Sky Flats Assistant MaximDL Plugin
This plugin for MaximDL allows the user to take a series of sky flats while automatically compensating for the darkening sky.
Usage:
Simply enter the initial exposure duration, the target ADU level and the number of flats you require.
The initial exposure duration should be chosen to be reasonable for the desired target ADU count – this reduces the iterations required to get to the starting level. It doesn’t need to be accurate though, the plugin will iterate to get to the correct value.
The target ADU selected should be about 33% of the saturation level of your camera. This will give the most accurate and noise free raw flats which will result in the best master flat once stacked. Going too high can result in pixels outside the linear range of the CCD and too low can result in poor signal-to-noise in the flat. The allowable error percentage allows you to control how close to the target ADU count the flats must be - exposures outside this range will not be saved, rather a new exposure time will be calculated and the exposure retaken. Making this too restrictive may result in a lot of exposures being thrown away.
The plugin will take the initial exposure and then calculate
the exposure
duration required for the next exposure, to get it near to the target
ADU
count.
It continues to take and discard exposures until the average ADU of the
image
is within the configured allowable error percentage of the target ADU.
The plugin then loops through the specified number of flat exposures, calculating the exposure duration for the next based on the results of the previous exposure, thus compensating automatically for the dimming of the sky over the sequence of the flats run. As of version 2.0, there is an option to use a predictive algorithm which compares the actual result with the theoretical result of the linear extrapolation, applying a dynamic scaling factor to the subsequent time estimates to compensate for the difference. This is especially useful for cameras which have a long download time.
The ideal target number of flats somewhat depends on the noise characteristics of your CCD, but after stacking about 10 flats the signal-to-noise ratio in the flat drops to a level which will be insignificant once applied to the light image.
The flats which meet the configured error range are saved in
the user specified directory.
Files are saved with a naming convention indicating the scope used,
filter and
ccd binning, e.g. SN8_IRBlock_1x1_skyflat0.fit
I discovered that by deleting the “FILTER” FITS keyword from the flat files, the calibration group created would be applied to images taken with any filter. This is useful if you don’t need different flats for each filter type since it reduces the number of flats runs required substantially. A filter independent flat is useful to compensate for dust on the ccd or uneven response over the ccd, but obviously not for dust on the filter itself. However, it’s generally easy to get to the filter to clean it, compared to the ccd.
Config options:
- Option to delete the filter keyword.
- Option to use the guide camera rather than the main camera.
- Option to use a predictive algorithm rather than the standard reactive algorithm.
- Path to directory in which to save flats.
Version history:
v1.0 - Initial release.
v1.1 – Fixed overflow bug with 16bit images. Plugin now finds correct
exposure before starting run.
v1.2 – Added binning support.
v1.3 – Changed default values for target ADU and number of flats.
v1.4 – Improved file naming to avoid runs with different filters
overwriting each other.
v1.5 – Allow filter independent flats to be captured. Increased default
number of flats to 10.
v1.6 – Add option to take a flat for the configured guide camera.
v1.7 – Filter out invalid characters from the filename.
v1.8 – Disable debug messages left on in 1.7.
v2.0 – Implement saving of config, user specified target directory, predictive duration estimation algorithm.
SkyFlats v2.0
If you download this to give it a go, please post feedback on
the MaximDL
Yahoo group, or email me directly at: winfij_AT_gmail_DOT_com
One final note - if you have any errors, please note the exact error
messages
and let me know!
Regards,
John