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:
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.
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.
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.