How Long Should I Expose?

[JohnSadlerAstro]

Hi,

In a period of enforced inactivity I'm doing a bit of research into getting good images, and I'm just wondering at what point should end my subs' expo length. It seems that localized glow from individual streetlights is more of an issue to me than the overall sky glow, but this can be resolved by observing after midnight when the lights in question are off. But I'm still not sure as to what properly exposed subs' histograms should look like. Should I expose until the "hump" of the RGB histogram has just moved away from the left hand edge? Or should I allow it to move a long way to the right? And which channel, R, G, or B, should I use to determine this?

John

[Starlight 1]

2/3rd in on the left .

[geordie85]

21 minutes ago, Starlight 1 said:

2/3rd in on the left .

I always thought it was 1/3 in from the left. 

[Starlight 1]

8 minutes ago, geordie85 said:

I always thought it was 1/3 in from the left. 

Yes you are right

[MARS1960]

I use the main histogram, not the RGB histogram and expose at 1/3 > 1/4 in from the left.

[JohnSadlerAstro]

Hi,

Thanks! I will take some test shots as soon as I can to work out the exposure lengths from that.

John

[wxsatuser]

As long as the histogram is fully detached from the lefthandside you should be ok.

I used to push mine as far as 40% on a good night, otherwise the norm was 25% for the peak.
This normally gave 5minutes at ISO 1600, lens at f/4, Canon 60Da with cls clip filter.

This is what your looking for.
See the white luminance peak is at roughly the 25% mark.
This was a 10minute exposure, must have been a real goodnight here.

[Waldemar]

Maybe this will help? : subexposurecalculator_V3.xls

[JohnSadlerAstro]

On ‎26‎/‎02‎/‎2017 at 05:56, wxsatuser said:

This was a 10minute exposure, must have been a real goodnight here.

I'D SAY!!!!  

That sub looks as good as my final processed version of M31! I guess that's the cls. Though I actually found that M31 seemed to "end" quite quick, unlike the Orion Nebula, which I get more and more of as I stack more; perhaps it's because there aren't really any faint outlying gasses of something....

John

[ollypenrice]

I would advise you to experiment. While I listen to other imagers very attentively, and take note of the theory, I don't always find that the consensus applies to my own circumstances. Those circumstances include the way we go on to process our own data. There may also be variables introduced by the nature of the target.

Working in LP may be different but for me there would be no 'one size fits all' answer to your question.

Olly

[RSM]

On 26/02/2017 at 05:56, wxsatuser said:

As long as the histogram is fully detached from the lefthandside you should be ok.

I used to push mine as far as 40% on a good night, otherwise the norm was 25% for the peak.
This normally gave 5minutes at ISO 1600, lens at f/4, Canon 60Da with cls clip filter.

This is what your looking for.
See the white luminance peak is at roughly the 25% mark.
This was a 10minute exposure, must have been a real goodnight here.

Based on the above image from wxsatuser of the canon histogram, I would say my best results have come from the histogram being between the first and second vertical lines from the left hand side (think that's 20-40%). At ISO 800 that is usually approx 2-3 min exposurevwith reasonably good conditions rising to 6-10 min with an LP clip in filter. 

Good Luck. 

[wimvb]

Not mentioned before, but also consider gain/iso.

I would recommend that you expose for the longest time your gear and local conditions allow. Then adjust ISO to keep too many stars and/or the target from becoming over exposed. If you can lower ISO, you'll gain dynamic range by decreasing noise. Up to a point.

For me, the sweet spot is ISO 400, which allows me to use exposures up to 8 minutes in my backyard, but much longer at my dark site.