I scratched my Soul (nebula) - but don't know how?

[Delboy_Hog]

Hello folks,

I wonder if anyone's seen anything like this before?  Focus not perfect though stars seem reasonably well aligned, but background looks like I clean my equipment with a brillo pad (I promise I haven't!).  This is only my second attempt at stacking Ha data in DSS, so I'm sure it's user error - though the other image (which was taken later) didn't show any sign of this, which confuses me, but reassures me it's not the kit.

About 2 hours of 5 minute subs, , ED80, modified Canon 600d and Astronomik 12nm Ha filter, no calibration frames.  Straight out of DSS - didn't seem to be much point trying to process it!

Looking back over the frames, there appears to have been a gradual shift of the object across the frame over the two hours, but nothing that caused any star-trailing in the individual subs, which makes me wonder if I've just got a stacking or alignment setting wrong in DSS?  Or is the lack of calibration frames the issue here - is this the dreaded "walking noise" that I've heard of?  Though I don't appear to see anything like this when just stacking light frames of other objects?

I'd be super-grateful for any thoughts / help with diagnosis!

Derek

[almcl]

It does have the appearance of walking noise and your comment about the gradual object shift tends to support that.

Are you able to dither between frames?  If so, you may find that helps particularly if you then stack with sigma clip in DSS (needs at least 20 subs and more is better).  There are software solutions as well but it would be better to eliminate at capture time, if possible.

Another question to ask is how tightly bolted down is your guide scope/imaging scope combination?  I found the rather floppy focuser on my ST80 was very good at generating this sort of gradual drift.

[Delboy_Hog]

On 30/08/2020 at 20:57, almcl said:

It does have the appearance of walking noise and your comment about the gradual object shift tends to support that.

Are you able to dither between frames?  If so, you may find that helps particularly if you then stack with sigma clip in DSS (needs at least 20 subs and more is better).  There are software solutions as well but it would be better to eliminate at capture time, if possible.

Another question to ask is how tightly bolted down is your guide scope/imaging scope combination?  I found the rather floppy focuser on my ST80 was very good at generating this sort of gradual drift.

Thanks Almcl, that's really helpful.  I've dabbled with dithering before, as an alternative to doing dark frames, so will try it again.  I hadn't noticed this kind of effect before when I didn't dither though, so I'm scratching my head as to why it's so apparent this time around.  I'm guessing that's because of the gentle creep across the frame - as you say, I'll check everything is bolted down properly, as something must either have been slowly slipping or possibly an unbalanced mount pulling gradually in one direction...

I really need to get a better understanding of the possibilities in DSS too, as the sigma clipping sounds very handy!

Thanks again, very helpful and thought-provoking information!

Derek

[johneta]

I haven't seen this type of streaking before without star trail.
Definitely give dithering a go if you are guiding with PHD etc.- pretty easy.

When I started dithering and stopped using darks on my images (DSLR), everything got a lot easier.
(except for Dec Backlash, but that's another story)

Edited September 6, 2020 by johneta

[ollypenrice]

Was it much warmer than on previous imaging nights?

The stacking has been done properly because the stars are aligned. Do you see the angled lines on individual subs?

Olly

[Delboy_Hog]

23 hours ago, johneta said:

I haven't seen this type of streaking before without star trail.
Definitely give dithering a go if you are guiding with PHD etc.- pretty easy.

When I started dithering and stopped using darks on my images (DSLR), everything got a lot easier.
(except for Dec Backlash, but that's another story)

Thanks for this Johneta, yes dithering sounds like the best way forwards on this one to clean up future images significantly!  I've seen a bit of debate on the dithering vs darks topic on here - you've found that you don't need darks if you've dithered?

[Delboy_Hog]

18 hours ago, ollypenrice said:

Was it much warmer than on previous imaging nights?

The stacking has been done properly because the stars are aligned. Do you see the angled lines on individual subs?

Olly

Hi Olly, hope you're well?

It was a little warmer, I believe (I was trying to see if there was any data recorded on the individual files that would tell me the sensor temperature at the time it was taken - sure I've seen that before but can't find it now).  Maybe I need to do some testing to see how the 600d reacts to different temperatures when it comes to noise.  We've probably not got many warm nights left here this year now, but if it's going to get this ugly I might have to hibernate this camera next summer!

You can see a sample frame below, which is fairly representative of the others - looking at it now in more detail, it's not pretty!!  But I'm not sure I see the angled lines here?

For comparison, a sample frame of the Heart and Soul taken with an 85mm camera lens.  It's quite a striking difference in smoothness.  Taken with the same camera settings (5 mins, iso 1600), but as you say Olly, different nights, so potentially different conditions.  I'm wondering if there was some high level cloud that crept over while I was sneaking a hot chocolate inside?

[ollypenrice]

9 hours ago, Delboy_Hog said:

Hi Olly, hope you're well?

It was a little warmer, I believe (I was trying to see if there was any data recorded on the individual files that would tell me the sensor temperature at the time it was taken - sure I've seen that before but can't find it now).  Maybe I need to do some testing to see how the 600d reacts to different temperatures when it comes to noise.  We've probably not got many warm nights left here this year now, but if it's going to get this ugly I might have to hibernate this camera next summer!

You can see a sample frame below, which is fairly representative of the others - looking at it now in more detail, it's not pretty!!  But I'm not sure I see the angled lines here?

For comparison, a sample frame of the Heart and Soul taken with an 85mm camera lens.  It's quite a striking difference in smoothness.  Taken with the same camera settings (5 mins, iso 1600), but as you say Olly, different nights, so potentially different conditions.  I'm wondering if there was some high level cloud that crept over while I was sneaking a hot chocolate inside?

Hi Derek, I'd say that there was evidence, here, for the 'rain' effect to be a stacking artifact. Have you tried an alternative stacking software? AstroArt offer a free trial which won't save but would let you see if it did any better.

Olly

[tomato]

It does look like ‘walking noise’ to me. I had this when taking short exposures on an uncooled CMOS camera, dithering between subs via PHD fixed it.

Another alternative stacking software is Astro Pixel Processor, this has a 30 day free trial, the calibration and stacking elements are very powerful and are widely recommended.

[alacant]

On 30/08/2020 at 21:14, Delboy_Hog said:

gradual shift of the object across the frame

Hi

I'm in agreement with @almcl and   @tomato Walking noise is easy to prevent by adding a dither to your sequence in between each frame. Calibrate your light frames with bias and flat but don't use dark frames, the latter will introduce more noise. Finally, stack using a clipping algorithm

Siril does an excellent job of all stacking work and is 100% free.

HTH

 

Edited September 7, 2020 by alacant

[m.tweedy]

ASTAP is a free stacking programme. It can be used for much more than Platesolving

https://www.hnsky.org/astap.htm

[johneta]

+1 for Siril free stacking programme

[Delboy_Hog]

On 06/09/2020 at 22:27, ollypenrice said:

Hi Derek, I'd say that there was evidence, here, for the 'rain' effect to be a stacking artifact. Have you tried an alternative stacking software? AstroArt offer a free trial which won't save but would let you see if it did any better.

Olly

Thanks Olly, I'll take a peek at AstroArt over the weekend and see what I can do!  Thanks again!

Derek

[Delboy_Hog]

Thanks everyone - I'll have a tinker with some alternative stacking softwares to see what I can do, and be sure to get my dither on (along with some proper calibration frames!) next time out!  Hopefully I'll be on here with a beautifully low-noise image in the not too-distant future..... (goes to check weather forecast)….

Thanks again 🙂