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HDR imaging question

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#1 LiquidMan

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Posted 21 June 2025 - 02:53 PM

Hi everyone, I hope you're doing well!

I'm planning on shooting an HDR image of the Cat's Eye Nebula. I got a lot of time on the core last year using just the UV/IR cut glass on my 2600mc and 15s exposures. However, I recently purchased an EAF, which seems to get me sharper images, so I'll be restarting this target.

 

Now onto my question: I'm planning on shooting my longer exposures with my ALP-T filter at 120s, but am contemplating between shooting the shorter 15s exposures with the same ALP-T filter, or with the UV/IR again. Should I shoot with the same filter for both my shorter and longer exposure images? I image from pretty bright Bortle 7-8ish skies, which can cause the backgrounds between images with and without filters to be VERY different. Particularly images without a filter can be tricky to get the background perfectly smooth. I can see this potentially causing problems when merging the images to create my final HDR image. Am I wrong in this assumption or should I shoot both images with the same filter?



#2 bobzeq25

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Posted 21 June 2025 - 04:40 PM

Why would you NOT want to use the same filter? Seems like the simplest and best idea to me.

#3 kathyastro

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Posted 21 June 2025 - 05:46 PM

I agree with Bob: use the same filter unless you have a compelling reason not to.  

 

However, the background will come exclusively from the longer exposures.  With normal HDR processing, only the parts that are overexposed in the long exposures will come from the shorter exposures.  So I don't believe that, in this case, the different backgrounds would be a factor.



#4 LiquidMan

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Posted 21 June 2025 - 07:16 PM

I wanted to shoot the 15s exposures mainly for RGB stars. Initially, I was unsure if this nebula was composed of anything more than Ha and Oiii, so I wanted to image it with no filter to capture everything at once. However on further research of this target, in addition to Kathy’s response, it doesn’t seem like shooting without a filter would be beneficial.

Anyways, thank you both for your responses, I’ll go with the same filter for both to make processing easier!

#5 kathyastro

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Posted 21 June 2025 - 07:31 PM

I wanted to shoot the 15s exposures mainly for RGB stars.

If you can get stars from the short exposures, great.  But that's not why you shoot them.  The Cat's Eye Nebula is one where you can't get the whole thing without HDR processing.  The long exposures will get you the outer, dim parts of the nebula, but will badly overexpose the bright core.  The short exposures are to get details of the core.  They will, of course, underexpose the outer fringes.  Your filters need to match for the two areas of the nebula to blend nicely.




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