HDR Merge for SBIG Aluma AC4040 Images

We're planning to add new algorithms to MaxIm DL that will make the HDR merge process less complicated. In the meantime, as of MaxIm DL Version 6.25 we've added an HDR Merge plug-in. It's in the Program folder; use the Add/Remove Plug-in command to add it.

Some notes on using it:

When capturing images in HDR mode with the Aluma AC4040, the High Gain image will appear at the top of the frame, and the Low Gain image will appear at the bottom.

I strongly recommend having the "overscan" dark pixels turned on during acquisition, because the HDR merge algorithm uses them to ensure an accurate bias level for the HG and LG images. The camera should have them turned on by default when imaging in HDR mode, though you can alter this setting in DL Config.

To calculate the HDR scaling factor, I recommend using a flat-field frame taken in HDR mode. The HG image should ideally have pixels in the 1500 - 2400 range. It's also possible to use a light frame to do this if there are a large number of pixels in this range. (I was able to successfully set the scale factor using a shot of M31.)

The process is a little complicated, because dark subtraction should be performed before HDR Merge, and flat-fielding should be performed after HDR merge.

First set up calibration:

1. Load your raw HDR dark frames into Set Calibration. You will need a set of darks, and a set of flat-darks (darks matching the exposure time of your flat-fields)
2. Calibrate all your flat-field frames
3. HDR Merge all your flat-field frames
4. Now add the flat-field to Set Calibration. Be sure to use Auto-Generate (Keep Old) so you don't remove the dark groups you previously added.
5. At this point it is recommended to use the Replace w/Masters button

So to recap, we now have a master non-merged Dark frame group, and a master HDR merged Flat-Field group.

Tip: Use the Batch Process command to calibrate the flat-field frames. To do this:

1. Open one flat image.
2. Bring up View / Batch Process Window.
3. Hit the Record button.
4. Hit Process / Calibrate
5. Hit Plug-in / HDR Plug-in
6. Hit Stop button
7. Turn off the Manual check box on the Plug-in step (it defaults on; needs to be off)
8. Now Hit the >> expander
9. Select all your flat-field frames
10. Select "Save result in" and for the Folder type HDR.
11. Hit the Play button and all your flat-field images will be calibrated and the result placed in the HDR subfolder

That set of flat-fields in the HDR folder should be added to Set Calibration using the Auto-Generate (Keep Old) mode, as discussed above.

Now to actually process your images - use Batch Process again to process each light frame image, and again deposit the result in the HDR subfolder. The batch process steps are as follows:

1. Calibrate (This does dark subtraction)
2. HDR Merge (Manual check box off)
3. Calibrate (Again! This does flat-fielding)

Now you can use the Stack tool to create your light frames. As usual I recommend dithering when capturing the images, and Sigma Clip or SD Mask (if small number of images) combine mode.

 

To add the Plug-in:


On the MaxIm menu bar,
Plug-in, Add/Remove Plug-in
[Browse]
C:\Program Files (x86)\Diffraction Limited\MaxIm DL 6
HDRMergePlugIn.dll
[Open]

Windows may then give you a User Access Control security prompt asking:
Do you want to allow this application to make changes to your system?
click [Yes]

Then click [Close]

 

It will produce a 16-bit image.

I can't guarantee whether it will work with images from a competitor's camera. The image needs to be saved in the same format as our cameras do.

 

It's not designed to do that. Maybe with some machinations it could be made to work, but I doubt it's worth the effort.

The dynamic range of the STC-7 is actually pretty good - very similar to the STF-8300 which has 16-bit readout. The reason is the very low read noise. As a result when you use StackPro the dynamic range is actually superior to the STF-8300 and the total read noise is similar.

 

Personally, I prefer High StackPro. It's produces nice clean results with far less effort.