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Comet Exposure Time

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#1 janame

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Posted 07 August 2020 - 08:24 PM

Imaging C/2020 F3 (NEOWISE) at an image scale of 1.29"/px (Orion ED80 600mm, ASI2600 3.76um), I've found that a 60s exposure makes the core slightly oblong, while 30s seems pretty round to my eye.

 

Using the optimal exposure time calculator in NINA, an exposure of 62 seconds (3x RN) - 207 sec (10x RN) is recommended.

 

I stacked about 40 minutes of 30sec exposures and am having a hard time bringing out the detail in the tail while keeping the background dark.  Presumably, the cure is more total integration time, but I'm wondering, with 30 second exposures, will my gains be eaten up by read noise?  Would I be better off guiding on the comet core? 

 

Guiding on a star, my RMS error is around my 1.5 arcsec (1 sec guide cam exposure).  Guiding on the core, I need 2.5 sec guide camera exposures, and my RMS error is around 2.5-3 arcsec.  Not sure if the poor guiding performance is a reasonable trade off.

 

For DSOs, I usually use a 3 minute exposure.

 

Best way forward?

1) Stack more 30s exposures (fighting read noise)

2) Guide on comet core (fighting guide RMS)

 

Thoughts?



#2 APshooter

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Posted 07 August 2020 - 09:11 PM

I'm imaging the comet right now.  Core was fine at 60 seconds, but you're right, thin tail.  I'm going for 3 minutes on the tail and just let the head blow out.  That, or image 60 seconds for the head, then 3 minutes for the tail and combine in post.


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