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Help with Darks & Flats

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#26 Forward Scatter

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Posted 21 October 2020 - 08:36 PM

I'm also considering capturing some flats, but am still unclear on a couple of things:
1) Using a cooled camera, do I need to cool to the same temperature for flats as I do for darks?
2) I see that the image train needs to be left unaltered from the time the flats are taken until they are used. I usually remove my FR/Spacers/Camera and store them separately. So I'm guessing my only options are to either a) start storing my optical system intact or b) take new flats during the day before an evening that I'll be imaging.

Thoughts?
David



Hi David
As the gain is set to 0 and the exposures may be 10s of milliseconds, cooling not really needed

#27 mikenoname

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Posted 22 October 2020 - 10:28 PM

So I'm guessing my only options are to either a) start storing my optical system intact or b) take new flats during the day before an evening that I'll be imaging.

 

Since 'a' is not an option for me, how does one take flats with a daylight sky when you don't know what your exact focus point is going to be until it's dark enough to focus on a celestial object? I must be missing something obvious...


Edited by mikenoname, 22 October 2020 - 10:28 PM.


#28 Cey42

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Posted 03 November 2020 - 08:34 PM

Following the advice of replies above and the steps on this post on the SharpCap forums about the ZWO294, I was able to resolve my issues and get a nice flat image.


As to what my problem was, I can only guess because I made several changes at the same time on how I did my flats and darks. These are

  • For darks, I captured them after sunset so the camera temperature would be closer to imaging temperature
  • For flats, I set brightness to zero.
  • I made sure the imaging train did not change. I think this was my original issue.

For my flats, I tried both with and without bias. For me, it seemed like the without bias produced slightly better results.

 

This is another good post on the SharpCap forums on how to capture flats/darks.

 

Thank you everyone for your help.


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#29 alphatripleplus

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Posted 03 November 2020 - 09:09 PM

Glad that it worked out for you.



#30 goldtr8

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Posted 08 November 2020 - 03:04 PM

Hi David
As the gain is set to 0 and the exposures may be 10s of milliseconds, cooling not really needed

Do you mean the brightness is set to zero.   Reading on the SharpCap Forum gain should not be set to zero, for the 294 you want to be above the high gain switch over point around 120 as the minimum gain settino.

 

I am also suddenly having issues so I am going to revisit this topic next time out.    I think it will just be a "flat" party.   I might to a live broadcast and go thru this stuff and then folks can comment back on my errors so I don't have to figure it out in the dark on a cold winter night LOL.



#31 goldtr8

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Posted 08 November 2020 - 03:11 PM

Following the advice of replies above and the steps on this post on the SharpCap forums about the ZWO294, I was able to resolve my issues and get a nice flat image.

As to what my problem was, I can only guess because I made several changes at the same time on how I did my flats and darks. These are

  • For darks, I captured them after sunset so the camera temperature would be closer to imaging temperature
  • For flats, I set brightness to zero.
  • I made sure the imaging train did not change. I think this was my original issue.

For my flats, I tried both with and without bias. For me, it seemed like the without bias produced slightly better results.

 

This is another good post on the SharpCap forums on how to capture flats/darks.

 

Thank you everyone for your help.

May I ask you share your final settings that fixed the flats with your 294 camera, I suspect its more than than the brightness to zero.    I would appreciate the complete workflow from camera settings as in brightness, gain, exposure time and if you pick mono or color flats and with or without bias.   Do you use a luminating panel or just do the early dusk with white T-Shirts or anything else. 

 

I went from not even thinking about flats to what just happened to mess up my stacking.    It was like a switch but I don't know what I did to mess it up or if was a SW update.   Anyway with 3.2.6448 dithering works great now with PHD2.

 

Thanks Don



#32 Cey42

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Posted 08 November 2020 - 05:11 PM

Don, I completely understand your frustration. I thought I had this figured out. Friday night, I went out and my images had horrible vignetting and I thought I had done everything the same. Then on Saturday, I redid my flats and tried something new. The night started off great. Then the stacks got vignetting again. I did figure what caused the mid-night issue which I think is related to my original post.

 

Tonight is clear again and I plan to try a couple more experiments. If everything works, I will report back my steps.

 

 



#33 donstim

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Posted 09 November 2020 - 10:14 PM

May I ask you share your final settings that fixed the flats with your 294 camera, I suspect its more than than the brightness to zero.    I would appreciate the complete workflow from camera settings as in brightness, gain, exposure time and if you pick mono or color flats and with or without bias.   Do you use a luminating panel or just do the early dusk with white T-Shirts or anything else. 

 

I went from not even thinking about flats to what just happened to mess up my stacking.    It was like a switch but I don't know what I did to mess it up or if was a SW update.   Anyway with 3.2.6448 dithering works great now with PHD2.

 

Thanks Don

For what it's worth, I make my flats with the all of the same settings that I am going to use for my lights (brightness, gain, color space, resolution) except for exposure, which is set such that the luminance is about 50% in the SharpCap flats capture tool histogram. With the 294 camera, I am not checking the bias and and am using mono. My flats are taken with a LED light panel covered by a stretched white t-shirt after focusing and deciding what gain(s) I will be using for the evening.

 

The brightness level I use varies with the gain level. The way I determine it uses a method suggested by SharpCap's Robin Glover -- with the camera covered as you would for taking darks, adjust the brightness level so that it just clears the left side of the histogram. You will find that the brightness level needed to do this increases as gain increases.



#34 Cey42

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Posted 10 November 2020 - 08:51 PM

Sunday night turned out to be successful night using the flats and darks. I ran some experiments to confirm what breaks things and what doesn't. I think I have figured out all the ways I can screw up and how to avoid them.

 

Here is my workflow as I have documented for myself. The values you see in parentheses are values specific to me and will probably be different for you. I am using ZWO 294MC uncooled camera and SharpCap. You will see some all caps, that is me a reminder to me to pay attention to this step.

 

 

Physical Setup


Set image train as plan to image
Make sure all spacers and camera are tight
If using filter, place it in
DO NOT CHANGE IMAGE TRAIN. If do so, will need to redo flats
 

 

Sizing


Determine the image sizes that want to use
  (4144x2822, 2816x2822, 1920x1080)

 

 

 

Lights


For imaging, determine values that will be using for
  ○ Exposure (6s, 12s, 24s)
  ○ Gain (300, 400, 450)
  ○ Brightness set to (10)
  ○ White Balance Red set to (50)
  ○ White Balance Blue set to (50)
  ○ Filter (No, Yes)

 

 

 

Flats


Using white t-shirt method during the day. Make sure no direct sunlight is hitting the front of t-shirt. Or can use dew shield.
For each combination of image size and filter of Light values need to capture a Flat.
In my case I have 3 sizes times 2 filter which gives me 6 flats.



STEPS
Make sure there is nothing in the Dark Subtraction "Preprocessing" area
Set Gain to between 200-300
Adjust exposure such that the white channel around 50%. The highest color no higher than 90%
BRIGHTNESS SET TO ZERO. Does not matter what doing for lights
White Balance Red set to value using for lights
White Balance Blue set to value using for lights
No Bias selected
Monochrome selected
(15) frames
Capture
Rename file with size and if using filter. Example with filter 'F4144.png'. With no filter '4144.png'
 

 

 

Darks


Capture after sunset so temperature is within 5-10C of imaging temperature. Camera temperature is shown in SharpCap. (Want to test to see if doing this at room temperature causes issues)
For each combination of size, exposure and gain, need to capture dark
In my case, I have 3 sizes times 3 exposures times 3 gains for 27 total flats

STEPS
Cover lens
Set exposure, gain, brightness, White Bal ® and White Bal (B) the same as you will use for capturing light frames
(15) frames
Capture

 

 

All The Things I Have Done Wrong


Here is a list of all the things I have done wrong while capturing flats and darks or when using them.


• When capturing flat, allowing direct sunlight on the t-shirt
• After capturing flat, forgetting to set brightness back to 10 when capturing dark
• After playing with SharpCap Brain tool, forgetting that it changes brightness and not setting it back to 10. Exposure and gain change but I remember to change those back.
• Not having my image train tight, so it loosens during the night
• After a failed plate solve, changing exposure or gain to plate solve. Then forgetting to change back
• While imaging, changing the exposure or gain for the new DSO and forgetting to change the dark and flat to match

 

 

I hope this makes sense and helps.



#35 Forward Scatter

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Posted 10 November 2020 - 10:50 PM

Hi Cey

To make things easier, you can save the different camera configurations for flats, darks, high gain/short exposure focus, lights, etc in the Camera Profile and just select the one you want at any given time without trying to remember the actual settings or readjusting over and over again. 

 

J



#36 goldtr8

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Posted 11 November 2020 - 06:50 AM

 

Flats


Using white t-shirt method during the day. Make sure no direct sunlight is hitting the front of t-shirt. Or can use dew shield.
For each combination of image size and filter of Light values need to capture a Flat.
In my case I have 3 sizes times 2 filter which gives me 6 flats.



STEPS
Make sure there is nothing in the Dark Subtraction "Preprocessing" area
Set Gain to between 200-300
Adjust exposure such that the white channel around 50%. The highest color no higher than 90%
BRIGHTNESS SET TO ZERO. Does not matter what doing for lights
White Balance Red set to value using for lights
White Balance Blue set to value using for lights
No Bias selected
Monochrome selected
(15) frames
Capture
Rename file with size and if using filter. Example with filter 'F4144.png'. With no filter '4144.png'
 

 

Did you try your flats with and without BIAS did it make any difference.   From some recent posts on the SharpCap Forum that Robin responded to me he suggests having the Bias on, I guess its easy enough to try.    I really appreciate your documenting your work flow.    One other thing that I am going to do is look at my flats with Fits Liberator to see the statistics for the ADU's across the face of the flat and also it any pixels were saturated.   That can be an easy quality check.


Edited by goldtr8, 11 November 2020 - 06:51 AM.


#37 alphatripleplus

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Posted 11 November 2020 - 08:35 AM

Just remember that you do not need to create new darks for each session if you have previously created a dark library of master darks for each exposure/gain/temp/binning combination that you use. Darks in your dark library are generally good for several months.



#38 Cey42

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Posted 11 November 2020 - 01:52 PM

Jay, thanks for reminding me about the camera configurations. I have been meaning use them and now I have a very good reason. It will help the process.

 

Yes I did try with and without bias. I could not tell any difference. For the 294 I have read both that you should and should not use bias. Since I am new at this, maybe my imaging skills are not good enough to tell the difference.

 

Has anyone tested how important it is when capturing darks to have the temperature near what you will be imaging at? It would be so much easier to capture darks whenever I want in the house.

 

Thanks,



#39 alphatripleplus

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Posted 11 November 2020 - 03:06 PM

From my experience, getting the temperature within 5C ( darks vs lights) is usually good enough for the short exposure darks typically used in EAA. At 10C difference, I can usually tell the darks are mismatched to the lights, particularly if it is warm and you are at high gain.



#40 Forward Scatter

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Posted 11 November 2020 - 07:06 PM

Jay, thanks for reminding me about the camera configurations. I have been meaning use them and now I have a very good reason. It will help the process.

 

Yes I did try with and without bias. I could not tell any difference. For the 294 I have read both that you should and should not use bias. Since I am new at this, maybe my imaging skills are not good enough to tell the difference.

 

Has anyone tested how important it is when capturing darks to have the temperature near what you will be imaging at? It would be so much easier to capture darks whenever I want in the house.

 

Thanks,

Fortunately bias frames offer really no advantage in EAA even with a 294 or similar cam (533); darks & flats are more important. The vignetting and dust mote donuts really pop up when stretching the livestack histogram. And those darned hot pixels! Always next the object one wants to image, even when using the master dark library. So I sometimes autoguide and dither, then get rid of them with sigma clipping.

 I agree with Errol on his previous post about the temps with darks. Not as much of a problem in winter, but in summer it can be a bit annoying. We'd all love to get to -35oC with the TEC cooling, but it's often a struggle to get to and maintain -10oC during the warmer nights. The darks help a lot, but the dithering helps to exclude the hot pixels the darks miss.

But with the 294, it's easy to make a master dark library at a variety of gain settings, binning, temps and exposures as mentioned before. It's what cloudy nights are great for. And we still get to play with our toys!

 

 J


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