Flats - Darks - Fits - PNG - Live Stack

[tmyers]

Brand new and first post, I have looked and have not found answers elsewhere so here goes.

If my lights are taken in .fits format should the flats and darks also be taken in .fits? Same question for .png?

For now I plan to stack my images in DSS, and not use Live Stack. Can I still use the Flats and Dark's Capture in SC, and then simply use the master flat and dark in DSS?

When taking flats I get the "Too Many pixels saturated", or something like that in the Flats window. I have tried lowing gain and or using faster shutter speed and can't get this alert to go away.

For the record I am using an ASI 1600MC Cooled through a SkyWatcher 100ED with white t shirt and a white screen on my tablet for ilumination.

Lastly, the Flats window never goes away and is hard to get to close, I am sure I am doing something wrong because my flats added more errors to the stacked image than was there to start.

Thoughts?

[tmyers]

Lots of people looking but none responding. Allow me to explain, I have been trying to apply flats and darks to some imaging from last weekend. The camera has not been moved, the focus has not been moved. I used a cotton shirt and white ilumination from a tablet, and I have used the tool in SharpCap for my Flats and Darks and have also taken a series (no masters) of flats to apply to my lights in DSS. My final stacked image has double the intensity of dust in it that the one with out the flats have. In other words it doesn't appear the flats are being removed form the lights, if that is the correct terminology. I am sure I am doing something wrong, just need to figure out what I am doing.

Any help would be appreciated.

[admin]

Hi,

When taking flats, you have to ensure that no area of the image is saturated (at maximum brightness) – if there are any areas at maximum brightness then they will not be corrected properly. Usually, it's best to ensure that all of the image is below 90% of maximum brightness to have a sort of buffer between the pixel values being used in the maximum possible value.

As to the file format to use to capture flat frames – I would suggest using fits format if you are going to process the flat frames in other applications. SharpCap can write 16 bit data to PNG format and read it back correctly, but some applications will quietly reduce 16-bit data to 8-bit when reading PNG files, so fits is probably a better bet.

For best quality flat correction you should use both dark frames (which should be taken at the same temperature, gain and exposure as the image frames) and dark flat frames (which should be taken at the same gain and exposure as the flats – temperature is less important for these as they are usually shorter).

Hope that helps at least a little bit – if you're still getting nowhere then consider sharing some of the raw data (upload it to Google Drive or similar and share the link) which would let others have a look to see if they can work out what is going on.

Cheers, Robin

[tmyers]

You may have hit the nail on the head with the saturation, it didn't seem to matter what I did I could not get the too may saturated pixels alert to go away. I will try dimming the tablet by a lot and try again.

[tmyers]

OK, I tried this again and started with the gain and exposure at very low settings. Started off with "Try to get more pixels above 10% Saturation" and "Too many pixels are Saturated", then it moved to "Try to get more above 20% Saturation, till that RED line was gone. But the "Too many pixels are Saturated never goes away, even if I lower everything to black. I even tried it with the illumination removed, same result.

I am thinking a re-load of Sharpcap may in order, Thoughts?

[turfpit]

Tim

A suggestion - summary of steps:

• download FITS Liberator from https://www.spacetelescope.org/projects ... load_v301/

• capture a single flat frame as type FITS (use the highest bit-depth available in the camera - e.g. RAW12, RAW16)

• take a screen snip and save to a file

• upload the saved file here

• post the Camera-Settings file for the capture

For a good flat, you should have something like this with a mean value of around 30,000:

flat-frame-in-FITS-Liberator.JPG (123.14 KiB) Viewed 2993 times

The detail:

I capture flats using an electro-luminescent panel. With a CMOS camera, I:

• turn on the histogram and select 'Logarithmic'

• capture lights

• adjust only exposure until the histogram is around 50%

A colour flat typically looks like this:

colour-flat.JPG (59.66 KiB) Viewed 2993 times

There will be 4 peaks - one each for Red, Green, Blue, Luminance. Depending on the sensor, they will be spread out. Aim to get the Luminance (white) peak around the middle in the histogram window by adjusting exposure. The other 3 must not touch the left or right hand sides. If this is the case then reduce brightness/dim the light source (one way is with a white cloth between the scope and the light source).

A mono flat typically looks like this:

mono-flat.JPG (58.79 KiB) Viewed 2993 times

For the mono case, there will be only one histogram.

Concentrate on taking a single flat. To keep things simple, don't get involved in trying to create a master flat directly in SharpCap (just yet). The histogram can be turned on via the Tool Bar.

Dave

[turfpit]

Note to the above.

I was using a 12V EL panel without problem. When I got involved with LRGB captures, I started using bin 2x2 for RGB filters. With the G & B I could never get the histogram in the middle as the frame was too bright. I used a piece of thick tee-shirt as a temporary measure. The solution was to construct another flat panel generator which used a 3V panel - less brightness.

teeshirt-sacrifice.JPG (75.81 KiB) Viewed 2992 times

MK2.JPG (87.05 KiB) Viewed 2992 times

Dave

[turfpit]

A one shot colour camera flat frame in FITS Liberator may look like the picture below. Note the 3 peaks. The grid (normal) is due to the frame not having been debayered. The histogram satisfies the 'not touching the left or right hand side criteria' as the minimum value is 10816 and the maximum value is 40912.

colour-camera-flat-frame-in-FITS-Liberator.JPG (145.83 KiB) Viewed 2986 times

Note that all the testing for producing flats can be done in the house, during the daytime - no need to wait for a clear night!

Dave

[tmyers]

I did reload to the latest version of Sharp Cap. Also noticed that it appeared to be attempting to take flats in RGB-24 that may have been the issue. When I switched to Raw 16 I got a Good Histogram note. This is the single flat you requested, and the settings were

Annotation 2020-03-08 125739.png (306.35 KiB) Viewed 2974 times

Following is the screen capture form SharpCap

Annotation 2020-03-08 130421.png (223.11 KiB) Viewed 2974 times

[turfpit]

Tim

Mean=28133 (surprised at the max=65535, as I would expect it to be less) and the histogram on the capture screen looks ok. I would say that is a decent starting point for a flat.

With the exposure at 1311ms, have you got any filters in the optical train or are you dimming the light source?

I cannot comment on RGB24 as I only ever use FITS for capture so as to use the FITS Liberator software for investigating frames to ensure settings are suitable.

For consistency I take all bias, flats, darks and lights in the same format. Mixing it is asking for trouble in my opinion. Also I do not mix frames taken with different software - seen a few come a cropper with that around the forums.

As to the vignetting, I got caught out once by having a narrow part in the optical train between end of scope and the camera. By swapping things around I ended up with the scope end widest and camera end narrowest which removed the vignetting (in other words, the light path was a cone). This was on an Altair 183C running at 5400x3400. You might want to check that.

Dave