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The Jackal

Light polluted ISO800 or ISO1600

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Hi Guys.

I would just like some advice on which ISO setting to use.

I shot these photos in heavy light polluted sky's and at the time this photo was shot 3 days ago it was 4 days from full moon, 60% humidity and 23deg at night. The moon was behind a building so it did give me some visibility.

I exposed both images 90seconds long as I felt longer exposure would be to bright so instead I tried to different ISO settings. The lighter image was shot in ISO1600 and the darker one ISO800. My question is with all the elements against me which would have been a better choice for stacking?

My final question is when stacking the image in Pixinsight it came out very green. I didn't shoot any FLAT FRAMES but did have darks and bias frames. Is this green normal or did I do something wrong?

Image details in Pixinsight: 21 Light frames ISO800 (90seconds), 15 Dark Frames (90seconds), 50 Bias Frames (Fastest Camera Speed).

I have also loaded my Final image after calibration.

post-39914-0-61755500-1448519412_thumb.j

post-39914-0-30491900-1448519479_thumb.j

post-39914-0-24703800-1448520508_thumb.j

post-39914-0-94933500-1448520623_thumb.j

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I'm a newbie at astrophotography but not photography in general. On both images it looks like the ISO is too high. As higher ISOs boost sensitivity you are also boosting unwanted light.

I would use the lowest ISO you have on your camera but maybe take longer exposures if you can.

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The ISO does'nt really matter but where on the histogram you expose to does.

Edited by wxsatuser

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If you have a residual green cast on your images, then Pixinsight has the very useful SCNR tool. Set the colour you want to bring back into line and then apply it to your image. :)

I'm a newbie at astrophotography but not photography in general. On both images it looks like the ISO is too high. As higher ISOs boost sensitivity you are also boosting unwanted light.

I would use the lowest ISO you have on your camera but maybe take longer exposures if you can.

There are more issue in AP to consider. One of the key ones is tracking the sky as the earth rotates. Longer exposures at lower ISOs do work, but then the tracking needs to maintain accuracy for longer. This generally involves more money. Much more.

As wxsatuser points out, it is where the histogram ends up that matters.

800-1600 ISO is a good starting point for a DSLR used for AP as it can produce very satisfying results without resorting to throwing silly money at the night sky.

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As Mike says. I tend to choose my ISO setting to complement my guiding or lack of it! For example if I feel I need 5 minutes to capture faint nebulosity at ISO 800 (my default) I may try 2.5 minutes at ISO 1600. To be honest I get very similar final stacked results anywhere between ISO 400 and 1600. Shooting in moonlight is not wholly recommended unless you go narrowband. That said my Flickr shows the veil shot in full moon conditions so it can be defeated by processing to some extent.

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Perhaps I'm just slow but I'm having difficulty working out how the four images shown relate to the text of the post. A caption next to each image would help, but maybe that's just me.

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The best way to remove the colour cast is to do some DBE then background neutralisation and then colour calibration after doing that the colour cast should be removed

Also if you unlink the channels on the stf tool then it will show you what the image looks like without the colour cast - note it doesn't change the image and you will still have to colour balance the image

Sent from my SM-G900F using Xparent Skyblue Tapatalk 2

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I must admit I found that a bit hard to tell as well. Anyway, thanks for the pointers, I suppose it does depend on what equipment you have and the means to ensure good tracking.

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ISO doesn't matter, as long as you don't saturate any of the pixels in the sensor.

ISO is effectively the amplification factor. The number of captured photons isn't affected by it.

Software will sort it out.

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Sorry for not labeling the pictures but they go like this.

The 1st image (bright green one) is when I stacked the images in Pixinsight.

The 2nd image with little red in the Galaxy is the final image after Processing in Pixinsight.

The 3rd image (darker one of the bottom or last two) is ISO 800.

The 4th image (brightest one) is ISO 1600

@Dave I do have a LPF in my camera.

@Mike or Pompey. Would you mind explaining the bit about the histogram? I dont know how to check this really with my DSLR or where it should be after one image.

@Pompey. SCNR is always part of my process in Pixinsight. The very green image just looked funny. As you can see in my final image it was removed.

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Just on another topic regarding the tracking. I have an Advanced VX mount and on this night my tracking was quite good I think. I would have been able to take much longer exposure but thought ISO800 should be the minimum. Next time I will try ISO 400 and instead of 90seconds I will try and expose 240seconds or more.

Here is the tracking results on PHD

post-39914-0-30448800-1448548062_thumb.j

Edited by The Jackal

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Hi

I am not an astrophotographer but am a keen photographer. 

Overall exposure is determined by three things

ISO                     how sensitive the sensor is to the light

aperture              how much light is allowed in

shutter speed      how long the shutter is open for

So if you decrease your ISO from 800 to 400 (1 stop)  then for example double your exposure from 90 seconds to 180 seconds (1 stop) you will get the same result.

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I have the same problem where I live (near Liverpool) even with a 30 second exposure the sky becomes washed out.

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Hi

I am not an astrophotographer but am a keen photographer.

So if you decrease your ISO from 800 to 400 (1 stop)  then for example double your exposure from 90 seconds to 180 seconds (1 stop) you will get the same result.

For astrophotography this is not the same, what matters most is total exposure time.

In your example the ISO 400 has a total time of 180 secs where as the ISO 800 has just 90secs.

The ISO 400 will have a higher SNR than the the ISO 800, to make them the same we need to

average 2 x 90secs at ISO 800.

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To get the correct exposure for astro with a dslr use the camera histogram.

Whatever other settings you use expose so the peak of the histogram is in the region

of 20 to 40%, This sets the exposure for your sky brightness.

Here is a 10minute exposure with the histogram peaak near 25%, this is what I personally aim for,

adjust for your camera as needed.

info.jpg

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Thank you Mike it explains it much better for me.

Here is to two different ISO's with Histogram. Is it normal for the Blue to be spread so far wide on the ISO 1600 image?

And also am I correct in saying the ISO 1600 image might have been the better image for the night is the histogram shows it around 3/1 or 4/1 from the left. The ISO 800 photo was little underexposed perhaps?

post-39914-0-71465000-1448606982_thumb.j

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Thank you Mike it explains it much better for me.

Here is to two different ISO's with Histogram. Is it normal for the Blue to be spread so far wide on the ISO 1600 image?

And also am I correct in saying the ISO 1600 image might have been the better image for the night is the histogram shows it around 3/1 or 4/1 from the left. The ISO 800 photo was little underexposed perhaps?

That's because the sky is blue. Whether it's scattered Sunlight, moonlight, starlight or light pollution, that's what we're stuck with.

Both of those subs look workable to me. Mike might know better. :)

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In theory, as long as the histogram is detached from the lefthandside, it should be ok.

Remember it's not so much the individual image that counts, it's the overall exposure.

Don't expect to increase ISO and reduce the exposure, you will not have the same SNR.

if you expose for 20minutes at ISO 800 then you need to expose the same amount at 1600.

To do this and keep the histogram the same, you need to break the exposure into equal subs.

These should equal the total exposure, in this case at ISO1600 2 subs of 10minutes.

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Thanks Pompey. I still think I should have shot my frames in ISO 1600 instead of ISO800 as the ISO1600 histogram shows the scaling closer to 1/3 on the image.

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For astrophotography this is not the same, what matters most is total exposure time.

In your example the ISO 400 has a total time of 180 secs where as the ISO 800 has just 90secs.

The ISO 400 will have a higher SNR than the the ISO 800, to make them the same we need to

average 2 x 90secs at ISO 800.

Yes, I was talking about a single exposure.

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I still think I should have shot my frames in ISO 1600 instead of ISO800 as the ISO1600 histogram shows the scaling closer to 1/3 on the image.

What people don't realise is that this 1/3 back-of-camera histogram statement is ISO dependent. If 1/3 is correct at ISO1600 then, as ISO does NOT change the sensitivity of you camera,  1/6 will be correct at ISO800 - basically you have to take into account the width of the histogram as well as its position.

NigelM

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What people don't realise is that this 1/3 back-of-camera histogram statement is ISO dependent. If 1/3 is correct at ISO1600 then, as ISO does NOT change the sensitivity of you camera,  1/6 will be correct at ISO800 - basically you have to take into account the width of the histogram as well as its position.

NigelM

You now confusing me a whole lot. So you saying both my images are fine as the ISO 1600 image is around 1/3 from the left and the ISO800 image around 1/6th

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