15 replies to this topic

[helloworld1234]

I just got a ZWO 294MM to play with. I found I am unable to fully calibrate out the Ha image with flats.

 

Light is 120 seconds with gain 120, temp -10C, and 7nm ZWO Ha filter

Flats are taken with the same filter, same gain, same temperature. Exposure time is 10 seconds with 30000 ADU exposure using iPad with diffuser.  (I also tried 3 seconds with 20000 ADU exposure, same result).

Dark flat of 10 seconds are also taken with the same gain, same temperature as light.

No bias frame is used

PixInsight WBPP with default settings is used to do the integration.

 

I have found that even with that, I can still see faint unevenness in the image showing the flat pattern.

Broadband and O3 images has no such issue and can be fully calibrated. Sii has the same issue though.

Manually calibrated single frame shows the same issue also.

 

Could anyone help with this? Thanks!

 

Flat image:

 

After integration:

Edited by helloworld1234, 27 June 2022 - 11:22 PM.

[hornjs]

Yeah, the 294 is a tricky beast.  Ironically most of the threads I have seen on this are with the color version.

I have trouble calibrating my 294 mm as well, but my problem is mostly broadband.  My Ha gives similar funky pattern to yours, but that one calibrates fairly well for me.

I do use 200 gain on my narrowband subs, though.

My LRGB was partially solved by going to 0 gain on the L and 50 gain on the RGB.  Someone on one of the facebook groups recommended that and trying for a lower ADU.  I can still see a bit of a pattern in my masters, but that is more easily dealt with by pixinsight DBE.  

So I suppose, you could try to increase your gain and see if that helps on your H?  Works for me.  

[CharLakeAstro]

I suspect the issue is present in broadband and Oiii also, but your histogram is stretched further with the Ha and Sii filtered subs, resulting in the issue being more obvious with these 2 filters. 

 

What is your light source for flats?

[helloworld1234]

The light source is diffused iPad 12.9 inch. Will also try ceiling projected led light panel later to see if it makes any differences.

For O3, I have extreme stretched it but didn't see the issue at all.

 

[helloworld1234]

The O3 flat also looks very normal.

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[helloworld1234]

Yeah, the 294 is a tricky beast.  Ironically most of the threads I have seen on this are with the color version.

I have trouble calibrating my 294 mm as well, but my problem is mostly broadband.  My Ha gives similar funky pattern to yours, but that one calibrates fairly well for me.

I do use 200 gain on my narrowband subs, though.

My LRGB was partially solved by going to 0 gain on the L and 50 gain on the RGB.  Someone on one of the facebook groups recommended that and trying for a lower ADU.  I can still see a bit of a pattern in my masters, but that is more easily dealt with by pixinsight DBE.  

So I suppose, you could try to increase your gain and see if that helps on your H?  Works for me.  

Definitely will  try to increase the gain. 200 gain still gives about 12 stops DR.

[Borodog]

+1 for trying 200+ gain on the flats.

[gabebalazs]

You might want to read about "etaloning" on the 294, where certain wavelengths, especially ones close to Ha have some sort of an internal reflection and interference going on, causing these uneven patterns.

[jonnybravo0311]

Don't use an iPad... when I first got my 294, I was using an iPad as my light source. The red (R, S2, Ha) channels all suffered. I changed light sources, S2, Ha, R calibrated fine afterwards. Then I had issues with G... LOL.

 

Once you figure out calibrating a 294, every other camera will seem like child's play

[gabebalazs]

Don't use an iPad... when I first got my 294, I was using an iPad as my light source. The red (R, S2, Ha) channels all suffered. I changed light sources, S2, Ha, R calibrated fine afterwards. Then I had issues with G... LOL.

 

Once you figure out calibrating a 294, every other camera will seem like child's play

So what light source did you switch to, just out of curiosity? I'm using my computer monitor (a pretty good one with no PWM) but I'm still open to better options for the 294. 

[helloworld1234]

Ipad flat calibrates well for my 2600mc using Ha filter. Guess 294 is way more picky.

[fuadramsey]

I use a cheap tracing panel with my 294mm pro and have never had a problem. 

 

I always use the same gain, offset and temp as the lights and do 30 flats per filter.

I shoot for 2.5+ seconds for my flats. And yes my HA flat look similar to yours but it calibrates well. I also use WBPP with default settings. 

[unimatrix0]

It looks to me like any sort of screen device, iPad, computer monitor, telephone is not a good idea to use for flats.   

 

2 Things work for me.  Natural skyflats and a $20 tracing board. Never had issues with either. 

I don't even do the t-shirt. If there are no clouds and the stars are not visible for the camera to pick them up, those flats work all the time. 

I don't have the 294 sensor camera, but if that has issues electronic screen devices, perhaps others do as well, just not as obvious as the 294 makes it to be. 

[helloworld1234]

iPad screen is slightly polarized so it is not that great. But good enough for 2600mm, 1600mm etc.

Sky flat is hard with 294mm sensor since it need to expose for more than 3 seconds. Needs lots of tshirts / paper

Let me try video LED panel + diffuser and see if it works.

[James Peirce]

You could try using diffusion with the screen (e.g. white t-shirt). Any sort of reflections can create their own issues. I also get an odd-looking flat with my Hα filter on the 294MM, but it calibrates perfectly. And make sure nothing else which could create the appearance of poorly correcting flats is involved, such as light shining or reflecting into the optics (moon, streetlight, light leak).

 

Adding to the above thought on gain, this has not been an issue for me at gain 108 (1x1) or 120 (2x2). I’ve also been fine, using diffusion, with screens from a tracing tablet, an iPad, or my flats panel. None of which, I understand, omits the possibility of another issue existing.

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[helloworld1234]

Thanks James and unimatrix0 for helping out. I finally able to calibrate out the 294mm sensor perfectly!

Here is what I did for the flat

1. Use a high quality CRI 95 LED panel video light to light my ceiling evenly.

2. Use t-shirt and diffusive glass as diffuser to achieve 5 seconds exposure for all narrowband filters.

3. Keep the ADU at about 15000 to 20000

 

What I have observed

1. Using iPad is causing issues for 294mm. Sii requires 3 times more exposure than Ha. But when using panel video light, Sii and Ha has the same exposure.

2. Glass diffuser has some reflections. Adding t-shirt layer before the diffusive glass helps eliminating the reflection.

 

See fully calibrated Ha and Sii.