M51 suggestions?

[Costas Soler]

Hi! Still new to Astro imaging... Here is a photograph of M51, JPEG version. It's a stack of 300ish-subs of ISO 800 and ISO 100, all of 30 second exposures. There are about 80 darks and 80 bias frames for each ISO. I've noticed that the image looks fairly grainy and the "grains" seem to run diagonally across the frame. Let me know what you think, and how I could fix or prevent any problems with it in the future. I've used deep sky stacker, photoshop c6. My camera is a Nikon d3200 on an 8" Orion Newtonian Astrograph. 

Thanks!

[wimvb]

Nice catch!

Why did you shoot at different ISO settings?

If you do different settings, you should calibrate each setting separately. For this one, I would only use the ISO 800 images, since you seem to have plenty of those.

Try increasing the exposure time for each sub (image). The longer each individual image is exposed, the more detail it will have. Generally 50 images of 2 minutes each are better than 200 images of 30 seconds, even if the total time on target is the same.

The streaks are due to pixels that were not rejected in the calibration process. They should have the same direction as the (RA) tracking of your mount. The best way to get rid of them is to dither (move the camera a few pixels between each exposure). You can reduce the effect by using cosmetic correction or hot pixel rejection in your stacking software.

Good luck,

[D4N]

Since your target is quite small on your chip I'd suggest using the custom rectangle in DSS to select just the galaxy the enable 2x drizzle in the stacking menu.  This should help clear out stacking artefacts.

[PeterCPC]

Have you tried adjusting the image in histogram and curves? Don't clip the black when you do.

Peter

[Stub Mandrel]

I've had worse effects on some images. Although hot pixels are a plausible explanation, I only get it rarely and it never follows the tracking direction. In your image the streaking is at 45 degrees to the ecliptic (RA tracking direction) so your polar alignment would need to be about 1/4 of the sky out to move hot pixels in that direction.

I suspect it's actually caused by light-pollution on very thin cloud when using relatively short subs.

I stand to be corrected!

[wimvb]

4 hours ago, Stub Mandrel said:

I've had worse effects on some images. Although hot pixels are a plausible explanation, I only get it rarely and it never follows the tracking direction. In your image the streaking is at 45 degrees to the ecliptic (RA tracking direction) so your polar alignment would need to be about 1/4 of the sky out to move hot pixels in that direction.

I suspect it's actually caused by light-pollution on very thin cloud when using relatively short subs.

I stand to be corrected!

I'm not convinced that it is anything optical or sky related. My images suffered from these streaks until I started dithering, and I haven't seen it since. If it were something in the sky, dithering shouldn't have had an effect on it.

Have a look at this image. It's a crop of a stretched, but otherwise unprocessed stack. Tracking was poor and streaks are in the direction of tracking.

There's also an example in the PixInsight tutorials:

http://pixinsight.com.ar/en/info/processing-examples/19/crux-southern-cross.html

Have a look at the second and third image in this tutorial.

[Stub Mandrel]

I don't argue that this effect can't be caused by tracking - I'm sure it can be. It's just that often it clearly isn't lined up with the tracking direction or any star-egginess so there must be other causes and I think faint drifting cloud would have a similar effect. The two components of M51 are pretty much aligned N-S so I can't see how a tracking error would give 45-degree streaks in the image above.

[wimvb]

 

3 hours ago, Stub Mandrel said:

I don't argue that this effect can't be caused by tracking - I'm sure it can be. It's just that often it clearly isn't lined up with the tracking direction or any star-egginess so there must be other causes and I think faint drifting cloud would have a similar effect. The two components of M51 are pretty much aligned N-S so I can't see how a tracking error would give 45-degree streaks in the image above.

Agreed, that would be strange. I have never measured or plate solved an image to confirm that they were in the direction of RA, and my exposures are too short for it to be caused by drift in DEC.

Anyway, the most important is that the effect can be reduced/eliminated by careful  processing.

[Astroboy239]

I think that's because of field curvature. Nice first image of m51 tho.