Help, please - some...
 
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Help, please - something is wrong with my integration 🙁  

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(@stephent)
Main Sequence Star Customer
Joined: 1 year ago
Posts: 27
October 18, 2020 21:53  

Hello all,

Any thoughts appreciated!!

My integration looks like this:

APP integration

Clearly something is wrong!

Am using a Canon 5d mkiv with a Skywatcher ED80, no filters. 

172x60s .CR2 lights, 37 flats (t-shirt, pale blue sky), 59 darks shot same night as lights, 33 bias & 31 dark flats. All ISO 3200.

Single session, default settings, Version 1.083 used.

Here is a sample Light:

APP1

Master flat:

APP master flat

 Master dark:

APP master dark

Master bias (note dark band at bottom 😫):

APP master bias

Master dark flat:

 

APP master dark flat

Any pointers would be much appreciated. Flats don't appear to be working as intended, there is a weird circular "rainbow" on the integration and an obvious issue with banding at the bottom of the image...

 

Hopefully it's something simple!!!

 

Many thanks

Stephen 


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(@guidoforrier)
Brown Dwarf Customer
Joined: 3 years ago
Posts: 8
October 18, 2020 22:37  

@stephent

did you use a 2" or a 1,25" adaptor to the camera ? a 2" focuser is OK for a full frame camera but a 1,25" will give you vignetting .

Guido


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(@stephent)
Main Sequence Star Customer
Joined: 1 year ago
Posts: 27
October 18, 2020 23:06  

@guidoforrier

 

Hi,

ED80, Feathertouch focuser, a field flattener and a standard 2" T adapter for EOS.

Kind regards,

Stephen.

 

P.S. here is the integration result with just darks. No circular rainbow effect, minimal banding, and fairly uniform vignetting. Puzzling!

 

APP darks only

 


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(@rkmvca)
Brown Dwarf Customer
Joined: 1 month ago
Posts: 6
October 19, 2020 07:09  
Posted by: @guidoforrier

@stephent

did you use a 2" or a 1,25" adaptor to the camera ? a 2" focuser is OK for a full frame camera but a 1,25" will give you vignetting .

Guido

Hi, I have the same problem as Stephen T, only worse!  This is the first I've heard about vignetting from a 1.25" adapter.  Can you elaborate on this?  

I shoot flats and the flats absolutely cannot get rid of my terrible vignetting -- and I have a 1.25" adapter for my Canon T7!

Any suggestions or help are appreciated! 

This post was modified 1 month ago by Rich Klein

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(@guidoforrier)
Brown Dwarf Customer
Joined: 3 years ago
Posts: 8
October 19, 2020 12:09  

@rkmvca

as your Canon T7! has a aps-C sensor the vignetting can not come from the 1,25" focuser/adaptor . so I can not find out what happened . is APP to blame ? I can not see why . I use a Sony A7R II = mirrorless and I have no vignetting with a 2" focuser .  I suppose there must be a physical obstruction .

Guido

 


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(@wvreeven)
Black Hole Admin
Joined: 2 years ago
Posts: 280
October 19, 2020 13:51  

What exposure time did you use for the flats? And I usually use a white wall, door or towel indoors where I can regulate the exposure time for the flats by opening and closing curtains and doors until the illumination is correct. I aim for the histogram peak to be between 40% and 60% when inspected with HistogramTransformation in PI (without actually performing a transformation of course).

 

HTH, Wouter


Stephen T liked
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(@stephent)
Main Sequence Star Customer
Joined: 1 year ago
Posts: 27
October 19, 2020 14:50  

@wvreeven

 

Hi Wouter.

Flats were shot at 1/1000s to get a nice peak in the centre of the histogram.

It looks to me as if the master flat is over-applied on one side of the image and under-applied on the other. The circular rainbow effect in the final integration is another mystery....

Regards,

Stephen.

 

 

This post was modified 1 month ago by Stephen T

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(@wvreeven)
Black Hole Admin
Joined: 2 years ago
Posts: 280
October 19, 2020 15:28  

@stephent

Hi Stephen,

In my experience with DSLR cameras you need to take flats with an exposure time of at least 1 second. The reason is mirror flip which takes a considerable amount of time compared to short exposure times, like the 1/1000 second that you use. This introduces an uneven exposure over the FOV resulting in artifacts like seen in your image. So, retake the flats and the dark flats and you should be fine. Just add more layers of white t-shirt if the sky is too bright, or take your setup inside and do it there. Just make sure to use a naturally illuminated room and try not to use artificial light since the spectrum of those sources messes up the flat field correction again.

Also, when using both dark flats and darks you don't need bias anymore. This is what I do with all my CMOS data (both Canon EOS and ZWO cameras): lights, darks, flats and dark flats. Works like a charm.

 

HTH, Wouter


Stephen T liked
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(@stephent)
Main Sequence Star Customer
Joined: 1 year ago
Posts: 27
October 19, 2020 16:44  
Posted by: @wvreeven

@stephent

Hi Stephen,

In my experience with DSLR cameras you need to take flats with an exposure time of at least 1 second. The reason is mirror flip which takes a considerable amount of time compared to short exposure times, like the 1/1000 second that you use. This introduces an uneven exposure over the FOV resulting in artifacts like seen in your image. So, retake the flats and the dark flats and you should be fine. Just add more layers of white t-shirt if the sky is too bright, or take your setup inside and do it there. Just make sure to use a naturally illuminated room and try not to use artificial light since the spectrum of those sources messes up the flat field correction again.

Also, when using both dark flats and darks you don't need bias anymore. This is what I do with all my CMOS data (both Canon EOS and ZWO cameras): lights, darks, flats and dark flats. Works like a charm.

 

HTH, Wouter

Many thanks for the advice. I will try what you have suggested. 

Regards,

Stephen.


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(@rkmvca)
Brown Dwarf Customer
Joined: 1 month ago
Posts: 6
October 19, 2020 16:49  
Posted by: @guidoforrier

@rkmvca

as your Canon T7! has a aps-C sensor the vignetting can not come from the 1,25" focuser/adaptor . so I can not find out what happened . is APP to blame ? I can not see why . I use a Sony A7R II = mirrorless and I have no vignetting with a 2" focuser .  I suppose there must be a physical obstruction .

Guido

 

I don't blame APP for the vignetting; this has been a consistent problem of mine and I was hoping that APP would help me get rid of it!

I'll stop hijacking this thread and start a new one in a couple of days with my setup and some images.  Thanks.  


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(@vincent-mod)
Universe Admin
Joined: 3 years ago
Posts: 2653
October 20, 2020 11:19  

The over correcting of the lights is because this seems to be an issue in the flats themselves. I can see a brighter part around the flat there which apparently is not there when you take the lights. Therefore it corrects those too much. There should be some hardware difference that causes that lighter ring around your flats to appear.


Stephen T liked
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(@stephent)
Main Sequence Star Customer
Joined: 1 year ago
Posts: 27
October 20, 2020 17:39  

@vincent-mod

Hi Vincent, thank you for your reply.

I have today taken some new flats using 4 layers of T shirt against my dell monitor set to white. Exposure time was 10 seconds each as opposed to 1/1000s previously and as recommended by Wouter @wvreeven above. 21 flats yielded this master flat:-

APP master flat 2

The same number of dark flats were created & used. Bias were omitted this time. Integration looks like this:

APP integration 2

I can't really say the result is better than the first attempt. Rainbow circle is still there and the vignette correction is still wrong...

I am wondering about trying to de-saturate the flats to see if that helps? Clearly I need to find a better light source as the monitor skews the histogram in the red channel

PXL 20201020 153007986

Regards

Stephen 

 

This post was modified 1 month ago by Stephen T

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(@wvreeven)
Black Hole Admin
Joined: 2 years ago
Posts: 280
October 20, 2020 17:55  

Hi Stephen,

I had a similar experience where I used an iPad for my flats. That also introduced all kinds of artifacts in my stacks. This is why I recommended you to use a natural light source. No light bulbs, especially led, and no tv or computer screens. Just a white wall or towel illuminated by daylight entering the room via a window is more than enough. Place the telescope at some 30 to 50 cm from the wall or towel and take your flats.

 

HTH, Wouter


Stephen T liked
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(@stephent)
Main Sequence Star Customer
Joined: 1 year ago
Posts: 27
October 20, 2020 18:00  

Here is the new master flat with background neutralised. The circular rainbow effect is here. Maybe that is a clue?

APP master flat 2 neutralised

 

Regards,

Stephen.


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(@stephent)
Main Sequence Star Customer
Joined: 1 year ago
Posts: 27
October 20, 2020 18:07  

@wvreeven

 

Thanks Wouter,

I couldn't find anything evenly illuminated by natural light and the clouds were very variable today, but I will keep trying - and looking at the histogram more closely!!

However the fact remains that the circular rainbow appears in both my old and new master flats both natural light and artificial when the background is neutralised.

Regards

Stephen 


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(@stephent)
Main Sequence Star Customer
Joined: 1 year ago
Posts: 27
October 20, 2020 18:20  

OK I have now got some flats with everything RGB in the middle of the histogram. Back later....


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(@stephent)
Main Sequence Star Customer
Joined: 1 year ago
Posts: 27
October 20, 2020 21:39  

Ok... New integration with better flats (24 x 1 second, white wall, natural light) new dark flats also:-

APP integration 3

Coloured ring still much in evidence.

Camera histogram for a sample flat

PXL 20201020 163256660 1

Master flat, neutralised BG

APP master flat 3 neutralised

The coloured ring is still there 😣  

Sample flat as APP sees it:

APP sample flat

Sample flat neutralised BG, again a slight ring

APP sample flat neutralised BG

and finally the highest score light frame, neutralised BG, no ring....

APP sample light neutralised BG

Any more thoughts much appreciated....

 

Stephen 


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(@vincent-mod)
Universe Admin
Joined: 3 years ago
Posts: 2653
October 21, 2020 12:15  

These issues can be very difficult to pinpoint, but this is starting to look like internal reflection in your setup. Not sure though, but what you could try is to take your integration without flats, then use the light pollution correction tool to get rid of the gradient as much as possible. Is a ring visible there (just fainter)?

I'm going to ask for the data soon. 😉


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(@wvreeven)
Black Hole Admin
Joined: 2 years ago
Posts: 280
October 22, 2020 09:35  

Is the camera modded in any way? Did you cover the view finder? Sometimes light enters through it and since you take the flats during daylight and the lights at night that may explain it if no covered. Just some guesses here.


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(@stephent)
Main Sequence Star Customer
Joined: 1 year ago
Posts: 27
October 23, 2020 15:34  

@vincent-mod

Thanks Vincent, no, the stretched integration without flats does not show the ring. I am reaching the same conclusion 🙂 

I switched to using a Canon 100-400mm lens last night for some subs. Haven't taken flats yet but will post the results back in here when I have. 

I perhaps mistakenly tnought that flats were only for controlling luminance so the addition of the rainbow is puzzling. I wonder if its some sort of diffraction from the Hotech field flattener I use.

Anyway, the flats with the 400mm should help point in the right direction.

Incidentally I tried stacking in DSS and you will be relieved to hear the results were the same 😉 


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