During unguided operation, the same benefits that accrue during Guided Operation can be achieved. Here, the mount is moved slightly between exposures.
Dithering can be done either totally randomly, as determined by a random number generator, or in a controlled manner to maximize separation between each sub-exposures artifacts while minimizing the overall guide star movement. Both options are provided. The amount of the dither is user-definable.
Choosing a Dither Factor Manually:
Max. Pixel Dither is used to set the maximum movement that is adjustable from 0 to 10 in arc-sec. for unguided imaging and 0 to 10 in guider pixels for guided imaging. In order to set the maximum movement most effectively, you need to know some values:
G = Guider image scale in arc-sec./pixel (asp)
I = Imager image scale in arc-sec./pixel (asp)
A good starting point is to dither at least 4 pixels on the imager. Even though a hot pixel is only one pixel, currently available alignment and combining techniques will smear out that hot pixel over as many as 4 pixels. In order to effectively eliminate the hot pixel, the dither must at least be that much.
Unguided Example: Assume you are taking LRGB images with the RGB binned at 2x2 and your unbinned image scale, I, is 0.5 asp. Your 2x2 binned sub-exposures will be 1 asp. Therefore, you should set your maximum dither at 5 arc-sec.
Dither method: Select Enhanced (preferred), Random or None.
Max. Pixel Dither: This is the peak dither value and can go +/- from the starting direction. As an example, suppose Enhanced Dithering is selected with a Max Dither of 3 arc-sec. The first sub-exposure will leave the mount position undisturbed. The second will move the mount +3 arc-sec. in RA. The third will move the mount - 3 arc-sec.from the first exposure. Thus the total movement between the second and third exposure is 6 arc-sec. but the movement relative to the first sub-exposure is ± 3 arc-sec. If dither is set to 0,the mount will not be moved between exposures.
Suggest: this button will enter a Max. Dither value, based on the parameters of your system entered on the settings page. This can be a starting point from which you can experiment if you desire.
Dither every <n> Exposures: In most cases, this setting should be left at 1, i.e. dither every sub-exposure. In some applications when taking a large number of exposures, there may be a desire to dither less frequently. This setting allow dithering to be used every <n> sub-exposures. For example, if set to 3, there will be an initial small dither for the first exposure, no dither for the second and third, and then a normal dither for the fourth exposure. This sequence will repeat for the number of exposures in the series.
When a long series of unguided exposures are taken, the mount will gradually track off the target. By using this feature, the telescope is periodically realigned to the target coordinates. If any periodic focusing using SkyStar is implemented, target realignment will occur during that process. If that is used, additional periodic realignment will occur only as long as the time between SkyStar focus runs has exceeded the realignment period.
Realign to Target Frequency (min.): If 0 is entered, this function is disabled. If a non-zero value is entered, the telescope will be realigned to the target coordinates via a precision slew after the current exposure completes and after the specified interval has elapsed. It can be disabled (set to 0) when guiding.