Dithering ? No guiding system ? No problem ! [Deep Sky] Acquisition techniques · Mohammed Hussain Khalifa · 1/24/2022 · 10 · 1405 · 2

Note: if you are a lazy person this is not for you! 

Hello folks,

I have been doing astrophotography for 3 years now, my current setup is (modified DSLR, star tracker, and L-enhance filter), Bortle 8-9. I started with a normal tripod and a camera then upgraded to that, with my final setup I can image in light polluted cities however that's not the problem.

Since I'm using a DSLR, the main struggle is the walking noise in the final stacked image. With a low budget, like most of the other astrophotographers out there, I couldn't buy a guiding system to dither, so I had to think of another way to dither my images. I asked local astrophotographers if they know any another way to dither, and someone suggested to manually do it.

Enough stories, let's get into the main topic.

Manual dithering 

I like to start with samples to let you know the difference between a dithered and non-dithered image. 


Both images are around 2 hours of total integration. 

As you can see the first image looks horrible, despite the fact it was taken with an unmodded DSLR, if it was dithered the final image must look cleaner.

When to dither ?

First of all, you need to know when to dither, I usually dither between 4-5 frames depending on the number of images I'm planning to shoot at one night. If my goal is around 60 or above, I dither between 4-5 frames. If around 100, between 5-6 and so on, the more images the less to dither.

How to manually dither ? Simple.

I use the sky-watcher star adventurer, on the tracker it self there are buttons where you can slightly change the pointing of your camera, I use these buttons to slightly offset my target from center in a random direction, (TRY NOT TO HEAVLY OFFSET YOUR TARGET). With these buttons you are limited, so I use the ball-head also to change in the other directions that the buttons can't do.

You can make it work with any mount, doesn't need to be the star adventurer.

Simple put, just randomly change the pointing of your camera, and you will be surprised with the results. I know it might be time-consuming but it's WORTH IT !

Note: This also works for cooled astrophotography cameras.

If you have any questions regarding this topic please do ask, I will be happy to answer them.

Clear skies !

Olaf Fritsche:
Since dithering only requires shifts of a few pixels, isn't it enough to just bump very lightly against the tripod every now and then?

Bumping the tripod will mess up the polar alignment and cause tracking issues later on.

However, instead of manual dithering you could use the "Skyadventure console app" if your skywatcher star adventurer mount supports it. And then you don't even need guiding if the mount controls the camera through a shutter cable.

Or use the ST4 port with a guider (if your mount supports it) So you could just be using guiding and dithering with the mount. But, also needing a way to control it, so guide camera, guide scope, laptop or asiair, things ad up in price, but it is possible.

You will only get dithering and guiding in the RA axis only.

How nice, I was just thinking of trying "manual dithering" tonight on my swsa!

I'm curious how would dithering only in RA workout?
Since dithering in RA with sam console would be very little effort for me because my setup is controlled through laptop which is on the network that my pc is on

I initially intended to dither every 5 frames by moving RA then after 5 more move the Dec a little, rinse and repeat.

If the forecast holds up I could post the results here?

A great post!

Since the rain pattern is induced mainly from RA drift from periodic error, manually dithering in just RA will only serve to worsen the pattern. A manual offset needs to be in a random direction and include DEC. In my experience, a large-ish (10's of pixels) offset every 15-20 minutes erases the rain pattern in a 1-2 hour stack. But this may not be feasible with very wide FOV lenses (like my 8mm) because the perspective change will cause distortion errors between offset subs.

More recently, I was able to salvage undithered data by attacking the other half of the cause: residual calibration error. The secret? Temperature matching the darks to the lights. Here's just 15 minutes of undithered data in 35C weather:

https://www.cloudynights.com/uploads/gallery/album_7587/gallery_273658_7587_4409.jpg

Cheers,

BQ

Olaf Fritsche:
Since dithering only requires shifts of a few pixels, isn't it enough to just bump very lightly against the tripod every now and then?

I had never seen walking noise in data sets I took with my Star Adventurer which I used in the field (mountain meadows) with PA done by the standard polar scope and with my eye sight and fingers only. At that time I always had a slight image drift over time, say 2 hours, so I needed to crop the stacking results more or less every time.

So I was very happy when I managed to setup my CEM26EC and Astroberry + Ekos + INDI such that – with Ekos' very nice PA tool (without Polaris in sight) – I got a very precise polar alignment. After downloading the RAW images to my desktop it was a joy to fastly scroll through 60-120 images in the image viewer without noticeable drift (or a very, very tiny bit only).

Curiously, I expected great stacking results ... which I also received. No or only very little crop around the borders was needed – what a progress!

... but then, after wiping, removing background and the green shift, I noticed walking noise more or less. Depending on the captured object it sometimes is tolerable and can be reduced a bit by StarTool's very useful noise reduction module which includes an option to mark and reduce walking noise. Unfortunately walking noise in my images does not appear in the exact same direction with parallel lines. They rather look slightly(!) curved and with uneven widths.

So, finally I get much better aligned image stacks with (generally) round stars to the corners – which is great, thanks to a good mount. But OTOH I have to deal with walking noise which is a pain. I have no guiding camera and really don't want to buy more and more equipment which I have to carry around, setup every single time (mobile) and which in addition needs 1 more communication cable + 1 more power supply.

Just my experience ...

Frédéric Auchère:
Hi all,

This requires a computerized mount, so I'm not sure it applies to a staradventurer, but in case you use Astro Photography Tool (APT) to sequence your acquisitions, you can dither your subs without a guiding system. There is an option called "APT dithering" that will dither every N frames by an amount based on the plate scale (arcseconds/pixel) of your system. See the relevant section of the user's manual.
It worked very well for me so far. I use that feature for all my acquisitions with my Az-EQ6, including for heaving drizzling, and I don't guide. I don't know if other applications (e.g. NINA) offer the same feature.

CS,

Frédéric

Obviously any kind of automated or computerized dithering needs BOTH axis motorized. SA relies on manual bumping along DEC, which needs you to be physically present every ~15-20 minutes.