Jump to content

sgl_imaging_challenge_2021_annual.thumb.jpg.3fc34f695a81b16210333189a3162ac7.jpg

Why do my tablet flats over-correct?


Richard_
 Share

Recommended Posts

Hi all,

I've run into a bit of a strange instance when calibrating my OSC subs using my master flat generated from a white tablet screen, especially with my L-eXtreme filter. One corner seems to overcorrect using these flats, however, when I use skyflats I don't see this overcorrection and the end result looks pretty good. Whilst the answer is clearly "take skyflats", I'm not always fortunate enough to take flats first thing in the morning (weather wise, it is the UK!) so it would be great to understand what's going on with my tablet and how to avoid this issue. Details are below, but my questions are:

  1. How can I prevent over-correction with tablet flats? Am I over-exposing them?
  2. If I change my light source, will I see this same issue if using an EL panel (e.g. like this one )or a bespoke flat field generator (like those provided by Geoptik, Pegasus Astro etc.)
  3. Is there anything in the PI WBPP 2.0 script which can help identify overcorrection and adjust the amount of correction applied?

 

Acquisition

For the tablet method, I capture my flats using the auto-exposure tool in my ASI Air Pro (it calculates the optimum exposure time based on the light source presented). The light source is my tablet display at 100% brightness, no blue-light/night time filter, and displaying this blank, white page provided by Covington on the screen. The display is large enough to cover the entirey of my Redcat 51. I attach 2 layers of t-shirt to the front of the telescope and the camera used is an ASI533. It's a small sensor, so I wouldn't expect to see a significant amount of vignetting compared to a full frame camera. For the skyflats, I used the same approach but used 4 layers of t-shirt isntead of 2 to ensure I don't end up with too short of an exposure time.

The exposure time has been adjusted to ensure the average value of the individual frame is ~30,000 ADU for a 14-bit image. For the tablet flats, this results in a 7 second exposure using 2 layers of t-shirt. For sky flats (overcast day), this results in 180ms flats using 4 layers of t-shirt to ensure the exposure time wasn't too low. The optical train had not been disturbed in this time, and both approximate exposures were determined by the ASI Air Pro auto-expose feature (I rounded them up when shooting). There were 30 x flats for each case. The tablet flats were taken at midnight, the skyflats were taken in the morning. Other than this, there was no difference in imaging conditions.

Processing

All stacking is performed in PixInsight using the WBPP 2.0 script. All settings under flat are left at default settings. The script was executed twice: the first was using tablet flats, the second was using skyflats. No other changes were made in the stacking process.

Flat Images and MasterLights

Below are four screenshot images. The top row shows debayered master flats (purely for illlustration purposes), the bottom row shows the final stack. The left side shows the tablet flats, the right side shows the skyflats. The statistics are from the master flat, pre-debayered, and show there there is a small sifference in mean values. However, the images show much brighter corners for the tablet flats, especially in the lower left where I see over-correction. You can see this clearly in the master light images. Looking at the master light which was calibrated using tablet flats, you can see a red cast over the lower left corner which I suspect is overcorrection from the flat. However, you do not see this on the master light which was calibrated using skyflats, the background looks fairly uniform.

image.thumb.png.dc9d7c7b3701dfbbf792504e6bc4cda7.png

 

Link to comment
Share on other sites

What scope are you using?

One thing to try would be to add long dew shield kind of shroud made out of very dark material (flocked on the inside) and take your flats like that.

Flat panels work well with baffled and flocked scopes. Some scopes can't cope with that much light hitting inside of the scope at different angles. Newtonians are prime candidate for this sort of behavior.

  • Like 1
Link to comment
Share on other sites

1 hour ago, michael8554 said:

I'm used to seeing Flats that have an even overall colour caste, not Flats that are red in the middle and green at the edges ?

Michael

I'm using a OSC camera (ASI533) with the dual narrowband L-eXtreme filter. I understand that flats don't require debayering for normal calibration and stacking, but the images above were debayered just to illustrate the brighter corners (for some reason, I struggled to show the brighter lower corner as a good comparison of the master flats without debayering). So perhaps the colour cast is a consequence of this filter type and the fact that the master flats have been debayered? I could try taking flats using just my UV/IR cut filter to determine if I see the same thing in the corner?

 

1 hour ago, vlaiv said:

What scope are you using?

One thing to try would be to add long dew shield kind of shroud made out of very dark material (flocked on the inside) and take your flats like that.

Flat panels work well with baffled and flocked scopes. Some scopes can't cope with that much light hitting inside of the scope at different angles. Newtonians are prime candidate for this sort of behavior.

Per my OP, the telescope is a Redcat 51. The dew shield is always extended during imaging and acquisition of flats. The dew shield has the original black flocking on the inside and has not been replaced. I couldn't comment on how effective the black flocking is as I have nothing to compare it to.

If the issue is too much light reflecting off the flocking, then surely the skyflats would be most affected since the lower exposure time means it's a brighter light source than what is provided by the tablet? 

Link to comment
Share on other sites

6 minutes ago, Richard_ said:

If the issue is too much light reflecting off the flocking, then surely the skyflats would be most affected since the lower exposure time means it's a brighter light source than what is provided by the tablet? 

Sky is very far away and tablet is very close - that is the difference. In ideal world - only rays of light that are few degrees off optical axis should reach sensor (depending on focal length and size of your FOV) - but flat panel close to opening of telescope emits light in all directions and all different angles.

In fact, now that I think about it - maybe issue with screen as flat panel is not because it is close - but with viewing angles.

Take your tablet and see if illumination and color changes with angle. That might be causing the issue.

In any case, you can get flat panel for very small amount of money nowadays. Don't purchase ready made flat panels for astronomy - those are ridiculously expensive. You can purchase simple LED panel light. I recently installed lighting in my new house and had opportunity to see what cheap led lighting looks like - and it struck me - ideal for flat panels for telescopes.

image.png.bae817d07d303f8b1d136da1863e7775.png

They come with AC/DC transformer that you can easily remove and hook them up to 12V.

  • Like 3
Link to comment
Share on other sites

  • 2 weeks later...
On 27/09/2021 at 09:44, vlaiv said:

What scope are you using?

One thing to try would be to add long dew shield kind of shroud made out of very dark material (flocked on the inside) and take your flats like that.

Flat panels work well with baffled and flocked scopes. Some scopes can't cope with that much light hitting inside of the scope at different angles. Newtonians are prime candidate for this sort of behavior.

Interesting that is because I was getting some horrid circle gradients using an led tracing pad on my 10 inch f4 Newtonian with the pad laying straight ontop of the scopes aperture even with a t shirt it must of still been too bright even though the scope was flocked and has baffles the light still must of been bouncing around in there 

Link to comment
Share on other sites

Thanks for the suggestion, Vlaiv, I'll look into that.

On 10/10/2021 at 21:21, newbie alert said:

Firstly , do you need to callibrate out dust bunnies or vignette?

14 bit camera's have a adu range of? 30000 seems excessive as over correcting shows...

It's mostly for vignetting caused by the filter. The sensor size is far smaller than the image circle so there is no significant vignetting like you see with full frame cameras. Q

Since my scope is a small refractor it's easy to blow dust off the objective lens. My image train does not get disassembled so the likelihood of introducing dust is fairly low. 

Although the camera is 14-bit, the ASI displays values representative of 16-bit imagos. Therefore, the maximum value is 65,536 ADU. My ASI Air automatically sets the exposure time to get into the 30,000 - 35,000 ADU range but of course I can choose whatever exposure time I want. 

Link to comment
Share on other sites

16 hours ago, Richard_ said:

 

Although the camera is 14-bit, the ASI displays values representative of 16-bit imagos. Therefore, the maximum value is 65,536 ADU. My ASI Air automatically sets the exposure time to get into the 30,000 - 35,000 ADU range but of course I can choose whatever exposure time I want. 

A friend also had an Air, and with his flats they were also over correcting...I don't know why zwo say  50% of the cameras range but if it's over correcting then it's obviously too much  .. I'd suggest a third, so a adu of about 22000  is about right..  so try it and see

But a pure white evenly illuminated light source  shouldn't introduce colour gradients 

Link to comment
Share on other sites

Your badly flattened version, in the problem corner, shows channel brightnesses in descending order, of red, blue, green, in the background sky. they are miles out of balance with green being far too low. This suggests that your panel has a corner which is brighter in green than elsewhere, no?

Olly

Link to comment
Share on other sites

+1 for using a flat light panel rather than a tablet screen. My flat calibration procedure was much more consistent when I moved from an IPad screen to a light panel.

Link to comment
Share on other sites

How do you calibrate your flats? Overcorrecting flats can be caused by poor flat calibration. With a cmos camera, you need to be very precise with calibration.

Keep the exposures such that the sensor is in its linear response region (for me that is the case when I aim for 25000 ADU).

Use a colour neutral light source if possible. If not, make sure that the three colour channels in an osc are all in their linear response region.

Keep the camera at a constant temperature, gain and offset.

Shoot flat darks at the exact same settings as your flats. Flat darks that don't match flats, can leave an offset in the master flat that will result in over- or undercorrection during light frame calibration.

 

Link to comment
Share on other sites

On 10/10/2021 at 22:21, newbie alert said:

Firstly , do you need to callibrate out dust bunnies or vignette?

14 bit camera's have a adu range of? 30000 seems excessive as over correcting shows...

The 14 bit output of a sensor is stored in the high bit range of the 16 bit image files. So, pitch black is 0 in 14 and 16 bit. The next light level is 1 in the 14 bit ADC output, and 4 (100 binary) in the 16 bit image file. Then 2 in 14 bit and 8 in 16 bit, etc. The brightest level is 16000 (rounded) in the 14 bit ADC, which becomes 64000 (rounded) in the 16 bit image file. So, 30000 in the output image file, is actually 7500 in the ADC output of the sensor. But depending on the camera or settings, that is not necessarily in the linear response range of the sensor. Especially since the 30000 value is an average of the entire image. The central region of the image (and local pixel response) may very well be a bit higher.

Link to comment
Share on other sites

Ok, so I've just ordered a light panel similar to the one Vlaiv posted so let's see how I get on when it arrives! For almost £6, it'll be worth a shot. 

Some good discussion and problem solving here, thanks for the suggestions all! 

On 17/10/2021 at 08:28, wimvb said:

How do you calibrate your flats? Overcorrecting flats can be caused by poor flat calibration. With a cmos camera, you need to be very precise with calibration.

Keep the exposures such that the sensor is in its linear response region (for me that is the case when I aim for 25000 ADU).

Use a colour neutral light source if possible. If not, make sure that the three colour channels in an osc are all in their linear response region.

Keep the camera at a constant temperature, gain and offset.

Shoot flat darks at the exact same settings as your flats. Flat darks that don't match flats, can leave an offset in the master flat that will result in over- or undercorrection during light frame calibration.

 

The images in my original post show the ADU readouts for my master flats, which are on average ~22,500 and ~20,700 for tablet and sky flats respectively. Granted, the AAP can over correct as it aims for just over 30,000 ADU, but I purposely reduced the exposure time to prevent this from happening. These are well below the ~30,000ADU range most people aim for and given the negligible difference in values between tablet and sky flats, I'm inclined to think that the problem is due to non-parallel light emitted from the light source, exactly as Vlaiv mentioned in his post. 

The tablet light source colour was as neutral as I could make it. Redlight/night light filters were disabled and I was using maximum display brightness whilst displaying a white image. 

Camera settings were exactly the same as lights (even the cooling temperature, which I'm told doesn't really make a difference). Dark flats were taken under the same conditions too using the exact same exposure time. With respect to the below quote, what exactly do you mean?

If not, make sure that the three colour channels in an osc are all in their linear response region.

 

 

Link to comment
Share on other sites

4 hours ago, Richard_ said:

With respect to the below quote, what exactly do you mean?

If one colour is stronger than the others, their average or median may be low, but the strongest colour may become saturated. Saturation in camera sensors is not abrupt but gradual. In an osc, the master flat isn’t debayered, and the average and median are calculated for all pixels together.

  • Like 1
Link to comment
Share on other sites

Ah got it, you mean to check that the histogram of each channel is centred and not clipping the right side at saturation, right? 

 

If I recall, a common issue with the ASI533 was that the green channel was shifted to the right when people compared their debayered flats. People were talking about changing the white balance in the camera driver settings but this sounds a bit complicated for me and given that I use an ASI Air I don't think I can change those settings for the camera. I didn't find any problem with my regular flats, just my l-extreme flats, so I decided I didn't need to tinker with my white balance. 

When my light panel arrives I'll be sure to run some tests using different filters and without filters to see if I can recreate my issue or whether it's resolved. 

Link to comment
Share on other sites

19 minutes ago, Richard_ said:

Ah got it, you mean to check that the histogram of each channel is centred and not clipping the right side at saturation, right? 

Basically, yes. But saturation effects, or rather, non linear response is subtle. A sensor’s response can become non linear long before a pixel is ”full” or saturated. In the linear response region, an increase of, say, 10% in the input signal results in a 10% increase of the output. In the non linear response region, the increase in output is less than that, but still more than 0%. In the true saturation region, any increase in input signal will not result in a change in output; the pixel is truly full. When taking flats, you need to be well outside of the non linear region, not just the saturation region. Usually being at half of a sensor’s maximum output (30 000 ADU) is ok, but lower can be safer.

Link to comment
Share on other sites

Thanks for clarifying, makes sense to me. Do you know if there's information out there that shows which ranges are linear and non-linear for a given camera/sensor or is your previous comment more of a rule of good thumb and/or experience? 

Link to comment
Share on other sites

55 minutes ago, Richard_ said:

more of a rule of good thumb and/or experience? 

Rule of thumb and experience from an earlier life in the semiconductor industry. So far, I haven't seen linearity diagrams for sony cmos sensors.

Link to comment
Share on other sites

Alright, the LED panel arrived today. The panel I ordered comes with a removable LED driver but the pictures didn't clearly show what voltage and current was outputted. The output of the driver is DC54-72V with a current of 280mA. So this is the voltage/current requirements for the light. See attached pictures. 

Call me an idiot, but when I remove the LED driver, I have no idea how I can connect this directly to a 12V DC output such as my ASI Air Pro to power the light. I obviously tried it but nothing happened. Surely I need some form of device to boost the voltage? Or perhaps I ordered the wrong light and that I should order a light which requires 12V DC outputted from the driver? 

DSC_1063_copy_3628x2721.jpg

DSC_1064_copy_3628x2721.jpg

Link to comment
Share on other sites

On 19/10/2021 at 20:25, wimvb said:

Rule of thumb and experience from an earlier life in the semiconductor industry. So far, I haven't seen linearity diagrams for sony cmos sensors.

Doesn't sharpcap sensor analysis measure this or is this some other number?

Not the same camera, or sensor but from the same Sony Exmor lineage of sensors so could be similar. Mine shows sensor linearity to 99,3%. Does this mean the last 0,7% experiences a nonlinear response, or that its overall 0,7%? Never really understood what that number stood for.

1934244143_sharpcapsensoranalysisRT571c.PNG.c754483f655807deb01602fe0313e4a7.PNG

 

I think i have some issues with light panel flats aswell, but unsure if its related to anything discussed here.

On 27/09/2021 at 11:44, vlaiv said:

Flat panels work well with baffled and flocked scopes. Some scopes can't cope with that much light hitting inside of the scope at different angles. Newtonians are prime candidate for this sort of behavior.

I think my issues lie here, i am flocking my tube to see if my flats improve.

Link to comment
Share on other sites

1 hour ago, Richard_ said:

Call me an idiot, but when I remove the LED driver, I have no idea how I can connect this directly to a 12V DC output such as my ASI Air Pro to power the light. I obviously tried it but nothing happened. Surely I need some form of device to boost the voltage? Or perhaps I ordered the wrong light and that I should order a light which requires 12V DC outputted from the driver? 

In that case look for DC/DC converter.

Something like this:

https://www.hycopower.com/product/12V-to-72V.html

for example or similar - just make sure it can output at least 0.5A

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

  • Recently Browsing   0 members

    • No registered users viewing this page.
  • Similar Content

    • By TakMan
      Still sorting the (new to me), Atik 16200 imaging train as I try to shift from my trusty SBIG 8300, Mac to PC for mount control/capture and from a separate guide scope to the Atik OAG....
      The camera needs to go back to Atik (awaiting the email from Vince) as there is dust inside the chamber, so this is a good time to get everything checked - ready for the autumn season.
      After some imaging/testing time at the rear of Leo in the last week, I noticed on my flat frames a strange half moon light - by the dust mote (that was over-correcting the lights).

      Eventually I worked out it was the screws surrounding the sensor cover window (or the 3x rounded cap screws that attach the EFW3), bouncing the light onto the back of the filter (Baader L in this example) and I suppose onto the cover window and onto the sensor. To test the theory, I opened the imaging train up and added a Sharpie pen to them. Couldn't get into the cross-heads with the pen, but with re-testing, the reflection had gone! Perhaps it would never be an issue with the actual light frames, but you never know with a bright star in the frame of a future target...?
      So today, after shifting slightly outwards the OAG stalk, I addressed the stainless steel screws 'properly', by (again), taking everything apart and lightly painting a cover of matt black acrylic paint over them and into the x-heads (a bit of overspill), nothing too heavy-handed as I didn't want to glue the things in with paint! The finished effect is duller than the pic here and the reflection has gone after another round of testing.

      Always something to catch us out, hey!? Why Atik can't use black screws is another matter.....
      Perhaps this may help others out at some stage....
      Damian
    • By astrobena
      Hey everyone,
      Over the past few days i've been gathering data on M31 due to the battery not lasting long and andomeda dissapearing behind a tree, therefore the mulitple imaging sessions. So far i've been out and me being me, only took dark, bias and light frames for the first 2 sessions but for the last one i also included flats... (The lights all have very slightly different settings cause i have been experimenting slightly... that being 45sec @ ISO 400, 45sec @ ISO 200 and 50sec @ ISO 100) Now DSS assinged all the correct bias and dark frames to the corresponding light frames but because i only have flats for my last imaging session it applied those to all the different light frames and not just to the last set...(and due to me moving the telescope / taking the camera off, the dust spots have obviously been moving around and that therefor dont work at all for the other light frames...) So my question: Is there any way in DSS to apply flats to only one set of light frames and if not are there any other apps with which i can do this with?
      Many Thanks!
    • By AstroRookie
      Hello,
      when applying the flats taken in my last session (to find out what is causing the strange diffraction spikes) with Siril, the final stacked result still shows the vignetting and the dust spots. I also did the whole preprocessing with Nebulosity, same result.
      I took the flats as follows:
      same iso as my subs camera and focus not touched I use a homemade flatbox combined with the a white t-shirt with Ekos took test shots till the histogram was half-way to the left checked all my flats, they all show vignetting and the same dust spots as in my subs I tried using them with and without using a bias frame, same result, the final result looks as if no flats were used.
      Anybody any idea what is going on? An other question I have, will the vignetting and dust spots also show in the master flat (flats stacked)?
      Thanks for your help,
      AstroRookie
    • By Jammy
      Hi all.
      I've created my own darks library for my ASI1600mm Pro.  Various gains and various exposure times all at -10 Celsius.
      I'm just wondering what people do with them?  Once you've taken darks and processed an image you get your master darks.
      Do you guys just keep the master dark, in a master darks library, or do you process your darks again each time you stack a new image?
      I'm just thinking to save storage, could I discard the dark sub frames and just keep the masters?  I don't want to throw them away if that is wrong though.
      Also, with this camera I take dark flats rather than bias frames.  Because the scope is covered, do I need to do dark flats each time I take flats?
      I have an even illuminated flat box which I always use at the same exposure lengths.  I guess the only thing that could vary is the gain value.
      I understand the need for flats every time you change focus, for each filter etc., but if I'm using the same exposure time can I just use a previous set of dark flats as long as the camera settings are the same?
      If this is possible, can I then create a dark flats library for any difference in gain values?  Not sure if I gain values affect dark flats though.
      I've read that back, and that's a lot of questions!  All help greatly received.
    • By Stargazer33
      I've finally got around to making my flats box.
      I decided to go for a cylinder rather than the normal square as I thought it would maximize the amount of reflected light and limit any 'dead' areas. I could also use the Celestrons dust cap retaining pins to lock the flats box onto the 'scope.
      I purchased some of the craft board that has a thin foam sheet sandwiched between two sheets of thick paper/thin card. In order to bend the card into a cylinder, I creased the board every 20 mm by pressing the edge of a steel ruler into the board. It took two of the sheets to make a cylinder big enough to fit my C9.25, with only a couple of cm trimmed off.
             
       
      I then made a reinforcing ring/defuser holder from two strips of the foam board; this time creasing them at 15mm intervals. I stuck these level to the bottom edge so the joins were 90° to the main cylinder joins. These strips were cut wide enough to ensure that the diffuser cleared the secondary housing.
        
       
      The cylinder was designed to lock into the C9.25s dust cap retaining pins so next I cut two keyways into the bottom outer side. They looked a little weak so I reinforced them with some Christmas chocolate reindeer plastic packaging!
       
       
      Although the foamboard is quite shiny, I wasn't happy with all the grooves, so I lined the inside with white A4 paper. The Perspex sheet was cut to shape and hot glued into place onto the ledge.
        
       
      Next, starting at the top, I notched the edge of the cylinder to run the LED string lights cable through and then started to spiral the LEDs around and down the cylinder.
        
       
      The top cap/reflector was made from two discs of foamboard. One to go inside the cylinder and one to sit proud of the edge. They were glued together before being hot glued onto the top of the cylinder. The LED light string that I bought has an integrated on/off button as well as both up and down brightness buttons with a 3M sticky pad on the back, so I stuck this to the top cap.

       
      As I had previously made myself a 'scope mounted power distribution box with aircraft sockets for power, I removed the 3 pin UK plug/ac-dc converter and soldered on an aircraft plug to match my 12 volt DC supply socket.
        
       
      The lightbox illuminated.

       
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.