Strange circular noise grain on image

[AstroNebulee]

Hi, whilst processing my deep sky images in ps cs4, I'm getting this strange circular grain.  I know that I haven't reduced noise or other editing processes, this is just after a couple of curves stretches and levels adjustments.  I stacked 110 lights, 15 darks , 20 flats and 20 bias for this image 30 sec exposures at 1600 iso using my sw 72ed ds pro az gti and canon 600d.  Does any other member know of this strange pattern or experienced this.  Sorry if i appear a but dim, I'm just getting used to producing a steady work reliable workflow.  Clear Skies

Edited April 17, 2021 by LeeHore7

[michael8554]

A combination of Walking Noise, and Field Rotation due to AltAz mount.

Dither.

Michael

[AstroNebulee]

Just now, michael8554 said:

A combination of Walking Noise, and Field Rotation due to AltAz mount.

Dither.

Michael

Thank you Michael, I have heard of walking noise and field rotation somewhere.  I was beginning to worry that my optics or camera were at fault.  As I say I'm new to all this, i have briefly looked into dithering but I'm just using a dslr attached to the scope and no laptop intervention where dithering can be set I believe (but as I like the quick set up of getting the mount, scope and dslr going as soon as cool down completed, sounds lame I know, but simplicity and portability are  key to me) Could I press the left or right arrow minutely on my synscan app during the delay in each shot to dither?

[michael8554]

Tiny Alt and Az pulses might work.

I would expect how to dither without guiding  has been covered in other posts.

Michael

[AstroNebulee]

Thank you again Michael, I shall have a look now I know what the issue with the noise is.

 

Clear skies

 

[Delboy_Hog]

Hey LeeHore7, that's a nice looking image you've got there!

Yes, as Michael says, I think it's walking noise - on an equatorial mount that doesn't have the field rotation, it appears in straight lines (I encountered this a while back, see: I scratched my Soul (nebula) - but don't know how? - Imaging - Image Processing, Help and Techniques - Stargazers Lounge

Dithering has indeed helped, and having also tried this on my Alt-Az mount where I don't guide or dither, I have tried moving the mount a tiny bit between frames.  Time consuming (can take a few seconds for the mount to settle afterwards), but definitely worth it.  Doesn't need to be a big movement, and you can get away with doing it once every few frames if you're taking lots of them (e.g. one dither every 3-5 frames). 

Be prepared for having to do a fair crop of corners / edges afterwards, to account for field rotation and moving the mount to dither, especially if you're taking lots of exposures over a long period of time, but otherwise it should work fine!

On a separate note (I also have a 600d), you might not need the dark frames for 30s exposures if you're dithering, especially if you're stacking 100+ frames as you did here.  While the 600d is a bit of a noisy beast if you start pushing it to 2+ minutes (especially on warmer nights!), I haven't noticed much of a difference with or without darks on shorter exposures - though I should stress I am no expert, so others may disagree!  Maybe have a look at the master-dark file and see how yours looks?

[AstroNebulee]

35 minutes ago, Delboy_Hog said:

Hey LeeHore7, that's a nice looking image you've got there!

Yes, as Michael says, I think it's walking noise - on an equatorial mount that doesn't have the field rotation, it appears in straight lines (I encountered this a while back, see: I scratched my Soul (nebula) - but don't know how? - Imaging - Image Processing, Help and Techniques - Stargazers Lounge

Dithering has indeed helped, and having also tried this on my Alt-Az mount where I don't guide or dither, I have tried moving the mount a tiny bit between frames.  Time consuming (can take a few seconds for the mount to settle afterwards), but definitely worth it.  Doesn't need to be a big movement, and you can get away with doing it once every few frames if you're taking lots of them (e.g. one dither every 3-5 frames). 

Be prepared for having to do a fair crop of corners / edges afterwards, to account for field rotation and moving the mount to dither, especially if you're taking lots of exposures over a long period of time, but otherwise it should work fine!

On a separate note (I also have a 600d), you might not need the dark frames for 30s exposures if you're dithering, especially if you're stacking 100+ frames as you did here.  While the 600d is a bit of a noisy beast if you start pushing it to 2+ minutes (especially on warmer nights!), I haven't noticed much of a difference with or without darks on shorter exposures - though I should stress I am no expert, so others may disagree!  Maybe have a look at the master-dark file and see how yours looks?

Hi DelBoy

Thank you for your advice.  Cropping the corners on a single small target such as M51, M101 wouldn't matter to much I expect.  It was just on the Leo triplet or Markarians chain that I'd probably lose something in cropping.  This is all the fun of trying to push how far you can take an al az mount and compete with the larger eq mounts. If i were to dither every 3-5 frames can I go in any direction as long as it moves a fraction, on the synscan app a tiny pulse should be sufficient I think.  I have tried using without darks with mine and did see some slight difference to my eye but it's all in how you see an image as we all like it a little different when processing don't we.  I'll attach what my master dark file looks like from this stack of M51 to see what you think.

 

Clear skies

 

 

[alacant]

5 hours ago, LeeHore7 said:

15 darks

Hi

600d? Lose the dark frames.

 

[AstroNebulee]

5 minutes ago, alacant said:

Hi

600d? Lose the dark frames.

 

Hi alacant 

Thank you 

So you think I don't need to take the dark frames atall for 30 sec subs. But if longer subs then take them? 

 

[alacant]

1 minute ago, LeeHore7 said:

But if longer subs then take them? 

Hi

No. Unless you switch an older DSLR, don't take them at all for any length of exposure.

[AstroNebulee]

8 minutes ago, alacant said:

Hi

No. Unless you switch an older DSLR, don't take them at all for any length of exposure.

Brilliant, that's going to save a whole load of time and shutter activations, that'll mean I can use those activations on more lights 😊

[Delboy_Hog]

On 17/04/2021 at 13:26, LeeHore7 said:

...This is all the fun of trying to push how far you can take an al az mount and compete with the larger eq mounts. If i were to dither every 3-5 frames can I go in any direction as long as it moves a fraction, on the synscan app a tiny pulse should be sufficient I think. 

Absolutely, I've had so much fun trying to image with the Skywatcher Mak 127 (which I see in your signature too!) on its alt-az mount.  Granted I'll never win any awards for the resulting images, and at 1,500mm focal length you have to be prepared to ditch 95% of your 30s frames, but it's still fun - and very rewarding too!  That said, I've hugely enjoyed having an HEQ5 and small refractor for those occasions where I need a slightly higher success rate!!

And yes, on the dither, any direction at random is good.  Although be aware that your mount might take more time to settle down when you nudge it in certain directions.  It might not of course, but where mine will settle almost immediately in two directions (apologies, I can't remember which), it will continue to move slowly in the others for 10 - 20 seconds after I've stopped pushing the button!  The dithering and settling process eats into your total imaging time, so I mean to experiment with how many frames I can get away with taking between dithers.

There's a skill in getting the size of dither right. Small is fine, I gather it only needs to be a couple of pixels.  I went too large to start off with and the stacking software rejected a few batches of images, though using the long focal length Mak there weren't many stars in the field of view to align - with the ED72 I think you'll be fine....good luck!

[grjsk]

I might have misunderstood something, but isnt the point of dithering to avoid that the same spot on the sensor lines up with the same spot on the object on all subframes? If that is correct, field rotation should take care of that on an alt-az mount?

[AstroNebulee]

Hi thank you, az you say I've had great fun trying to image with my Skymax 127 and had quite some success with double stars (they are on this forum somewhere now) I really enjoyed that and was only taking 8 secs subs for that. I will try a little nudge of the button at stages and see how it goes but as you say the mount does take a while to settle back into its stride. I'll try it in EQ mode when my SA arrives to use the cw bar and wedge, quite a few different setups to olkat with and all the fun of a nice portable setup and imaging in minutes, clear skies 

[alacant]

23 minutes ago, grjsk said:

field rotation should take care of that

Hi

For dither -and the subsequent clipping algorithm- to be used to advantage, the field of view should only be changed between frames. Not constantly during them.

 

Edited April 18, 2021 by alacant

[grjsk]

4 minutes ago, alacant said:

Hi

For dither -and the subsequent clipping algorithm- to be useful, the field of view should be changed between frames only. Not constantly during them.

 

That makes sense, but as long as you choose objects wisely (low to the east or west) and keep each of your exposures short enough, field rotation during a frame will be small enough not to matter, but the field rotation will make sure that your first frame is significantly different than the last?

[alacant]

7 minutes ago, grjsk said:

field rotation during a frame will be small enough

Yeah, absolutely. Do it. There are good shots to be had when using altaz mounts. Just keep it simple.

Edited April 18, 2021 by alacant