Drizzling with NSG?

Ken Bates

How would one go about incorporating NSG into Drizzle? Since NSG takes the registered images and creates _nsg files, and since registering with drizzle creates drizzle files that point to the original registered (non nsg) files, it would seem that there is a more complicated workflow that is needed. To wit:

1. Register normally without drizzle


2. Execute the NSG script, which creates nsg files from the registered files


3. Register the nsg files  using drizzle. Nothing really done here, but it does create drizzle files that point to the nsg files instead of the original registered images.


4. Integrate nsg files with no normalization


5. Drizzle integrate the nsg files

Am I overthinking this? Seems like all the above is needed, but it also seems like there might be a simpler way that takes advantage of both nsg and drizzle. Not always needed, of course, but could be useful in some circumstances unless I’m way off base (which seems to be a common occurrence!)

 - Ken

John Murphy

NSG is not currently compatible with drizzle. To make it compatible, NSG needs to output .xnml files. This is not easily done from JavaScript. Hence, this functionality will have to wait until I have ported NSG to C++. That needs doing anyway for performance reasons.

Is there a way of getting the current script to work with drizzle? The short answer is no, there is no good way of doing this. I have not tried it, but there might be a very hacky way of doing it. This hack (if indeed it works) would require mono files.

• If there is a meridian flip, use 'FastRotation' to rotate the images by 180 degrees either before or after the flip.
• Run NSG on the unregistered files. To make this work, increase the 'Photometry Star Search' -> 'Star search radius' to its maximum (20). Provided the shift between images is less than this search window, it should still manage to match the ref / tgt stars. Reduce the 'Star flux tolerance' to reduce the risk of invalid star matches. Try 1.2
• Use the 'Photometry stars' dialog to check that the ref / tgt stars have been correctly matched. Test on the image with the greatest shift from the chosen reference image.
• Increase the sample size to at least 1.5 times the 'Auto' default size.
• Only use smooth gradient corrections. The default of 2 should work well. Don't use less than 1.0
• Deselect 'ImageIntegration'.

Then, do your drizzle processing as normal, starting by registering the _nsg files with the 'Generate drizzle data' selected. Make sure you set up the ImageIntegration process with 'No normalization' and NWEIGHT. Note that DrizzleIntegration will then go back to the unregistered, but scale and gradient corrected, _nsg files.

As you can see, it is a very hacky solution... You would also need to be very careful to get the ImageIntegration settings correct (see help documentation). The scale correction should be good, but the gradient correction will be more approximate because the reference and target samples will be offset from each other. It is probably better to wait for the C++ version that will do the job properly.

Regards, John Murphy