Making mosaics

Making mosaics was done by hand in the past. With the new guider there are several possibilities. The easiest (but least flexible) one is to draw a rectangular area in the guider window using the middle mouse butten (which doubles as a scroll wheel on most mice, including the one at the DOT). Move the mouse to one corner of the rectangle you want to draw, press and hold the middle mouse button, move to the other corner you want, and release it. If you are not satisfied, just draw a new one, the old one will then automatically disappear.

A yellow rectangle will be shown, with a grid of dots inside. The spacing between the dots is such that if you move the guider target from one dot to the next, there is approximately 15% overlap between the fields of view.

You can then change the script to automatically cycle through grid positions. There are two script commands that are relevant:

exec guidercmd move mosaic start
exec guidercmd move mosaic next

The first will move the telescope to the first (bottom left) grid position, the second command will move the telescope to the next grid position. Positions are visited in a zig-zag pattern to minimize the distance travelled. Note that the guider has to be set to “Track”, otherwise it will not move the telescope to the next mosaic position. An example script:

filter HaFilterTune origin 6562.703
exec guidercmd move mosaic start

repeat 180
    duration 20
        filter HaFilterTune relative  0.0
        burst 40 1
        filter HaFilterTune relative -0.5
        burst 40 2
        filter HaFilterTune relative +0.5
        burst 40 3

        exec guidercmd move mosaic next
    end
end

When starting the script, it will move to the first grid position, take three bursts at three Halpha wavelenghts, go the next grid position, and so on, for one hour. The number of complete mosaics covered depends on how big you draw it of course.

Suppose that you have p no. of total points for making a complete mosaic, or a grid of m rows and n columns, in which case p = m·n. If you want to make r no. of images of the complete mosaic, then you need to specify repeat as p·r (p multiplied by r). repeat depends on the no. of points selected for making a mosaic, while duration only decides the cadence of observations. For example, in the image shown at the top there are 25 points (m = 5, n = 5, hence p = 25) to make a complete mosaic of the yellow box shown. With repeat set to 180 in the script given above, there will be 7 (= 180 modulo 25) complete mosaics of the yellow box, and 5 more images will be taken at the 5 points on the bottom row.

Some tips:

Another option is to give commands during the script to move the telescope to absolute or relative positions. This will result in longer scripts, but you can make any shape mosaic. The commands are:

exec guidercmd set position x y
exec guidercmd set soho x y
exec guidercmd set helio x y

exec guidercmd move position dx dy
exec guidercmd move soho dx dy
exec guidercmd move helio dx dy

The first three commands set the position, either in raw coordinates (1 unit = 1 solar radius, x- and y-axes as seen from earth without P0/B0 rotation applied), SOHO coordinates (in arcsec), or heliographic coordinates (in degrees). The last three commands make relative changes to the target position. So for example:

exec guidercmd move soho 50 0
exec guidercmd move soho 0 50
exec guidercmd move soho -50 0
exec guidercmd move soho 0 -50

Will move the telescope in a small square loop. You can try these commands out, without “exec” in front, in a shell. Other commands are:

exec guidercmd stop
exec guidercmd track
exec guidercmd dark
exec guidercmd flat
exec guidercmd center

In the shell the following can be useful:

guidercmd get mode
guidercmd get position
guidercmd get soho
guidercmd get helio