables us to track the stars using one axis. The axes in an EQ mount are called DEC (short for Declination) and RA (Right Ascension). DEC and RA are used to specify the coordinates of stuff up there, so that we can find what we're looking for. Think of DEC as the equivalent of Latitude and RA as the equivalent of Longitude - if you don't know what they are, google them :) We track the stars in RA only (east to west as they "move" across the sky). Happy with all that? Good :)

How much does an EQ mount cost? I hear you ask. How long is a piece of string? A top of the range mount will cost you an arm and several legs. An old EQ5 - probably between £100 - £200, depending on whether it comes with a scope. That's used by the way - new ones are a bit more. When I started this stuff, I bought a Skywatcher 200p 8 inch reflector with a Skywatcher EQ5 mount for £200 on ebay. I had to add motors (about £100), a polar scope (about £40) and a few other bits and pieces, but on the whole, not hugely expensive. So you can find this stuff if you look around.

Polar Alignment
This is by far the most important thing you will have to do if you want to end up with decent images. That bears repeating:  Polar Alignment is by far the most important thing you will have to do if you want to end up with decent images. Got that? I kid you not guys - without a good polar alignment you WILL get star trails - sausage shaped stars - and probably field rotation. The better your polar alignment, the longer the exposures you will be able to take (up to a point) without star trails. I say up to a point because the chances are your motors won't be precision made and will cause errors in the tracking (called Periodic Error or PE). All motors have PE to an extent. Some mounts are equipped with Periodic Error Correction (PEC) - they memorise the errors and make allowances for them - clever stuff. Probably not within our price range though - certainly not within mine! :)

There are some excellent resources on the internet on Polar Alignment, so I'm not going to cover it here in any detail. I've put a link in below to point you in the right direction, but you really need to get to grips with this stuff if you're serious about taking decent images.

Polar Aligning the HEQ5 for Idiots by Astro Baby

Astro Baby's article on Polar Alignment is excellent, and is a must read, but it contains a broken link. She refers to a program called PolarFinder, but the link provided doesn't work. Have no fear, you can download the thing here :)

Who said this stuff was easy?

Drift Alignment
A black art. The work of the devil. And stuff like that. Most beginners would rather stick a wasp's nest up their backside than attempt this. It ain't that bad guys - it just takes a bit of practice and perseverance. When you crack it, you'll have about the most accurate polar alignment it's possible to have, which makes you feel good, even if it doesn't improve the snaps :)

When I was trying to improve my polar alignment, I spent ages on the internet reading up about drift alignment. Most of the stuff I read was confusing to say the least, and then I found Peter Kennett's website. It's a great website, but it's no longer maintained due to Peter's work commitments, and most of the links don't work. Plus it's concerned mainly with film astrophotography, which isn't as popular as it used to be. Concerned that his unique explanation of drift alignment would be eventually lost, I sought his permission to reproduce his pages here, and he very kindly agreed. If you click on the new tab labelled "Drift Alignment" at the top of this page, and I strongly recommend that you do, all will be revealed. :)

Now there is a way, apparently, to drift align with just a camera i.e. no scope, which would be very useful for you guys intending to do widefield. The article is here, but I must admit I haven't tried it, although I will soon hopefully. If anyone gives it a go, I'd be extremely interested to hear how you get on :)

OK guys. I was going to add all about actually taking the images, calibration frames etc. here, but this blog would then be a bit too lengthy, so that will have to wait until the next one. I'll try my hardest to get it done a bit quicker though :)

As always, may your skies be forever cloudless



  1. An informative read Doug, i even got a good laugh out of the sticking needles in eyes bit haha! Hope to see a new image off you soon if you're skys clear up. Its not much better here if that makes you feel any better!

    1. Hello mate :) I'm halfway through the next post would you believe? Should be up within a week or so (I've said that before!) Nothing else to do here (astro wise) at the moment. I'm guessing the moon is back now, although I can't tell. That will mean during the "no moon" phase we've had one clear night. One of the many joys of living in the UK. :)

  2. Haha yeah, the Moon has returned unfortunatly. You dont realise how inconvenient it is until you start AP. Thats a real shame you've had so much cloud, with you having you're new scope and not being able to use it. I'm sure it will clear up. Look forward to the next post!

  3. Glad I stumbled across your blog! I'm also in the budget astrophotography game but you seem to have a leg up on me with the tracking mount. I'll be sure to check back when I finally get a mount and hopefully a small scope for the holidays, until then I've linked you in my sidebar: Cheers from Bowling Green, Ohio in the USA!

    1. Thanks Eric, and thanks for the link :) Good luck for when you get the scope and mount - if you have any problems, don't hesitate to get back to me :)

  4. Hello Doug,

    May I suggest you have a look at a program called Stellarium, it's completely free and shows you exactly what the sky will look like at any given time in any location. Fantastic for planning shots.


    1. Hi Evan. Thanks for that. I actually use Stellarium myself - wouldn't be without it :) Apart from seeing what's up there at any given time, I use it to find the DEC of my planned target so that I can put this in the setting circles of my mount and then just slew backwards and forwards in RA. At least I know I'm in the right place, more or less, in one axis. An excellent program, and a great price :)

  5. Doug, I've really enjoyed reading the blog posts. It seems that a lot of your post processing work is done in CS5. For an astronomer on a budget, can this be purchased at a relatively cheap cost? Any other recommendations?

    1. Hi Steven. Must get some more stuff up on this blog :)Glad you've enjoyed the limited material that's up at the moment :)

      If you have a child at school, you can purchase a student version of CS5 (CS6 now) for a huge discount. There's no way I could afford the full fee! For a really good price, you could try The GIMP - that's free! Also, Photoshop Elements is a lot cheaper, and includes most of the functions you'll need. Then there's Paint Shop Pro (that used to be free years ago) for a reasonable price. And of course the astrophotography dedicated packages, which ain't cheap and are very non-intuitive, so it might be an idea to steer clear of them. I could go on....

      Hope that helps a bit :)