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What causes these elongated stars?

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#1 yg1968

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Posted 20 October 2013 - 11:36 AM

On 20% of my raw images, I get elongated stars as if the image moved while I was capturing it. What causes this movement? Is it the guiding or the motors of my EQ-6? What can I do about it. I was using an ED127 (15 pounds) and guiding with an ST80 (3 pounds). Time of exposure was 4 minutes.

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#2 yg1968

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Posted 20 October 2013 - 11:38 AM

Here is a normal image without the elongated stars.

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#3 Madratter

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Posted 20 October 2013 - 11:54 AM

Hard to really say without seeing the guiding graph. But it isn't a pure N-S or E-W problem. My GUESS is that guiding there was a flip in direction of where it thought it needed to correct declination. Then backlash kept it from guiding it out sucessfully. Meanwhile, since the declination was out more than the RA axis, periodic error in the RA axis was occurring as well.

Another alternative is you get flexure that is somewhat jumpy in nature.

Another alternative is wind gusts.

I have put these in order of what I consider most to least likely.

#4 yg1968

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Posted 20 October 2013 - 01:04 PM

Thanks. If guiding is the issue, is there any setting that I should change in PHD? Aggressiveness, perhaps?

#5 Madratter

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Posted 20 October 2013 - 01:45 PM

If in fact the problem is changes in direction with declination guiding with backlash holding things up, you can figure out what direction the guiding needs to go in (North or South) and then allow only corrections in that direction. Also, make sure your min-motion parameter is not less than what your seeing will support. (In general, your minimum motion parameter should be around the size of your RMS error for RA axis in PHD). It certainly helps if your polar alignment is real good too.

#6 Alex McConahay

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Posted 20 October 2013 - 02:02 PM

Before doing anything fancy, try the following:

Put a star in the crosshairs on PHD guiding. Start an exposure on the main camera, and watch as PHD Guiding tracks the star. (Also, "Enable Graph" and watch thing bounce around.) Do the crosshairs in fact stay on the guide star? (Are the graph lines close to zero?) If they don't then something is wrong with the guiding.

If the crosshairs don't stay, is the motion up or down, back and forth, or what? Are the deviations from zero on the graph large or small? Is the motion in one direction and increasingly larger?

If the motion is increasingly larger, then maybe you need more aggressiveness, a longer max correction, or some other adjustment. If the motion is back and forth, then increase the exposure (although that is probably not the problem) on the guidestar to avoid chasing the seeing. Or decrease the aggressiveness.

If the crosshairs stay on the guide star BUT the stars are elongated, then you have either flexure or field rotation. If the elongation of the stars is in an arc, then it is field rotation. If the eongation is straight line, then you have flexure.

Apply fixes to flexure or polar align better. Then try it all again.

If it happens on one frame out of many, (and I suspect it happens more often than you think. I doubt the stars in the second post are as round as you think) then it is probably a periodic error. TO find these, you could use one of the fancy programs for this purpose ((There was a free one once called PEC Prep, I think--google it), of PemPro, or simply watch your PHD graph for a while. You will notice at some regular time, your mount goes off by itself. and then comes back where it should.

Give these things a try.

Alex

#7 GTog

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Posted 20 October 2013 - 02:54 PM

Level? I used to underestimate the importance of leveling the mount, putting all my efforts into getting the alignment just right. But I have found that if I just can't seem to get nice round stars, especially if there's an east-west component, I need the relevel.

#8 Raginar

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Posted 20 October 2013 - 03:38 PM

It's guiding. You're probably over-guiding is my guess. Make sure both your telescopes are lined up roughly to point at the same point. For starting settings, try 80, 15, .15 and a max movement in RA and declination of around 1000. Your calibration steps should equal out to about 6-12 max. How's your polar alignment? If it's working on one picture and not anohter, did you decide to try dithering and you're not letting it settle enough? What are you capturing with and are their any automation things going on while you're guiding?

You need to show us your graph and your settings to get better advice.

#9 yg1968

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Posted 20 October 2013 - 08:15 PM

A lot of good suggestions. I will try some of those. My polar alignment was likely not perfect. I did level but I will pay even more attention to it next time. I was capturing with the SSPro2 and my guide scope and imaging telescope likely weren't pointing at the same spot. I will try the PGD Guiding settings that you suggested. I was using the default settings for PHD. Nothing else was going on while I was guiding.

#10 Raginar

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Posted 21 October 2013 - 09:35 AM

The default settings aren't good. There is an article in here from Craig a Stark called 'When Push here Dummy Isn't'. There is also a useful FAQ in their Yahoo group. The big issue you probably had is the default for dec guiding is like 400... And that's probably too little.






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