Author Topic: Strange noise pattern  (Read 8244 times)

Offline EorEquis

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Re: Strange noise pattern
« Reply #15 on: 2014 August 29 20:28:37 »
Out of curiosity then, is there a "preferred" amount?  Depends on image scale?  Image size?  Target?  All of the above?

Even the largest PHD dither of 5 pixels is not really adequate to avoid this sort of effect.  Aim for something around 15 pixels, so that's 'very high' with a scale of 3x.  Clearly that might cause more issues with guiding settling post-dither so you may need to adjust the PHD / capture application parameters to allow more time between frames.

This sort of streaked background  pattern crops up all the time on various imaging forums.  The diagnostic is the hot pixels though - if your stars are nice and round (which they are) and you have little dotted trails of hot pixels in the same orientation as the background streaks then it's lack of dithering.  You can usually find a way to reject the hot pixels themselves, but the background streaks are much harder to suppress.

Excellent insights, thanks.  Much obliged!

Offline EorEquis

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Re: Strange noise pattern
« Reply #16 on: 2014 August 29 20:30:51 »

So now the question becomes...why did I only start getting drift and rotation 2/3 of the way into the night...hrmmm.
Probably because the balancing of your scope/mount was not good.

Hrmm, certainly possible I suppose, though I'm typically pretty fastidious about that sort of thing.

Can you expand on this?  What sort of balance issue could track/guide exceptionally for 3+ hours, and then go sideways in 20 minutes?

Offline EorEquis

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Re: Strange noise pattern
« Reply #17 on: 2014 September 09 03:56:23 »
Carlos and Ian,

Weather finally gave me a chance to image again last night, the increased dither did the trick!  Much more random noise pattern, much better results after integration.

Thanks for the insight on the amount of dither required to truly be beneficial!

Offline gvanhau

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Re: Strange noise pattern
« Reply #18 on: 2014 September 10 09:28:23 »


So now the question becomes...why did I only start getting drift and rotation 2/3 of the way into the night...hrmmm.
Probably because the balancing of your scope/mount was not good.

Hrmm, certainly possible I suppose, though I'm typically pretty fastidious about that sort of thing.

Can you expand on this?  What sort of balance issue could track/guide exceptionally for 3+ hours, and then go sideways in 20 minutes?


Hello, Sorry, very late on this answer. first of all, I am glad you resolved the issue.

As an example on my theory: suppose you have your telescope on the east side of the mount and that your balance is a little heavy on the east (as many recommend) the tracking is good, because the motors are pushing the scope. If now, during the session, the scope passes the highest point, and now is on the west side, the balance of your mount is now west heavy, and the scope is now falling and being braked by the motor instead beeing pushed. This motion is far more irregular than at the first stage, because it depends on friction, backlash, etc. This can cause trailign, double stars, and other guiding problems.

Geert
Geert Vanhauwaert

Offline MikeOates

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Re: Strange noise pattern
« Reply #19 on: 2014 September 12 04:06:42 »
Hi, I agree with Geert about the balance issue. It may seen counter productive, the scope actually needs to be unbalanced not only in RA but also the Dec axis. The dec axis is easy, just make the camera end or the top of the tube heavier so that no matter which orientation the scope is in, one side is always heavier.

The RA is more difficult and if you have a permanent setup there are ways of using weights on cords to always make the east side heavier.

I do mine manually as I always set up all the equipment each session and I stay with the scope and perform a manual meridian flip when it suits. I then move a weight so that the East side of the pier is always heaver. The weight difference I use is about 0.75 kg measured with a hand held luggage scale, using the scale as a simple tension readout. For example, I attach it to the end of the counterbalance rod and see what effort I have to make to move the RA axis with the clutch off, do this pulling up and down on the east side and it should be harder to move it pulling up.

Since doing this, my keeper images have gone from about 50/60% to about 98%, just every now and then one sub gets trailed not 100% sure why, could be a sudden move of the mount as it’s not on solid ground or gust of wind or just me being too close and disturbing the ground.

Glad you sorted out the noise issue.

Mike

Offline dayers

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Re: Strange noise pattern
« Reply #20 on: 2014 September 12 08:26:47 »
Mike and Geert,

Your comments on good mount balance were very interesting to me. I have been trying to come up with a consistent balance procedure that I can use at the beginning of each imaging session.

Geert, I read your comment on 10 September as referring to RA balance with the counterweight rod horizontal and the OTA on either east or west, heavier on the east side. Is that right?

Mike, Am I right that you are saying that it doesn't matter whether the  top end or the camera end is heavier, as long as it stays the same wherever the beast is pointing?

One of these days I will get this all figured out. I started to make progress (I think) when I started using PHDLab to look at the RA and DEC traces after each time out.

Thanks, guys,

Dave
Dave Ayers
  Stellarvue 80 mm refractor on CG-5 mount, Orion 50mm guide scope. Imaging camera SBIG STF-8300M, guide camera ASI120mm. PHD Guiding. Sequence Generator Pro, PixInsight.

Offline MikeOates

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Re: Strange noise pattern
« Reply #21 on: 2014 September 12 08:53:37 »
Dave,

As far as I am aware the dec axis can be biased either way because unlike the RA which under normal use will be driven in one direction, just at different rates, the dec axis may be driven either way depending on how good your polar alignment is. You may find it works better one way than the other, that you will only find out by trial and error. For me I have a weight at the objective of the scope, just because it was easiest to fit there. Alternatively, just slide the scope along the tube rings or what ever you use to fit the ota.

Mike

Offline dayers

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Re: Strange noise pattern
« Reply #22 on: 2014 September 12 08:58:22 »
Thanks Mike. Makes sense. Dave
Dave Ayers
  Stellarvue 80 mm refractor on CG-5 mount, Orion 50mm guide scope. Imaging camera SBIG STF-8300M, guide camera ASI120mm. PHD Guiding. Sequence Generator Pro, PixInsight.