Creating Flats

Discussion of using SharpCap for Deep Sky Imaging
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mike77345
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Joined: Sat Sep 08, 2018 7:08 pm

Creating Flats

#1

Post by mike77345 »

I have a question about flats creation. I read the users guide but wanted to get some comments on the following advice on taking flats posted in Cloudynights ( https://www.cloudynights.com/topic/5797 ... adu-value/ ). The advice was as follows:
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"The ideal ADU value for flats varies by camera. You want about the highest ADU value that you can get before pixels either saturate or, in the case of the ASI1600 or any anti-blooming camera, you want to be just below where the pixel response goes non-linear.

You can find this yourself by taking increasingly longer exposures and graphing the number of ADUs per exposure length. The graph should be an increasing diagonal line that starts straight, but eventually levels off. The ideal ADU value is just before the line levels off."
_______________________

I have an ASI290MC planetary camera. SharpCap is great since it shows the ADU units up in the right hand corner. I created plots as suggested as shown below. This camera is an 8 bit one so 256 max ADUs.
Flats ADU Plot.JPG
Flats ADU Plot.JPG (48.55 KiB) Viewed 3406 times
You can see that depending on the brightness of my light box I get fairly linear plot up to 23 msec on the medium bright setting of the light box and 500 msec on the low brightness setting. This is not close to the 50% setting (128 ADU). I was thinking of looking at all my color histograms and keeping the right most setting of the highest ADU histogram (white, red, blue, green) reading just below the 23 or 500 msec reading depending on the light box setting.

Am I on track with this procedure? Would appreciate thoughts on this.
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turfpit
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Re: Creating Flats

#2

Post by turfpit »

Mike

This is how I take flats. I use a home made flat panel generator cost around £20 - parts all off ebay, powered by 2x1.5V AA batteries. See the @turfpit posts in this thread viewtopic.php?t=1376.

For settings, I leave gain, black_level/offset/brightness (depends on camera) the same as for lights. I adjust exposure down until the log histogram shows about 50% (or just less). I am doing traditional capture in RAW format so don't care about colour balance. All I ensure is the luminance channel is around 50% and none of the other channels touch either the LHS or RHS of the histogram. Mono is easier as just the luminance to deal with. I take 100 flats and 100 bias frames - the time for capture is minimal.

colour-camera-flats.JPG
colour-camera-flats.JPG (62.54 KiB) Viewed 3403 times


Final image here https://www.astrobin.com/full/0ffje8/0/. This is an Altair 183C V2, using RAW12.


mono-camera-flats.JPG
mono-camera-flats.JPG (54.92 KiB) Viewed 3403 times


Final image here https://www.astrobin.com/full/393472/0/. This is an Atik 314E using RAW16.


I have 2 flat panels, a 3V one and a 12V one. When doing LRGB with the 12V panel and using bin 2x2, I couldn't get the exposure low enough to avoid saturating the histogram. This was quickly solved by placing a white handkerchief over the end of the scope. I have the parts to build another 3V flat panel which has been tested to work.

These are the ADU values (red box). I would use the same process for 8-bit, although for lunar imaging I never bother with flats as a 10,000+ frame SER video usually loses the dust bunnies (if not then it is a clone/stamp job in Gimp) when stacked in Autostakkert.



colour-ADU-RAW12.JPG
colour-ADU-RAW12.JPG (72.18 KiB) Viewed 3400 times

mono-ADU-RAW16.JPG
mono-ADU-RAW16.JPG (63.4 KiB) Viewed 3400 times

Dave
mike77345
Posts: 7
Joined: Sat Sep 08, 2018 7:08 pm

Re: Creating Flats

#3

Post by mike77345 »

Hi Dave,

Thanks very much for your post. It helped me understand the process a lot better. Just getting started so really on a steep learning curve. I like your custom light box frame. I did have some really basic questions.

1. For the moon shot flats using Gimp clone/stamp in Autostakkert what did you exactly mean?
2. I assume the black backing is vinyl in the light box? How did you attach it to your EL panel. Also how did you cut the circle out of it? I guess you
could use a razor knife with a guide tracing on the backing?


Mike
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turfpit
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Re: Creating Flats

#4

Post by turfpit »

Mike

Thanks for the comments.
Just getting started so really on a steep learning curve
Just coming up on 3 years and still on a steep learning curve. The problems used to be mount alignment, focusing, settings but the boundaries get pushed. For me LRGB, narrowband and guiding are the next ones to conquer. I am in no hurry. I see a lot of issues on forums because people take on complexity before the basics are mastered.

Question 1.
I stack lunar videos with Autostakkert. Sometimes the resultant stack shows dust bunnies from the optics (can depend on how much dew there is). To remove these black blobs from the image I use the clone/stamp tool in GIMP (which I use for all my post-processing) - think of it as a copy/paste operation for graphics. The technique is to 'steal' some similar data adjacent to the blemish and paste it over the blemish. The steps are zoom in, copy suitable material (control/click), click on the blemish to paste, zoom out. Done at 200% zoom, I have never been able to find the joins. The same facility is available in graphics software such as Photoshop, & FastStone Image Viewer.

dust-bunny.JPG
dust-bunny.JPG (25.74 KiB) Viewed 3369 times

GIMP-clone-tool.JPG
GIMP-clone-tool.JPG (15.48 KiB) Viewed 3369 times


An alternative way to deal with the dirty optics is to capture some flat frames (maybe 20), create a master flat in Autostakkert and have the software apply the flat as part of its processing. Flat frame acquisition would require the use of a light panel. I have successfully used a de-focussed moon as the light source (it has to cover the sensor completely) for flats.

Question 2
I use plasticard - it is easy to drill/file/cut and can be purchased in sheets of various sizes in 1mm, 2mm, 3mm .. thicknesses. This can be found on eBay from engineering suppliers or model shops.

The flat box is a sandwich
  • plasticard
  • EL panel
  • plasticard with circular hole
held together with plastic bolts/nuts/washers.

To cut larger holes I use these cutters which can be attached to a drill. This set gets me up to 127mm. If not an exact match then I use next lowest size and file to fit. The plasticard is easy to work.

cutters.JPG
cutters.JPG (116.26 KiB) Viewed 3353 times

For my 8" SCT I had an engineer cut the hole with a radial cutter (an adjustable arm with a blade on the end). This type of tool can be found on Amazon.

Some people use graphics tracing tablets to good effect. Of course you need a spare USB port to power the tablet. I started off with the tee-shirt and sky method.

IMG_1259a.JPG
IMG_1259a.JPG (89.69 KiB) Viewed 3368 times


My view on that one is 'we used to live in caves but progressed'.

Some of the commercial flat panels are quite sophisticated and have adjustable brightness either through hardware or software. I have seen £120 - £250 as typical costs.

I have found the EL panel combined with the histogram to be a cost effective and simple way to acquire flat frames. The only issue I ever hit was the 12V panel was too bright for binned RGB but I resolved that with a 3V version. The EL panel requires an inverter and the inverter voltage will be dictated by the size of the panel. Up to 100mm diameter seems to work with 3V but the larger A6/A5/A4 panels need 12V. the advice is not to use rechargeable batteries as they do not make the required voltage.

Hope this helps. Good luck on the journey.

Dave
mike77345
Posts: 7
Joined: Sat Sep 08, 2018 7:08 pm

Re: Creating Flats

#5

Post by mike77345 »

Hi Dave,

Thanks a lot. I should be able to handle it from here on. I got a nice Tictek EL panel for $22 on Amazon that is dimable. So really cheap to rig something up now that I have your information. Sure beats buying a $350 flat field generator. Thought I would mention I found the HP Spectre battery to be a great alternative for an astronomy portable battery. You can get a USB-C to 5mm pin adapter to give 12 volt output for my dew heaters or mount and use the USB-A to power the EL panel. Has 20,100 Mamp-hr capacity which is pretty amazing.
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