How to get the true color of bright stars?

nathanm

New member
Jun 28, 2020
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I am new to PixInsight . I am mostly a landscape astrophotographer. I mostly use Canon DSLR, including a full spectrum modified 5D Mark IV, but I also recently acquired a QHY 128c chilled color camera.

I would like to make wide angle starscapes that are close to "true color" for the stars, even the bright ones.

Some stars deserve to be white. PixInsight has a photometric calibration feature where it finds stars or galaxies that are within your frame, and which are close in spectrum to our sun and thus serve as a white reference.

Many bright stars have strong colors. However, all of the bright stars in my photos are massively overexposed. It's easy to see why - a DSLR and decent aperture lens with stacked exposures can record stars down to 11th or 12th magnitude, but that is far wider dynamic range than the 14 bits of range that the sensor records.

In normal photography terms, stars are intrinsically a high dynamic range subject. So, I have tried HDR techniques but there are several problems:

An exposure short enough to capture very bright stars without saturation has few stars in the photo so it can be hard to align. But worse, when you do align it you find that the small, correctly exposed star must fill a huge gap. That is because over exposed stars create a spot much larger than unsaturated stars. That's because the point spread function for the stars drops off with distance, but intrinsically has much larger radius as the exposure goes up.

So, even if you correctly align the exposures, you have a large diameter saturated hole to fill. A conventional HDR merge leaves the overexposed ring and halo around the smaller, correctly exposed core.

One could imagine techniques to get around this, but so far I have given up on trying to make all stars the same size. As it stands, we have all gotten used to astrophotos with bright stars that are huge - a bit like the convention on maps where big cities are shown with a dot size proportional to their population.

But I still want the stars - whatever their size - to be the correct color. So this means something like blurring the short exposure stars to make them as big as the overexposed versions, and then combining that with a mask of the long exposure.

A separate short exposure is one way to sample the correct color, but there might be another way.

The slopes or wings of the point spread function often record the color of the star as a fringe. The core is saturated in one or more RGB channels, but as one moves radially away from the core you typically reach an area where the pixels are not saturated. This raises the possibility that one could do a single-exposure where one reconstructs the color from the fringe

It could be tricky because the fringe or edge might interact poorly with Bayer array.

So this leaves two ways to make true-color bright stars:

1. Use short exposures to capture the color, then use that to color the over-exposed star image. This is using a capture at different exposures, like HDR, but it really isn't HDR.

2. Use the unsaturated fringe around bright stars to sample their true color, then propagate that to the saturated portions.

I am interested in whether anybody has experience doing either one of these things in PixInsight ? Or a different way to solve the problem?
 

Juan Conejero

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Sep 2, 2004
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1. Use short exposures to capture the color, then use that to color the over-exposed star image.
This is the way to go. And yes, this is exactly a high dynamic range problem and must be handled as such. Use our HDRComposition tool to merge the images. Then use our PhotometricColorCalibration tool to apply a photometry-based white balancing function.
 
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pfile

PTeam Member
Nov 23, 2009
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you aren't limited to 2 exposures; you can do as many as you like and HDRComposition will throw out the data that it doesn't need, taking only the properly exposed data from each exposure. so you don't need to worry about the gap as long as you've got enough images to fill everything in.

where it finds stars or galaxies that are within your frame, and which are close in spectrum to our sun and thus serve as a white reference.
the white reference galaxies need not be in your frame. the stars in the frame are matched to a selected white reference, one of which is average spiral galaxy.

if you are doing wide angle shots hopefully there are so many stars in each frame, no matter how short the exposure, that StarAlignment can find them. if not, there is always DynamicAlignment, where you point out a few stars as references.

rob