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Elongated Stars - Any Ideas?

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#1 geneva_min

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Posted 11 September 2019 - 08:44 PM

Before I start randomly messing around with my imaging train and make things worse, I thought I'd reach out to the experts.

 

I started working on a narrow band image of the eagle nebula last night by shooting HA.  I noticed elongated stars at a weird angle in the top right portion of the image.  The rest of the image looks OK to me.  I've had some flat field issues in the past with the stars streaking radially outward, but this is different.  I checked the individual subs and they show the same issue.  Do I need to tweak my field flattener / imaging train distances?  Could my filter stink?  Any ideas?

 

My equipment is:

Orion Atlas mount

William Optics ZS73 with the WO non-adjustable field flattener

ZWO EFW with the basic 31mm filters

ZWO ASI1600mm

5 min subs, with dark, flat, & dark flat calibration all taken last night.

 

Guiding was good with a 50mm scope and ASI120mm.

 

Here is the uncropped image (calibrated and stacked) and a zoom in on the upper right and lower left.

 

Eagle_HA_CN.jpg

Eagle_HA_RC.JPG

Eagle_HA_LC.JPG



#2 Stelios

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Posted 11 September 2019 - 08:58 PM

I've had similar problems, and I assumed that it's tilt. The top right image would be typical (if observed all around) of the spacing to the reducer or flattener being too much, but the bottom left doesn't mirror it, so I assume that the sensor is not parallel to the flattener focal plane, resulting in the distance to the sensor top right being a bit longer.

 

What causes the tilt is another story... Are you threaded all the way down? Compression rings can compress unevenly and cause this effect. Any spacer rings improperly threaded might do it. 

 

OTOH, that's an uneducated guess, I hope someone with more experience sheds some light on how to attack this annoying issue. 



#3 rkinnett

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Posted 11 September 2019 - 09:04 PM

If you had radially outward or tangential pancakes in all four corners, then I would suspect your camera sensor was too close to or far from your flattener.  Since you only have the pancakes in one corner, looks like a tilt problem.  Stelios beat me to it!

 

This page describes how to diagnose tilt, but it relies on CCDInspector which is very expensive:  

https://astronomy-im...o-solve-it.html

 

This discussion points to some free or cheap alternatives:

https://www.cloudyni...-ccd-inspector/



#4 rockstarbill

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Posted 11 September 2019 - 09:05 PM

Focus on a star in the center, then on a star in all four corners. Then compare the change in the focuser position from the center to each corner. That will tell you what the problem is. 


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#5 geneva_min

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Posted 11 September 2019 - 09:28 PM

Makes sense guys and I love rockstar's simple and brilliant idea.  All my spacers / components seem tight and properly threaded.  In fact I'm more worried about them being stuck together than being loose.  smile.gif

 

After reading your responses and thinking a little more about last night I have a possible culprit.  The flattener has a built in rotating device with one set screw.  Last night when I was setting up I bumped it and it rotated slightly.  I then loosened it, twisted it around and generally played with it for a minute............God knows why but I'm sure you can understand.    Anyway.  I then re-tightened it to death and moved on with my night.   I didn't stop to think that I could be seating it askew.  Heck, maybe the single set screw design itself puts an axial torque on it when death threaded.

 

Next time the skies clear (probably just as the eagle sets) I'll make 100% sure the rotater is squarely seated and see what happens. 



#6 bobzeq25

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Posted 12 September 2019 - 01:24 AM

Focus on a star in the center, then on a star in all four corners. Then compare the change in the focuser position from the center to each corner. That will tell you what the problem is. 

But how much of a difference will you be able to see?

 

If a stepper motor focuser is used, you can count the steps.  If one is focusing manually, I don't see how you could measure the position accurately enough..  The typical focuser scale is 1mm increments.  I suppose you could try to estimate the rotation of he focuser knob.



#7 rockstarbill

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Posted 12 September 2019 - 09:15 AM

The important bit is the direction the focuser moves to come to focus at each point vs the center. You're overthinking this, Bob. If the focuser moves in, the chip needs to move out and vice versa.
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#8 Astrola72

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Posted 12 September 2019 - 10:11 AM

Set screws and USB connections, the bane of our hobby.




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