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Exposure time for light polluted skies

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#1 Aquat0ne

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Posted 16 October 2021 - 09:18 PM

Hi all,

Interested to hear peoples thoughts on the following:

I am currently shooting NGC7293 Helix Nebula. I have made the following image based on:

https://flic.kr/p/2mBvK41

- Around 4 hours of exposure time in total
- 267 lights ( 60 secs each with a few 30's as well)
- 50 odd flats
- 50 odd darks
- 20 odd bias
- Bortle 7/8 skies
- Sony A6000 with kit 210mm lens on an AZGTI mount.
- Guided with ASI120mm

- Stacked in DSS and then processed in Siril and GIMP

Questions for conisderation:
- I am planning to add more exposure time. Probably another 2 hours. Is it worth it or am I getting to the law of deminishing return?

- If I was to add more lights should I stick with 60 seconds or try for slightly longer exposure times such as 90 or 120 seconds? Is this worth it considering it's a fairly light polluted sky.

My thinking is to stick with 60 second exposures and add another 2 hours. I worry that longer exposure will just wash out due to light pollution.

The other issue is the target will go past the roof line of my house so I probably will only have a few hours of shooting time left.

#2 vidrazor

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Posted 16 October 2021 - 10:04 PM

In Bortle 7/8, 60 seconds is good. Yes you can risk washing out shooting any longer. You need 2 more hours at least, and probably 4-5. I'm shooting an experiment in Bortle 9 right now on the Cygnus Loop, and I don't go beyond 30 seconds. I'll need at least 10-12 total hours minimum, and probably more like 15. So far I've only been able to capture 2.5 hours. What ISO are you shooting at? I take it you're at f/6.3?

 

Just keep going. More time, short subs.


Edited by vidrazor, 16 October 2021 - 10:10 PM.

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#3 Aquat0ne

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Posted 17 October 2021 - 12:39 AM



In Bortle 7/8, 60 seconds is good. Yes you can risk washing out shooting any longer. You need 2 more hours at least, and probably 4-5. I'm shooting an experiment in Bortle 9 right now on the Cygnus Loop, and I don't go beyond 30 seconds. I'll need at least 10-12 total hours minimum, and probably more like 15. So far I've only been able to capture 2.5 hours. What ISO are you shooting at? I take it you're at f/6.3?

Just keep going. More time, short subs.


Thank you. Good to know I need a lot more exposure time. How many hours have you shot on the Cygnus loop?

I am shooting at iso3200 at f/6.3. I would be shooting at 1600 but I have a Hoya Red enhancer on the lens. Perhaps I should ditch the filter.

Should've posted about this earlier. With the weather a bit dodgy here in Melbourne I suspect I will only get another few hours at the target before I loose sight of it.

#4 vidrazor

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Posted 17 October 2021 - 03:03 PM

Well you can shoot at 3200, but you'll need a lot of subs to minimize the noise. Your read noise is pretty good at 3200, as you can see in the chart below, but I think I would shoot at 1600 anyway to give you an extra stop of dynamic range. You can process the subs separately in DSS, just shoot calibration frames for each session, and put them in separate subgroups in DSS, shoot a blank image with the same camera and stick it in the main group, but don't select it. This way DSS processes each set of subs separately, then combines them into one. As I mentioned, so far I only have 2.5 hours on Cygnus, I need to wait until clear weather and no moon to grab some more.

Attached Thumbnails

  • a6000.jpg

Edited by vidrazor, 17 October 2021 - 03:04 PM.


#5 Aquat0ne

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Posted 18 October 2021 - 02:50 AM

Thank you. I will switch to 1600ISO. I was always unsure about pushing high with this camera.

With regards to the DSS processing so is it the blank imaging that separates the different batches of pictures. I assume it doesn't matter what sort of frame you add the blank pic to the file list as?

#6 vidrazor

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Posted 18 October 2021 - 09:05 AM

Thank you. I will switch to 1600ISO. I was always unsure about pushing high with this camera.
With regards to the DSS processing so is it the blank imaging that separates the different batches of pictures. I assume it doesn't matter what sort of frame you add the blank pic to the file list as?

Yeah the blank is just a place holder, you can shoot it in your room. Just make sure it's not checked when you go to stack. Then place your separate sessions into each of the subgroups, along with their calibration frames.



#7 bobharmony

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Posted 18 October 2021 - 09:12 AM

Yeah the blank is just a place holder, you can shoot it in your room. Just make sure it's not checked when you go to stack. Then place your separate sessions into each of the subgroups, along with their calibration frames.

 

Thank you. I will switch to 1600ISO. I was always unsure about pushing high with this camera.

With regards to the DSS processing so is it the blank imaging that separates the different batches of pictures. I assume it doesn't matter what sort of frame you add the blank pic to the file list as?

As an alternative you can place your bias frames in the first DSS tab and check them.  They will then be applied to all the tabs that contain each nights lights, darks, and flats.  Might as well do something useful with the tab, since DSS insists on having it there.

 

I shoot from a heavy LP site, and have found it easier to process my data if I don't use filters.  This is strictly a matter of preference, not a suggestion for you.  I find it easier to deal with the LP than to deal with the color cast I get from filters.

 

Bob


Edited by bobharmony, 18 October 2021 - 09:13 AM.


#8 vidrazor

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Posted 18 October 2021 - 09:44 AM

As an alternative you can place your bias frames in the first DSS tab and check them.  They will then be applied to all the tabs that contain each nights lights, darks, and flats.  Might as well do something useful with the tab, since DSS insists on having it there.

I shoot from a heavy LP site, and have found it easier to process my data if I don't use filters.  This is strictly a matter of preference, not a suggestion for you.  I find it easier to deal with the LP than to deal with the color cast I get from filters.

Bob

Yeah I always shoot separate calibration frames because the sensor temperature will be different on different nights (and in this case different ISOs), so I keep everything separate, but that's personal choice. It takes less than a minute to fire off a set of bias frames too. I agree LP filters are not really advantageous. I tried one and it seemed to create more post processing problems, and hurt more than helped. Lots of subs, good calibrations frames, and effective gradient removal to me works much better.
 


Edited by vidrazor, 18 October 2021 - 09:45 AM.


#9 Aquat0ne

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Posted 19 October 2021 - 04:42 AM

Yeah the blank is just a place holder, you can shoot it in your room. Just make sure it's not checked when you go to stack. Then place your separate sessions into each of the subgroups, along with their calibration frames.

As an alternative you can place your bias frames in the first DSS tab and check them. They will then be applied to all the tabs that contain each nights lights, darks, and flats. Might as well do something useful with the tab, since DSS insists on having it there.

I shoot from a heavy LP site, and have found it easier to process my data if I don't use filters. This is strictly a matter of preference, not a suggestion for you. I find it easier to deal with the LP than to deal with the color cast I get from filters.

Bob


Thank you both.

I don't always shoot calibration (darks and bias) frames if I have frames from another session that matches the temps. So it's good to know there is another option of listing files in DSS to use a single set of calibration frames.

#10 bobharmony

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Posted 19 October 2021 - 08:16 AM

Thank you both.

I don't always shoot calibration (darks and bias) frames if I have frames from another session that matches the temps. So it's good to know there is another option of listing files in DSS to use a single set of calibration frames.

Happy to share what I have learned over the years.  I totally agree with reusing darks that are temperature matched, and I use a master bias for about 6 months before shooting a new one.  Flats are more of an every session thing for me, as I tend to rotate the camera when I move to a new target to get the framing I want.

 

Bob




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