ASI 294MC / Optolong L-Extreme Dual band Filter producing weird Flats

[PadrePeace]

I'm shooting Sh2-129 (Squid Nebula) with a ZWO 294MC and the L-Extreme Dual Band filter.  Scope is a WO71GT flattened and reduced to 336mm FL (0.8). When stacked in APP I'm getting OIII flats (first image) and Ha Flats (second image) looking as below. Any ideas why the is so much brightness on the right hand side of the OIII and why the Ha looks so different?  The Flats where shot in NINA's Flats tool at 5.63s exposure and the same camera settings as the lights.  I have discounted light leaking into the sensor during FLAT production.  Any other comment on what others may be getting with the same combination would be appreciated. 

 

Ha Flat

This is the OIII stacked using the Master Flat above:

This is the Ha stacked with the Master Flat above:

 

Finally, this is the Master Flat in RGB. Hope the colours might give a clue to what is going on here.

 

This is only 10.5hrs or integration so plenty left to do I guess.......

regards

Padre

Edited September 17, 2021 by PadrePeace

[Adam J]

As discussed something seems wrong about this but I have no first hand experience with this camera or filter. I know that you can get blotchiness in the flats with this camera though. Someone must have experience of this combination. 

[Budgie1]

I found with ASI294MC Pro and the L-eXtreme that if the exposure of the flats was too short then you get some strange results. But with 5.63s that shouldn't be the issue, what ADU are you aiming for?  

I normally aim for an ADU of 25000 and exposure of 1.5s to 3s, then replicate the Flats with Dark-Flats as well. I use APT's Flats Aid to produce my Flats and point the scope at a cloudy sky with  a white t-shirt over the scope. I then add layers of t-shirt until I get the exposure and ADU I'm looking for. I've also started taking the flats with the camera cooled to the same temp as the lights, this may not have a bearing but I thought it wouldn't hurt.

This is one of my last Flats taken with the ASI294MC Pro with the L-eXtreme fitted, 1.5625s exposure, 120 gain, offset 20 and sensor temp on this one was +17°C. You'll see I also have the light corners on the right hand side of the image but it doesn't seem to effect the final stack.

 

[PadrePeace]

Budgie

Thanks for chiming in with you experiences. I also cool to -10 to match my subs though I know that’s not necessary. ADU was 23000, Gain was 120 with the offset at 8. I used a white screen on an iPad as the source which has been fine for other CMOS cameras but could be the issue here. I’ll try sky flats tomorrow and report back. 

[Budgie1]

2 hours ago, PadrePeace said:

Budgie

Thanks for chiming in with you experiences. I also cool to -10 to match my subs though I know that’s not necessary. ADU was 23000, Gain was 120 with the offset at 8. I used a white screen on an iPad as the source which has been fine for other CMOS cameras but could be the issue here. I’ll try sky flats tomorrow and report back. 

I used to use a blank MS Word document on my PC screen as the light source with my DSLR, but that never really worked with the ASI294MC. Sky flats seem to be work fine, so I've stuck with those.

I started this season by reducing the cooling from -10 to -15°C (to reduce the dark current), gain 120 and went for offset 20. But found that offset 20 seems to give more noise on the image, so I've gone back to the ZWO default of 8. I still need to re-do my Darks for these settings though.

[sharkmelley]

It's quite normal to these these large scale background patterns in H-alpha.  See here for instance in a long Cloudy Nights thread discussing this very point:

https://www.cloudynights.com/topic/661685-word-of-warning-asi294mc-pro-and-opt-triad-and-nb/?p=9458529

Mark

[GoldTop57]

I'm getting similar looking flats with this combination too. I initially thought it might be the filter but when I removed it from the imaging train and recreated the flats with the same exposure time I couldn't see any difference. I've seen the long thread on Cloudy Nights but to be honest most of it was beyond me. I think I'm getting reasonable results with dark flats and no bias frames.

[PadrePeace]

I don’t use bias either. 
Took some sky flats today at 2300ADU. Then took some more with my flat panel (not the IPad). Interesting outcomes. 

Sky Flat

Flat Panel (not the iPad)

 

seems the iPad and the sky flats have significant glow down the rhs which my flat panel does not. The Histogram on the latter looks a lot better as well. 

[Adam J]

4 minutes ago, PadrePeace said:

I don’t use bias either. 
Took some sky flats today at 2300ADU. Then took some more with my flat panel (not the IPad). Interesting outcomes. 

Sky Flat

Flat Panel (not the iPad)

 

seems the iPad and the sky flats have significant glow down the rhs which my flat panel does not. The Histogram on the latter looks a lot better as well. 

To me this says that the filter is leaking something through that's in the sky flats but not emitted by the panel. So UV or IR maybe.