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Egg shaped star on only bottom left and right?

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#1 nhassey48

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Posted 03 March 2021 - 03:37 PM

Hello everyone. I recently got a Skywatcher Evostar 72ED scope with the 72ed dedicated flattener/reducer and use a ASI533MC Pro. I noticed the other night that my raw images had egg shaped stars but only on  the bottom left and right. Both the field flattener and the camera both say to use 55mm of back focus which I have achieved, I think. not quiet sure where on the field flattener to measure but from the sensor to the end of the extensions is 55mm. Does anyone have any advice or things to check?

 

OPT had a article about egg shaped stars and according to  their pictures my stars are all egg shaped point towards the center which they say the flattener it too close. Any help would be greatly appreciated!



#2 Tapio

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Posted 03 March 2021 - 04:15 PM

Egg shaped stars pointing towards center does indicate camera being too close to reducer.

But all corners should be like that, ie symmetrical.

If you have only some corners then that would indicate tilt somewhere.

How is your camera attached (and reducer)  ?



#3 nhassey48

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Posted 03 March 2021 - 04:52 PM

Everything is screwed together. M52-M48 adapter to the reducer, reducer is M48-M42 and then every extension is M42 to the camera.

#4 nhassey48

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Posted 03 March 2021 - 04:54 PM

Could over torqueing the adapters to camera sauce anything like that?

#5 petercoxphoto

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Posted 03 March 2021 - 05:55 PM

Could be tilt in the camera itself - the sensor may not be totally parallel to the body of the camera.

 

To test, take an image. Now rotate the camera 180 degrees and take another. If the problem area did not move relative to the camera sensor (i.e. if it was in the bottom of the frame before and it's still there after the rotation) then the tilt is in the camera itself. If it moved (so it's now in the top of the frame), then the tilt is elsewhere.

 

Camera tilt is not unusual, and there are a few adapters you can buy that will let you correct it - ZWO makes one, Gerd Neumann another. 

 

You should definitely not be overtightening the adapters, but unless you cross-threaded them then it shouldn't cause tilt. If everything is threaded together and the tilt is not in the sensor, then it may be a collimation problem with the scope. Refractors don't typically need to be collimated, but you may have gotten a bad sample. Or the problem could be with the flattener. You could try removing that and taking test images - if the tilt is gone then you know that's the cause.

 

But check the sensor tilt first, it's going to be the easiest to correct unless there's something else obviously wrong.

 

Cheers,
Peter



#6 cybermayberry

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Posted 03 March 2021 - 07:57 PM

As other have pointed out this is most likely tilt.



#7 nhassey48

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Posted 03 March 2021 - 10:34 PM

I just used the CCD inspector, but I don’t really know what it is supposed to look like. Anyone ever use that program?



#8 Tapio

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Posted 04 March 2021 - 01:31 AM

Show us what you got.

#9 nhassey48

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Posted 04 March 2021 - 10:05 AM

https://i.imgur.com/QOIBOp5.png

 

This was just using one of my 5m Orion subfrmaes.


Edited by nhassey48, 04 March 2021 - 10:06 AM.


#10 petercoxphoto

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Posted 04 March 2021 - 11:00 AM

That shows exactly what we've been suggesting. A combination of tilt and spacing problems. The curvature is a high number, that's your spacing. Then the tilt is self-explanatory. Fix the spacing problem first, then do the tilt troubleshooting I posted above.

 

Cheers,
Peter



#11 Tapio

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Posted 04 March 2021 - 11:01 AM

That doesn't seem too bad.
But you should have several images in CCDI to get more reliable results.

#12 nhassey48

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Posted 05 March 2021 - 12:02 AM

That shows exactly what we've been suggesting. A combination of tilt and spacing problems. The curvature is a high number, that's your spacing. Then the tilt is self-explanatory. Fix the spacing problem first, then do the tilt troubleshooting I posted above.

Cheers,
Peter


For the ccd tests, should it be multiple exposures and stacked first? Or just go off of individual frames?

#13 nhassey48

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Posted 05 March 2021 - 01:17 AM

Ok so I don't know if this fixed it but I wanted to share with you all my discovery tonight...

 

I was pretty frustrated tonight and started transferring everything over to my 8" F/4 Newt for galaxy stuff. Upon taking my camera off of my Refractor, I took a look at it and noticed that the very first M42 spacer after the cameras 11mm ring had a light gap through one side but not the other...further investigation. I had used the 1.25" threaded ring adapter to screw my 1.25" L-Pro filter to be close to the sensor so I wouldn't have vignetting. It turns out, that little ring cannot be used with the M42 extenders because it will bottom out on it and cause a slight tilt in the extensions. I have not yet taken test shots but tomorrow I will try again with the standard 55mm back spacing, and see if I get those weird egg stars again. I will post back here, if anyone cares lol.




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