reticule/image not centering

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cbbella
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Joined: Mon May 11, 2020 6:19 pm

reticule/image not centering

Post by cbbella »

hi, i'm trying to collimate my skywatcher 200p newtonian and i'm following Astronomy Shed's video on "Advanced Newtonian Collimation"
https://www.youtube.com/watch?v=8ROvNH5uwDo&t=922s

when i turn on the crosshairs or circle reticule, the image of my focus tube doesn't seem to be centred. can anyone explain why this is please. i have tried the zoom on auto and 100%

i have done this previously in the past and it used to centre ok
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admin
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Re: reticule/image not centering

Post by admin »

Been a while since I have done Newtonian collimation, but if I remember rightly you have put the camera into the eyepiece holder and then you see a view of the internals reflected first off the secondary mirror.

Three immediate reasons come to mind as to why everything is off center:

1) The camera is not sitting square or not central in the eyepiece holder
2) The focuser is not square or is sagging - therefore holding the eyepiece off line
3) Secondary collimation is off, so that the reflection off of the secondary is throwing things off centre.

I think that if the primary collimation was the only problem then the view would look OK (centered) in the image of the primary, but the image of the secondary (reflected off the primary) would be off center, but as I say it's a long time since I tried this sort of thing (I had a Newt, but got rid of it due to the high maintenance it required in terms of collimation etc).

cheers,

Robin
cbbella
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Joined: Mon May 11, 2020 6:19 pm

Re: reticule/image not centering

Post by cbbella »

hi Robin,

the primary mirror is blocked off by a white sheet of paper down the tube and a brown piece of paper is opposite the focus tube for contrast so what the camera is seeing is the white sheet of paper in the secondary mirror and the outer ring is the focus tube. everything seems to be sitting square in the focus tube. i just thought that the crosshairs or circle reticule would be centralised around the image of the focus tube as that is the image from the camera being sent to sharpcap

regards

Colin
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Re: reticule/image not centering

Post by admin »

Hi,

yes, that makes more sense now I understand what I'm seeing in the picture.

To go back to my original list, some of the possibilities could explain this - the alignment of the focuser or droop in the focuser drawtube. Another thing that comes to mind is that the sensor may be off center in the camera (I've seen some camers like this). I think that if you rotate the camera in the focuser then everything in the image will appear to rotate around one point (maybe not the center). That point that things rotate around is the pixel at the center of the drawtube, so place the reticule center over that pixel.

cheers,

Robin
cbbella
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Joined: Mon May 11, 2020 6:19 pm

Re: reticule/image not centering

Post by cbbella »

ok i'll give that a try Robin

thanks
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