Streaks in integrated output of Ha lights

@rjchuri Hi Bob,

Those streaks are known as walking noise and cannot be removed with post-processing. The only way to not have them in your integrated images is by preventing them by dithering. Let me know of you need more information about dithering.

Wouter

Thanks for the quick response.

Yes I am interested in any technique to reduce this noise.

Also, why does it show up only when using an Ha filter?

Bob Churi

@rjchuri Hi Bob,

Walking noise gets introduced when an object systematically moves a little bit in the frame. This is usually caused by a good but not perfect polar alignment. If you do three imaging sessions with a mobile setup then the polar alignment will be slightly different each time, causing the streaks to appear in different directions. They are visible in your Ha images since the signal to noise ratio in those images probably is lower than in your RGB images. What exposure time do you use for Ha?

In order to counter these streaks you need to dither at least 15 and preferably at least 20 pixels in the imaging train. This means letting your image acquisition software introduce a random offset of said number of pixels every few (say, 5) images. This will spread out any systematic noise differences of the sensor in a way that APP (or any other image processing software) can identify it and reduce it.

Note that all image acquisition software will typically allow you to specify the dither offset for the guiding train. This means you'll need to calculate the plate scale of both trains so you can calculate the dither offset for the guiding train assuming a wanted dither offset for the imaging train of 20 pixels. you can use this tool

https://astronomy.tools/calculators/ccd

to do that. If, for instance, your imaging train has a plate scale of 2 arcsec/pixel and your guiding train 5 arcsec/pixel, then you will want to configure your image acquisition software to dither 20 * 2 / 5 = 10 pixels.

This is assuming you use image acquisition software of course.

HTH, Wouter

Thanks for the explanation.

My exposure time has bee 45 seconds but I could bump it up to 60 seconds. Not much above that. Should that help?

I will try that in the next few days.

I do not use acquisition software, just the simple Moveshootmove.

Bob

@rjchuri 45 seconds and even 60 seconds is short for a 12 nm filter bandwidth. You should try to increase that to 90 or even 120 sec. With filters like this it becomes essential to use astro imaging software to be able to guide and dither. Only increasing the exposure time with not be enough.