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Dither


woodblock
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I wasn't sure if I should put this in mounts or in software.  I'm just getting into the groove again after a midsummer break.

 

I'm using  HEQ5, ASI1600 camera, PHD2 guiding, APT and EQmod.

 

I took some pictures and was looking at the star shapes by blowing them up and I saw that the stars were not round but elongated slightly like a very small dash. The effect was the same over the whole picture and the orientation  of the lines was the same over the whole picture. The length of the dash was not dependent on exposure time.  PHD2 guiding was working well as far as I could see.

 

Then one of the pictures had a satellite trail across it but it had a tiny sine wave superimposed on it and I realised that the sine wave was oriented in the same direction as the star dashes. At this point, by chance, I turned off the guiding, took a  picture and lo and behold the stars were perfectly round. So it appeared as if PHD2 was causing the problem.

 

I think it might be something to do with Dither but I don't know much about how it works. I found some settings in PHD2 but there seems to be no way of turning it off completely. Is dither done by PHD2 or by EQMOD or by APT.

 

Any ideas would be welcome.

Cheers

Steve

 

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Dither is done by PHD2 Steve - it instructs EQMOD to move the mount very slightly. However it dithers between exposures. It moves a few pixels in a random direction so making it easier to find hot pixels which don't move.

Can you post a stretched picture or two of the images? It might be something else causing the problem.

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Thanks Terry, I think I'm talking rubbish. 

 

I've attached the picture with the satellite trail (At least I think that's what it is). The picture is actually a screen grab from MaximDL because the original file is too big. If you zoom in you can see how the stars are all elongated slightly. The picture is otherwise completely raw and unprocessed. The picture is about 1 degree across.

 

I've looked at the PHD2 guide log which tells me that the total RMS error was 0.61 second over the session.

2022-08-11_19-46-23.jpg

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Thats a very odd image. I see what you mean about the zigzag sine wave of the satellite trail. It is like a high speed oscillation somewhere in the system. The satellite will have passed through the field of view fairly quickly. To be honest I'm not sure what is going on there as I've never seen anything like that before. Maybe somebody else on the forum will know.

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What have you got your guiding exposure set to?

I don't use PHD2 any more but use guiding within EKOS (as I use Raspberry Pi4's as capture computers) and have my exposure set to 2.5 seconds. I did the same exposure when I used PHD2. I have it set like this otherwise you end up chasing the seeing in the atmosphere.

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There are a couple of things you could try. Increase the mount settle time to a couple of seconds after each dither. You should find that setting in APT. Run the Guiding assistant in PHD2, check the measure dec backlash option. Run the asistant, then apply any recommendations it makes. Also check your mount settings in EQmod, make sure pulse guiding is enabled, and set the pulse guide to .5 in both Ra and Dec.

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I've made some progress with this. I don't think it's anything to do with dither or guiding.

 

The telescope is a skywatcher Newtonian and I found that one of the locking screws on the mirror was loose. I found it while checking the collimation. I've collimated again and made sure that the three locking screws on the mirror cell were all tight. It is now vastly improved although there is still a very slight wobble in satellite trails.  I think the wobble is caused by a slight vibration in the mount but I can live with it for the kind of work I do.  I've always known that a Newtonian tube is an ideal oscillator. A light weight rod with a weight at each end on a springy mount.

 

Also it turns out that I'm not the only one. I found this on another forum called INDI Open Astronomy Instrumention

 

https://indilib.org/forum/gallery/10074-who-knows-what-is-this-oscillating-line-satellite-trail.html

 

By the way, it's astonishing how many satellites trails I get. My field of view is about 1 degree. Last night I did 60x1minute exposures and had 18 with trails including one which had two trails, a bright one and a dim one crossing. I am pretty sure they are satellites.

Edited by woodblock
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