Can't Get Past Wipe

[danweis1]

Hello,

I am very new at all of this and StarTools looked like a good way to post process my astrophotography attempts. I keep getting the same problem with every image I'm trying to process. Following some videos I've watch and after reading a few articles, here is the process I've been trying. First, Autodev, Optional Bin 50%, then Wipe. At Wipe, I always get the same thing no matter what image I'm processing. Pic attached.

Equipment is: SE6 on and AVX mount, Hyperstar, and ZWO ASI183MC camera. SharpCap using Live Stack. No darks, flats or bias frames (yet). If anyone can give some advice I'd appreciate it.

Thanks.

[admin]

Hi,

By the looks of it, all you need to do is Crop away your stacking artefacts at the borders of your image. Do that and you should be on your way!
As you can see here in the documentation, your are running into a textbook case of gradient/bias remnants due to Wipe backing off close to the darker-than-real-background anomalies.

Preparing data for the Wipe module

^ Leaving stacking artifacts in will cause Wipe to interpret the anomalous data as true background, causing it to back off near the location of the artifacts.



It is of the utmost importance that Wipe is given the best artefact-free, linear data you can muster.


Because Wipe tries to find the true (darkest) background level, any pixel reading that is mistakenly darker than the true background in your image (for example due to dead pixels on the CCD, or a dust speck on the sensor) will cause Wipe to acquire wrong readings for the background. When this happens, Wipe can be seen to "back off" around the area where the anomalous data was detected, resulting in localised patches where gradient (or light pollution) remnants remain. These can often look like halos. Often dark anomalous data can be found at the very centre of such a halo or remnant.

^ Halo around a simulated dust speck dark anomaly.



The reason Wipe backs off is that Wipe (as is the case with most modules in StarTools) refuses to clip your data. Instead Wipe allocates the dynamic range that the dark anomaly needs to display its 'features'. Of course, we don't care about the 'features' of an anomaly and would be happy for Wipe to clip the anomaly if it means the rest of the image will look correct.

^ Masking out the dust speck in order to make Wipe ignore that location.



Fortunately, there are various ways to help Wipe avoid anomalous data;

• A 'Dark anomaly filter' parameter can be set to filter out smaller dark anomalies, such as dead pixels or small clusters of dead pixels, before passing on the image to Wipe for analysis.
• Larger dark anomalies (such as dust specks on the sensor) can be excluded from analysis by, simply by creating a mask that excludes that particular area (for example by "drawing" a "gap" in the mask using the Lassoo tool in the Mask editor).
• Stacking artefacts can be cropped using the Crop module.

^ The result of making Wipe ignore the anomalous data. No halo-like remnant is left.



Bright anomalies (such as satellite trails or hot pixels) do not affect Wipe.

Hope this helps & wishing you clear skies,

[danweis1]

Thank you for your reply. I couldn't see any problems with the image, so I just cropped the middle 90% out (removed the edges) and everything went smoothly. I just paid for the software this morning.

No more problems, except... after a wipe, after I hit "Keep", the image goes back to the original unmodified version (at least it looks like the original version). I have had this happen on four different images. Any ideas what may be wrong?

Thanks,
Dan

[admin]

Thank you for your reply. I couldn't see any problems with the image, so I just cropped the middle 90% out (removed the edges) and everything went smoothly. I just paid for the software this morning.

No more problems, except... after a wipe, after I hit "Keep", the image goes back to the original unmodified version (at least it looks like the original version). I have had this happen on four different images. Any ideas what may be wrong?

Thanks,
Dan

That's great Dan, and thank you for supporting StarTools!

The reason why your image seemingly "goes back" to the original version, is because of what Wipe does;
Its job is to get rid of gradients in your linear (unstretched) data. Gradients take up a range of brightness levels (aka. "dynamic range") in order to show/describe them (e.g. this is a stretched image showing gradients). Once Wipe had removed these gradients from the linear data, these brightness levels can now be re-allocated to show/describe real celestial detail (e.g. a stretched image without showing gradients). To do this, a new stretch is in order. Within Wipe, Wipe applies a courtesy and temporary(!) "AutoDev"; an automated stretch of your linear data that is meant to show any remaining issues. However, once Wipe is done, it will return your data to its linear state, so you can re-do your stretch using AutoDev or Develop. This time, that stretch will not have to contend with gradients and can therefore show more real celestial detail.

Hope this helps!

[danweis1]

Thank you very much. So helpful.

Dan