Imaging Source DMK 21AF04.AS

Camera review for solar system imaging

Page 1 - Introduction

The DMK 21AF04 camera by Imaging Source became one of the most used cameras out of the range of "fast interface" cameras (IEEE1394 firewire or USB2) for planetary / solar system imaging including it's predecessor DMK 21F04 and and sister model DMK 21BF04. Originally intended for industrial vision and scientific purposes Imaging Source recognised the growing astronomy market segment like a few other competitors and offers this camera among other models as astrophotography versions as indicated by the .AS extension in the ordering code.

The Imaging Source astronomy cameras basically come in three flavours:

• Monochrome CCD versions - DMK models
• Color CCD versions with RGB output - DFK models
• Color CCD versions with raw Bayer output - DBK versions

Currently each flavour is available as 640 x 480 1/4" CCD, 1024 x 768 1/3" CCD and 1280 x 960 1/2" CCD version, all equipped with Sony CCDs.

The 21AF04 models using the Sony ICX098 1/4" 640x480 CCD variants should be the most popular models. The CCD color variant 098BQ is well known from the famous Philips ToUcam Pro models and the monochrome version 098BL was often used to modify the ToUcam. 640x480 pixels are enough to image solar system planets, more pixels are only usefull for lunar and solar imaging. Compared to the ICX204 (1024x768) and ICX205 (1280x960) the ICX098 offers the highest pixel sensitivity in relation to pixel size. Unless a higher resolution than 640x480 is required the 21AF04 models are best planetary performers out of the range.

The "BF" models (not in the range of AS astrocameras) include external trigger capabilities not required for astro-imaging. The "F" models feature a longest integration/ exposure time of 1/30s - too short for most astronomical purposes, so that the AF models should be the astronomers choice. The longest integration/ exposure times of the .AS models has been extended to 60 minutes compared to 30s of the regular models so that extended deep sky imaging is possible although this should be limited in practise by the missing active cooling.

Monochrome, color or Bayer ?

Color CCDs contain a Bayer color microlens matrix - each pixel features a green, red or blue filter only, colors are arranged in the Bayer pattern. The missing two colors for each pixel are calculated from adjacent pixels having the required color. The DFK models do this debayer calculations in the camera, the camera output is complete RGB for each pixel. The DBK models output the raw bayer data to the host machine where the debayer calculation will be done instead of in the camera, the main advantage is speed: Only one third of the full RGB data has to be transfered and higher frame rates are possible.
The principles of the Bayer matrix limit the true color resolution to one third of the resulting RGB data since only one color is sampled per pixel. To aquire the full color resolution a monochrome CCD with a 3 pass RGB or CMY capture is required. Of course the capture using RGB filters and the processing of the data are more timeconsuming than a single color camera capture pass and might be critical for the recordings of a very short time window.

A monochrome CCD is more sensitive than the color equivalent since the color matrix is not present. Captures using special filters like IR-pass or UV-passfilters require a monochrome CCD without a color matrix for practical results.
Most solar system imagers should opt for the monochrome CCD variants of the cameras. Lunar and solar imaging is usually done without using color information (the usage of bandpassfilters is highly recommended, though). Imaging planets is recommended with mulltiple monochrome passes and filters unless the convenience of a color camera is favored - the Bayer models should be prefered then.

Why "fast interface" cameras ?

Cameras using a slow bus like USB1.X (e.g. the good old Philips ToUcam Pro) have to compress image data with lossy compression algorithms to deliver higher frame rates - the higher the frame rates the more image quality is affected. Solar system imagers try to gather as many raw image frames as possible in the recording time frame. The best frames affected least by seeing will be selected and stacked/averaged to minimize random noise of the raw frames and increase the effective bit depth. The more frames are used the better the resulting signal to noise ratio and the effective bit depth.
Cameras connected by firewire IEEE1394a (or even faster 1394b) or USB2 can deliver high frame rates without image compression - more raw frames with better quality can be captured.

Personally I prefer IEEE1394 to USB2 because cameras supporting the IIDC/DCAM protocol can be used with any capture software and operating system supporting the standard instead of requiring proprietary drivers.