Horsehead Nebula

universe24 · Post by **universe24** » Thu Dec 06, 2007 4:50 am

An encouraging image. Maybe need more frame to over come the noise level...and dark frame substraction?

wucheeyiun · Post by **wucheeyiun** » Thu Dec 06, 2007 8:13 am

I think so too...
btw, the horse head looks different for this, looks like it is eating something.
Actually, ir should all look the same. Could it be that there is some changes taking place up there?
Experts please comment..

check this pic and compare

http://images.google.com/imgres?imgurl= ... image&cd=2

wucheeyiun · Post by **wucheeyiun** » Thu Dec 06, 2007 8:19 am

I mean there looks like an increased in dark matter....

kingkong · Post by **kingkong** » Thu Dec 06, 2007 10:56 am

universe24 wrote:An encouraging image. Maybe need more frame to over come the noise level...and dark frame substraction?

generally how many frames do you take?

i shot this using iso1600, so it's rather noisy, even though it's an eos...

i think i'll improve alignment on my mount again, i'm noticing some drift... the last drift alignment was more than 6 months ago...

after that hopefully, i can shoot at lower iso and longer exposures.

thanks for all the inputs.

Meng Lee · Post by **Meng Lee** » Thu Dec 06, 2007 11:09 am

As mentioned in my primer, do not shoot at ISO 1600, its not just about noise, there will be artifacts also. Make sure shoot in RAW mode. Take 50 frames as a guideline, but in general, themore the better, no limit. Don't overexpose also, shoot at optimum exposure length.

Take at ISO 400 or 800, at optimum exposure length, RAW mode, 50 frames, and take about 20-30 dark frames.

Tachyon · Post by **Tachyon** » Thu Dec 06, 2007 11:12 am

Nice color!

universe24 · Post by **universe24** » Thu Dec 06, 2007 11:47 am

For both image case, image calibration should be need. How many frame need for stacking is depend on the level different between noise and signal. Considered the background noise, maybe a couple 100 frame need...just estimate...might not need too much.

mch3898 · Post by **mch3898** » Thu Dec 06, 2007 2:36 pm

Hi Meng Lee,

You mentioned "take at optimum exposure length". How do you estimate what is the correct exposure length? I am using the EOS400D. I tend to get too much skyglow. Is this due to the street lights?

Meng Lee · Post by **Meng Lee** » Thu Dec 06, 2007 2:55 pm

As mentioned in the primer, to determine the optimum exposure length, you take in large JPEG mode first, look at the histogram and if the 3 peaks, Red, Green, Blue lie 30-50% of the range of the histogram. That is the optimum exposure length.

If skyglow is severe, like my case, 30 sec ISO 800, the peaks are already too far right, then I will use a Light Pollution filter to be able to take longer exposures. If optimum exposure length is too short, you will need hundreds of frames to stack, which is a disaster to the computer. Thats why the filter helps to lengthen the optimum exposure length. But normal LPR filters will shift the colour balance to a problematic level, thats why the IDAS LPS filter is highly recommended because its colour balance is quite good.

aquillae · Post by **aquillae** » Sat Dec 08, 2007 7:40 am

nice HH shot, MengLee. It's really nice to see you guys still able to take nice pictures during these cloudy months

.

btw, I was bored (haven't done any astro-related stuffs lately), so I downloaded Kong's "unprocessed" jpeg and played around with it (I hope you don't mind).

I used Photoshop. I made a fake flat field by applying max gaussian blur twice and substract it from the original image. I tweaked "Curve", "Level" and "Color balance". I used NoiseNinja to reduce the noise and soften the image.

I boosted the red channel in purpose to show more nebulosity, but I think it made the pic's background looks a bit reddish though.