PI stacking - not enough stars found in stacking

[masjstovel]

Hi, 

I have the ZWO ASI 1600 and did a Ha, LRGB session on the M81 last night. I am new and have only used PI a week, but have stacked successfully on 2 other occations. - following youtubevideos on how to do things.

So i did 30x200s subs with Ha, and 15x200s subs with L, R, G and 15 darks. 

Then when i do batchpreprocessing (first thing i do in PI) i get this error message "4 stars found - need at least six". It varies if its 4 or 3 or 2, but same message.
I've tried BPP several times now and It happens every time on Ha subs, but not the same sub each time - so i ask myself what is different from my Ha and LRGB subs? They are much noisier (why? and are they supposed to be noisy in the pre-editing phase?)

Why is this happening and what do i do? 

 

Edited November 9, 2019 by masjstovel

[masjstovel]

I also get this sort of error alot, but it doesnt abort the stacking process and the results look fine to me. Is this something i just live with, or? 

[masjstovel]

Tried removing noise from each Ha sub with ACDNR tool - but now i got this:

[vlaiv]

Maybe just do visual examination of particular sub that is causing error?

Second error that shows sometimes is completely unrelated - PI reports that it cannot successfully parse FITS header keyword for SITELAT / SITELONG - which should contain latitude and longitude of your observing site (I'm guessing here but it would not be hard to check those FITS keywords to see what they represent really).

This could be because of either software used for acquisition is not writing proper format for these fields in FITS header, or PI can't interpret what is written according to FITS standard for some reason (not properly implemented in PI or part of specification not implemented at all). In any case, it should make absolutely no difference to stacking result - that is just metadata that you can go without when stacking subs.

As for issues with stars - there could be number of reasons why these are not detected. Could be that there is too much of guiding error and stars are shaped like trails rather than circular - algorithm just does not recognize them, or maybe they are too large to be considered stars (depends on sampling resolution and how much "search area" is configured in PI when detecting stars), or any other number of reasons.

Best thing to do is to first visually inspect said subs to see what the star profiles look like (round or not, etc) and then we can further think what would be good course of action.

[masjstovel]

1 hour ago, vlaiv said:

Maybe just do visual examination of particular sub that is causing error?

Second error that shows sometimes is completely unrelated - PI reports that it cannot successfully parse FITS header keyword for SITELAT / SITELONG - which should contain latitude and longitude of your observing site (I'm guessing here but it would not be hard to check those FITS keywords to see what they represent really).

This could be because of either software used for acquisition is not writing proper format for these fields in FITS header, or PI can't interpret what is written according to FITS standard for some reason (not properly implemented in PI or part of specification not implemented at all). In any case, it should make absolutely no difference to stacking result - that is just metadata that you can go without when stacking subs.

As for issues with stars - there could be number of reasons why these are not detected. Could be that there is too much of guiding error and stars are shaped like trails rather than circular - algorithm just does not recognize them, or maybe they are too large to be considered stars (depends on sampling resolution and how much "search area" is configured in PI when detecting stars), or any other number of reasons.

Best thing to do is to first visually inspect said subs to see what the star profiles look like (round or not, etc) and then we can further think what would be good course of action.

Thanks alot @vlaiv! I have checked them, there are not any obvious differences or errors. If i zoom about 200% i can see the focus is worsening from sub 1 to 30, but i would say an neglichable amount (?). That means not visible at 100%, slightly at 150% and "some" at 200%. 

Does DSS stacks get the same result in form of quality? could try that

[masjstovel]

And its not 1 particular sub, its different subs between the times i tried - but Ha every time. Thats why I was suspecting the noise

[vlaiv]

5 minutes ago, masjstovel said:

And its not 1 particular sub, its different subs between the times i tried - but Ha every time. Thats why I was suspecting the noise

I don't think it is noise in question.

I had something similar in different software, and it might be the case with PI as well.

Are you trying to align Ha frames on their own or are you trying to add them to some other stack?

Issue that I was having that can be related to this is when software tries to match the stars not only by coordinates and relative spacing but also by star brightness. Ha subs will have significantly lower ADU values in stars then regular subs. Maybe PI tries to match subs with star intensity taken into account and fails for that reason?

On the other hand it could be SNR ratio of Ha subs - how long are they in terms of exposure?

[masjstovel]

200sec all of them. Well that could be. But i have tried both Ha and L as reference image. I did it with Ha only now, and 1 of 29 suceeded. All the others got under 6 stars - and got left out...
I do not understand why

[vlaiv]

1 minute ago, masjstovel said:

200sec all of them. Well that could be. But i have tried both Ha and L as reference image. I did it with Ha only now, and 1 of 29 suceeded. All the others got under 6 stars - and got left out...
I do not understand why

Is there any sort of threshold for star brightness in PI? DSS has that, and if you lower it - it will find more stars, but if you lower it too much it will start mistaking noisy pixels for stars and alignment will fail.

I don't use PI so don't know exact setup of it, but maybe PI help files could provide a clue, or list of stacking options?

[masjstovel]

I use these settings for the light frames. tried with percentile clipping as well, and no luck

[masjstovel]

3 minutes ago, vlaiv said:

Is there any sort of threshold for star brightness in PI? DSS has that, and if you lower it - it will find more stars, but if you lower it too much it will start mistaking noisy pixels for stars and alignment will fail.

I don't use PI so don't know exact setup of it, but maybe PI help files could provide a clue, or list of stacking options?

We posted simultaneously. Im not steady at all on the technical side on this, but do these settings tell you anything regarding this?

Edited November 9, 2019 by masjstovel

[vlaiv]

That is integration of already aligned images. From PI tutorial, there is this section:

I'll check to see what options are listed in help to see if we can change something to aid detection process

[vlaiv]

I can't seem to find relevant section in help file - maybe you can make a screen shot of that section opened to see what options are available?

[masjstovel]

Lovely! That window is unfamiliar to me - I've used the BatchPreprocessing script? 

[masjstovel]

You know where this screenshot is taken from? script that is.
In batch preprocessing there is only a "Registration" and an "Integration" tab in which i can change settings

Edited November 9, 2019 by masjstovel

[vlaiv]

Have no clue - quick search on the internet gave me this page:

https://www.lightvortexastronomy.com/tutorial-pre-processing-calibrating-and-stacking-images-in-pixinsight.html#Section6

In section 6 it deals with registration / alignment of images and it shows that window so I assumed that is readily available option for image registration under PI.

[masjstovel]

I found it, its just a different approach. I calibrated only in BatchPreprocessing script, and now ill do the alignment in "StarAlignment" script, which you screenshotted it looks like. Here are the settings. You know what the "Distortion model" could be?

 

I 

[vlaiv]

Distortion model is probably related to lens / wide field images. When we are imaging, we are mapping spherical "surface" to flat surface, or rather angles to distance on the sensor. Larger the angle (or smaller the radius of sphere compared to sensor size) - more distortion there will be in the image. When you try to align subs with very large offset this can cause issues as star distances will not be equal if stars are in center of the field and on the edge.

Maybe easiest way to explain that would be to observe "north pole" and 4 points on equator. You can arrange them such that lines connecting them along the surface of the earth are - equal length and always at 90 degrees to one at north pole. Try placing such 5 points in the plane with those properties Angles don't map ideally to distances in plane and you have distortion.

Not sure if you need to concern yourself with that unless you are using wide field lens or fisheye lens or something like that.

As for star detection - I would try adjusting sensitivity, peak response and upper limit - these settings seem related to what you need but I have no clue what each does solely based on their names (try increasing sensitivity, lowering peak response and probably leave upper limit alone - not sure if it will help changing that).

 

 

[masjstovel]

I see, so its like the profile of the "lens" , like "Lens correction" in photoshop? When you take a photo of a bridge from a low standpoint and the bridge pillars points inwards, when they really are vertical? Same but different ?
I tried changing those settings but didnt work. What is RANSAC? Thats displayed in the "Starmatching" tab. 

[Jokehoba]

When I first started using PixInsight, I also had issues with star detection. I recall the key is the RANSAC tolerance /iterations values. As Harry Page says "Experiment!"

These are the settings I use and haven't changed them for several years. I now tend to use the Batch Preprocessing script simply for calibration and continue manually with StarAlignment, Normalisation, Image Integration and Drizzle to complete the stack.

John

[vlaiv]

Just now, masjstovel said:

I see, so its like the profile of the "lens" , like "Lens correction" in photoshop? When you take a photo of a bridge from a low standpoint and the bridge pillars points inwards, when they really are vertical? Same but different ?

Pretty much the same thing - it corrects for "geometry" of image. Imagine that you have two images of pillars shot at different sections and at different perspective points. Both show curved pillars, but you can't put together two images because curve is not uniform, so you "straighten" pillars in the images, join images and then "return" the curve to some extent - that is what will happen with wide field lens shots that are offset by large distance, in order for stars to overlap you need to "bend" each image ...

8 minutes ago, masjstovel said:

I tried changing those settings but didnt work. What is RANSAC? Thats displayed in the "Starmatching" tab. 

RANSAC is algorithm used to determine which stars in one image align to which stars in another image. It is short for "Random sample consensus". Star centers will not always perfectly align in images, partly due to above distortion but also due to noise and seeing (you can't pinpoint exact center of the star to great precision and there will always be some error).

In addition to this - some stars in one image might be missing from the other image and vice verse (those that are just next to the edge and you dithered your sub and it moved FOV and some stars are no longer in the image but some other appeared on the other side). RANSAC tries to find best match in all available stars by removing "outliers" (those stars that don't have their match). It tries to find mathematical solution for transform between one image to other with respect to minimizing alignment error in all the stars. If you are more interested in this, here is wiki article on algorithm (it has nice visual explanation with fitting a line thru a set of points and choosing to discard outliers in the process):

https://en.wikipedia.org/wiki/Random_sample_consensus

 

[vlaiv]

8 minutes ago, Jokehoba said:

When I first started using PixInsight, I also had issues with star detection. I recall the key is the RANSAC tolerance /iterations values. As Harry Page says "Experiment!"

I agree with experiment part, but from what I know about RANSAC - tolerance is related to how much a star can move to be included in the match, or rather what is average displacement value. This can help if you have distorted stars in one or more subs and PI complains that it can't find enough descriptors to make a match or similar.

RANSAC is process that happens once star detection is finished and PI already has a list of stars it is going to try to match over frames. It will not have impact on star detection.

[masjstovel]

Ok, thanks! I've been trying alot of different things now, and couldnt get it to work. Same thing in DSS - So i have to lay flat down, cry and realize its the bad focus that does it 😪.

 

[vlaiv]

18 minutes ago, masjstovel said:

Ok, thanks! I've been trying alot of different things now, and couldnt get it to work. Same thing in DSS - So i have to lay flat down, cry and realize its the bad focus that does it 😪.

 

Yes, poor focus can do that, but then again what would be the point of stacking subs with such bad focus in the first place?

Here is something that you can do to test if it is indeed due to poor focus.

Take all of your subs prior to registration and after you have calibrated them and do integer resample on them in PI (bin them in software). Bin them x3 or x4 and then try again to register and stack them. Binning will reduce resolution of the image and blur size due to poor focus will be less in relative terms. This could help star detection algorithm do it's thing.

[blinky]

Something has changed in the latest version of Pixinsight ith regards to star registration/stacking - I think it might be the 'Compute PSF Fits', try disabling that.  The only other thing I can suggest is to ask on the Pixinsight forum or save some of the subs to one of those file lockers places and let us have a look and see if we can get it to work.