Ha Imaging Issues

[Likwid]

Hello, I just bought an Ha Clip Filter and I am having some odd issues with it. Other than the standard focusing issues, I am getting images with an insane amount of noise. I have only tried this in my modified Rebel T3i, but I have an old modified Rebel XS I am going to test it with as well on the next clear night, in case this is a camera issue. I have tried this filter twice now and it seems like both times I attached this filter, everything goes to crap for the night. I am using an Orion ED80 on an Orion Sirius Mount, with an Orion ST80 and ZWO ASI120MM Mono Camera for guiding. My T3i is very finicky and will start go get unbearable noise around 35C+, so I try to keep it under 30C when I can. Both of these frames were taken in similar conditions (30C, 300s Exposures, 400ISO on Ha, 800ISO on RGB), a few days apart on the same rig. Is it just me or are they vastly different in terms of image quality? I had what appeared to be almost perfect guiding conditions in PHD according to the graph for the Ha frames, so I surprised to see little streaks instead of normal stars. I had a total of 30 minutes of integration with Ha and when I stacked them, it was painful to look at. Is there anything I can do to improve this? Thank you!

 

[wimvb]

I think that the reason for this is that your Ha images are under exposed. The clip filter will filter out any light other than true wanted signal (Ha), while the RGB image also contains sky glow or light pollution. The RGB image also has more amplification than the Ha image, because you have a higher ISO value. Your RGB image is therefore a lot brighter than the Ha image, and you have to stretch that (Ha) more to reveal the data. This causes the noise to also show up.

You'll see that once you process the RGB image and apply background neutralisation (aligning the colours in a background sample) and white point balancing, that your RGB image may actually be noisier than your Ha image. 

[Starwiz]

6 hours ago, Likwid said:

My T3i is very finicky and will start go get unbearable noise around 35C+, so I try to keep it under 30C when I can

Noise is an unfortunate product of higher temperatures.

I usually image at -10C with my ASI1600mm, but once forgot to cool it, so it was operating at +28C.  The amount of noise I saw on the first image was striking.

Obviously, you can't cool the DSLR, but more frames and dithering will help significantly.  I'm usually working with hours of data for Ha even with the cooled camera.

As Wim said, you may need to expose for longer too.  Part of this is to overcome the read-noise of the camera. i.e. the signal needs to be much higher than the read noise.

John

[Likwid]

Awesome, that actually makes a lot of sense. I was a little underwhelmed at the amount of nebulosity in the Ha pic, so I figured I was a little underexposed. I just didn't realize it would have that effect on the images. Thank you both for your time!

[MarsG76]

The noise is an unfortunate side effect of DSLRs with Narrowband filters.... thats is why I cooled my 40D.. the other reason I use a 40D for my astrophotography is because it's only 10Megpix, and has big pixels, resulting in very good noise control....the higher megapixel cameras have smaller pixels and much higher noise.. I found this to be the case with using my 7D comparing to 40D....

Once I cooled my 40D, the Halpha subs are near noise free until I go over the 15 minute subs length mark.. even than its very minimal... last time I imaged narrowband my 15 min subs had only sporadic noise pixels here and there but the 40D sensor was running at 4-5 degrees C.

The trick is to dither your subs and capture many subs... and before stacking your subs, run them all through a single pixel removal in a program like PS or Gimp.

Dithering will regain some resolution, negate the mottle and cancel out the remaining noise.

 

 

[Likwid]

Great, thank you! I have actually considered going back to my Rebel XS because the T3i noise is unreal sometimes. Back when I was just using a Star Adventurer, I was getting a lot of subs out of necessity. Now I have started using longer exposures and less subs. I'll have to break that habit, or just focus on one specific target for longer than a night or two. It is hard to do when there is a whole sky to explore