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Collimation and defocusing

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#1 turki

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Posted 04 May 2021 - 09:51 PM

Hello folks,

if i am doing a collimation for my Edgehd 8”
And i need to defocus is there any different if use the telescope main focusing system? Because i have a second focuser on the same telescope “moonlite focuser”
Does it make any difference in the collimation procedure i f i used the scope focusing hand rather than the moonlite focuser?

 

one more thing!

 

if the collimation is done and very perfect”the star in the center of my Ccd is ok

but the corner star is not is this is ok or should all the star in the field of imaging should be circular when i defocus !


Edited by turki, 04 May 2021 - 10:10 PM.


#2 macdonjh

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Posted 05 May 2021 - 09:48 AM

turki,

 

Welcome to Cloudy Nights.  In theory, it shouldn't make any difference whether you use the stock focuser or your added after-market focuser when collimating.  However, the stock focuser may allow your mirror to shift making collimation more difficult.  Mirror shift was one of the reasons I added a Moon Lite focuser to my C11 when I had it.  When I collimate an SCT I use the added focuser if the scope has one.



#3 dcaponeii

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Posted 05 May 2021 - 02:20 PM

Expanding upon Post #2, always focus your manual focuser in the CCW direction as the final motion when you get close to focus.  Then using your Moonlite focuser to go near, far and through focus is fine.  The CCW motion allows gravity to keep the mirror in contact with the focusing mechanism.  Also remember that final collimation should be done at focus by assessing the Airy Disk.  A good option is to use Metaguide with your imaging camera.


Edited by dcaponeii, 05 May 2021 - 02:22 PM.

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#4 turki

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Posted 06 May 2021 - 06:53 PM

turki,

 

Welcome to Cloudy Nights.  In theory, it shouldn't make any difference whether you use the stock focuser or your added after-market focuser when collimating.  However, the stock focuser may allow your mirror to shift making collimation more difficult.  Mirror shift was one of the reasons I added a Moon Lite focuser to my C11 when I had it.  When I collimate an SCT I use the added focuser if the scope has one.

many thanks macdonjh

 

what a bout the second part should all the stars in the field collimated and looks circular or only the center star during the defocus procedure?



#5 BalconyAstronomer

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Posted 07 May 2021 - 08:45 AM

The focus plane of an SCT is not flat. So, when the centre is in perfect focus and your camera's chip is big enough the stars in the corners will be slightly out of focus and produce the "egg shape" which you are seeing.


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#6 Endymion

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Posted 07 May 2021 - 04:31 PM

You should make sure the scope comes to focus at the correct back distance using the main focus knob to ensure stars are corrected across the field. I would recommend you start your collimation from that position and then either way of focusing should be ok.
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#7 turki

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Posted 08 May 2021 - 11:58 AM

You should make sure the scope comes to focus at the correct back distance using the main focus knob to ensure stars are corrected across the field. I would recommend you start your collimation from that position and then either way of focusing should be ok.

Thats good, many thanks




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