Testing increasing integration times (OSC, Bortle 8)

[Lee_P]

I thought it would be interesting to see the difference increasing integration times makes using my kit and sky conditions. So, I took 20 hours of data I'd collected on the Iris Nebula and produced images of different total integration times. Then I cropped in very close, and performed some simple edits in PixInsight, just to allow for comparisons. I thought I'd post the results here in case anyone could find them interesting or of use 😃

 

 

 

 

The earliest hours make the biggest difference, perhaps unsurprisingly! I'd say that the nebula's structure is there by 10 hours, and after that adding more time helps to smooth out the noise, but we're definitely into diminishing returns. It's hard to see much difference in the final few images, but there is a slight improvement. So, I'm thinking that 20 hours is a good target time for me to aim for.

 

Info about image acquisition:

* April 2021
* Bristol, UK (Bortle 8 )
* Telescope: Askar FRA400 f/5.6 Quintuplet APO Astrograph
* Camera: ZWO ASI 2600MC-PRO
* Mount: Orion Sirius EQ-G
* Guide: William Optics 32mm; ZWO ASI 120MM Mini
* Software: PixInsight, Photoshop, Lightroom
* ASIAIR PRO, ZWO EAF

[CloudMagnet]

Thanks for putting this together, as you said looks like 10 hours onwards is the start of diminishing return. Definitely useful to see it in practice. Have you done anything similar for other types of targets like galaxies or emission nebula to see if the same integration time holds true?

Would be interesting to look at how different surface brightness could affect the "best" integration time as well.

[Lee_P]

5 minutes ago, CloudMagnet said:

Thanks for putting this together, as you said looks like 10 hours onwards is the start of diminishing return. Definitely useful to see it in practice. Have you done anything similar for other types of targets like galaxies or emission nebula to see if the same integration time holds true?

Would be interesting to look at how different surface brightness could affect the "best" integration time as well.

Well, I do happen to have 24 hours of data for M81 / M82, so perhaps I'll repeat the experiment for those! Once some nebulae are up in the sky again, I'd like to try using my L-eXtreme filter.

[CloudMagnet]

Sounds like a plan, its worth doing to find what the practical limit is. With limited clear nights spending 20+ hours as a big commitment if its not really giving much benefit.

[DaveS]

Well, S/N goes as the square root of integration time, so you are definitely into diminishing returns.

Having said which, I have several images / projects with more than 20 hours, and a couple with more than 30. Always a temptation to add just another few hours.

[SteveNickolls]

Thanks so much for posting this experiment, very helpful to see how the final result varies with overall exposure time. Bortle 8 too. Have you by the way seen the IKI Observatory data for the Iris Nebula on SGL, few of us could aspire to the times pent imaging and in far better dark conditions?

Cheers,

Steve

[Lee_P]

35 minutes ago, DaveS said:

Having said which, I have several images / projects with more than 20 hours, and a couple with more than 30. Always a temptation to add just another few hours.

I'd be interested to see a similar experiment to mine, using your 30 hours!

[Lee_P]

12 minutes ago, SteveNickolls said:

Thanks so much for posting this experiment, very helpful to see how the final result varies with overall exposure time. Bortle 8 too. Have you by the way seen the IKI Observatory data for the Iris Nebula on SGL, few of us could aspire to the times pent imaging and in far better dark conditions?

Cheers,

Steve

Good shout, I'll download that and will have a play. Might make me a bit depressed about the quality of my data though 😅

[Avocette]

Interesting experiment - thanks for making the results nicely accessible! 

Did you use any filter during image capture?

[Lee_P]

9 hours ago, Avocette said:

Interesting experiment - thanks for making the results nicely accessible! 

Did you use any filter during image capture?

Thanks, I'm glad you found it interesting. No light pollution filter used. I haven't found them to be effective from my location. I posted another experiment, on this topic, here: 

 

 

[Dazzyt66]

Another great Bortle 8 test from @Lee_P - I’m just getting into imaging (from Bortle 8 skies) so all this info is VERY useful and as I’m able to automate my sessions now 10 hours of subs seems doable (although I’d be pretty much happy with the images after 5! 😂).

Good work sir!

 

[TerryMcK]

Another very interesting experiment. Great work.

[Lee_P]

10 minutes ago, Dazzyt66 said:

Another great Bortle 8 test from @Lee_P - I’m just getting into imaging (from Bortle 8 skies) so all this info is VERY useful and as I’m able to automate my sessions now 10 hours of subs seems doable (although I’d be pretty much happy with the images after 5! 😂).

Good work sir!

 

Great! I'm tempted to make a website dedicated to OSC imaging from city skies. We're a particular niche. Could be a project for the Summer...

[Ouroboros]

A very interesting project. Thanks for publishing your data, @Lee_P.  I am surprised you can get anything worth using at Bortle 8 with a colour camera and no LP filter.   I struggle at Bortle class 5 with a DSLR, but then I suppose the advantage of your ASI cmos camera is that it is both more sensitive and uses a shorter optimum integration time than a DSLR. There’s hope for us all in your example.  

Did you do much background subtraction/colour correction to your data or is this just stretched data? 

Edited May 2, 2021 by Ouroboros

[Lee_P]

2 minutes ago, Ouroboros said:

A very interesting project. Thanks for publishing your data, @Lee_P.  I am surprised you can get anything worth using at Bortle 8 with a colour camera and no LP filter.   I struggle at Bortle class 5 with a DSLR, but then I suppose the advantage of your ASI cmos camera is that it is both more sensitive and uses a shorter optimum integration time than a DSLR. There’s hope for us all in your example.  

Did you do much background subtraction/colour correction to your data or is this just stretched data? 

Thanks! The new-generation CMOS OSC cameras are very good. And work well with filters like the L-eXtreme. I'm increasingly coming to think that conventional wisdom like "don't use OSC from a city" and "you need a LPS filter" is outdated. 

To answer your question, for each image in this experiment I followed the same few steps in PixInsight. They were mostly to get a set of images that could be displayed side-by-side for fair comparison.

* Crop
* ColorCalibration
* AutomaticBackgroundExtraction
* EZ SoftStretch
* SCNR (remove green)
* Slight CurvesTransformation

[Dazzyt66]

44 minutes ago, Lee_P said:

Great! I'm tempted to make a website dedicated to OSC imaging from city skies. We're a particular niche. Could be a project for the Summer...

I think that would be brilliant and even though I use a DSLR I’ve had to trawl through loads of info with lots of trial and error as LP/lighter skies makes such a difference on what’s possible with AP (although I’m happy to try anything your thread on LP filters saved me some time AND money! 🙏) - and I do it mainly so my wife and kids can see what’s out there from our back garden too rather than me just looking through a lens 😄

Edited May 2, 2021 by Dazzyt66

[Lee_P]

3 minutes ago, Dazzyt66 said:

I think that would be brilliant - I’ve had to trawl through loads of info with lots of trial and error as LP/lighter skies makes such a difference on what’s possible with AP (although I’m happy to try anything your thread on LP filters saved me some time AND money! 🙏) - and I do it mainly so my wife and kids can see what’s out there from our back garden too rather than me just looking through a lens 😄

Super, well feel free to suggest what you'd like to see on a website dedicated to OSC from a city, Knowing that the effort will be useful to at least one person might motivate me to do it!

[Ouroboros]

Far be it from me to pour cold water on the idea of a dedicated website, most of us in the UK image in light polluted skies so whatever is relevant to you in cities is relevant to most of the rest of us. So personally I’d rather see astronomy info and discussion shared on one site like SGL. Maybe a special sub-section could be started on Stargazers Lounge?   

[Avocette]

4 hours ago, Lee_P said:

Thanks, I'm glad you found it interesting. No light pollution filter used. I haven't found them to be effective from my location. I posted another experiment, on this topic, here: 

Another very interesting post - thanks for pointing me to it! I suppose I am relatively lucky with my light pollution then, since it is yellow streetlamps which shine on my telescope. However there are four of them, all in the S to SW direction, so for any targets which I need to image near the Meridian, we’re in LP trouble. The attached photo shows the Moon,Jupiter and Saturn a couple of days before conjunction......

[ollypenrice]

A helpful post. As your signal builds, so does the need for some differential processing to preserve the core while stretching out the dust. In PI you'd do this by masked stretching. I'd do it in Photoshop using layer masking. They key thing, though, is that it would be well to process the long set differently from the short.

Olly

[Lee_P]

15 minutes ago, ollypenrice said:

A helpful post. As your signal builds, so does the need for some differential processing to preserve the core while stretching out the dust. In PI you'd do this by masked stretching. I'd do it in Photoshop using layer masking. They key thing, though, is that it would be well to process the long set differently from the short.

Olly

Absolutely! That would make side-by-side comparisons rather difficult though. Not impossible I suppose, but too tricky for me 😅

FYI this is the final edited photo with all the data.

[Ouroboros]

Absolutely superb. I wouldn’t know this had been obtained at a severely light polluted site. 

[Dazzyt66]

4 hours ago, Lee_P said:

Super, well feel free to suggest what you'd like to see on a website dedicated to OSC from a city, Knowing that the effort will be useful to at least one person might motivate me to do it!

For me, the useful stuff was:

How long to image (‘optimal’ sub length and amounts before diminishing returns)

What to expect with various imaging combinations  - ie really faint DSOs may not be an option with LP, what LP images look like at different subs etc.

Gear lists (especially the stuff around filters (LED v Sodium pollution)

‘Easy’ step processing for simple results - for me personally this was a biggie. I suspect most starters just want to get a ‘decent’ image. Bringing out really faint nebulousness wasn’t a key thing for me.

I think positioning is key. The need to get the darkest position to hand and making the most of that probably limited view

Theres probably lots more! 😂

[Dazzyt66]

2 hours ago, Ouroboros said:

Far be it from me to pour cold water on the idea of a dedicated website, most of us in the UK image in light polluted skies so whatever is relevant to you in cities is relevant to most of the rest of us. So personally I’d rather see astronomy info and discussion shared on one site like SGL. Maybe a special sub-section could be started on Stargazers Lounge?   

I like that idea!! 😄

[ollypenrice]

1 hour ago, Lee_P said:

Absolutely! That would make side-by-side comparisons rather difficult though. Not impossible I suppose, but too tricky for me 😅

FYI this is the final edited photo with all the data.

That's great. No question.  Personally I'd have another look at the core, though. I suspect that, with a dedicated stretch carefully blended in, you could resolve the central star as a round star with bright nebulosity around it. As your stack has increased, so has the saturated area around the progenitor star, but that can be prevented in processing. It's well controlled as it is but I bet you could perfect it.

Olly