Plan of Action (manual dithering)

[Iem1]

Morning guys,

I am going to head out tonight to a bortle 3-4 sky in an effort to tackle the Triangulum galaxy again. Will be my first deep sky outing in a few months as I am back in Uni and have been doing some nightscape instead.

This was attempt one:

 

I was super pleased with the detail I managed to grab, but as you can see, the walking noise absouletly kills the image! That and a suspected light leak damaged the image.

 

Anyway, tonight's plan is to try fully manual dithering. I use a Sky guider pro and a WO Z73 with an astro modded 600D, somewhat luckily, the Triangulum galaxy is fairly small in the frame and relatively easy to find/center so I am not too worried about losing it in the frame or shifting it too far between frames, I have a bit of room to spare. I realise an automated system dithers by just a few pixels, think mine will be a touch more than that  

My plan is simple, but arduous. 45 second subs (1 minute is doable, but requires almost perfection, might struggle while dithering too) while dithering by way of simply pushing either the left or right R.A. movement keys on the SGP between frames.

Is it likely that taking the time to do this between frames is going to have an impact? I usually have ~15 seconds between each image, so the rig should have ample time to settle down before the next 45 second shot starts.

 

Cheers guys, Excited to get back out there!

[Iem1]

Sidenote: The image above was around 2 hours of 30 second subs with no dithering 

[Grant93]

Nice picture!

I also own a SGP, and I also did this once with a 600d and SMYG 135mm Lens, with 10 seconds between exposures. I also only did it every 4-5 frames, and probably every 15 frames I would fiddle with the DEC bracket a bit (slightly loosen one side and tighten the other), so you're not only dithering in RA but DEC also. You will find the 10 or 15 seconds between frames eats up your imaging time massively, when you've been sitting out for 4 hours but got 3 hours 15mins of data (but very worth it if you're willing to do it).

However, in this weather I cannot be doing with sitting out there for hours on end haha (unless I'm out with the binos or dob🤓). I would rather set up, spend 15 minutes making sure its running ok, then go and sit inside where I can see the rig running on my backgarden from the patio door, whilst watching TV 😅. But the sad thing is, when you have done it once, and you see the difference, it means more money to be spent on a mini PC and guiding set up to be able to automatically dither 🥶 Which I am planning on making before christmas 😅.

But to answer your question: Yes 15 seconds should be plenty 😃.

Good luck & wrap up warm

Grant

[ONIKKINEN]

For it to be effective you want to dither in both axis, so move RA and DEC a bit. It will make a big difference to your noise levels.

 

Another way i dithered with a DSLR was to tilt the camera 90 degrees every now and then. Only works with small targets that can be cropped though. Not sure how you would do this with the SGP though.

[Iem1]

17 minutes ago, Grant93 said:

Nice picture!

I also own a SGP, and I also did this once with a 600d and SMYG 135mm Lens, with 10 seconds between exposures. I also only did it every 4-5 frames, and probably every 15 frames I would fiddle with the DEC bracket a bit (slightly loosen one side and tighten the other), so you're not only dithering in RA but DEC also. You will find the 10 or 15 seconds between frames eats up your imaging time massively, when you've been sitting out for 4 hours but got 3 hours 15mins of data (but very worth it if you're willing to do it).

However, in this weather I cannot be doing with sitting out there for hours on end haha (unless I'm out with the binos or dob🤓). I would rather set up, spend 15 minutes making sure its running ok, then go and sit inside where I can see the rig running on my backgarden from the patio door, whilst watching TV 😅. But the sad thing is, when you have done it once, and you see the difference, it means more money to be spent on a mini PC and guiding set up to be able to automatically dither 🥶 Which I am planning on making before christmas 😅.

But to answer your question: Yes 15 seconds should be plenty 😃.

Good luck & wrap up warm

Grant

Yes! It is demoralizing how much imaging time is lost due to rest bewteen shots and to make matters worse, I used to take dark frames too. So another 30-40 minutes would be lost taking those. Thankfully I do not bother with those now  I just use Bias & Flat.

I have my thermals ready for tonight, it is going to be very cold indeed. Hoping my little USB Dew heater manages, it doesn't get particularly warm so I am not sure it will! That and DSLR batteries die remarkably quick when it is sub 5 out!  

13 minutes ago, ONIKKINEN said:

For it to be effective you want to dither in both axis, so move RA and DEC a bit. It will make a big difference to your noise levels.

 

Another way i dithered with a DSLR was to tilt the camera 90 degrees every now and then. Only works with small targets that can be cropped though. Not sure how you would do this with the SGP though.

Thank you ONI, I shall try as you and Grant suggested and do both RA & Dec via key pad + tightening/losening screw.

I will for sure have to check PA often if adjusting the DEC. The SGP is very easily knocked out of alignment.

A heck of a lot of extra work, hoping all the effort pays off and I can get a nice image. I do love the Triangulum galaxy  

The walking noise is nowhere near as prominent in my 60 second subs; This around 2 hours of 60 second subs of the Cygnus Wall

Is this reflective of a longer exposure time? are 30 second subs more susceptible to walking noise than 60 second subs? Or might there be something else causing this?

Cheers guys 

[ONIKKINEN]

2 minutes ago, Iem1 said:

Is this reflective of a longer exposure time? are 30 second subs more susceptible to walking noise than 60 second subs? Or might there be something else causing this?

Longer subs will have more signal to noise ratio so noise is less apparent. With DSLRs the read noise of a single frame can be quite significant and is always the same, regardless of exposure time. So if read noise is the same between shots but one has 2x the time compared to the other it must have better SNR. But also, the cygnus wall is a busy target with detail everywhere, so it might just be that walking noise is less apparent because you stretched the image less. Hard to say, because of the difference in resolutions.

Speaking of resolution, binning is also an effective way to reduce noise. Try resampling your triangulum to 50% resolution, it will wipe out a lot of the noise.

[Grant93]

I wouldn't try to answer that last question as I am a newbie to this myself, I have had pictures where walking noise is far more evident than others, for no known reason to myself. I'm just commenting to say these are some nice images!

What do you use to process? Can't wait to see your m33 😀

Grant

[Iem1]

1 hour ago, ONIKKINEN said:

Longer subs will have more signal to noise ratio so noise is less apparent. With DSLRs the read noise of a single frame can be quite significant and is always the same, regardless of exposure time. So if read noise is the same between shots but one has 2x the time compared to the other it must have better SNR. But also, the cygnus wall is a busy target with detail everywhere, so it might just be that walking noise is less apparent because you stretched the image less. Hard to say, because of the difference in resolutions.

Speaking of resolution, binning is also an effective way to reduce noise. Try resampling your triangulum to 50% resolution, it will wipe out a lot of the noise.

Thank you for the info mate.

I might test 60 seconds, see how I fare with them. The longer the better. Want to get as clean of an image as possible.

Will look into resampling too!

1 hour ago, Grant93 said:

I wouldn't try to answer that last question as I am a newbie to this myself, I have had pictures where walking noise is far more evident than others, for no known reason to myself. I'm just commenting to say these are some nice images!

What do you use to process? Can't wait to see your m33 😀

Grant

The first Triangulam image was done in Photoshop, soft stretches and colour balance. Then I use starnet to remove the stars and do a harder stretch and saturation if I remember correctly.

 

I believe this was the final Triangulum image I ended up with after I tried to heal the image with a mix of SiRiL and PS work (Removing banding and trying to remove the light leak)

Not sure how it comes out posted here as I am uploading from my phone

[wimvb]

The recommendation for dithering with a DSLR is 12 pixels. When I dithered manually I went for 15 pixels, this way:

1. Determine your pixel scale (arcsecs/pixel = 206 * pixel size (um) / focal length (mm) )
2. Set the hand controller to 1 x sidereal speed
3. After the first exposure push the RA+ button for as many seconds as your pixel scale is.
4. After the second exposure press DEC+ for as many seconds
5. After the 3rd exposure: RA- for the same time
6. After the 4th exposure RA- again

Then in order:

DEC-, DEC-, RA+, RA+, RA+, DEC+, DEC+, DEC+, RA-, RA-, RA-, RA-, DEC-, DEC-, DEC-, DEC-, etc

This will give you a spiral pattern which will break the walking noise. To keep track of the movements, jot them down on a piece of paper. I did, and I believe I still have that piece of paper somewhere.

This is what the registered sub frames will look like:

https://wimvberlo.blogspot.com/search/label/dithering

Edited November 3, 2021 by wimvb

[Iem1]

Thank you Wimvb for the info mate

But tonight did not go to plan!  

I forgot a screw that attached my RDF to my scope, a single screw which put a halt to the deep sky imaging session! I did not fancy trying to find anything by eye without the RDF.

Luckily though I did pack my Samyang 14mm! So between clouds I grabbed a few shots, stacked them and (very) crudely blended the separate foreground/background in PS. It is not particularly good, but at least I did not leave empty handed