Dithering questions

[Moonstruck]

Hello,
I have never tried dithering before, and would like to give it a try. I am having a problem with noisy images, per my last post, and others have said dithering may help to solve that problem. I have some questions on how to set up dithering in Sharpcap. I am using PHD2 and ASCOM (EQMOD). I have Sharpcap version 4.0. I did some reading on the subject from previous posts and hope I set it up right.

1. In the sharpcap settings/guiding tab, I set it as follows:
-max dither step=1pixel
-settle threshold=2 pixels
-minimum settle time=5 sec
-maximum settle time=60 sec
Are these settings ok?

2. In the sequencer tab, deep sky sequence planner sub-tab, guiding sub-tab:
-checked box for "use guiding and/or dithering while capturing light frames."
-checked box for "require guiding while capturing light frames (pause if guiding lost), leave checked for dither only guiding." QUESTION- I want to be able to always dither while guiding, but take light frames without guiding sometimes. Will checking that box prevent my being able to do that?

-Should I check dither while capturing for a number of seconds (say, 300) or a certain number of frames (say, 10)? What do I base that decision on?

3. In PHD2:
-Brain icon: advanced settings, global tab; under dither settings, I checked "random" and scale of 1.0.
-in tools menu: "enable server" is checked.
Are both of these settings OK? Is anything else needed in PHD2?

4. Is there anything else I need to do in either sharpcap or PHD2? Is there another way to set up dithering in sharpcap without using the sequencer function? Note that I'm only interested in dithering when taking light frames, not livestacking.

Thanks,
John

[admin]

Hi,

the guiding settings are not really valid. The dither step is how far PHD2 will try to move the guide star, whereas the settle threshold is how far from the target position PHD2 will accept as 'good enough'. You almost certainly want the first to be bigger than the second - maybe 10 and 2 rather than 1 and 2.

For the guiding options, ticking the 'Require guiding' checkbox does mean that you must have guiding active to capture, so if you want to capture without guiding another time, you will need to remember to uncheck that. It's probably best to set the dither to a number of frames, but to be honest it doesn't make much difference except in cases where you might lose time due to guiding lost. If the guiding is lost then SharpCap will try to get it going again and wait until it is going, which means that capturing a number of frames will take longer than usual, which might lead to a difference between 'Every XX frames' and 'Every YYY seconds'.

I think the PHD2 settings are OK - mind you, it's a while since I've bothered with guiding

Finally, yes, the only two places in SharpCap that you can use dithering is the sequencer and live stacking, and since you don't want to use live stacking, you are left with the sequencer.

cheers,

Robin

[Moonstruck]

Thanks for all the info! I'll give dithering a try.

[Moonstruck]

HI again Robin,
Sorry to bother you again but I have another question. If you don't have time to answer I completely understand as I have been asking a lot of questions lately.

Anyway, I imaged the fireworks galaxy again last night, taking 4-min frames for about 5 hrs. I tried dithering for the first time but I am not sure it worked. The noise in the image was greatly reduced which is good, but I had egg-shaped stars. I am not sure if the dithering worked since I didn't see any annotations saying "dithering" in the PHD2 graph. I also noticed that over the 5 hrs of capture, the image moved over an inch on my screen from where it started. Is that normal, or should it have stayed perfectly stationary the whole time?

Two questions:
1. Was I dithering? I couldn't tell from the PHD2 log or graph during capture.
2. How do I get rid of those egg-shaped stars in the future? I think I polar aligned it correctly (I used the sharpcap polar alignment tool as usual and had an "excellent" result). Was it the guiding? Or maybe my mount (EQ6)? What are some common reasons for this?

I included the sharpcap capture file, PHD2 log and the final processed image below if that helps. Again, if you don't have time for a detailed analysis that's ok- I appreciate any general comments as well.

John

Processed image:
https://drive.google.com/file/d/1q3ciIf ... sp=sharing

PHD2 log:
https://drive.google.com/file/d/1oWgG0i ... sp=sharing

Sharpcap capture file:
https://drive.google.com/file/d/1gCIUg8 ... sp=sharing

[admin]

Hi John,

the only real information to go on there is the PHD2 log file, which only has one 'dither' action in it - you can find this line which represents the dither

Code: Select all

INFO: DITHER by -9.975, 1.272, new lock pos = 844.793, 959.933

I'm not sure why - there might be more info in the SharpCap log or in the sharpcap capture settings saved next to the saved stack image (the one shared doesn't have the live stacking parameters in it).

Looking at the stars in the image, I think I can see a bigger companion to every star located below and to the right of the better 'point like' version of the star. I wonder if this could be a collimation issue as it is consistent across the frame. If you have the individual raw frames saved then look at those - if you are seeing the same isse (just fainter) in all the raw frames then it is probably an optical effect. If the raw frames mostly look good then it may be the result of stacking.

cheers,

Robin

[Moonstruck]

Thanks. I looked at two of the raw frames and it is also apparent in them, so it isn't the result of stacking. Any idea what, other than collimation, could cause this? I just collimated it very recently and i think that is good. Maybe the mount (EQ6) is out of adjustment?

Also I cleaned this telescope recently and in the process took off the corrector plate. It seemed to be snug when I put it back in place, but maybe it wasn't... I'll check!

[admin]

Hi,

to be honest, what I could see in the image didn't look like a mount issue - that would involve trailing or similar. It looked more like optics. Perhaps you could upload a cropped section of one of the individual frames for myself and others to look at in detail to see if we can work anything out.

When you cleaned the corrector, did you make sure that you replaced it in the same orientation relative to the tube as it was originally? I seem to recall reading somewhere that SCT correctors are matched to the rest of the optics and need to keep the same alignment.

cheers,

Robin