1600MM Low ADU

[matt_baker]

Just had my first light with my 1600MM.

Looked at the suggested ADU values for lights and they're around 850 ADU at gain 139 offset 21

I live in a Bortle 5 area, so you would expect I'd be able to reach the desired ADU rather easily in 240s with my Ha filter.

I tried on two targets. The Eastern Veil and the NA Nebula. Maybe I should've tried a different, brighter target but my median ADU on a 240s sub was 400 and a 600s sub was 540

Am I doing something wrong or do I live in a darker sky than I think and can exposure much longer?

My Ha filter is the ZWO 7nm

 

Matt

[symmetal]

The recommended ADU values are primarily used for LRGB or wideband imaging where there are many more photons landing on the sensor in a given time compared to narrowband imaging. The suggested ADU just avoids unnecessary long exposures which add little to the overall image or too short an exposure where camera read noise is an issue.

For narrowband imaging the ADU values quoted are still valid but unless you're in a very light polluted zone you won't achieve them in your exposures unless you expose for a very long time. Where I am in Bortle 3 skies I estimated I needed around 2 hours in Ha to achieve the background ADU recommended with my 1600. So for narrowband imaging it's just best to expose for as long as you can manage. I generally take 480s or 600s narrowband exposures. The background is still way below the recommended ADU level though. It just means that the camera read noise still adds a little to your image compared to it being insignificant if you expose to the recommended sky background ADU level.

Alan

[matt_baker]

Thanks for this, for a sec I thought I was having some kind of issue. I'll try 480s subs but doesn't amp glow almost become a problem at that point, possibly even with darks?

[symmetal]

Yes, amp glow is very noticeable at those exposures but a set of matching duration and temperature darks will take care of it. Any exposure over around 30s will have amp glow creeping in. It takes a while building a set of darks at those durations but once done they will be good to use for a long time. A new set of darks can be taken every year or so if you wish. 

Alan

[david_taurus83]

Hi Matt. I used to take 15m exposures at unity gain with an Ha filter and though the amp glow is significant, its removed completely with darks. This is 16 x 900s subs.

 

[matt_baker]

Just had a chat with some people from cloudy nights.

I knew about the min ADU equation by Jon Rista but I was using it wrong. For the gain input, I was putting 139 instead of 1 electrons per ADU. xD

For gain 139 offset 21, my min ADU should be at least 514 ADU, which for my Ha subs equates to about 600s. This is if my swamp factor is my read noise squared x 3, which equates to 7.2075

[symmetal]

54 minutes ago, matt_baker said:

Just had a chat with some people from cloudy nights.

I knew about the min ADU equation by Jon Rista but I was using it wrong. For the gain input, I was putting 139 instead of 1 electrons per ADU. xD

For gain 139 offset 21, my min ADU should be at least 514 ADU, which for my Ha subs equates to about 600s. This is if my swamp factor is my read noise squared x 3, which equates to 7.2075

I assume when you say your min ADU you mean the average sky background ADU which is close to the image median value. The read noise at unity gain is nearer 1.75 e- rather than the 1.55 e- you used to get 7.2075. This makes 3*RN^2 around 9.2 e-. Adding offset 21 and multiplying the result by 16 to convert from 12 to 16 bit gives a sky background ADU of 483.

3*RN^2 is 483 ADU,   5*RN^2 is 581 ADU   and 10*RN^2 is 826 ADU.

I tend to use the 10*RN^2 figures myself but it's just a personal preference. 

1 hour ago, david_taurus83 said:

Hi Matt. I used to take 15m exposures at unity gain with an Ha filter and though the amp glow is significant, its removed completely with darks. This is 16 x 900s subs.

I've tried 15m and 20m exposures but for some reason I run the chance of slightly eggy stars over 10m so I tend to limit it to 10m. It's an AZ-EQ6 which I thought had a worm period of 8m. 

Alan