Focusing a Digital Camera for Astrophotography

After doing astrophotography with film for some years, I started to take astronomical pictures with the Coolpix 950 in 1999 and since 2002 with the Coolpix 4500. I have a 130mm (5.1") Newton telescope on a Super Polaris mount as my primary instrument.

Because of the fixed length of most Digital Cameras they only can be used in an 'afocal' configuration at the telescope. To be able to record an image the camera lens has to look in to an eyepiece quite the way a human would do.

I couple my camera to the telescope with a 'Vixen Digital Camera Adaptor', but other solutions are also availble.

I have been asked quite a few times how I focus my astrohphotos with the Coolpix cameras. After sending the n´th e-mail containing this information, I decided to put up this page...

Settings:

for astronomical images I use the following settings on the Coolpix 4500:

• Noise Reduction ON
• ISO 200
• Manual Exposure
• time: Bulb
• f/number: minimum (f/2.6 in wide angle)
• White Balance: Sunny
• Focus: Infinity

To control the exposures I either use a self built bracket which presses down the shutter-release button mechanically or when near 220V electrical power, I use the very convenient Snappix program on my laptop computer connected to the camera with a SC-EW3 cable.

Focusing:

• set the camera zoom to the widest possible angle, to make finding and centering stars easier.
• center a bright star in the eyepiece
• set the camera to infinity focus (the mountain symbol)
• zoom in on the bright star, and refine the focus with the telescope´s focuser(!). use the digital zoom to get the biggest possible image of the star. When zooming in you possibly have to recenter the star
• lock the telescope focuser
• zoom back to wide angle
• remove the camera, find and center the object to photograph in the eyepiece (without changing the focus!!) **
• reattach the camera with the desired object in the field of view
• zoom back to the desired image size
• take the picture!!

**) With the Vixen Digital Camera adapter it is possible to attach and remove the camera without screwing it off, you just loosen or tighten the three screws connecting the digital camera adapter to the DG ring...

Assembly instructions for the Vixen DG Digital Adapter (pdf), look for DG digital ring and Digital adapter at Vixen North America's web-page.

With a screw-on type of adapter (like Scopetronix) you have to recenter the object without removing the camera, which is not so easy without precise GoTo capability...

out of focus	sharp!
The digital zoom can be used as a kind of focusing magnifier...

You can find some of my digital astrophotographic shots in my gallery pages. If you have any questions, corrections or suggestions, don´t hesitate and send me an e-mail!

M 13, Globular Cluster in Hercules