Probably simple question regarding light pollution... - Forums

Topic starter

Dec 16, 2022

Is it correct to assume that no matter how long of an exposure I take, if I am sitting in Bortle 6/7 I will never get the faint dust structures that surround the neighborhood of M42? I get hints of it but in the best images I see here it takes on it's own reddish glow (is the dust glowing in h-alpha?). I assume there just isn't enough contrast due to light pollution for my camera to pick out the structure and faint color of the dust. Am I correct or is there something else at play here. Here is my 3+ hour image...

M42 - The Orion Nebula

Thanks for the knowledge...

Benjamin

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Boyan Stiliyanov

Dec 16, 2022

I read this on wikipedia regarding the limiting magnitutde:
Telescopes at large observatories are typically located at sites selected for dark skies. They also increase the limiting magnitude by using long integration times on the detector, and by using image-processing techniques to increase the signal to noise ratio. Most 8 to 10 meter class telescopes can detect sources with a visual magnitude of about 27 using a one-hour integration time.

so if I understand correctly the more integration time you have the fainter stars you will see. Of course if we talk about nebulosity and details you also have to see your atmospheric conditions.

By the way I found this picture taken from bortle 9:
https://www.reddit.com/r/astrophotography/comments/tfrzvv/ic434_horsehead_in_shh/

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Ed Dixon

Dec 16, 2022 · Edited: Dec 16, 2022

This is a post from 8 years ago that might help:

https://www.cloudynights.com/topic/486089-m42-and-filters/

In there Starman1 says:

I can't tell you how many times I've viewed the Orion Nebula over 51 years of observing, but it's a few.
I have tried every filter on M42--notch filter, broadband, narrowband, O-III, H-B, even H-aIt depends what details you view in the nebula.
If viewing faint stars in the nebula no filter is best,
If viewing small details in the Regio Centralis, no filter is best.
If viewing the outer envelope of the "ring" a narrowband works great.
If viewing the "arches" or "bow" parts on either side of the Regio Centralis, both narrowband and O-III filters work well.
If viewing the M43 area, I find either no filter or an HB filter works really well.
Higher powers (10-20X/inch) work very well on the high surface brightness of this nebula, and that means no filter used.

This is just about the only nebula that I've noticed hurts your night vision. After viewing through a scope, my eye's sensitivity isdefinitely lower.

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andrea tasselli

Frequent contributor

Dec 16, 2022

Benjamin DeHaven:
Is it correct to assume that no matter how long of an exposure I take, if I am sitting in Bortle 6/7 I will never get the faint dust structures that surround the neighborhood of M42? I get hints of it but in the best images I see here it takes on it's own reddish glow (is the dust glowing in h-alpha?). I assume there just isn't enough contrast due to light pollution for my camera to pick out the structure and faint color of the dust. Am I correct or is there something else at play here. Here is my 3+ hour image...

M42 - The Orion Nebula

Thanks for the knowledge...

Benjamin

With the right tools you can go much deeper than that, assuming long integrations.

Torben van Hees

Dec 24, 2022

More integration needed. 9 hours will very noticeably increase the depth of the image. In that region there‘s both dust and thin hydrogen filaments. The dust is usually more difficult to catch, but if your camera is unmodified, it‘ll hold you back on the h-alpha emissions.

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JohnHen

IOTD Staff

Dec 24, 2022

Benjamin DeHaven:
Is it correct to assume that no matter how long of an exposure I take, if I am sitting in Bortle 6/7 I will never get the faint dust structures that surround the neighborhood of M42? I get hints of it but in the best images I see here it takes on it's own reddish glow (is the dust glowing in h-alpha?). I assume there just isn't enough contrast due to light pollution for my camera to pick out the structure and faint color of the dust. Am I correct or is there something else at play here. Here is my 3+ hour image...

M42 - The Orion Nebula

Thanks for the knowledge...

Benjamin

Light pollution decreases SNR (signal to noise ratio). In order to get same SNR as in less polluted skies in theory you will need more integration time. In practice there are all kind of gradients in the image such that it will be hard to reach a result that is same in quality as in less polluted skies. But it is certainly worth trying more integration time. A typical technique for light polluted skies is to shoot subs with far lesser exposure time because if sub exposure time is too large then light solution signal is so strong that the faint signal cannot be recovered.
CS, John

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dkamen

Dec 24, 2022

Benjamin DeHaven:
Is it correct to assume that no matter how long of an exposure I take, if I am sitting in Bortle 6/7 I will never get the faint dust structures that surround the neighborhood of M42? I get hints of it but in the best images I see here it takes on it's own reddish glow (is the dust glowing in h-alpha?). I assume there just isn't enough contrast due to light pollution for my camera to pick out the structure and faint color of the dust. Am I correct or is there something else at play here. Here is my 3+ hour image...

M42 - The Orion Nebula

Thanks for the knowledge...

Benjamin

Hi,

Hα tends to be a more pinkish shade of red, the stuff you are talking about is good ole' "space dirt" with small quantities of Hα, so colour is brownish-red when you stretch it.

Anyway, 3 hours at f/5.6 on a Bortle 6+ is too little for the dust, as evidenced by the noise in your image. You need at least 12 hours. You probably also want to crop the edges and do a mosaic of the central parts because your lens does not perform too well at the edges (look at star shapes) and this is very detrimental to SNR at the dusty places which happen to lie there.

Cheers,
Dimitris

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