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Over Exposure?


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Just a quick question to confirm my utter dumbness.

A few nights ago I was getting forst light with my Asi5785MC on Andromeda.

Working in NINA and running a sequence of exposures.

Equipment is  EquinoxED80 Pro, flattener, IR cut filter, Aso585MC running at native gain.

Guiding a CEM60 with an OAG using an original ASI120MM camera

Guiding was lovely and inital sequence was going quite well.

As it was a test I knew that my FOV wouldnt fit M31 in the frame but I wasnt too worried and I knew that the sub lengths even when stacked wouldnt reveal too much.

So I started with 10 x 30sec exposures

Moved to 10 x 60 second exposures

Moved to 10 x 120 sec exposures

Moved to 10 x 180 second exposures

All was going well until I got to the 180 sec exposures.

When NINA showed the satretched image on screen it was completely blown out with a very bright green hue to the image.

Akll the others showed a stretched image of a fairly nice background with the image not blown.

Am I expecting too much at 180 sec on this object or do I need to adjust camera settings for the longer runs

Thanks

Nick

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Andromeda is quite a bright target and can usually be captured in 10-30s quite well.

When taking images you need to do a few tests as you've done and assess which is best. Looking at the image histogram curve helps to assess if your black or white levels are clipped (something you don't want) but you can usually tell by looking at the image if it's too bright.

If using shorter exposures, take more images to improve the signal and to better average out the noise when stacking. Shorter exposures also helps reduce the potential frequency of any guiding errors affecting the number of subs.

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Perhaps it's just NINA's stretching visualization. Check the linear images in your processing software. Each exposure lenght will require a different stretching intensisty. M31 is a tricky target since the core of the galaxy might blow out when you try to bring out more detail from the outer parts. A combination of short and long exposures using HDR integration might be required in post processing.

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Something funny must have happened with your 180s image (perhaps only in the stretch), as you wouldn't get a change like that just from 120s to 180s - you're certainly not going to blow the background out. Or perhaps there's the chance a bit of cloud came over?

If you're using NINA, I would look at the statistics in the Imaging screen, rather than try and interpret what the stretched image looks like. I use this as my monitoring tool when imaging.

Most important is the Median value - essentially this tells you the level of your background. A "good" value will depend on your camera and its' settings, including offset. It's relatively easy to work out a range of values that work well for your set up, so that you are swamping the camera read noise (say in the x5 to x10 range). See Jon Rista's contribution to this thread on CN in particular - https://www.cloudynights.com/topic/597682-i-need-a-primer-on-read-noise-calc-asi1600/#entry8200280

I turned the formula in this around to get a range of values - if you're interested in this approach and want more detail of what I did, just reply, and I can explain.

Another thing to keep an eye on is the Max value - this shows how many clipped high pixels you've got. This gives you an idea of how much your brighter stars are clipping (or not). With M31, you're also in danger of blowing out the core of the galaxy as it's so bright (it's one of the few targets that can benefit from taking a set of much shorter exposures alongside longer ones and then combining with HDR techniques - but that's another story!). I'm not going to say what a good value is here - I often clip into the low thousands of pixels, but my RASA is so fast that I can't really avoid this unless I shoot very short exposures (ay 5s) and I can't face processing thousands of subs at a time!

 

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Thank you all for the detailed explanations.

As I said it was just an experimental run and, yes, potentially I could have had some cloud cover over the 2 shots at 180secs, but the image was so blown out in green in the stretched view that it confused me somewhat.

Its a learning curve and potentially on these bright targets I should refrain from subs longer than say 60sec and concentrate more in the 15-45 second realm and just take a lot more.

The experiment was fun though as it was my first time out successfully with N.I.N.I, Platesolving, centering, sequencing, getting co-ordinates via Stellarium and PHD2 guiding at the same time. I cant wait for the next clear sky to try something again.

Nick

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On 16/02/2023 at 08:35, nickarp2000 said:

Just a quick question to confirm my utter dumbness.

A few nights ago I was getting forst light with my Asi5785MC on Andromeda.

Working in NINA and running a sequence of exposures.

Equipment is  EquinoxED80 Pro, flattener, IR cut filter, Aso585MC running at native gain.

Guiding a CEM60 with an OAG using an original ASI120MM camera

Guiding was lovely and inital sequence was going quite well.

As it was a test I knew that my FOV wouldnt fit M31 in the frame but I wasnt too worried and I knew that the sub lengths even when stacked wouldnt reveal too much.

So I started with 10 x 30sec exposures

Moved to 10 x 60 second exposures

Moved to 10 x 120 sec exposures

Moved to 10 x 180 second exposures

All was going well until I got to the 180 sec exposures.

When NINA showed the satretched image on screen it was completely blown out with a very bright green hue to the image.

Akll the others showed a stretched image of a fairly nice background with the image not blown.

Am I expecting too much at 180 sec on this object or do I need to adjust camera settings for the longer runs

Thanks

Nick

Honestly 30-60seconds is all you need at unity gain more likely 30s, just lots of exposures. 

If not the the core will be blown out. 

Adam 

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