The M7 Open Star Cluster in Scorpius

[iceman (Mike)]

Hi all

This image of M7 (Ptolemy's Cluster) was taken at IISAC2009, early on the Saturday night before it clouded over. The Milky Way was setting in the West, and I captured 50 minutes of data on this object before it set behind the mountain.

This image is my first real deep space (prime focus) image in some time (since January this year!!), so while it’s not in the league of Louie’s beautiful version of M7 that appeared on APOD last week (congrats Louie!), I’m reasonably happy with the image given the lack of data, no flats calibration and the guiding problems I was having at IISAC2009. It really needs to be much deeper and longer to get the golden star field of the Milky Way.

The image was captured with my normal Deep Space Imaging equipment – the unmodded Canon 350D with the ED80, riding on the EQ6 and guided with an 80mm refractor using my DMK21AU04 and PHD guiding.

Exposures were 5 minutes each, with dark frame subtraction in camera. 10 exposures @ ISO800, FL ~ 480mm, F/6.

Larger version here:
The M7 Open Cluster in Scorpius

Thanks for looking.

[bmitchell82 (Brendan)]

I don't mind it at all. very natural colours and you have hints of the nice dust lanes. welcome back to this type of imaging

[multiweb (Marc)]

Looks cool Mike. Lovely colors for such short data. I know what you mean about lack of imaging. The weather hasn't been very cooperative lately and it's a good feeling to stick some subs in regardless.

[AlexN]

Yea mate its looking the goods, but definitely needs an hour more exposure to bring out the real golden look of the milkyway and to make the dusty regions stand out more... Nevertheless, its a lovely imaging... Welcome back to deep sky imaging Mike!

[Paul Haese]

I would prefer to see a much larger image of this. The dust lands are visible on my screen but you need to get more of the star colours to gold as Alex has suggested. Processing looks really nice.

[iceman (Mike)]

Thanks guys - It's been a while but was fun to process a deep space image again.

Paul,there's a 1200px wide (600kb) version here: 1200px wide version of M7

[DavidU (Dave)]

Nice one Mike, faint dust there........the bigger version is great

[Phil Hart]

nice image.. would love to have been at IISAC.

any reason why you're doing dark frame subtraction in camera? isn't that making life harder and wasting dark sky time? you can let the camera take darks in the boot of the car once you've gone to bed and get better processing as well? you're a smart guy and you know what you're doing so maybe you had a good reason!

[iceman (Mike)]

Thanks David and Phil.

Phil I wish you could've been there too - hopefully next year?

Phil I tried dark frames a couple of times, but my darks never properly matched the lights for some reason - temperature difference? So when I subtracted them, I ended up with holes in my nebula.

So since then, I've always done them in-camera. I really love the results of it, but like you say - it's a whole lot of sky time where the camera isn't taking exposures.

Maybe I should give separate darks another try.

[Phil Hart]

Originally Posted by iceman

Thanks David and Phil.

Phil I wish you could've been there too - hopefully next year?

Phil I tried dark frames a couple of times, but my darks never properly matched the lights for some reason - temperature difference? So when I subtracted them, I ended up with holes in my nebula.

So since then, I've always done them in-camera. I really love the results of it, but like you say - it's a whole lot of sky time where the camera isn't taking exposures.

Maybe I should give separate darks another try.

Do you know the dates for IISAC next year yet? I need to plan some other committee meetings around it so I can be there!

And you should definitely investigate this darks issue further.. something is wrong if you can't take a series of darks and subtract them with a better result. try sending me some light and dark frames (eg yousendit.com) if you want me to test the processing with ImagesPlus.

you should also be able to do an extreme stretch of a master dark frame and see if it looks right or contains other artefacts before you subtract it.

For the benefit of others.. in camera darks subtract a single dark frame from the light frames. While this will remove the hot pixels, it will actually contribute more random noise to the image, making it [very slightly] worse than using no darks at all. By taking a large number of darks at a convenient time at close to the same temperature (i have never had any serious problem with only 'approximately' the same temperature), averaging them all together and then subtracting those, you should get a better result than in camera dark frame subtraction. and you won't waste half your dark sky time with dark frames - doubling your exposure time is more important than the dark frame calibration anyway!

phil

[Phil Hart]

and of course, now you could try it with your new 40D.. that should pretty quickly identify whether the problem is with your processing routine or something specific about the frames you're getting from your 350D.

[Hagar (Doug)]

Nice clean image Mike. Images such as this benifit from difraction spikes for the brighter stars in the cluster. They just seem to highlight the cluster so well. Others probably disagree but I like them.

[iceman (Mike)]

Thanks Phil - I will definitely give it another try. I've been too stubborn and didn't want to risk losing a nights imaging (since they're so rare for me these days) to try the darks again.

But I will definitely experiment again.

I'll send you an email re IISAC2010.

Thanks Doug - I agree, spikes would've helped this image especially given the lower magnification. I used to use some wire held on with blue tac but I seem to have lost it and couldn't find it at Lostock when I looked for it.