"Weight" in SubFrame Selector, Additive, Multiplicative

[Alex McConahay]

I am having a problem understanding the usage of a few terms.....

"Weight" as used in the Subframe Selector. What is "Weight?"  Who does it show in an image?

In the background extractor(s), I see that there are additive and multiplicative variations. OK, I see how you apply the inverses (subtraction and division). I even see the examples (gradients is one, vignetting is the other)....but what makes something additive, and what makes something multiplicative? I am having a difficulty getting my brain around that choice of words.

Alex

[mschuster]

Hi Alex,

Weight in SubframeSelector is for you to define however you want. You can find some examples in the documentation and also in this thread.

An example of an additive gradient might be light pollution, i.e, it adds light in various parts of the frame.

A multiplicative gradient might be due to optical vignetting. It is multiplicative because it scales all light by some multiplicative factor near the corners of the frame.

I don't use DBE so I am not positive about this.

Thanks,
Mike

[pfile]

yes, mike has it. the vignetting that is normally corrected by flats has a multiplicative nature.

light pollution gradients are additive. the LP photons add to the signal photons.

if you are trying to correct a frame by computing a "synthetic flat" with DBE then you want to use multiplicative correction. if you are trying to remove an LP gradient from a properly flattened image, you should use subtractive correction.

mike - wasn't the point of the weight computation in SFS to compute the proper noise weighting of a sub before registration? if i remember right you had noticed that the noise profile of a registered frame is 'corrupted' by registration.

rob

[mschuster]

mike - wasn't the point of the weight computation in SFS to compute the proper noise weighting of a sub before registration? if i remember right you had noticed that the noise profile of a registered frame is 'corrupted' by registration.

Yes. I believe RC7 now does pre-registration weighting, so using Weight for just this purpose is no longer necessary.

Some other weighting experiments use combinations of SNRWeight and FWHM for weighing, and SubframeSelector Weight can be helpful for this.

One example is this: Do two integrations. The first integration integrates all frames (except the extreme outliers) weighted by SNRWeight. This integration gives the best SNR in dim areas. The second integration integrates only a subset of frames with smallest FWHM, weighted by a combination of both SNRWeight and FWHM. This integration gives the best FWHM in bright areas, at a cost of SNR. But the loss of SNR tends to not matter in bright areas because SNR is high enough anyway. Now blend the two integrations together linearly using a luminance mask as a blending coefficient. The result has better SNR in the dim areas and better FWHM in the brighter areas.

Regards,
Mike

[Ignacio]

Interesting approach, Mike, I will try it in my next image. I have experimented in the past with Starsupport as weight (usually divided by a fix number of star, to some power<1), which is, I think, better than FWHM since it factors in background levels/noise.

Ignacio

[EorEquis]

I've taken to using :

(2 * SNRWeight) - FWHM

As my weight.

I then write this to the FITs keyword MYWEIGHT when the SFS frames are copied to new destinations (note here, they must be copied, not moved, for the new keyword to be written...my hunch is that moving it to another physical drive would also write the new keyword, since new data is being written, but I haven't tested this)

Then, in image integration, there's a setting asking how to weight frames...tell it to use a FITs keyword, and enter MYWEIGHT (or whatever you used).

I've had excellent results with this method.  YMMV, of course.

[Alex McConahay]

Thanks, All, I still have not wrapped my head around additive and multiplicative, but let's talk about weight......

Mike sauys weight is something I define. Okay.... but if weight is something I define and assign, what is being reported in the "Weight" column when I first push the "measure" button? I have not assigned any weights yet, have I. Or is there something working in the defaults.

Alex
 

[mschuster]

Hi Alex,

The default Weight equals SNRWeight. Weight is defined by the Expressions > Weighting field.

Weight can be used to weight subframes in integrations. Or for other things also.

For example, the measure of star elongation Eccentricity may be hard to understand. Use (1 - Eccentricity^2)^0.5 as a weighting expression and Weight will equal star aspect ratio.

Regards,
Mike

[Alex McConahay]

Thanks, Mike,

I shall experiment.

Alex