DITHER IN THE AEQUENCER

Discussion of using SharpCap for Deep Sky Imaging
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Desbarry
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Joined: Thu Feb 06, 2020 7:09 pm

DITHER IN THE AEQUENCER

#1

Post by Desbarry »

Hi there, I tried out dithering in the sequencer the other night. A couple of issues. Firstly having plate solved and got my target bang on central M101 Pinwheel, as I dithered the target moved off from the central spot more than I would have liked. Is this because I had to auto select a new star every time it got lost due to clouds? Should i have just let phd2 use the same star and keep on guiding on the star it had just lost? When i stacked all my 70 images the target was off the centre so much that i actually had to crop the photo. What am i doing wrong? Ive attached the unprocessed stacked image and it should have been in the centre of the frame but its off. Ive altered a screenshot to better show the galaxy being off the centre as it wasnt that clear on the original screen grab, you can also see a big dither spike on the guiding graph, my guiding was quite good too at 0.67 ish

Thanks Derek
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off target.jpg
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dither graph.jpg
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m101.jpg
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admin
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Re: DITHER IN THE AEQUENCER

#2

Post by admin »

Hi,

I'm presuming that the target gradually drifted away from the center as the imaging progressed - moving a little each dither (at least that is how I would expect it to happen).

As far as I know, PHD dithers randomly, so while sometimes it will move back the way it just came, other times it will move in the same direction again. Over time the drift will tend to add up. I don't think this depends on how you have set PHD up in terms of guide stars.

You may want to look at how far each dither moves your main image - for instance if the dither is set up to move the guide star 10 pixels, that may move the main image a much larger number of pixels. If that's the case then you might reduce the dither size in PHD to a smaller number of pixels.

cheers,

Robin
Desbarry
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Re: DITHER IN THE AEQUENCER

#3

Post by Desbarry »

Robin , thanks for the reply. Its the AMOUNT it was moving off centre that kind of alarmed me, so I can reduce this in PHD2 NOT in Sharpcap?
Derek
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Re: DITHER IN THE AEQUENCER

#4

Post by admin »

Hi,

no, it's in the SharpCap guiding settings - reduce the 'Max Dither Step' setting.

I also had a look in the PHD2 settings and there is an option for 'Spiral' dithering, which may work to help limit the total drift. I must admit I've never tried this.

Robin
Desbarry
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Re: DITHER IN THE AEQUENCER

#5

Post by Desbarry »

Robin, I had another go and reduced the RANGE setting in PHD2 from 1 to 0.5 I dont know if this worked but the drift in this plate solved target is less but still its not bang on centre like it was when I plate solved it prior to running the sequencer. I will try spiral as it is currently on RANDOM I think.
Cheers Derek
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Re: DITHER IN THE AEQUENCER

#6

Post by admin »

Hi Derek,

sounds good - did you also reduce the 'max dither step' in the SharpCap settings?

cheers,

Robin
Desbarry
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Re: DITHER IN THE AEQUENCER

#7

Post by Desbarry »

Yes Robin, Ive reduced it from 20 to 12 pixels(is this ok?) and Ive increased the minimum settle time from 10 to 20 seconds just to give it a bit more time to settle. Havent tried it yet so fingers crossed, just downloading the latest beta 4 Incidentally how far are we fromm the full release?

Cheers Derek
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Re: DITHER IN THE AEQUENCER

#8

Post by admin »

Hi,

yes, I think you could potentially turn it down even more - remember the number of pixels here is measured in the pixel scale of your guide scope, so 10 pixels movement on the guide scope might be 50 pixels or even more on the main scope (depending on relative focal lengths).

cheers,

Robin
Desbarry
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Re: DITHER IN THE AEQUENCER

#9

Post by Desbarry »

Robin its a SW 9x50 finder modified to be a guidescope so using it with a ZWO178mm 2.4 microns and focal length is 181mm entered all this into PHD2 scope is an Esprit 120 with 0.8 reducer so working at F5.6 and 672mm fl.
Im not sure what my pixel scale for this setup is might need to search google lol

Derek
Desbarry
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Re: DITHER IN THE AEQUENCER

#10

Post by Desbarry »

Ok so main scope is 1.15 pix and imaging guidescope is 2.73 so just over 2,27 times as much.
Thanks Derek
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