Weird Lines after Levels Adjustment

[Pinarello87]

Hi forum. 

Need some advice from the seniors and pros. I just started AP for 2 months and am enjoying the process. 

I use a Skyrover 60ED apo with FF and a optolong moon filter. Use an unguided EQ3 goto mount. Dslr is a nikon d3100. 

I was imaging Centaurus A with about iso 400, 30x 45" subs. Flats biased and darks use in my image stacking with DSS. I use Gimp to do final editing. 

I observed lots of uni directional lines if i pull up the levels high enough. Its all across my image. Im wondering is this some kind of fault within my image train or my dslr sensor? 

P/s i was cleaning the filter and flattener so i wonder is this a smudge also? 

Any ideas or experiences to ecplain? Below is a pic sample of part of the image. 

Thanks much. DC

 

 

[wimvb]

This is so called walking noise. It's caused by offsets between exposures. You can get rid of it if you dither between exposures (= move the mount slightly in a random ra/dec direction). For the subs you already have, use more aggressive pixel rejection while stacking.

[michael8554]

Dither would do, it but I see you aren't guiding, which makes random 10 to 15 pixel moves difficult.

Michael

[Pinarello87]

On 16/05/2019 at 04:36, wimvb said:

This is so called walking noise. It's caused by offsets between exposures. You can get rid of it if you dither between exposures (= move the mount slightly in a random ra/dec direction). For the subs you already have, use more aggressive pixel rejection while stacking.

 

On 16/05/2019 at 05:47, michael8554 said:

Dither would do, it but I see you aren't guiding, which makes random 10 to 15 pixel moves difficult.

Michael

Thanks guys for the advice. 

Cheers

[newbie alert]

If you do a 10 sec delay in the exposures you should get a natural dither, if not then a 20 sec delay..

[michael8554]

DC is tracking but not guiding, so no RA dither unless his tracking rate is way out .....

Michael 

[Pinarello87]

21 hours ago, newbie alert said:

If you do a 10 sec delay in the exposures you should get a natural dither, if not then a 20 sec delay..

 

8 hours ago, michael8554 said:

DC is tracking but not guiding, so no RA dither unless his tracking rate is way out .....

Michael 

So does this mean to manually dither in RA using this 10-20sec delay is pointless? If i dither manually in dec would it be different? 

Im not upgrading to guiding so soon yet. So i guess i need to do more work i guess. 

[newbie alert]

10 hours ago, michael8554 said:

DC is tracking but not guiding, so no RA dither unless his tracking rate is way out .....

Michael 

If he's not guiding which is the way I'm seeing it and just using the tracking of the mount it won't be that pixel perfect otherwise no-one would have the need to guide.. the idea of a dither is so that you don't put a green pixel on top of a green  pixel on top of a green,  and the same for red etc causing colour mottle ..if you dither it averages it out.. unless he's using some top of the range mount it's not going to be that accurate

If you look at his image the walking noise is going in the same direction as the drift in the stars

[michael8554]

1 hour ago, Pinarello87 said:

manually dither in RA

To me this means you are pressing the RA button ?

If set to a very slow Slew Speed you might be able to move the mount (in Dec too) the tiny amount required to give a 12 pixel Dither to the dslr. 

1 hour ago, Pinarello87 said:

dither in RA using this 10-20sec delay

This delay won't give you a dither for the reason I supplied. But If you are shooting untracked, then yes each exposure will be shifted due to the earth's rotation. You'd have to work out how long to pause to give the correct amount of pixel shift, and after a number of exposures field rotation would start to be a factor.

Michael 

[wimvb]

To manually dither using the hand set (hs):

Suppose you have a imaging scale of 2 "/pixel if you want to dither 15 pixels = 30 arcsecs:

- Set hs to 1 x sidereal.

- Press RA+ button for 2 seconds to dither 15 pixels in RA+.

- Same for RA- DEC+ DEC-.

- Dither in an outward spiraling pattern.

In other words, slew at 1 x sidereal speed for as many seconds as the numerical value of your imaging scale. This will give a 15 pixel dither.

I've done it this way until I automated the process; it works. Because you don't change the dither direction in DEC every time, the spiral pattern is less sensitive to backlash than a random dither.

[wimvb]

If polar alignment is anything less than perfect, your mount is rotating around another point than the true NP. This results in a drift in RA and DEC, plus field rotation. It is this drift that is used for polar drift alignment, and is the cause for walking noise. That is why walking noise follows the same direction as the offset between stars when you stack.

http://wimvberlo.blogspot.com/2016/07/dithering-in-hardware_7.html?m=1

http://wimvberlo.blogspot.com/2016/07/the-effect-of-dithering_21.html?m=1

[Pinarello87]

Really lots of tech talk here. I know its important to get good alignment etc. 

from where I am imaging on my balcony i dont have Polaris in view and much reference stars to do a really good drift alignment (being at equator 3 degrees north). I have to take exposure subs of the brightest star i can find  between 30secs to 60secs long to see where the drift goes and adjust my azimuth/altitude accordingly (this after proper leveling of mount) until i dont see any drift. Ive managed to push 90'' subs with very minimal drift noticed. 

Thats the drifting alignment i do now. So far it works enough for me and i get a rather decent image of M13 like below (you will still see the walking noise there)

 

 

So with whatever i do and learn now until i get better and better I hope to digest the technicalities as I slowly progress on. 

At least i know the root cause of the problem, understanding the solution to it, and then practice it every time I do my imaging.