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Quick Crescent Neb from under a white street lamp


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Part 1

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As many of you know, I always like to show the "behind the scenes" with all of my DSO images. I'm not particularly a fan of showing just the resulting DSO image and then saying "that is it".

Anyway, I had another short session on NGC6888 back on 26-June. OK it is a bit grainy, but what some of you might find interesting is the local conditions that I face in my back yard. I hope this will inspire more people like "the rest of us" with urban skies and poorly designed street lights, to try and do something similar.

First I'll show the result of one night:

-20x3 min subs (for the colour)

-12nm H-Alpha for the HA data ( 4x10 min subs )

-QHY8 one shot colour for the colour data and H-Alpha

-IDAS filter (for the broadband data)

-Skywatcher 190MN scope on EQ6 with EQMOD

I thought it was OK for a 2-hour session, obviously would benefit from lots more data and the absence of street lights !

I'd like to add some OIII data later.

Anyway here's a little on how it was done (posts below).

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Edited by PortableAstronomer
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Part 2

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Imaging environment:

My backyard is about 2km from Amsterdam Schiphol airport.

We have the A4 motorway and lots of tower blocks around.

The worst offenders are

- the street lamp next to my house. It's a compact-fluourescent type.

The emissions from the street lamp make the IDAS filter quite ineffective.

It also spills almost into the OIII wavelength. So much so that it penetrates an Astronomik OIII filter, but recently I purchased the Baader ultra-narrow passband OIII CCD filter, and that actually does the job of filtering out the street light, at the expense of cutting out a lot of fainter 495.9nm line.

- the greenhouses about 5km away that illuminate the low southern view with a tungsten light that is not possible to filter out. Fortunately this doesn't affect objects that are high in the sky.

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Edited by PortableAstronomer
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Part 3

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The sky above.

This is how the sky looked, approximately as the eye sees it.

The atmospheric conditions seem better this evening than normal.

I could see some of the stars in Lyra, probably down to about mag +4, but that is about it.

On a cloudy night, the sky is very bright, like the 3rd image here of the street.

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Edited by PortableAstronomer
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Part 4

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The main stack of colour images (see below) comes with some very noticeable gradients, mostly from the LP.

I used PixInsight to remove the gradients using several iterations of

- Dynamic Background Extraction tool.

- Histogram transformation

I used PixelMath to subtract only about 70% of the background each time, before re-stretching.

I find this method works well to protect the very faint signal of the nebula.

Below you can see the result of the background extraction (followed by re-stretch in Histogram)

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Edited by PortableAstronomer
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Part 5

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The resulting colour image appears too saturated with background stars.

Obviously this is in part of the Milky Way so you should expect lots of background stars, but to put more emphasis on the Crescent Nebula, I used a Morphological Transformation in PixInsight to tighten the stars and make the background less 'busy'.

I then merged this with about 25 mins of H-Alpha data, using the Max function in the Red channel. There are much better ways to merge H-Alpha and RGB but this was just a quick result.

Hope you enjoyed reading!

Clear skies

Chris

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Edited by PortableAstronomer
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The result, although quite grainy, is not bad considering I had 2-hours of imaging time with a one-shot-colour camera, under a very offending white street lamp.

The other thing you don't see very often is the combination of nebulosity detail, and keeping some good star colours in the background. I think this result is a reasonable balance between showing some nice colourful star backgrounds ( I used a chromiance boost in PixInsight to do this ), while being blended the nebula itself.

I'd like to try using Atik 314L , but to get the wide field I would have to use the smaller 80mm scope, and then lose out on aperture and have a slower f/ratio.

The other thing about the result here is that it capitalises well on the ridiculously low noise that you get from the Sony Super-HAD CCDs ( requiring no dark frames ). This session required some very aggressive stretching, so I would hesitate to use a noisier chip, but would still be interested to see how a KAF-8300 chip would (or not) cope.

Well, a medium-sized mono chip is certainly something for another day!

Edited by PortableAstronomer
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That's an excellent result and very well presented.

I have one of those 'white light' street lamps right outside our front door. Providing you can shield the light and the air is not moist, they do make a difference. Otherwise they are a pain in the butt.

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Hi Chris,

I admire your effort and hard work producing an image - and a very nice result it is, too!

I used to have a white street light entirely filling my back garden with light - I complained to the site developers about it and eventually they fitted a light shield, improving matters vastly. Have you complained to your council? I believe they should be obligated to at least reply to you.

PS: Failing that, you could fit a fence extension to block it out.

Edited by Shibby
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What a fantastic thread this is, thanks for taking the time and effort of explaining everything. It just goes to show what results can be gained with some hard work and know how.

Thanks for sharing and well done!

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Super write up Chris. nice to see/read about the circumstances in which others capture their images.

And certainly an inspiration to those of that have the 'street lamp' plague.

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Very enjoyable article.. the challenges you have are an inspiration to anyone with moderate skies to get out and go for it :)

Am a big fan of pixinsight myself and will try the math..

Great stuff...

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Hats off to you Chris.

Lovely detail in that report, and quite amazing what you can get from such a light polluted place.

My street light gives my entire house and garden a sickly orange glow and I will be talking to the council at some stage to task for a shield if possible.

Cheers and thanks for sharing.

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Well thanks everyone for the very complementary responses!

It seems the following topics got particular interest

* Imaging DSOs from very light-polluted environment (and seeing the environment)

* Seeing the images being transformed into their final (rather modest) results ; i.e. seeing the "before" and "after" which is not very common on threads at the moment.

I did a similar format here, showing lots of 'before' and 'after', hopefully to add some inspiration

http://stargazerslounge.com/imaging-tips-tricks-techniques/93728-examples-extreme-processing-extreme-light-pollution.html

http://stargazerslounge.com/imaging-deep-sky/98940-urban-m81-m82-combo-30-mins.html

I'm only mentioning these threads because some of the feedback on here showed some particular interest in that kind of "behind the scenes" format, and newcomers may have missed them.

I also got some messages that showed interest in "behind the scenes videos".

I did record some "behind the scenes" videos, that show the actual telescopes and the capturing and the imaging environment, then show the processing, which is always nice to see how it was done...

Astro processing of M51 and M81/82 with DeepSkyStacker PixInsight LE | ChrisLoran on blip.tv

M42 processing with DeepSkyStacker, PixInsight LE, GIMP | ChrisLoran on blip.tv

Processing of Owl Nebula M97 using PixInsight LE | ChrisLoran on blip.tv

Thanks to Harry's Astro shed for the inspiration on the processing side.

From some of the PMs I've received, I will try and get out there and do some more videos. Seems to be good re-live a good night out under the night sky.

Clear skies! Chris

Edited by PortableAstronomer
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