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AstroAndy

Which is better, incremental shorts subs for cores on DSOs, or one short set, one long one?

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Hello all

I am wondering whether it is better to shoot, eg., 1m, 3m, 5m, 10m subs incrementally, or whether it is enough to shoot, eg. 1m, then 10m for the cores of DSOs.

I suppose in PS the short ones can be boosted (as one only needs the core element); also, DSS has a high dynamic range function, so far it worked well on the incremental subs, will it average out very short and long subs? Also, would it be ok to shoot, say, 10 short ones, and 30 long ones, or would the number of short to long have to be about even?

Any advice appreciated.

Andy

 

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I wouldn't bother with more than 2 different exposure lengths, it just makes processing more complicated, there are very few objects where this is a problem. Just take one set of exposures for the core and bright stars, and then another for the rest of the object.

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I find that it is hardly ever necessary to take short subs for cores. M42, certainly, and the Cat's Eye Nebula. Just maybe for M31 and, quite honestly, that's about it. Occasionally I'll use the RGB layer in greyscale as a 'short' for covering luminance cores, as in M51. Generally, however, just look at your linear stack first. If the core is not overexposed at the linear stage it can be stretched carefully and not allowed to saturate in the final image.

Olly

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@MikeODay has achieved very impressive results with incremental exposure times, especially on globular clusters. As usual in AP, there is no single right answer. It's about how much time and effort you are willing to put into an image. An astro image is created in two steps; data collection and post processing. One depends on the other.

Otoh, as to what is necessary, as Olly wrote, it's usually enough to image at one exposure time (which may differ for each filter in lrgb imaging).

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M81L_10m_singl_sub.thumb.png.1d865ea7633f89c8e56140d982b4f73a.png265827669_Autosave001copy.thumb.png.f07e60c7c6fbbb03715e34704bfbd4d5.png

 

21 hours ago, wimvb said:

As usual in AP, there is no single right answer. It's about how much time and effort you are willing to put into an image.

Hi Olly and Wim,

thanks for your input, maybe I'll check into using RGB as lum. cores. As it is, I seem to get overblown cores on such objects as M81 in as much (little?) as 10 min subs ( have attached a linear stack with only levels done , and one 10min. sub straight from my Atik 314L, which illustrates the point [using selective vision, you won't see the star shapes 😞]). So far, w/ incremental subs (say 10 at 1m, 10 at 3, etc.), I get good cores, either by HDR in DSS, masking it in in, or layering it in. A real extreme would be using ha lum. cores.

@ Wim, don't I know it, I've been chasing color and more lum. for this M81 since abt. 3 1/2 months now, between job, clear nights, travel to dark sites, and Moon, I'll have it in 3 more months. 😑

- Andy -

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One thing's for sure: you'd only need a handful of shorts to fix that core. The signal you're going to keep out of them is right at the top of the brightness scale and you won't be stretching to any extent so noise is minimal.

Olly

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On 14/01/2019 at 16:28, ollypenrice said:

One thing's for sure: you'd only need a handful of shorts to fix that core.

Aye Olly, a few minutes of short subs isn't going to eat into my lum. run.

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