Question about using Wipe

[jopangel]

Hello all,

I am working with an image of M31 and I am having some issues using the Wipe routine. Should I be able to get the gradients and vignetting removed in one pass or would multiple passes be better? Still learning the program and getting a little frustrated at times.

Joey

[admin]

Hi Joey,

A single Wipe, very early on (before final stretch etc.) is all that is needed.

StarTools discourages multiple passes of any module.

[LanFeusT]

Hi Ivo,

I'm surprised by this response. How would you deal with an image that clearly has a gradient (say light pollution) and some vignetting from the lense then? I would have imagined that running a Gradient wipe, followed by a dev/auto dev and a vignetting wipe would be a good idea, it seems to be giving good results for that case. Is there a reason why it shouldn't be done, and how would you resolve that kind of issue?

Thanks!

[admin]

Hi Ivo,

I'm surprised by this response. How would you deal with an image that clearly has a gradient (say light pollution) and some vignetting from the lense then? I would have imagined that running a Gradient wipe, followed by a dev/auto dev and a vignetting wipe would be a good idea, it seems to be giving good results for that case. Is there a reason why it shouldn't be done, and how would you resolve that kind of issue?

Thanks!

No problem!
It's best to apply the most aggressive of the two operations (in this case the Vignetting preset), as the less aggressive (for example the Default settings) one is "incorporated" into anything that's more aggressive.

The Wipe module operates on the linear data by reversing any stretching that has been carried out by Develop or AutoDev, so Develop or AutoDev should not make any difference for the effectiveness of the gradient removal (here's where the Tracking feature comes into play where StarTools attempts to apply operations at the best possible stage in the flow - in this case when the data is still linear).

Wipe will reapply the stretch that was performed as best as it can, but often the characteristics of the data will have changed a fair bit, so that a new stretch is appropriate/required after Wipe has done its thing.

If you have a particular data set that seems problematic and/or seems to get better results with repeated Wipes, I'm happy to have a look.

I'd also like the take the opportunity to say that flats are a very important part of astrophotography and Wipe's Vignetting removal is - at the end of the day - just a crutch. You will always get much better result from properly flat-correct data!

[LanFeusT]

Ah I see! Thanks for the explanations!

And yeah flat... unfortunately also known as "the hardest thing to do after a guiding" -_-
I've yet to manage making flats for my 8" scope, not to mention that since I don't have a pier, I would have to redo flat everytime as the camera is never in the exact same spot between exposure nights :/

[admin]

Ah I see! Thanks for the explanations!

And yeah flat... unfortunately also known as "the hardest thing to do after a guiding" -_-
I've yet to manage making flats for my 8" scope, not to mention that since I don't have a pier, I would have to redo flat everytime as the camera is never in the exact same spot between exposure nights :/

Maybe you could use the white-t-shirt-pulled-over-the-tube-around-dusk method? (yes, that's its official scientific name )

[davidjbillo]

...I would have to redo flat everytime as the camera is never in the exact same spot between exposure nights :/

I set up and teardown every time. Maybe I'm not as fussy as you, but I only use two camera orientations...long dimension E-W and long dimension N-S. The only other camera requirement is that it be in the focused position. So I have made two sets of flats, one for each of those two orientations, and I just keep reusing them. For vignetting correction, that should be fine. For dust donuts, well, I don't have many, they are not very visible, but I will probably have to make another set of flats before too long, but certainly I don't feel the need to do them every time I use the camera.

OK, that was a bit OT, but I thought I'd mention it.

[LanFeusT]

Next time I go out imaging I'll try and add some marks so my camera is always in the same position, the two position idea is a really good one thanks!

The problem I've had mostly is just finding a way to make a good flat, the "white-t-shirt-pulled-over-the-tube-around-dusk" (TM by Ivo!) has not been very successful for me ^^

[davidjbillo]

I made my flats on the dining room table. I have a laptop with a 17" screen, just large enough to cover the end of my 8" Newt. I set a pure white desktop, no icons, and a sheet of paper to help make it evenly lit. You could probably use the t-shirt method indoors as well if you can have a large evenly lit surface/wall in front of it.