Posts tagged 'Nikon D50'

Observing Report / Lens Test 22nd September 2009 (Big Dubyah)

Posted by on September 23rd 2009 in Astrostuff, Observing Reports, Pics, Shiny new kit

I'd not planned to go out observing, but there was an hour or so of cloudless sky yesterday evening and I had a little time to spare. This was an opportunity to sky-test the Nikon AF-S DX NIKKOR 35mm f/1.8G lens to assess how well it is suited to taking widefield pics of the night sky.

In particular I was looking to see:

  • how much sky would be captured in the frame;
  • if a reasonable focus could be achieved using the camera's auto setting;
  • how quickly the camera sensor would become saturated at f/1.8 and ISO 200;
  • how much vignetting would be caused by using a 52-48mm step-down ring and Light-Pollution filter;
  • how much coma would be present at the edges and corners of the image; and
  • how much star-trailing would be caused when using a static tripod as opposed to using a driven equatorial mount.

The challenges would be to see if I could find a reasonable exposure time, get a small number of images for stacking, and then process-out the vignetting, star-trailing and any light-pollution.

Jupiter was in the southern sky so I pointed the camera at it to get the autofocus set. I then turned off the autofocus and fixed the position of the lens focus ring by the judicious application of duct-tape.

Next, I pointed the camera at the Cassiopeia / M31 Andromeda area and took a few test-frames of various durations. Eventually I settled on an exposure duration of 30 seconds and took 12 pics.

These were stacked in DSS and hurriedly processed in PSCS3, and here's the result:


Mouse-over the pic for details, click it for a bigger pic.


I have to say that I'm quite impressed with the raw images that this lens produced. Shooting with the aperture wide open captured a lot of light, and for a change I had to tone down the details rather than go through the usual routine of having to drag the details out of the murk. I'm damned if I can find any coma in the raw frames, which means that there'll be no need to crop off any bad areas. The expected vignetting was dealt with in PSCS3, the compensation isn't perfect but to be fair I've only done a quick fix, no doubt I could get it better if necessary. The amount of star-trailing was acceptable, and I'm confident that I could get it processed out if necessary and if I had enough time.

And the framing? I'm happy with it for targets the size of Cassiopeia, but the capture-area might be a tad small for meteor-work. Time will tell - between now and the end of the year there will be plenty of opportunities for snapping a few meteor trails (more on that in another post very soon (but don't hold your breath)).

Processing Andromeda

Posted by on October 2nd 2008 in Astrostuff, Pics

Alistair (Stravaiger) asked me how I got that recent pic of Andromeda, so I promised him this "how-to" post. Now I'm aware that there are other and better ways of acquiring and processing frames, so please bear in mind that I'm fairly new to this astrophotography malarkey, and that this is an account of the techniques used for this image only.

My basic kit:

  • Camera: Nikon D50 body with remote IR shutter-release
  • Telescope: Celestron C8-N Newtonian reflector 200mm (8.0") aperture, focal Length = 1000mm, focal ratio: f/5
  • Tripod and Mount: Celestron CG-5 Equatorial with retro-fitted Skywatcher Synscan motor-drives and control

The purpose of this little lot is quite simple - the mount is set up so that its axis of rotation is aligned with the axis of rotation of the Earth. The motors drive the telescope around the mount-axis at the same rate as the rotation of the Earth, but in the opposite direction, so that the telescope stays pointing at the same place. The telescope is there to grab the light and direct it into the camera. The camera needs no lens attached, as the telescope, attached to the camera by a T-ring adapter, is acting as a lens of 1000mm focal-length and 200mm aperture.

The basic method is to find the target and take long exposures, these are "light" frames. For this first attempt at Andromeda I took 22 RAW exposures, each 30 seconds long, at ISO 200. The number and duration of exposures required isn't set in stone, but the general rule is "more is better" - longer exposures capture more light, more exposures allow better processing, more about that later. Lower ISO settings give a better the "signal-to-noise" ratio, and hence cleaner, finer images. Other folk use higher ISO settings, and rely on image-processing to get rid of the excess noise. RAW exposures allow post-processing with minimal data-loss, but almost any format will do at a push. The other important exposure is a "dark" frame - taken with the same settings as the others, but with the telescope cap on, so that the only signal detected by the CCD is that from the camera itself. Take it just after you've taken the "lights", the camera should have warmed-up by then. This should provide a black frame which shows up any "hot pixels" and other camera-induced noise, the effects of which need to be negated. I should have taken more "darks".

This method provided 22 light frames like this:

Single light frame, originally in Nikon NEF format, converted to .jpg format to allow uploading

and one dark frame like this:

Single dark frame, originally in Nikon NEF format, converted to .jpg to allow uploading

Most folk then plug these frames into image-stacking software "as is", but I prefer to enhance the light frames beforehand, using Photoshop to tweak the brightness, contrast and saturation before stacking. It's very much a "suck it and see" type of adjustment, I like to see if I'm going to be able to brighten the background stars without burning-out (overexposing) the bright areas. Don't worry if the background gets a bit "noisy", the stacking process deals with that. After tweaking, the light frame above ended up like this:

Single ENHANCED light frame, originally in Nikon NEF format, converted to .jpg format to allow uploading

Now that dark frame - it just looks black, but buried deep within the RAW file data is a lot of info about the performance of your CCD. There's no call for this step during the processing, but this is what it looks like when enhanced to the max - lots of digital noise from "hot pixels" and conditions within the camera, stuff that's there in all of the other frames too, stuff that we want to get rid of in the final image :

Single OVER-ENHANCED dark frame, originally in Nikon NEF format, converted to .jpg format to allow uploading

The next trick is to use image-stacking software to process the frames. For Andromeda I used Deep Sky Stacker because it's simpler to use, but there are others including Registax (lots of tweaking options) and K3CCDTools (great when you get used to it), all of which have free versions available. Just open the program, load the light frames, load the dark frame, tell it to check all of the input frames then tell it to stack them. Then it gets on with the stacking process, finding the stars, subtracting the CCD noise (that you found on the dark frame) from each light frame, and stacking the resulting frames together. This process looks at each frame, and, broadly speaking, averages the data for each pixel across all of the light frames, so if there's the same data in a particular pixel on every frame, it's kept, as the average equals the original, but if it's random noise (in that pixel on one or just a few frames) it gets averaged out and hence is less noticeable. That's why having more frames is good. There's a better explanation here. Cool stuff, eh?

Anyway, after a while (it depends on the processing power of your PC/laptop), it produces a cleaned, stacked image, which it autosaves. This is the one to work with, not the one that it puts up on screen, which has all sorts of auto-adjustments done to it, and which can be dumped. From now on, save your work in a low-loss format, I saved as a .tif file. It's not quite the finished article, it looks fair, but it's full of good data that needs bringing out with a little post-processing:

Stacked image, originally in .tif format, converted to .jpg format to allow uploading

I do the final tweaks Photoshop CS3, but I suppose any decent equivalent will suffice. Small adjustments to contrast, exposure, brightness and saturation are what are required to achieve a pleasing result, the aim being to bring out as much fine detail as possible without overdoing the larger features - it's all too easy to end up with nice bright stars but an overcooked galaxy-core and a grainy background. Colour-casts can be removed, but this can lead to problems with false-colours. Experts tend to use the Levels and Curves functions to tweak the result, (they call it "stretching the histogram") but I only resort to that when all else fails, and I didn't use them on Andromeda. The final action is a touch of sharpening, PS CS3 can do this but I prefer to use the Focus Magic plugin. You'll find the results in the next couple of related posts (or just click the "Andromeda" tag in the post header above).

And that's it. Simple, really!

The good news is that decent results can be obtained with lesser kit. I've stacked pics taken with just camera+zoomlens+tripod, i.e. without a telescope or a tracking mount, and so long as the field of view is wide and the exposures are kept short, any star-trails are minimal and the result can be quite pleasing.

Final tips:

  • Make a note of what you do - it's a pain in the ass when you find a method that gives a good result, then can't remember how to replicate it next time out
  • Progress slowly, learn from everything you do
  • Experiment a lot
  • Try using a CCD webcam instead of a dSLR - it's a cheap way to get into imaging planets and the Moon
  • Don't go out and spend a fortune - there's no end to the amount of money that you can throw at this game. Second-hand gear is usually very good, especially scopes, which can be amazingly cheap
  • Get yourself onto an astronomy forum, they're great repositories of information imparted by folk that have usually learned the hard way themselves. I recommend Stargazers' Lounge, even though I'm banned from there (don't mention my name there - you might regret it!)
  • Make full use of the free software available on the web - there are some great packages out there, such as WinStars, Cartes du Ciel, Celestia, Starry Night. There are many other options out there, but you might have to pay for them.

Back in Blighty

Posted by on August 18th 2008 in Great Escapes

The skies are overcast, there's rain about and the wind's getting up a bit, so we must be back in England. It's a sharp contrast to the (mostly) glorious weather that we've been having during our summer hols over in Norway and Denmark.

There's plenty to blog about, so I'll have to split the report into sections to be posted over the next few days. You might have to wait for the pics though - I've got 831 of them to sort out. Here's one of them to tide you over, just in case you've forgotten what a clear sky is:


The section of the Milky Way in the eastern sky. Denmark, August 2008.

(Nikon D50 on fixed tripod, 18-55mm lens @ 18mm, f/3.5, ISO 400, 6 x 30s exposures stacked with DSS, processed in PS CS3)

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