"Virtual Equatorial" digital imaging
Carrying an equatorial mount is not always possible. The opportunity of a clear sky in the french Alpes mountains and the presence of the Machholtz comet allowed me to test the new Iris feature of the Virtual Equatorial with a very, very minimal instrumentation.

For french version, click here

Instrument and Acquisition
When you travel for a ski week-end, you usually do not take your equatorial mount with you. Nevertheless, the attraction of a clear mountain sky and a comet bright enough decided me to test what I will call "the astro minimal kit".

- Digital camera: sony DSC-W12

- optional but very handy: a remote trigger flexible, "Hand-made" by Christian

- Small table tripod

The digital camera

Sony DSC-W12 has a 1:1.8" CCD sensor, of the 5Mega pixel class. It has a very large LCD screen to ease image review and pointing but was in this case useless as the comet was high in the sky, close to zenith.

The acquisition parameters

Set in mode M, image quality in "Fine", focus locked at infinity, exposure duration at 30s, zoom at min (7.9mm) and Iso at 400.
It shall be noted that there is no "raw" format accessible, and for long exposure the camera automatically take a dark exposure and subtracted after the real exposure. No way to alter this setting, so let's go with it...

The acquisition site

A ski resort is a not the best site for astro-imaging... lot of city lights. The first thing was to find a spot where those lights are dimmed by pinetrees. The next drawback was that my tripod beeing very small, the camera was very close to the ground, which was snow. To point the comet, the only way was to lay-on the snow and try to direct the camera body toward what was supposed to be the faint comet.

To prepare observation, I use Planetarium software from AHo software on my palm, a sony clie. You can enter comet and asteroids elements, make your own list of object and upload it, the color screen and graphics are easely readable and all basic functions are accessible in few clicks. The beauty of Handled-device like Palm or Pocket PC is that they boot in a second...

Acquisition sequence

A sequence of 11 images, each of them of 30 sec. This was a little bit too long to not have star motion blur.

Also, the use of a flexible was really practical in this case. The alternative would have been to use the delay button. The risk is that you screw up the pointing when arming the next exposure and reselect the delayed mode on the rear of the camera. Also, was quite nice to be able to keep the gloves during the sequence...

Image processing

You have to first get the Iris 4.30 or above version on Christian's site.

The first operation is to convert the jpg images in .pic (or .fit) format. Unfortunatly there is no batch command in iris to do this unless you rename first your 11 images with a name like name1.jpg, name2.jpg, etc. With DSC00023.jpg, DSC00024.jpg type sequence, the index does not start at 1 and Iris command are not able to process it with the jpg2pic command...
So, after the 11 manual "load and save", the 11 pic images are ready for the registration.

One individual image, 30sec, 400ISO, "Fine" format, automatic dark subtraction. Size reduction 50%

As all images have a dark subtracted, there is no need to process dark here. No flat either, but it could have been a good idea to make one.

To register the sequence you can then either perform the manual star selection as explained on the web site of Iris or use the "re-discovered" powerful coregister function. The requirement is to preset the polynome degree of the distorsion to 2 to accomodate with field rotation.

For a sequence of 11 images, m1...m11 - registration done on the first image.

The sequence is as simple as:

To configure the algorithm of registration with a degree 2 polynome





Repeat up to last image of the sequence... here I have 11 images


To complete the K sequence with the first image

ADD2 k 11

Change display thresholds adequatly to increase the image contrast - a background correction has been performed: command POINTON, manual clicks on the image background (several dozens), POLY, SYNTHE - POINTOFF to return to normal pointing mode. The automatic SUBSKY function here does not give good results because of the presence of the dark areas of the pinetrees

One can see on the image background a fixe pattern noise which probably comes from the CCD sensor. Un fortunatly as the star motion was almost parallel to this pattern, the registration did not eliminate it. It would have been better to have either change the camera orientation or moved slightly the camera randomly between each exposure. large image.

The 11 images added with no registration
Animated gif of the sequence not registered, it is nice to admire the sky rotation Animated gif of the registered sequence, now the pinetrees are moving... the distorsion is perfectly computed, the stars are kept at the same position all along the sequence
1:1 area around the comet with M34 - 11x30sec - 330sec total exposure - 400 ISO
2:1 Black&White, inverted - we can guess the 2 tails of the Machholtz comet - the stars "square shape" is probably due to a combination of too long exposure time and jpg compression effect
So, is it the revolution ?
  • Yes ! - Taking direct images of sky by simply putting your camera on the ground is so easy... the sensitivity of the new digital camera allows sky image recording with short exposure, short enought to record something within the exposure duration limit where sky rotation is not or almost not visible. Then the addition of images to increase the contrast is only limited by your patience and the sensor background noise.
  • But, do not dream... this is for wide field images as the limitation is the maximum exposure duration to get star images with no motion due to sky rotation visible, this being not compensated as you do not have an equatorial mount.
    You can compute the max exposure time for each focal configuration if you know some basic facts on the sensor.
Exposure duration computation

To not see star motion on the image, this means that the star shall not move of more than one pixel during the exposure.

The angle on the sky seen by a pixel is:
p/F, in radian, considering a small angle, with p pixel size and F focal of the lens
On the sky, the star will move by 15 degrees by hour. The apparent angle is more important if the star is close to the equator and at the extreme null at the polar north. So, we shall multiply the 15 degrees by hour by the cosinus of the declinaison.
In my example p=5.6 10-3 mm, F=7.9mm - if we consider shooting at 45°, the maximum exposure shall have been 14 sec. With 30 sec, the star motion is in fact visible and cross 2-3 pixels as expected.

Sensor Pixel Size ?
The Sony Super CCD HAD is a 1/1.8" - this means a Horizontal by Vertical dimension of 7.176mm x 5.319mm (for detailed explanation on sensor dimension click here). The number of pixel in the "Fine" image is 1280x960. By dividing the horizontal dimension by 1280 we find a pixel size of 5.6 microns. If you do the same on the vertical dimension, the pixel appears to be square.