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Recommended DSLR exposure starting point?

DSLR Astrophotography Beginner
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#1 lphilpot

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Posted 24 July 2021 - 10:24 AM

A simple question and maybe the answer is trial and error, but...

 

I want to take tracked wide field / Milky Way DSLR images, but when looking for a suggested exposure starting point, all I find are recommendations for avoiding star trails on static mounts. It's currently hot, hazy, cloudy and / or rainy (i.e, very poor transparency at best) so it's academic for now -- maybe until fall -- just trying to get my ducks in a row.

 

Relevant:

  • Star Adventurer 2i
  • Canon T8i / 850D, 24mp
  • Sigma 17-70, 17mm @ f/2.8; I'll close down a stop and / or rack in to ~22mm if necessary, but not unless needed
  • Typical night time temps are upper 70s / lower 80s F right now
  • I plan on eventually stacking, flats, bias, etc. I realize there'll be lots of mistakes along the way, but that's the direction.

I took some static test shots from my Bortle ~6 patio back in January just to see noise vs. ISO, but that's a lot different. ISO 1200 - 1600 looked not too bad on 15 second exposures, IIRC, but they weren't stacked. Where I'll be shooting might be Bortle 4 to the ENE, brighter elsewhere.

 

The T8i is relatively clean (but it's hot out) so I have no idea whether to start at ISO 800, 1200, whether to expose for 10, 30, 120, 300 seconds, etc. I know there'll be trial and error, but I probably won't be able to tell much at all until the images are stacked (i.e., not in the field). I'd like to avoid being initially so far off I can't tell what's happening.

 

Taking trailing out of consideration, does anyone have a rough starting point?

 

Thanks.



#2 james7ca

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Posted 24 July 2021 - 10:35 AM

The general recommendation for exposing with a DSLR is to place the peak of the histogram (as displayed by the camera) at about the one third point between black and the fully saturated white. So, if black (underexposed) is shown on the left of the histogram you want the peak to show about one third toward the right (toward fully saturated or overexposed). As for ISO, I'd try 800. That peak in the histogram represents the sky background and any stars, the Milky Way, and any other nebulae will be above that reference.

 

This means that your base exposure depends upon how bright your sky is from light pollution, etc. However, if you have really dark skies you may find that your exposure time is limited by the tracking accuracy of your mount, not by your histogram level.


Edited by james7ca, 24 July 2021 - 10:38 AM.

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#3 B 26354

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Posted 24 July 2021 - 10:36 AM

Trial-and-error are what will ultimately solidify the matter... but I'd start with 30 seconds.  grin.gif


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#4 JCDAstro

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Posted 24 July 2021 - 10:44 AM

+1 for using the histogram.  Its the easiest advice I have been given for "in the field" measurements to make sure that you have data that you can at least work with.  


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#5 lphilpot

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Posted 24 July 2021 - 10:54 AM

The general recommendation for exposing with a DSLR is to place the peak of the histogram (as displayed by the camera) at about the one third point between black and the fully saturated white.

That makes sense. Looks like some sample shots are in order. Even static shots will point me in the right direction, although I suspect they might tend to be a little more left on the histogram since they're smeared and less concentrated. Probably a very small amount, though.

 

I'm not new to DSLRs, etc. and use the histogram every time I shoot in daylight. But the advice for astrophotography is appreciated. Just looking for a little traction.  :)

 

Thanks.


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