15 replies to this topic

[andrevilhena]

Hello everyone,

 

I have just acquired the Optolong L-Extreme and when using it, I noticed some strange flats frames and I was wondering if someone can help understanding them.
The camera I use is the ZWO ASI204MC Pro and the scope is the RedCat51.

 

The first time I used this filter I noticed the flat frame had a dark area in the middle, instead of a radial uniform gradient:

 

I went to analyze previous flats with the same setup but with the Optolong L-Pro and situation seems similar.

 

In the meantime I noticed that I was not using the correct backfocus. William Optics states a BF of 59.7mm and I was using 34 mm. I don’t have adapters for the correct BF and only managed to get 56.5mm and went shooting the Horsehead Nebula that way (using the L-Extreme).

The light frames showed a kind of reddish frame around the photos and that frame is also visible on the flats:

 

 

For me this flat is very weird, since it has alternating layers of brighter and darker areas. Besides, the inner dark “rectangular” shape matches the reddish frame I mention in the light frames but does not disappear after calibration.

 

To dig a bit more I made the following four flat frames:

 

BF = 35.5mm, No filter     >> 
BF = 35.5mm, L-Extreme >> 
BF = 56.5, No filter           >> 
BF = 56.6, L-Extreme       >> 

 

Looking at these images, it seems to me:

 

  >> Flats without filters seem to have a standard radial gradient
  >> Flats with L-Extreme show inner darker areas

 

It seems clear that the filter considerably changes the flat frame but I’ve never seen any mention to it, despite being a quite popular filter. Does anyone have any clue on what may be happening here and how to correct it?

 

Cheers,

 

André

[ZL4PLM]

if thats a 294 then yes some filters do show up this is which is why the flats are important 

 

I noticed some similar mottling on my 294mm but not all bands HA shows it more 

 

but it calibrates fine - some of what you see looks like dust motes too

 

cheers

 

Simon 

[Dynan]

Can you post your Flat Stats? And maybe a Histogram?

 

ASI 204MC-Pro...not familiar with that model. ZWO seems to be down right now so no specs are available. Is it 14 or 16 bit?

[andrevilhena]

Can you post your Flat Stats? And maybe a Histogram?

 

ASI 204MC-Pro...not familiar with that model. ZWO seems to be down right now so no specs are available. Is it 14 or 16 bit?

 

Attached you can find the image that showed problems (flat calibrated only), as well as data on the stacked flat. The camera is the ASI294 (sorry for the typo).

 

 

if thats a 294 then yes some filters do show up this is which is why the flats are important 

 

I noticed some similar mottling on my 294mm but not all bands HA shows it more 

 

but it calibrates fine - some of what you see looks like dust motes too

 

cheers

 

Simon 

 

I understand that but the thing is that the flats don't seem to correct this issue. And in the meantime, I found a thread on a similar topic (https://www.cloudyni...c-pro-blotches/). The pattern here is the same and in a related topic (https://www.cloudynights.com/topic/661685-word-of-warning-asi294mc-pro-and-opt-triad-and-nb/?p=9374592), the pattern found was photographed on the sensor.

The conclusion was that it seems to be a defective sensor.

 

What can I do if this is the case?

Attached Thumbnails

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[WoodlandsAstronomer]

Did you say what the light source is? Broad continuous spectrum light source required when shooting flats with Lextreme

[Tayson82]

View of flats is not important. Important is how the works.

In stacking process You need also bias, but maybe for Yurs camera better will be use darkflat. 

Bias=DarkFlat

Do not forgot add Darks

 

first screen it maybe some fog on camera or inside tube.

Edited by Tayson82, 23 December 2020 - 05:14 AM.

[tjugo]

Hi, my issues with the Lextreme filter are limited to halos on the green/blue channel. Flats work just fine.

 

Cheers,

 

José

[pathint]

I get the same central dark spot from dew on the optical window if I cool the camera too fast. This is independent from filters. Raising the temperature to ~0C and then recool takes care of that.

 

On the other hand, I have an ASI294MC with L-Enhance. Flats also give me a lot of problems. I see uneven color distribution between red and green around the edge of the sensor like yours. Not only it shows up in the flats, but also in actual images as well. It is then an art to adjust the method of taking flats so it can cancel out the color pattern in the calibrated image.

[Xentex]

The circles are clearly dust specs.  Based on the size, they're on the glass window on the camera and not on the filter.

 

The darker blotches are likely from the AR coating on the same glass window on the camera.

 

You're looking at stretched flats, so keep in mind that whatever difference exists in the data is going to be stretched to cover the full black to white spectrum.  Based on the scale shown to the right side of the first flat, there is about 3% less sensitivity in the dark central blotch than the areas around it.

 

On my ASI6200 I get similar blotches of light falloff in Ha and Sii.  (Much less in Oiii).  The L-extreme is dual narrowband, so your L-extreme flat should look about like the blend of my Ha and Oiii flats if we have the same AR coating.  It does.

 

If you want to verify that this is what's going on take a flat, rotate the camera relative to the rest of the optical train, take another flat, and compare them.  No need to do anything difficult or fancy.  Just unscrew the camera a 1/4 turn from whatever is screwed directly in front of it.  You only have to leave it loose like that for one flat.

 

If the blotches keep the same orientation they're on the glass window (or the sensor itself).  Given that yours are similar in intensity to mine, and with both have ASI cameras, I feel pretty confident it's the AR coating.

 

This is what flats are for and in my opinion there's absolutely nothing wrong or unusual about yours.

Edited by Xentex, 24 December 2020 - 08:45 PM.

[andrevilhena]

The circles are clearly dust specs.  Based on the size, they're on the glass window on the camera and not on the filter.

 

The darker blotches are likely from the AR coating on the same glass window on the camera.

 

You're looking at stretched flats, so keep in mind that whatever difference exists in the data is going to be stretched to cover the full black to white spectrum.  Based on the scale shown to the right side of the first flat, there is about 3% less sensitivity in the dark central blotch than the areas around it.

 

On my ASI6200 I get similar blotches of light falloff in Ha and Sii.  (Much less in Oiii).  The L-extreme is dual narrowband, so your L-extreme flat should look about like the blend of my Ha and Oiii flats if we have the same AR coating.  It does.

 

If you want to verify that this is what's going on take a flat, rotate the camera relative to the rest of the optical train, take another flat, and compare them.  No need to do anything difficult or fancy.  Just unscrew the camera a 1/4 turn from whatever is screwed directly in front of it.  You only have to leave it loose like that for one flat.

 

If the blotches keep the same orientation they're on the glass window (or the sensor itself).  Given that yours are similar in intensity to mine, and with both have ASI cameras, I feel pretty confident it's the AR coating.

 

This is what flats are for and in my opinion there's absolutely nothing wrong or unusual about yours.

The pattern follows the camera frame, which means they're camera related. I absolutely agree that flats should correct but actually they seem to overcorrect. I have tried flat exposures froms 2 to 10 seconds, mean ADU from 30 to 75%, sky flats, screen flats and nothing seems to solve.

I'll keep working on this to check if I find any setting.

[Rasfahan]

I just had a similar problem with overcorrecting flats on my ASI183MC. I redid my dark library (including flatdarks) and that fixed it. Some weeks before the old library had worked perfectly.

[mralpaca]

I had the EXACT same problem with my 294mc-pro and L-extreme. Very similar pattern to you too. The way I fixed this was to nearly overexpose my flats. I create my flats in sharpcap, which has a handy histogram. I push the exposure all the way to the right to JUST before it clips. I use an iPad with a white PDF open and a white tshirt over the end of the scope. Doing it this way fixed my issue entirely- before this I was getting completely unusable data. 

 

I also took 50 flats and averaged them, as less flats created some weird artifact in the middle of the image. Here are my approximate settings:

- Cool camera to the shooting temperature (I am not sure why this is needed, but this fixed the problem for me)

- Find the proper exposure in the flat wizard in sharpcap (for me, it was around 3.75s exposure at 121 gain (my shot gain)) [I am using a redcat and another scope, both at around f4.5 or so]

 

That is it, it fixed my issue entirely. 

Edited by mralpaca, 05 January 2021 - 05:12 PM.

[andrevilhena]

I had the EXACT same problem with my 294mc-pro and L-extreme. Very similar pattern to you too. The way I fixed this was to nearly overexpose my flats. I create my flats in sharpcap, which has a handy histogram. I push the exposure all the way to the right to JUST before it clips. I use an iPad with a white PDF open and a white tshirt over the end of the scope. Doing it this way fixed my issue entirely- before this I was getting completely unusable data. 

 

I also took 50 flats and averaged them, as less flats created some weird artifact in the middle of the image. Here are my approximate settings:

- Cool camera to the shooting temperature (I am not sure why this is needed, but this fixed the problem for me)

- Find the proper exposure in the flat wizard in sharpcap (for me, it was around 3.75s exposure at 121 gain (my shot gain)) [I am using a redcat and another scope, both at around f4.5 or so]

 

That is it, it fixed my issue entirely. 

 

Hello mralpaca,

 

Thanks a lot for the advice - I'll try to use it next time.

 

Actually, in the meantime I found that integrating subs from both sides of the pier cancel this pattern in the final image with a "regular" flat. But if you integrate separately each side, the pattern will appear. This explains why the first time I used this filter I haven't noticed.

 

André

[Alexl007]

I had the EXACT same problem with my 294mc-pro and L-extreme. Very similar pattern to you too. The way I fixed this was to nearly overexpose my flats. I create my flats in sharpcap, which has a handy histogram. I push the exposure all the way to the right to JUST before it clips. I use an iPad with a white PDF open and a white tshirt over the end of the scope. Doing it this way fixed my issue entirely- before this I was getting completely unusable data. 

 

I also took 50 flats and averaged them, as less flats created some weird artifact in the middle of the image. Here are my approximate settings:

- Cool camera to the shooting temperature (I am not sure why this is needed, but this fixed the problem for me)

- Find the proper exposure in the flat wizard in sharpcap (for me, it was around 3.75s exposure at 121 gain (my shot gain)) [I am using a redcat and another scope, both at around f4.5 or so]

 

That is it, it fixed my issue entirely. 

 

 

Unfortunately, I have the same problem! L-eXtreme and asi294Mc Pro, redcat51 .. The red shades on lights frames .. I'm pretty new to astro photography with ccd camera and do not know exactly how to remove those red shades.
Some write that you should take Flats frames WITHOUT l-extreme filters (?), Some say you should NOT take Bias frames, some say you should take ALL frames (ie Flats, Darks and Bias) WITH l-extreme, some say WITHOUT l -extreme!

Maybe you could write a little more about how exactly to do?
Fold program You use to stack them. DSS? APP? Siril? Pixin?

 

thx

[FiveByEagle]

Isn't this a known issue with the 294MC Pro?
 

I started a thread on the ZWO group asking between the new cameras and was informed of it. I could be mis-remembering but Trevor Jones, Astrobackyard, experienced the same

[rnshagam]

Has there been any resolution on the issue?  I'm using an ASI294mc pro and an L-extreme filter in two different OTAs and using a tablet as the light source with each OTA setup.  My flats look wonky in the red Ha channel, but more or less ok in green and blue OIII. I'm playing around in SiriL with constructing a synthetic flat, in which I take the green channel flat and copy it over to the red and blue channels.  So far, not great results, but I'll keep trying to fiddle with the levels in the file, which would correspond to changing ADU values.

 

Any other suggestions are appreciated.