avandonk
05-05-2009, 03:00 PM
Thought some of you may wonder what is involved with my HDR method.
Here is a quick précis of the method subject to change as it is a work in progress.
Take sets of exposures doubling the time for each successive set.
EG 15, 30 sec. 1, 2, 4, 8, 16 ...minutes. It is better to have more exposures for the longer exposures to minimize noise by median stacking. You could also double the ISO setting if the time gets to long. That is an 8 minute exposure at 500 ISO is equivalent to a four minute exposure at 1000 ISO.
The shorter exposures are only needed if there very bright stars in the image. These can saturate in a few seconds.
What this means that for a Canon 12 bit sensor and 8 sets of exposures you will end up with an image with a dynamic range of 19 or 20 bits! Or 19 or 20 stops.
Old method or if you use INCR.(Convert all RAW files to TIFF and correct for flats etc with ImagesPlus. )
I now use the fridge which keeps my camera at a constant temperature. I also correct for darks and flats in uncompressed fits as the corrections are then done before interpolation to 16 bit tiffs. I also digitally develop the fits ie stretch before converting to tiffs as it gives much better data. I obviously use the same parameters on the different exposure sets. For 12 bit data I use a breakpoint of 4228 and background wt of 0.55.
You should now have tiffs that can be stacked with RegiStar.
I then use RegiStar to median stack each exposure set. RegiStar will stack the dim images even when you can only see a few stars.
You now should have a set of images differing only in exposure.
This is the tricky bit.
Run registar on all these images and make an averaged image but tick the intersection box in Combine. This is most important as you need to have an image where all the images have a common area.
Lets call this com_av.
I then trim by cropping com_av in PS to get a nice regular shaped image without any black bits due to drift or rotation. Lets call this com_av_crop
Move all the images into a new directory with com_av_crop.
Now run RegiStar again using com_av_crop as your starting image.
Now it is just a matter of combining com_av_crop with each registered image in turn.
Again tick intersection and set the weight of com_av_crop to 0.0001 and average. What this does is excise an exact area out of the exposure corresponding in size and content to the reference image com_av_crop.
Always check that only two images are ticked for combination. That is com_av_crop and the relevant exposure.
I then save these as 15srg, 30srg, 1mrg, 2mrg etc
Repeat for each exposure and you will end up with perfectly aligned and identical in size set of images differing only in exposure.
I then use Photo Shop to adjust levels. Get the R G and B histograms to the left. Stretch further by sliding the RGB thingy to the left about to 1.5. I also then drop the R to 0.85 to give an initial colour balance. This will depend on the camera and or filters.
These can then be opened in EasyHDR as it will only take identically sized images.
Generate the HDR image and when this is done check that the EV differences are 1 EV for doubled images.
Tick the jpg box as your images are already stretched.
Now generate the LDR image
There are a lot of controls on the right and these should be adjusted to get what will look like the best final image. It is better to adjust here as you are drawing on the data of all the images. It will take trial and error to get this bit correct. I am still doing trial and error myself.
I hope this is clear if not feel free to ask any questions. It is a lot of work but the results are worth it.
Bert
Here is a quick précis of the method subject to change as it is a work in progress.
Take sets of exposures doubling the time for each successive set.
EG 15, 30 sec. 1, 2, 4, 8, 16 ...minutes. It is better to have more exposures for the longer exposures to minimize noise by median stacking. You could also double the ISO setting if the time gets to long. That is an 8 minute exposure at 500 ISO is equivalent to a four minute exposure at 1000 ISO.
The shorter exposures are only needed if there very bright stars in the image. These can saturate in a few seconds.
What this means that for a Canon 12 bit sensor and 8 sets of exposures you will end up with an image with a dynamic range of 19 or 20 bits! Or 19 or 20 stops.
Old method or if you use INCR.(Convert all RAW files to TIFF and correct for flats etc with ImagesPlus. )
I now use the fridge which keeps my camera at a constant temperature. I also correct for darks and flats in uncompressed fits as the corrections are then done before interpolation to 16 bit tiffs. I also digitally develop the fits ie stretch before converting to tiffs as it gives much better data. I obviously use the same parameters on the different exposure sets. For 12 bit data I use a breakpoint of 4228 and background wt of 0.55.
You should now have tiffs that can be stacked with RegiStar.
I then use RegiStar to median stack each exposure set. RegiStar will stack the dim images even when you can only see a few stars.
You now should have a set of images differing only in exposure.
This is the tricky bit.
Run registar on all these images and make an averaged image but tick the intersection box in Combine. This is most important as you need to have an image where all the images have a common area.
Lets call this com_av.
I then trim by cropping com_av in PS to get a nice regular shaped image without any black bits due to drift or rotation. Lets call this com_av_crop
Move all the images into a new directory with com_av_crop.
Now run RegiStar again using com_av_crop as your starting image.
Now it is just a matter of combining com_av_crop with each registered image in turn.
Again tick intersection and set the weight of com_av_crop to 0.0001 and average. What this does is excise an exact area out of the exposure corresponding in size and content to the reference image com_av_crop.
Always check that only two images are ticked for combination. That is com_av_crop and the relevant exposure.
I then save these as 15srg, 30srg, 1mrg, 2mrg etc
Repeat for each exposure and you will end up with perfectly aligned and identical in size set of images differing only in exposure.
I then use Photo Shop to adjust levels. Get the R G and B histograms to the left. Stretch further by sliding the RGB thingy to the left about to 1.5. I also then drop the R to 0.85 to give an initial colour balance. This will depend on the camera and or filters.
These can then be opened in EasyHDR as it will only take identically sized images.
Generate the HDR image and when this is done check that the EV differences are 1 EV for doubled images.
Tick the jpg box as your images are already stretched.
Now generate the LDR image
There are a lot of controls on the right and these should be adjusted to get what will look like the best final image. It is better to adjust here as you are drawing on the data of all the images. It will take trial and error to get this bit correct. I am still doing trial and error myself.
I hope this is clear if not feel free to ask any questions. It is a lot of work but the results are worth it.
Bert