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Square Frame on Image

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#1 elrico

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Posted 01 June 2020 - 01:00 PM

Hi, can anyone advice please, I do mostly EAA and had firstlight with my new camera ZWO533 OSC using my Skywatcher espirit 80mm in

Alt/Az mode, I noticed this when I later looked at my jpg image of IC5146 Cocoon Nebula, lots of LP Moon 34% Bortle 6 Sky.

Does anyone know what is causing this 'square' Please.

Regards

Eric



#2 D_talley

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Posted 01 June 2020 - 01:15 PM

It would be best if you uploaded an image for us to see.



#3 elrico

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Posted 01 June 2020 - 01:49 PM

Hi, it wouldnt accept my image for some reason so here is another attempt, just trying my new set up under very poor LP , just need help in identifying this problem square.

Regards

Eric

IC5146 Cocoon Nebula, I do EAA saved on the screen JPG with the new Software ZWO ASI Studio

Attached Thumbnails

  • IC5146 COCOON NEB.JPG

Edited by elrico, 01 June 2020 - 01:56 PM.


#4 Alien Observatory

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Posted 01 June 2020 - 02:34 PM

Alt Az Field Rotation...Pat Utah :)


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#5 Rickster

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Posted 01 June 2020 - 02:40 PM

I don't see a problem, other than vignetting.

 

Edit to add.  The 533 has a square sensor, right?


Edited by Rickster, 01 June 2020 - 02:44 PM.

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#6 elrico

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Posted 01 June 2020 - 02:58 PM

I don't see a problem, other than vignetting.

 

Edit to add.  The 533 has a square sensor, right?

Yes it is square, would a Field Flattener hekp do you think ?


Edited by elrico, 01 June 2020 - 02:59 PM.


#7 Rickster

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Posted 01 June 2020 - 03:02 PM

Sorry elrico, but help with what?  It is supposed to be square.



#8 Alien Observatory

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Posted 01 June 2020 - 03:04 PM

You will need a Field De-rotator or an EQ mount to fix it...Pat Utah :)

https://www.optecinc...pyxis/index.htm



#9 Alien Observatory

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Posted 01 June 2020 - 03:59 PM

I don't see a problem, other than vignetting.

 

Edit to add.  The 533 has a square sensor, right?

I downloaded the image and stretched it...Field Rotation is very apparent with the 533 square sensor cam...This is completely normal for an Alt Az mount and looks like about 30 to 60 minutes of stacking...Pat Utah smile.gif

 

I will add my .02 worth of advice...Your 80 mm telescope combined with a fairly large sensor if very well suited to AP (using an EQ mount), not so much for EAA.  You have a huge FOV and that is why all 4 sides of the "square in question"are showing.  With a smaller sensor maybe only one side of the square will reveal itself.  I have used a ZWO 290 mono cam at 540 mm and there is very little apparent field rotation.  Field Rotation is a complex subject and the magnitude depends where you are pointed to (N,E,S,W) and what your Attitude is.  Here is a link with a lot of information...

https://calgary.rasc...ld_rotation.htm

Attached Thumbnails

  • post-258207-0-56280600-1591037320strech.jpg
  • field_rotation_calgary_2.gif

Edited by Alien Observatory, 01 June 2020 - 08:41 PM.

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#10 Rickster

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Posted 01 June 2020 - 05:33 PM

Ah, I see, said the blind man.

 

Yet another reason to use an Eq mount.


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#11 roelb

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Posted 01 June 2020 - 06:23 PM

Use SharpCap instead of ASILive, you shouldn't have a inclining square.


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#12 Alien Observatory

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Posted 01 June 2020 - 10:07 PM

Field Rotation is a mechanical / mathematical issue..nothing to do with Software...Pat Utah :)


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#13 elrico

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Posted 02 June 2020 - 03:31 AM

Thanks to all , I will stick with ASI STUDIO as I like its easy to use interface, maybe shorten my subs to 20 secs and have to do a bit of cropping/orietation of the JPG after saving for my library, its no problem on screen as the ^ Square* doesn't appear . 

Conditions were poor that night so maybe the longer exposures 45sec contributed to the field rotation although I never had this issue with my Atik Infinty so maybe the square chip contributed, time will tell no doubt and as is usual in this game time will tell

eric


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#14 roelb

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Posted 02 June 2020 - 08:34 AM

Field Rotation is a mechanical / mathematical issue..nothing to do with Software...Pat Utah smile.gif

Yes I fully agree but SharpCap handles better so you don't notice the field correction. 


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#15 Alien Observatory

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Posted 02 June 2020 - 09:26 AM

Yes I fully agree but SharpCap handles better so you don't notice the field correction. 

Roel, I will just say with a Rectangular Sensor only the "sides "will show rotation as the top and bottom of the sensor are closer to the center of rotation and will not show rotation like the square sensor.  We will just have to agree to disagree about the SW contribution to field rotation....  Pat Utah :)


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#16 donstim

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Posted 02 June 2020 - 02:12 PM

Thanks to all , I will stick with ASI STUDIO as I like its easy to use interface, maybe shorten my subs to 20 secs and have to do a bit of cropping/orietation of the JPG after saving for my library, its no problem on screen as the ^ Square* doesn't appear . 

Conditions were poor that night so maybe the longer exposures 45sec contributed to the field rotation although I never had this issue with my Atik Infinty so maybe the square chip contributed, time will tell no doubt and as is usual in this game time will tell

eric

Shortening your subs won't matter. It's the length of your total integration time and where you are pointing in the sky that determines how much the field will rotate during an image stacking session. See the nice graph Pat posted in one of his responses above. Imaging an object in the East or West will have much less field rotation than an object in the North or South. And imaging an object low in the sky in the North or South will have less field rotation than an object higher in the sky.

 

By the way, that graph is for one latitude (I believe it is 37 degrees). The field rotation rate will be greater as you get nearer the poles and less as you get nearer the equator.

 

Cropping the image is an easy and handy way to deal with it.


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