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Question about workflow for combining different ex...
Scott Vanaman

Last night I took 4 different length exposures of M43 (Orion) and was wondering the best workflow for processing these together. I have 30+ subs for each exposure. I took 30s, 60s, 90s, and 120s photos.

Should i stack each length in DSS, then use the 4 outputs together? I want to try to preserve the core of the Nebula, since the 120s photos started to appear to have blown it out.

I have been using Photoshop, but hopefully the techniques are transferable.

Any guidance would be helpful.

- Scott V

Hari Doraisamy
I would use the group feature in DSS. Each exposure set would be in a different group. DSS takes care of it.

Scott Vanaman

Quoted Text

I would use the group feature in DSS. Each exposure set would be in a different group. DSS takes care of it.

I have not used that feature of DSS before, so I'll look into doing that.


- Scott V

Lou Varvarezis

Scott,After stacking in DSS you can create separate stacks and use layer masks in PS when you combine to reveal the detail core. It can be a lot of work to combine all of those layers together using masks. You might be able to get away with just using the longest and shortest exposures if you don't want to make more than one mask to combine all of your subs. This technique is handy especially if the core was blown out while trying to expose for the fainter features of the nebula.Alternatively, the dynamic range of your camera might be wide enough to reveal the detail you are looking for in the core after stacking your longest exposure. In PI, I would start with the HDRMultiscaleTransdform, I assume PS has a similar process. You may decide this method is sufficient and avoids using masks with this target if the data in the core is not over saturated or clipped.


Canon EOS Rebel XS (full spectrum mod)

Astronomik CLS CCD

Orion thin off axis guider

QHY5L-II M guide camera

Celestron CGX, Edge HD 8, 0.7x focal reducer

Optec TCF-Leo focuser

Gary Trapuzzano

Hi Scott,

I ran an experiment recently with combining multiple exposures of M42. I was curious to see the difference between the resultant combination of the same data using layer masking verses using weighted layer averaging. My take on the outcome was pretty much what I suspected in that the layer masking technique gave a better result because it preserved more of the low level data present in the longer exposures.

Here's the workflow I used for the weighted average experiment (see attached sample image: M42_Composite_Flat_Weighted_Layers_Average.jpg).

As mentioned above, I've used masks before when combining the trapezium of M42, but tried a different method this time to see if it would reduce the (sometimes) noticeable transitions between the different exposures. This alternative hybrid technique goes back to the way stacking was done years ago using Photoshop before auto stacking software was available. The workflow I used was the following:

  • Take a series of exposures over the desired range. For that M42 image I took 20 shots each at 30 sec, 15 sec, 7.5 sec, 4 sec, 2 sec, 1 sec, and 0.5 sec.
  • Align & stack each set of exposures using automated software. For that M42 image I used Images Plus software for auto alignment and stacking.
  • I saved each image in the set to uncompressed TIFF (32bit float) and opened each image as a layer in Photoshop.
  • The Photoshop layers were then organized (ordered) with the longest exposure time image as the bottom layer to the shortest exposure time as the top layer.
  • The Photoshop layers were then manually aligned to one another using the stars in each image as reference points.
  • The Opacity of each Photoshop layer was then set as follows:
  • 0.5 sec layer, Opacity = 14%
  • 1 sec layer, Opacity = 17%
  • 2 sec layer, Opacity = 20%
  • 4 sec layer, Opacity = 25%
  • 7.5 sec layer, Opacity = 33%
  • 15 sec layer, Opacity = 50%
  • 30 sec layer, Opacity = 100%
  • (Note: The Opacity setting is 1 divided by the number of total number of layers in the stack at that point in the stack, starting from the bottom.)
  • The Photoshop layers were then flattened into one layer and Levels and Curves were used to stretch the histogram of the composite image.

I've also attached a sample image from the same data but this time using the using layer masking technique (see: M42_Composite, Flat, Layer Masking, w-Txt.jpg). Note: I tend to use a very soft brush for the masks and sometimes I reduce the opacity of the shorter exposure layers to soften the blending transitions.

Please excuse the elongated stars in the sample images. The M42 data was taken while testing a new planetary camera (ZWO ASI485MC) to see how it would do with a bright DSO. The mount was only roughly polar aligned and I wasn't tracking.

Clear skies,


Gary Trapuzzano

Quick follow-up Scott... I'm guessing only one file can be attached to a reply since it looks like only one of the attachments actually made it through.

Anyway, here's the sample image with the weighted layers average (see attached: M42_Composite_Flat_Weighted_Layers_Average.jpg).

Clear skies,


Scott Vanaman


Thanks for the information, I'll give that a try.

And your oblong stars test image still looks better than any other image of taken, so you're excused!

- Scott V

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