Background artifact issue: Page 3

Background artifact issue

jonesdee

I am waiting for a clear night to try Dunc's Offset setting. My concern would be that the blacks could be clipped with such a low offset value (?)

I have rectified the artifact by generating artificial flats from my stacked images before applying any other post-processing, however it would be better to eliminate it at the capture stage if possible.

Dave

Support@ZWO

Looking forward to test result.

jonesdee

Hi Sean

I am cautiously optimistic but have not been able to do conclusive testing so far thanks to the UK weather. A few nights ago I tried the same target (M51) but seeing was poor and the moon was bright, so my exposure length was very limited. Using the standard Unity Gain setting with Offset 30, 10 second exposures gave a histogram range of 2k to 65k (saturated highlights). Reducing the Offset to 14 allowed me to increase the exposure to 20 seconds, with a range of 800 to 65k, so better dynamic range than the standard setting.

I shot 150 subs and although the image quality was poor (due to bad seeing and insufficient target data), the purple/green artefact was definitely less noticeable than usual in the stacked image, but could still be seen by stretching and increasing saturation, so it was not entirely eliminated. I checked Dark frames shot at this setting and the bottom end was not clipped as I originally feared, so calibration was not impacted.

I'm definitely encouraged by this, but would like to repeat the test on a better night, first using standard unity gain setting, then an appropriate combination of low Offset, 120 Gain and exposure to suit. This should give a good comparison of the impact on the colour issue. Sadly the moon is full this week, so it will be a while before I can complete the experiment :(

Regards

Dave

Support@ZWO

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franksd

Hi--

I've had my ASI294 almost two years but only used it infrequently. Now I have time. . .but I'm getting weird mottled color through the image after I remove the background using Pixinsight ABE. From this thread I'm now suspecting my offset is too high. My camera doesn't seem to have it locked down but is variable; at least, in APT I can call up the camera settings and adjust it. I've been shooting at 40 offset, unity gain. These subs were 4 minutes in Bortle 6 skies of San Diego, CA. The subs look OK; an autostretch of them looks OK. But when I fully process with darks, flats and bias frames things get weird. Even before the stretch the histogram is pretty far to the right; I can't get a good black and the weird color artifacts show up. I've tried some screenshots that may help (or confuse).

My impression from this thread is that it's all in my shooting. I don't think 2-minute subs will get me much at unity gain, but should I try that? Or should I change the Offset from 40 to something less? Thanks for whatever advice you can provide.

- Frank

wsg98

M94+small.jpg

ZWO asi294 MC Pro: 400@ 30 seconds, unguided, darks only, Bortle 4-5.

gain: 121

offset/ brightness: 4

per

I think the artifact issue is addressed by 3 things

1) no flat calibration frames under 1 second I’ve heard even 3 seconds with max of 8 seconds. No bias frames.

2) offset should be low, I use 4 some others use 10 and 14

3) dither light frames to reduce background mottle. I dither every second sub, frequency will vary with exposure length.

I’m not an experienced astrophotographer but I’ve picked this up these 3 strategies from forums and YouTube and have put them into practice.

Support@ZWO

We have two ways to reduce color patterns of 294pro camera.

1. DBE or SCNR function in Pixinsight, for long exposure frames.

2. Shorter exposure time and stack more frames, this is helpful, some users has tested.

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