Astrophotography without equatorial mount
I describe here how to shoot deep sky objects, nebulae and galaxies, using a Pentax DSLR coupled with the O-GPS1 unit (reviewed here and here). Thanks to its ASTROTRACER function, this unit is able to drive the camera image sensor in order to compensate the earth rotation for a few minutes. Thus it allows long exposures on celestial bodies. Here is my dedicated gallery.
All you need is:
- a O-GPS1 unit and a compatible Pentax camera: K-3, K-5, K-5II(s), K-r, K-30, K-50 or K-500(?) with fully charged battery,
- or a K-3 II or a K-1 with fully charged battery
- to be sure about the camera firmware version (at least 1.03 if you are using a K-5),
- a remote control to trigger the exposures,
- a solid tripod,
- a free (as much as possible) from light pollution place,
- some coffee (or tea) for a few hours ;-)
Preparing the camera and the GPS module
Please follow these steps:
- put the GPS unit on the camera. One GPS battery capacity is about 4 hours, you will have to change it before the camera battery,
- switch to RAW mode (it's mandatory),
- turn off the automatic noise reduction for long exposures,
- choose the 3sec. with remote control triggering mode,
- choose 1600 ISO sensitivity (800 if sky conditions are very good, you will benefit from an higher dynamic),
- choose the maximum lens aperture or step down 1 speed depending of the lens quality (but remember that stepping down one speed means doubling the total exposure time to obtain the same result),
- switch the camera to manual focus mode,
- wait for the GPS unit to receive signal from satellites,
- perform a precise calibration at the same place you will shoot the stars (it take less than one minute, retry if it failed),
- put the camera on the tripod,
- switch to 'M' shooting mode, take aim at a bright star, and perform precise focus using the liveview at x10 magnification,
- switch to 'B' (bulb) mode,
Note that the calibration have to be performed again each time the Camera or the GPS module is switched ON.
You are now ready to shoot galaxies and nebulae with the lightest and cheapest astrophotographic system existing on Earth!
The most exciting part: shooting!!
The maximum exposure time depends of the lens you use: from my experience you can expect up to 1 minute with a 200mm lens for the best case (close to the north pole), 30 seconds for the worst (near zenith or near the celestial equator). With a 50mm lens you can expect up to 5 minutes exposures.
In order to improve the S/N ratio and because of the limited exposure time, it's necessary to perform more than one single shoot (I recommand at least 6 to 8 images). The S/N ratio progresses with the root square of the shoots number (it's multiplied by 2 with 4 shoots). It's important to notice that you have to re-adjust field every 2 to 4 shoots (with 200mm lens).
Dark frames: Usually dark frames are necessary to get the read-out signal and subtracting it from the images. But thanks to the Exmor sensor the read-out signal is very low and dark frames may be avoided when shooting (relatively) bright objects or under medium to strong light pollution (sky glow). If the sky conditions are very good, I think you should shoot dark frames, but I've never experienced such condiftions.
I use Lightroom for the demosaizing process and for lens corrections. It 's very important to obtain a uniform background because the subject's dynamic may be less than the background variations, so thanks to Lightroom to offer very fine corrections for the DA*200. Otherwise you need to add your own flat field images in the pp.
Reject images where stars are notably stretched. So just perform lens corrections in Lightroom and export images in 16-bits TIFF format.
I use DeepSkyStacker. It's a freeware, very easy to use. It can load 16-bits TIFF or even RAW format data. It will also automatically align the images. Save the stacked images in 16-bits TIFF format.
Import the TIFF image into Lightroom and adjust the levels using (mainly) the tonal curve tool.