My filters and first results of lunar petrographic imaging
Some time ago I wrote about petrographic Moon imaging. I managed to collect a set of quite cheap filters for this type of imaging using eBay and Edmund Optics shop. In this article I'll present those filters and some first results I got - and do look to be correct.
Filters
766 nm EO Infrared Bandpass Filter
Filter for the original 750 nm band.
- Center Wavelength CWL (nm):766.0
- Center Wavelength CWL Tolerance (nm):±2
- Full Width-Half Max FWHM (nm):10
- Full Width-Half Max FWHM Tolerance (nm):±2
- Minimum Transmission (%):≥50
905 nm EO Infrared Bandpass Filter
Filter for the original 900 nm band in the mafic image set.
- Center Wavelength CWL (nm):905.0
- Center Wavelength CWL Tolerance (nm):±2
- Full Width-Half Max FWHM (nm):10
- Full Width-Half Max FWHM Tolerance (nm):±2
- Minimum Transmission (%):≥50
940 nm EO Infrared Bandpass Filter
Filter for the original 950 nm band in the mafic image set.
- Center Wavelength CWL (nm):940.0
- Center Wavelength CWL Tolerance (nm):±2
- Full Width-Half Max FWHM (nm):10
- Full Width-Half Max FWHM Tolerance (nm):±2
- Minimum Transmission (%):≥50
425DF40 OmegaOptical / eBay
Filter for the maturation images (415 nm in source papers)
745AF30 OmegaOptical / eBay
Extra filter, a broader one for the original 750 nm band.
1000LP OmegaOptical / eBay
A 1000 nm longpass filter to be used as the 1000 nm bandpass filter in source papers. Cut on - 986 nm.
Results
Maturation ratio image:
Mafic ratio image:
Red channel from the mafic ratio image (you get this after divising two images):
Mafic and maturation ratio images, C11; f/10; UI-1245LE-M-GL:
Although the filters aren't exactly on the source/original wavelengths they seem to work quite good. It's hard to validate if the colors do represent what they should represent (It's hard to find any other images of this type on the net). I described those image above in my gallery. To make those photos I used the filter set described above with the 745AF30 and not 766 nm EO filter.
After stacking and processing each image they need to be aligned vs each other. I use Nebulosity for this (Batch - Align and Combine Images - Save each file) by selecting small feature on the frame that may work as a star for the Nebulosity aligning algorithm. When frames are aligned then they may be divided - by applying divisor image as a flat to the divided image. If you imaged Moon edge clip the background in levels so that the black background is really black and has no fuzzy noise (that would create a quite bright signal after division). To enhance colors of a mafic/maturation image limit the gamma noticeably.
900-1000 nm bandpass filters are very dim and will require max gain and longer than usual exposure. They are to dark for high resolution imaging so don't think about Barlows for this type of imaging. If you use a DS camera (like Atik 314L+ in my case) use exposures longer than 1-2 sec to get 30-50 frames with nicely filled histogram. For "planetary" cameras those exposures will be shorter, but more frames will be required.
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