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Backfocus or tilt?

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6 replies to this topic

#1 odehjas

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Posted 22 June 2024 - 03:27 AM

I'm seeking help from experienced observers. Please look at the two images below and notice the star trails in some areas. Are these trails due to a backfocus issue or sensor tilt?

 

Details:
Camera: ASI2600MM Pro
Telescope: 14” Celestron EdgeHD with 0.7x reducer.
Focal Length: 2737mm.

 

Thank you for your assistance!

 

Best regards,

 

gallery_346168_26694_2305223.png

 

 

gallery_346168_26694_324973.png



#2 Tapio

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Posted 22 June 2024 - 05:26 AM

Looks pretty good to me.

Only slight tilt, in first image lower and top right.

ASTAP inspection confirms this.

Attached Thumbnails

  • tilt_346168_26694_2305223.jpg

Edited by Tapio, 22 June 2024 - 05:26 AM.


#3 james7ca

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Posted 22 June 2024 - 06:28 AM

These images don't look anything alike. Are these crops from a larger field or are they reduced, full-frame images? Also, what is the image scale in these reproductions?



#4 odehjas

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Posted 22 June 2024 - 06:58 AM

Greetings,

 

Thank you for your replies.

 

Yeah, I had to provide more details. Both images are full frame without scaling or reduction, they were taken on two different nights, I’m using a 10 Micron mount with accurate model and without guiding.

 

Below are the details:

 

Image 1 (Top):
BIN Value: BIN4
Image Scale: 1.12”.
Exposure Time: 10s
Gain: 400
Filter: Ic.

 

Image 2 (Bottom):
BIN Value: BIN4
Image Scale: 1.12”.
Exposure Time: 10s
Gain: 100
Filter: C.

 

Regards,



#5 odehjas

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Posted 22 June 2024 - 07:14 AM

Hello Again,

 

The below image was taken on a different night as well, with the following details:

 

BIN Value: BIN1
Image Scale: 0.28”.
Exposure Time: 180s
Gain: 250
Filter: Ic.

 

Regards,

 

https://www.cloudyni...694_6811835.png



#6 michael8554

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Posted 22 June 2024 - 07:47 AM

The middle of the latest image shows drift, so go back to evaluating 10 sec exposures.

 

But the amount of drift is the same everywhere, so may indicate good backspacing.



#7 james7ca

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Posted 22 June 2024 - 12:49 PM

[quote]...Both images are full frame without scaling or reduction...[\quote]

But, they were apparently binned at 4x4. Your first sample image is only 1562x1044, not at your camera's native 6248x4176 resolution.

 

Your latest image doesn't look too bad although in terms of image quality it can be somewhat difficult to judge something that has been processed and stretched and reduced/binned by about 4X from the original capture scale. To me, the stars look a little bloated in your first two samples and that could indicate poor seeing, poor optics (collimation?), poor focus, or poor tracking (or some combination of all four). But, apparent star size also depends upon your processing and thus you should measure some of your original subs while they are still in a linear state, before any post processing. It would also be useful to know whether the subs in a given series of captures all look the same. Are the above single exposures or stacked/integrated masters? They kind of look like individual subs (i.e. not a stacked master integration).

 

That said, your latest image seems to look better than the first two.




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