Stacking images questions

[luka]

Hi everybody,

I am using a DSLR (Nikon D80) on a tripod to make some 50mm wide field images with 10s exposure which I would like to stack. Several questions:

1. When using Deepsky Stacker, the stacking of images is relatively straightforward but there are some kind of "dark trails" behind stars, as in the attached image which I have brightened to emphasize the trails. What am I doing wrong and how do I get rid of them? Darkening the image gets rid of them but it also gets rid of details.

2. Stacking RAWs instead of JPGs with Deepsky Stacker creates much darker images with less detail. The detail is not there when I adjust levels afterwards in photoshop. Deepsky Stacker also detects less stars with RAW files. I thought that RAW images should give higher quality???

3. Is Registax used only for stacking planetary photos? I could not find any tutorials for non-planetary stuff.

4. Are there any other good and free stacking programs that I should give a go?

Thanks

[Dennis]

Hi Luka

I get similar “streaks” with my cooled ST7 CCD camera when I take short exposures, less than 20 secs at F10. I figured that it was due to noise as when I take longer exposures, over say 60 secs, the artefacts disappear as the Signal to Noise Ratio is better with longer exposures.

Cheers

Dennis

[floreatfocuser]

interesting problem with the trails, i have something similar with my 350D, but lines are purely vertical.

My understanding with DSLR's s shorter exposure is better to avoid noise.

What ISO setting are you using?

[luka]

ISO 800 and sometimes 400.
I have also tried stacking with iris and then no trails are visible. Weird...

[Astroman]

do you take darks as well? leaving the lens cap on and taking exposures at the same time exposure as the lights? If it is in camera noise the Darks could eliminate the lines, wont hurt to try. RAW files are not necessarily better, the RAW files are just that, raw. No processing on them so you are working with all available data, including the noise. There is more processing to do with raw images and it takes practice to find what works and what doesn't. If you adjust the curves in PS you can get a darker background as well as bringing out star detail. Try the dark frames, if that doesn't work, then we can rethink what is happening.

[luka]

I am already taking darks and flats and also offset files.
The posted image was brightened to emphasize the lines, I can (mostly) get rid of the lines by making the image darker but then the stars go away as well :(

I will try longer exposure to see if better signal to noise will fix things... once the clouds go away...

[JohnH]

I have seens these diagonal streaks in some of my images too, they seem to be most common in stacked frames with poor signal to noise ratios. The cure is longer exposures/more exposures.

Darks do not get rid of these streaks, this is due to the dark subtraction routine not compensating for all hot pixels...(ie not those below a cut off value). These uncorrected pixels then appear to move in a stacked image as tracking is never perfect (non-existant in your case). I think compressed images will have worse impacts than RAW but the best answer is to improve signal to noise...

[Terry B]

Try Iris for a free imaging reduction programme that stacks deep sky images very well.
http://www.astrosurf.com/buil/us/iris/iris.htm