How I got this image !

Getting ready
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- Apart the GPS unit and the camera you need a (solid) tripod and a remote control.
- Prepare the camera: mount a lens, switch to RAW mode (mandatory), choose 1600 ISO (800 if sky conditions are very good, you will benefit from a higher dynamic) and 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).
- Put the gps unit on the camera with full charged batteries. One GPS battery capacity is about 4 hours, you will have to change it before the K-5 battery.
- After switched on the gps, you wait less than 1 minute the gps receives signal from satellites
- Perform a precise calibration (it take more or less one minute, retry if it failed)
- Switch to 'M' shooting mode, take aim at a bright star, and perform precise focus using the liveview at x10
- Switch to 'B' (bulb) mode
- Choose the 3s with remote control triggering mode

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.

The most interesting part: shooting !!
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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 (8 for this image). The S/N ratio progresses with the root square of the shoots number (it's multiplied by 2 with 4 shoots) you will take more than. It's important to notice that you have to adjust field every 2 to 3 shots (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

Processing:
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Pre-processing:

I used Lightroom for the demosaizing process and applied the 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 vary 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 less corrections in Lightroom and export images in 16-bits TIFF format.

Stacking:

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

Post-processing:

Import the TIFF image into Lightroom and adjust the levels using (mainly) the tonal curve tool.

Enjoy https://www.pentaxforums.com/gallery/images/smilies/smile.png

Brilliant.
Wonderful image. Excellent, clinical technique.
Well done and congrats.

Thanks for the detailed explanation. I'll be buying a O-GPS1 ASAP and putting my DA*200 to good use.

Wow! Great shot, great write up. I've been thinking about getting a O-GPS1, and now I'm thinking that I will sooner rather than later.

Very good image!