Trying to make sense of the colimation screen

[cyrille.de.brebisson]

Hello,

I am trying to use the colimation screen to help me collimate a scope as the mirror has a center hole and I therefore can not use a laser...

However, the information displayed seems to "jump all over the place", and I do not understand why.
Here are 2 set of images that I took:
https://drive.google.com/file/d/1zq6ZZ5 ... drive_link
https://drive.google.com/file/d/1iyyR_C ... sp=sharing

I would really appreciate if you could have a look at them and tell me why Sharpcap is having issues extracting colimation information from them.

Thanks,
Cyrille

[admin]

Hi Cyrille,

that tool has never really been as successful as I had hoped, which is why it is still marked as experimental after all this time. By relying on effectively the larger star sizes away from the optical center, it struggles when you have something like a coma corrector that is designed specifically to reduce the effect that the tools is trying to measure.

The problem you are seeing with the pattern moving around a lot is made worse by having a relatively small number of stars in view to take measurements from. It should be more stable if you move to an area of the sky where there is a rich star field - that lets the pattern get calculated from the sizes/positions of hundreds of stars not just 10 or 20, meaning that random seeing fluctuations have less impact.

To be honest though, I think there are better ways to collimate... Some you could try include

* A tri-bahtinov mask and the free SkyCal software
* A defocused star and live planetary stacking in SharpCap to stack multiple frames to help stabilize the 'average' shape in the face of seeing. Reset the stack after each adjustment

cheers,

Robin

[cyrille.de.brebisson]

Hello,

Thanks for the info.
I will try making a tri-bathinov...

But I will also definitely try the planetary idea, which I like...

Is there a way in SharpCap to measure pixel distances on images so that I can see if things trully progress when I change the settings?
I am not sure exactly how to express it, but when you change the collimation the center "shadow" will move and the problem is to measure by how much and in which direction. Is thre any tool that will help me do so?

Thanks,
Cyrille

[Jean-Francois]

Hello Cyrille,

Look at this topic ... viewtopic.php?p=52497#p52497

It is what you want for the center "shadow" alignment.


Jean-Francois

[cyrille.de.brebisson]

Hello,

That sounds perfect! Thanks...

BTW, it looks like you made an all sky polar alignement script also. Can you point me to it?

Cyrille

[Jean-Francois]

Hello Cyrille,

Yes, I tried in the last 20 minutes to upload it.

But very strange behaviour of the forum ... I need to login again and again for each action.
I tried to upload the files ... but not possible to insert in the text region.

Note that the old version is no more available ... I wanted to modified an old message and I deleted the old script ... but now I cannot insert the new version

Jean-Francois

[Jean-Francois]

Hello,

Concerning my script for the polar alignment ... It was now possible to modify the old message.
The last version is now here: viewtopic.php?p=52608#p52608

Regards,
Jean-Francois

[admin]

Hi,

to go back to the original question about measuring things in SharpCap, yes, you can do that...

One option is to use the 'Pixel Value Readout' tool - that shows the pixel colour/brightness values and X and Y positions for the pixel you put the target on.

Another, if you can plate solve, is to use the 'Pixel Position/Click to Recenter tool'. In this tool, you click on a point in the image and it shows the RA/Dec/Alt/Az positions for that point. If you right click somewhere else in the image it will show the distance between the two points, both in terms of pixels and in degrees/minutes/seconds of arc.

cheers,

Robin