@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

@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!

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! 

@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

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

@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.

@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

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.

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.  

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.

@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:-

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

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

Regards

Stephen 

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

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

Regards,

Stephen.

@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 

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

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

Coloured ring still much in evidence.

Camera histogram for a sample flat

Master flat, neutralised BG

The coloured ring is still there 😣  

Sample flat as APP sees it:

Sample flat neutralised BG, again a slight ring

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

Any more thoughts much appreciated....

Stephen 

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. 😉

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.

@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 😉