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Importance of centered secondary

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#1 StarMich

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Posted 18 October 2020 - 03:05 AM

Hi, let me first briefly introduce myself: I'm Michael, 27 years old and currently 10 years active in astrophotography.

My main setup consists of an HEQ5 pro, a SW 8" f/5 newtonian, an ES PN208 f/3.9, a Canon 1100D modified and I recently switched to mono CCD imaging with a secondhand SX Trius-35. For coma correction I use a GPU comacorrector.

 

I'm currently aiming for the best possible field illumination for my Explore Scientific PN208 f/3.9 imaging newtonian. The scope is 5 years old now and while doing maintenance I wanted to do something about the design flaws as well.

 

With the current secondary holder design it's impossible to center the mirror beneath the focuser.

 

My question is: is this really an issue when the entire primary is visible? I read about optimal field illumination when the secondary is centered, but this isn't achievable with this telescope.

 

I attached an image showing the (very bad aligned and already degrading) secondary at the moment. This is as far down as I can go, I would need to replace the holder screws with longer threads to go further but I don't know if this would actually improve things?

 

 

 

Thanks in advance,

Michael

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#2 Steeveaux

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Posted 18 October 2020 - 10:25 AM

On-axis you see the whole primary.
Can you tilt the focuser to center it?

#3 Vic Menard

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Posted 18 October 2020 - 10:34 AM

My question is: is this really an issue when the entire primary is visible?

Yes--it really is an issue.

 

To help, you'll need to provide an image through a centered pupil (peep sight/collimation cap/Cheshire eyepiece) so we can see the focuser and primary mirror alignments. It would also help if you could provide pictures of your secondary holder and spider so we can all see the design flaw you've discovered.

 

And welcome to Cloudy Nights!


Edited by Vic Menard, 18 October 2020 - 10:35 AM.



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