Should I dither

[Richie092]

Morning all.

Have captured my first set of subs this week and to be honest I am really pleased with them. I only have Ha & OII at this point with 10 minute subs, guiding looks like it is good and the stars don't seem to stretch in the corners (to my newbie eye) but they are a little square.

As I understand it my imaging resolution is 3.2 (Atik 383 and Star 71 4.9) so I think that is undersampled? Probably making the really small stares square?

Does dithering cure this? If so, will I need to stack them differently if dithered? I haven't stacked anything yet but was just going to use default settings.

All I have at the moment is 15x10 min Ha and 12 x OII but hoping to add more this week.

Thanks in advance

Richie

[vlaiv]

Yes, you should dither.

No, you are not probably not undersampled.

What you perceive as little squares are likely to be in fact astigmatic stars in the corners.

When dithering you don't need to stack differently, but you can - dithering enables some algorithms to work properly - like sigma clip rejection.

[endless-sky]

If you are undersampled, dithering certainly helps. It also helps with walking noise, so to me dithering is a win-win.

If you use PixInsight for your pre and post-processing, you can process your dithered subs by checking "Generate drizzle data" in the StarAlignment process. This will create drizzle files, that then you will add to the regular ImageIntegration process: you integrate the image like you normally would, but click on "Add Drizzle Files" and check "Generate drizzle data" again, in the integration settings. Then, as a final step, you will use DrizzleIntegration process and select the drizzle files again and this will generate a final, drizzled image. Stars should appear a lot rounder in this image, with respect to the one that came out of the regular ImageIntegration.

[Richie092]

Thanks for the replies. I was going to use DSS to start with. I already have PS  but have been reading up on peoples opinions of PixInsight and it does look good it also does everything on the MAC. I will download the trial.

My subs I have now aren't dithered so clear skies permitting I will capture enough data this week to process both and compare. Dithering is just a tick box in Kstars so that bit is easy!

 

[endless-sky]

I use KStars as well! Are you using an auto-guiding setup? Because there are two settings for dithering, in EKOS: one for guided dithering and one for unguided dithering. Just make sure you select the correct one: I made the mistake of checking both of them, and the next time I tried opening EKOS it kept crashing. I had to manually edit the config file to remove one of the two settings (in my case, the guided dithering one, as I don't have a guiding camera, yet).

[Richie092]

I use the internal guider (not PHD2)

through a separate guide scope/cam. Would you call that an auto-guiding set up?

Edited July 19, 2020 by Richie092

[endless-sky]

Yes. Then I believe the box that needs to be checked is under the "Options..." (lower right of the screen) of the EKOS "Guide" tab, and it will be the first one ("Dither"). Then you need to specify how many pixels, how many frames to dither, etc.

The box that you do not want to check, if dithering while guiding, is the one called "Non-Guide Dither Pulse".

Hope this helps!

[Richie092]

Thanks. Will go in and check that now. Appreciate your help.

[MarkAR]

Ekos has a default of 2x2 dithering, should work well.

If you dither then make sure you set the same in Pixinsight for drizzle.