Double Cluster NGC 884 and NGC 869

Hi,
This photo was taken during the night of the 15th August, it was a clear night and my intention was to take photos of the meteor showers. Anyway, last night, I had a chance to copy all the photos taken, about 500. Each exposure was 10 seconds, ISO 400 and 55mm. I guess I forgot that I had it on full zoom. I only had about 6gigs of disk space left, so could only get 131 photos into DSS. I then got it all set up, it took almost 3 hours to deal with it. I had 2 darks that I took at the beginning of the session. DSS chose 111 photos in total, so about 18 minutes of lights.

I used the following, as best as I could for post processing: http://astrochat.co.uk​/forum/viewtopic.php?t​=13241

I hope it looks decent, I noticed that it looked a bit green, and so tried to correct it. Please do criticise it. Also, need to tell you that I live in a place with some light pollution. I also don't know why it looks a bit out of focus.

Thanks.

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Hi there,

several things I'd mention:

First, judging by the looks of the stars, the focus is a bit off - but that's easily correctable. Take several shots before and after the infinity mark on your lens and pick the best one. Do keep in mind that variations in temperature will shift the focus, so if you have long sessions you'll need to check it quite frequently. The other thing, your stars are not dots, but rather ellongated. Two things can contribute to this - either a tracking error (missaligned mount) or lens distorsion known as coma. I notice that the stars in the lower left of the image appear more circular than those on the upper right, so i assume this is a crop of the upper right corner, given the difference in the shape of the stars.

Second, you have a lot of noise. This you can correct by lowering the temperature of your camera (difficult), by incerasing the ISO and thus decreasing the exposure time (but higher ISO also means noise) or by stacking hundreds and hundreds in order to increase the signal to noise ratio.

Colours are present, but you have a blueish/green dominant tint. The blue comes from the stars not in focus, the green seems to be noise. Some software (like IRIS) needs to have the green channel adjusted, as there are twice as many green piyels as there are red & blue. Again, easy to correct. the easiest is to find a refernce image on the internet of the particular field you're photographing and then twitch the colours to match that.

Speaking of colours, on a 350D I suggest you don't go over ISO 200 if you're looking to achieve aestheticallly pleasing photographs. Noise and above all a very narrow dynamic range will give you very little to work with: hard to get differences in colour, burnt highlights and black shadows. If you're interested in identifying objects rather than make nice pictures, push it to 1600 - up to you to decide which you need.

I've used and abused the 350D for years, it's an indestructible beast.

Enjoy

Thanks for the comment and advice. I did notice on some astrophotos that I took in the past when uploading to Flickr, the astrometry goes a bit out on the stars near the edges. I guess that is the coma in the kit lens then. I updated my signature to say that I only have the kit lens.

I guess, this means I have to try and keep the interesting sections closer to the centre of the lens. I have a feeling that the dominant blue-green tint is due to me attempting to deal with the colour processing, and failing badly on that. When I get home, I will upload the original converted to jpg file; I cannot upload the tiff file due to file sizes and all that.

If I use the ten second exposures, would stacking hundreds increase the amount of information that I get? Or, would it only just reduce the amount of noise I have?

I am practising all that I can at home from the window so then when I do finally get a chance to go out to the mountains for no light pollution, I can actually put what I practised into practice, and have a less wasted effort.

Once again, thank you for the advice.

Hi there,

A given exposure - say 10s - will record a finite amount of photons, say 40% of the total nebulosity present in the sky. Stacking them allows you to exploit to the maximum the information you have, by eliminating noise. One 10s photo has the same amount of info as 200 10s photos, only that in the latter you are able to exploit more.

You are right about practising from the window - shoot as much as you can, regardless of bad conditions. And as a rule, try to frame the interesting things in the middle, it's always the part of the picture with the best quality (if you don't buy the uber-expensive field flattners and so on).

the green tint is always a bare with a stock camera and no filters. You can hide it with layers in PS but still hard to PP out without messing up the color balance too much. Given that this was 10days ago, the moon wasn't up high yet so either you have allot of light pollution causing the graining effect or you need more darks. I'd lean on the 2nd one, my general rule is 30% dark to light ratio. You can always add more darks another night, but these are temp based so make sure its about the same temps it was when you imaged.

Keep it up and more you "practice" the more you learn. 75% of the battle is in post process. Also if you're using the Kit 18-55 i find that F6.3 makes the field pretty darn flat and can use the entire frame.

ECCE_LEX.... very good information. Thank you.

Anytime ejicon, anytime.

I am probably the stupidest man alive, I've lost my 7D with its 85mm 1.4. Dead drunk, left it in a cab. I seriously deserve a public whipping.

Saving up for another - lost it the same day my uber cool go-to motor drives arrived for my mount.