Need help : Amp glow or light leak Sony nex-3n modified

David_M
22 Apr, 2017 22:11

Hi,

I just received my nex-3n modified astrodon inside.
It is much better with Ha.
But on the left side of the sensor, there is some kind of light leak or amp glow or electroluminescent thing.
See the picture. Same thing with the master dark. 11x2,5 minutes at ISO 800.




It appears at 30 sec exposure time too, but softer.

This artefact wasn't there, or not visible before the mod.

Where does it come from ?
Is there a way to fix it ?

Thx
David
dagoldst
22 Apr, 2017 23:06
What happens if you wrap the camera up light tight, say in a heavy towel, and take the same dark frame?
David_M
22 Apr, 2017 23:15
Thx for you answer, I will try this.

Actually, the darks were taken with the camera cap, no lens attached. So it is not an internal reflexion of the lens.
dagoldst
23 Apr, 2017 15:27
David
Thx for you answer, I will try this.Actually, the darks were taken with the camera cap, no lens attached. So it is not an internal reflexion of the lens.
I understand, but I have seen light leaks in cameras from other than the usual suspected places.  For instance, some years back, Canon had one on one of their DSLRs that  light came in around the top LCD. Give it a try with the lens cap on, wrapped in a thick dark towel.  If the pattern remains, it is not a light leak.

FWIW,

David
AMultiverse
23 Apr, 2017 22:53
Because the pattern is the same for both your lights and darks it does look like amp glow. Most cameras have amp glow. It is more of a problem for cameras without cooling, and those built to be compact in camera size like general purpose cameras.  It is also more visible on warm nights. For my cameras that have this problem I try to have at least 30 each light and dark subframes. This is a stochastic process, so enough samples are required to get a true mean of the fixed pattern noise, and the rule of thumb from statistics is 30-40 samples are needed. It is important to match light and dark subframes. The same ISO (aka ASA, gain). The same exposure times. The same camera. The same sensor age within a year (sensors change with time). The same ambient temperature within +/-10F. The same binning. The same camera settings (e.g. automatic noise reduction settings, white balance, etc.). The same image file format (e.g. jpeg, RAW, etc.) The same image format ( e.g. small, medium, large, etc.). Of course at least 30 samples (subframes). This takes a lot of time and disk space, so it is best to try and standardize on a few combinations so you don't have to take darks for every session.

BTW, Goldstein is correct in pointing out that light leaks are a problem, and that a lens or body cap does not guarantee good darks. The camera needs to be in a similar environment for both lights and darks. You cannot use a towel around your camera (except for testing) for your darks, unless you are using a  towel for your lights.
David_M
23 Apr, 2017 23:24
I've made darks with the cam in a box in a dark room this morning.
Same result. Not a light leak.

Thx for your detailed answer Gilbert.
Before the cam was modified, this amp glow wasn't present, or visible at least (same exposure time or conditions).
I'll try with more lights and dark next time. This version was just a little try.
If I understand well, to fix this I need as much darks as lights?
Once calibrated, this amp glow will disappear, or be reduced ?

CS, David.
AMultiverse
24 Apr, 2017 00:27
No matter how many lights, you should have 30-40 darks. However, if you also have at least 30 lights, then the darks will be the most effective. My experience is as long as the darks are well matched with the lights, you will have all trace of amp glow removed. If you are using flats the same rules apply. Flat darks must be matched with flats, and there should be 30-40 of each. People with expensive equipment tend to use fewer control subframes because expensive cameras have fewer problems, but everyone could benefit with 30-40.

While flat darks must be matched with flats, flat + flat dark sets do not need to be matched with light + dark sets except for the camera's ISO.

I find that for many of my scopes/lenses I don't need flats+flat darks, but darks always make a big positive difference.

I make a set of darks with my set of standard settings for each of my cameras once a year and use that set with any lights I make that year. I have found 32 subframes each for darks, flats, and flat darks to be a convenient number.

To help your choose a standard setting for ISO, use this camera testing site to help. You want as much dynamic range as possible, but also you want the highest ISO to get the shortest exposures. That is where the dynamic range curve starts being non-linear; for most Sony cameras it is at ISO 100. However, depending on your tracking or guiding you may need higher ISO to get shorter exposure times; in that case choose your ISO based on your desired exposure time.
Marcus1968
24 Apr, 2017 18:32
Hi David,

I am quite sure that this is not amp glow.
1) it would be at the border of the sensor most, and not start in the middle of the sensor.