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Basic widefield with a Camera and Tripod


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After various discussions in a photo forum, thus the references to lens sweet spots etc, I wrote this up.

All you need for this is a Camera and a Tripod. I've done these with my Z2 and the 450d. The Z2 sensor is really too small and the ISO handling not good enough, but it works nonetheless.

You want to use a wideish lens. I typically use the 18mm end of the 450d kit. Set the lens aperture to wide open, forget about the sweet spot, you are looking to gather as much light as possible, if you want to get as many stars as possible in the end picture. Set the ISO to the highest you can, that the camera will be able to control the noise reasonably on, for the consumer Canon this appears to be ideal at 800. Set the Shutter to 30 seconds, this appears to give the ideal exposure time at 18mm, before star trailing becomes noticeable (it'll be there, but it's likely to be so small, you won't see it). Make sure you are set to shoot in RAW, then take at least 10 exposures at these settings, as the number of individual shots increases, the more detail you get, but... the more the resulting image will be affected by field rotation (appears as smearing in the top right and lower left corners). I'd recommend using mirror lockup and a remote or the camera self timer (I've used both).

Once you have captured the image frames (Lights), you need to then capture some Dark frames. These are exactly the same exposure settings, so 30 seconds at ISO800 in the same temperature conditions (don't take the camera in) but with the lens cap on. Go for 5. These are used in the stacking process to remove sensor noise and hot pixels. This is the equivalent of having in camera long exposure noise reduction turned on but allows you to spend more time capturing light frames, as the exposure time for each frame doubles if you turn it on in camera.

Load all the images, lights and darks into Deep Sky Stacker using the appropriate option when you load them in. These are called subs (or sub frames). Then click Register, taking the defaults and allow DSS to stack after registration. There are a lot of options in here, and it takes quite a bit of time to work them all out, but the defaults will get you going. Once the stack is finished, DSS will load the resulting file and it'll look possibly quite odd. This is DSS carrying out it's own algorithms to stretch the data and pull out the detail. This does work, but requires some playing around and experimenting, and to be honest, is not the easiest thing to do. So for now, save the image, and select the embed adjustments but don't apply option.

Load up the TIF in your editing package, and prefer for some fun... This is where things get tricky. You need to now use curves (or levels for those who don't have a curves tool, there's a filter for Elements Widows that provides curves) to stretch the histogram. You're looking to change it from an almost vertical bar on the left hand side to a broad shape.

This is the sort of thing you are looking to do (I'm not very experienced at this bit as yet, and it takes me a lot of messing to get to a result I'm happy with, I'm sure there are people here who'd be able to make a far better job of this step in much shorter time)...

But basically you are looking to apply curves something like these. Not necessarily in this order or severity, maybe stronger curves would work better, it depends on what details are in your image, but it gives the general idea.

curves1-1.jpg

curves2-1.jpg

curves3-1.jpg

curve.jpg

When you have finished this, you should get a reasonable resulting image. Here's a couple I did in exactly this way in August (the moon was down, and the skies were pretty dark).

jupiterinmw.jpg

cygnusmw.jpg

Hopefully it'll be useful to someone.

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After various discussions in a photo forum, thus the references to lens sweet spots etc, I wrote this up. All you need for this is a Camera and a Tripod. I've done these with my Z2 and the 450d. The Z

For shots like these it is often worth looking out for earlier Canon EF prime lenses. The older ones without usm are often optically superb and were designed for larger 35mm-sized sensors but sell for

Hi John Thanks for the write up. I have been using something similar to remove light pollution from my widefields. I wrote it up on my website, but your write up is much better and more comprehensive

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Hi John

Thanks for the write up. I have been using something similar to remove light pollution from my widefields. I wrote it up on my website, but your write up is much better and more comprehensive than mine. Try playing with the curves for the different colour channels. You should get a darker sky and reduce the orange glow. I found there's no perfect curve, just trial/error and personal taste.

Look forward to seeing more

Cheers

Danny

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Glad you like it. Hopefully someone can make use of it.

Danny, processing is still not a strong point, and there are a lot of tricks I've not learnt yet. That curves on colours sounds good. I wrote this for a photography forum as I kept being asked what I was on about with stacking and how to get started in astrophotography without spending silly money. If nothing else, it's good practice at capturing and processing data.

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Cheers Steve. The images above were taken with the 18-55 kit lens wide open. You may be right, but it doesn't seem to degrade the image any... The kit lens has some serious barrel distortion issues at 18mm though.

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The kit lens has some serious barrel distortion issues at 18mm though.

For shots like these it is often worth looking out for earlier Canon EF prime lenses. The older ones without usm are often optically superb and were designed for larger 35mm-sized sensors but sell for a song because of their age.

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Good luck Paul. It's all practice. There are many better imagers on here but there's also plenty of encouragment from those that have gone before. Give it a bash and see how you get on.

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Quick question- I gave this ago last night- my first proper attempt at imaging! How do you focus the camera? I switched it to manual and set it to infinity, but the resulting image shows the stars as disks rather than points of light, which i suspect means that the lens is slightly pastthe true point of focus?

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Ah.. now that's one of the vagaries of some of the cheaper lenses, they focus past infinity. I cheat and use the liveview and zoom in all the way. Otherwise, if the moon is up, use that with the AF, then switch the lens to manual focus. I've not really got the experience of focusing using other techniques, but I think you're best bet, is to set the lens to the end stop, back off a little, take a test exposure. Zoom in all the way on the preview and check it. Adjust the focus a little, repeat the test, until it's as good as you can get. A bhatinov focus mask would make this much easier to do.

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Otherwise, if the moon is up, use that with the AF, then switch the lens to manual focus.

A distant streetlight will do, if it's more distant than 5000 times the focal length of the lens it's at "infinity" for the purposes of imaging. An ordinary streetlight 250 metres away is plenty bright enough for the AF to lock in to if using a 50mm lens.

With longer / faster lenses, bright stars are enough. I've found with a 300mm f/2.8 that a 4th mag star is enough to work the AF, provided that the centre (most sensitive) point is selected for single point AF and that the camera is accurately aimed.

Keep checking the focus. If using a zoom lens, the focus needs to be reset if you twist the zoom ring. Focus can drift with temperature too, this can be quite a serious issue when using longer & faster lenses, or when using a scope as a lens ... unless you have Zerodur optics and a carbon fibre tube!

Most camera lenses - even the very best ones - give significantly better images if they're stopped down a bit (e.g. use a f/4 lens at f/5.6), and this also makes them more tolerant of small inaccuracies in focusing.

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Thanks for the ideas folks, i'll give them a go if weather permits. Interestingly its not a cheap lens, but a Canon L series! I stopped the lens down as well. I'll try focusing on the moon/ jupiter/ streetlight then switch to manual as you suggest. Pity i dont have liveview on the 400D- it'll have to be trial and error!

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I have a very simple "bridge" camera atm, but am hoping to progress to DSLR in the not too distant. If the property market gods are kind, I am also hoping to purchase a shiney new scope that will enable me to start imaging with said DSLR. I am inspired by this thread and will be seeking advice etc. when the time comes.

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