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North America mosaic

Bob Andersson

By Bob Andersson
in Imaging - Deep Sky

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Bob Andersson

Bob Andersson

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Hi folks,

Having captured this widefield shot of the NGC7000/IC1318 region last year with a 165mm focal length Pentax MF lens I wanted to try my hand at a four panel mosaic using my TEC 140. The 980mm focal length obviously meant a much narrower field of view and operating at f/7, as compared to f/2.8 with the Pentax lens, meant much more time spent capturing the subs. Here is a thumbnail of the final result at one eighth size:

NA_Neb_Mosoac_Web_v1.jpg
Click here for the full-size version (8MB download)

Each panel required 16 x 1,000 second H-alpha subs and 5 x 200 second subs in each of red, green and blue for a total exposure time of a little over 21 hours. PixInsight was used to produce both an H-alpha and an RGB mosaic. The H-alpha image had the stars removed (in Photoshop) and was a blend of that "no stars" image together with processed versions using PixInsight's LocalHistogramEqalization and HDRMultiscaleTransform tools. The RGB image was split into "no stars" and "just stars" versions. The "no stars" image, while considerably less deep than the H-alpha image, was used to modify the colour of the brighter regions of the nebulosity while the fainter regions were coloured with my default "red plus a bit of blue". The RGB stars were then added to the colourised H-alpha data. Lots of tweaks along the way but those are the key steps used.

I think I've learned a lot while processing this image and I've come away with a huge respect for those who routinely do mosaics involving more than two panels. When I look at my own effort closely I see all sorts of defects and I rather doubt that I'll be jumping back into a similar four panel project for a while given the amount of telescope time required, time which has been particularly precious here in the UK this summer.

Bob.

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Bob Andersson

Bob Andersson

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Thanks for the nice words!

One thing I have noticed is that some browsers (Internet Explorer for one) don't cope well with the image, producing way too saturated colours. Firefox, with colour management enabled, works well as does IrfanView, and naturally enough Photoshop displays what I intended as that was used to compose the image. Strange that some browsers have such trouble as the embedded image profile is flagged as sRGB which should be the default for unmanaged browsers.

Bob.

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Shibby

Shibby

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I like this a lot! And I do like your treatment of the colour. Ha is obviously red and if our eyes were as sensitive as our cameras, this is probably just as this nebula might appear to us with its overwhelming emission nebulosity.

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Bob Andersson

Bob Andersson

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Hi Mark,

The silly thing is that it's the same image - no desaturation or any other changes. It's just that some browsers couldn't cope with the sRGB profile that Photoshop attached. I'll have to dig into this further as it may well affect other images I have (or will) produce. Firefox (v15) gave no clue that there might be a cross-browser issue as it was correctly displaying the image in just the same way as it looked when it left Photoshop!

Bob.

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Gina

Gina

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Wonderful image - truly magnificent - beautiful :) I use Firefox so don't have any problems. In fact I use Firefox on all three OS platforms I use :)

Glad you found some clear skies - been dreadful down here. Some clearish spells last night and I got a few subs but not enough to be of much use before the the clouds came back!

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Bob Andersson

Bob Andersson

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Very nice! Love the clarity and focus spot on!

Hi Paul,

Wish I could claim the credit but the focusing was handled by my MicroTouch focuser together with the bundled FocusMax Software plus my copy of Maxim DL. I have to manually focus when I use my Pentax 165mm lens rather than the 'scope and it's a right royal pain! Autofocus is so much cooler and, while expensive, probably pays for itself when one considers the time saved against the capital outlay of the rest of the imaging system! :laugh:

Bob.

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Bob Andersson

Bob Andersson

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Hi folks,

Thanks for all the kind comments. I was definitely out of love with this image shortly after I finished processing but the pain is fading now and maybe I will be willing after all to tackle more large mosaics as and when the need arises! But not for a few months as I have more modestly sized targets in mind for now...

Bob.

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