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Single HA frame review/question

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#1 flyingg103

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Posted 08 December 2019 - 09:06 PM

OK, so i know this has been discussed a million times but still find myself confused. I think i am grasping more as i read so hear goes my question.

 

I have been imaging for about 1.5 yrs with the ASI1600MM pro mono camera and NB filters and like my results for the most part. I have recently been told that my exposures might be way underexposed and after doing some further reading that may be the case. I have always tried to keep my mean ADU displayed in SGP around 800 or so ( still not sure if that is a good number or not) since the SGP displays the 16 bit values. So converted to 12 bit that would be 50 ADU in 12 bit. I have read in several exposure conversations that we should keep around 550 or so using 76 gain settings. I do not know what the offset is as i have been using the ascom driver for the camera. I want to start using the native ZWO driver so i can control the gain/offset values and try different settings. 

 

When looking at my subs they appear to be somewhat overwhelmed by light pollution even at around 800 indicated ADU in SGP. My fear has always been not to go longer exposures to keep the light pollution down. 

 

My typical subs are below:

 

Orion ED80 

Field flattener ( no focal reducer)

ASI1600MM cool 

IDAS LP 2 filter at the beginning of the image train before the field flattener

ZWO NB filters

180 sec exposures

gain settings 76 ( not sure of the offset as this is not controllable using the ascom driver)

Skies Bortle 5

 

I am attaching SGP screenshot of image statistics and histogram. ( where should the histogram be? should it be further from the left that what i show ?)

Also attaching the same single sub and stack of HA/SII/OIII for review.

 

Guidance is appreciated.

 

https://www.dropbox....ODZS3ATaa?dl=0 

Attached Thumbnails

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Edited by flyingg103, 08 December 2019 - 09:09 PM.


#2 2ghouls

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Posted 09 December 2019 - 08:43 AM

I think you are likely swamping read noise at 3 min and 800 ADU, so then the question is how long can you expose before clipping bright stars? Next time you are out imaging try taking the histogram sliders and move the right one all the way to the right and the left one all the way to the left. That is the unstretched (linear) image. I am guessing yours will be completely black unless there is a very bright star in the field. Personally, I have found I am comfortable clipping 10-20 bright star cores. Others may be comfortably clipping more or less, it’s a personal decision. I wouldn’t worry about light pollution overwhelming your image with narrowband. It’s usually just slight gradients that are easy enough to to remove with PI. Personally, I try to use 5 minute subs for everything (adjusting gain accordingly) and dither every frame. I find I get the best results with my limited Sky time that way.

#3 flyingg103

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Posted 09 December 2019 - 06:17 PM

Overall is the 800ish ADU reading seen in SGP enough of exposure? Once divided by 16 to get the 12bit ADU is is only 50ADU. Based on the estimated 550ADU recommended for 76 gain it would seem that I am off by a factor of 10 being way underexposed.

 

In the single HA frame you can barely see the nebula even though this is a reasonably bright object. Most of the brightness is due to the LP and If i continue to expose for longer wont the LP overwhelm the image?


Edited by flyingg103, 09 December 2019 - 06:19 PM.


#4 PhotonHunter1

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Posted 11 December 2019 - 09:32 PM

I have the same camera and have complied the below ADU scale. I typically image using the 76/15 or 200/50 offset/gain settings so the -e/ADU values are derived from my FITS data. Your values may vary based on read noise values as well.  
 

76B7EE68-374B-4CD5-9794-B97CDC657A4D.png


Edited by PhotonHunter1, 11 December 2019 - 09:32 PM.

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#5 Drakkith

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Posted 12 December 2019 - 08:42 AM

I do not know what the offset is as i have been using the ascom driver for the camera.

 

Your background looks to be around 830 ADU's (16 bit). Your dark current is roughly 0.01 e-/p/s at the temp recorded on the fits file, so around 1.8 e- per pixel over the 180 sec exposure. Converted to 16 bit that's about 29 ADU's of dark current. Since you're using narrowband filters the contribution from light pollution is likely negligible. So just looking at your raw image file and the camera's specs, I'd guess the offset is 800 ADU's in 16 bit, or 50 ADU's in 12 bit.

 

At a gain of 76 there's about 2 e-/ADU (12 bit). Measuring the outer edges of the nebula gives me an average ADU of about 840 in 16-bit. Convert to 12 bit and remove the offset and you get 2.5 ADU's. Multiply by two and you come out with about 5 e- per pixel in 180s as your target's signal. At the wavelength of HA your camera has somewhere around a 50% QE, so those 5 e- turn into 10 photons, meaning you're getting about 10 photons/pixel in three minutes in the outer edges of the nebula, or one photon every 18 seconds per pixel.

 

Your sensor is absolutely starved for photons.

 

I highly recommend increasing your exposure time to 10 or 20 minutes.

 

Most of the brightness is due to the LP and If i continue to expose for longer wont the LP overwhelm the image?

 

Even if we assume that all of that 800 ADU background contribution is from LP instead of bias offset you'd still be able to image for 10x as long and still not come close to overwhelming your image. That would only be a background ADU of around 8k and a 'core' ADU of about 8700, not even a fifth of the way to maxing out your image.

 

Furthermore, given that you're shooting narrowband, it is very likely that you have no more than a handful of ADU's as contribution from LP.

 

In the single HA frame you can barely see the nebula even though this is a reasonably bright object.

 

At 1 photon every 18 seconds, it's no wonder you can barely see the dimmer portions of the nebula!



#6 astronate

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Posted 13 December 2019 - 12:08 PM

Flyingg103,

If the ADU math is getting confusing, take a dark frame using SGP at the same gain, offset, temp, and exposure time and see what the mean is. Then subtract that dark mean from your light mean. I think you will see that you are barely getting any signal over the dark current/noise/background - regardless of 12 or 16 bit, etc.

I’m betting you are way under exposed.

-Nathan

#7 JukkaP

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Posted 13 December 2019 - 12:51 PM

I wouls like to point, that you are using 2 filters for no reasons. They can reflect, loss some signal. Let say the idas let 90% ha trough. And the again you zwo filter is letting trough 90%+ ha light. So you might be loosing extra 10% of the light before it gets to the sensor.
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#8 Drakkith

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Posted 13 December 2019 - 01:53 PM

I wouls like to point, that you are using 2 filters for no reasons. They can reflect, loss some signal. Let say the idas let 90% ha trough. And the again you zwo filter is letting trough 90%+ ha light. So you might be loosing extra 10% of the light before it gets to the sensor.


Ohh, I didn't even catch that when reading the original post. That's almost certainly not helping. The NB filters themselves should be all that's needed to deal with light pollution.


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