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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

Attached Thumbnails

  • IMG_20201017_112851.thumb.jpg.0ae0935cb126216247a2c23f8c22650d.jpg


#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.


#4 StarMich

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Posted 27 October 2020 - 04:15 PM

Hi sorry for the response delay. While collimating the best I could to further illustrate the issue the compression springs of the secondary mirror screws started to come loose and fall out. Based on your responses I decided to take the assembly apart and to look for a solution right away.

Here are some pictures of what happened and to further illustrate the issue:

 

ES PN208 secondary assembly 1
 
ES PN208 secondary assembly 2

 

I found some screenshots of when I first examined the problem when I bought the scope in 2016, these clearly show the offset of the secondary from the tube center:

 

Offset 1
 
Offset 2

 

I tried to find longer screws to push the mirror further down as the screw that holds it is long enough, only the set screws are to short. The threads seem to be very specific and even a specialised supplier I went to didn't recognize the threads.


Another solution without drilling into the tube to shift the spider or focuser was to find a thicker spacer to fit between the mirror and the set screws. I did find some stainless steel ones online which did almost match the diameter of the original. The only thing missing was a hole of 5mm in the center, went to buy an HSS Cobalt drill bit to be able to drill through this stuff.

It turned out to be extremely hard to drill through, after half an hour of drilling I was barely one mm trough.

 

Steel spacer

 

 

I eventually gave up on this solution and decided to give 3D printing a try. The local observatory printed a spacer in PLA with a thickness of 1cm:

 

Printed spacer

 

 

It seems to hold collimation and solved the centering issue, here is the view through the concenter eyepiece:

 

Concenter 1
 
Concenter 2

 

 
 

I'm almost there I think, the primary could be centered a bit better. smile.gif

 

@Vic: Out of curiosity, what are the consequences of not being able to center the secondary like before? Light being trimmed when moving focus away from the secondary?

 

Best regards,

Michael


Edited by StarMich, 27 October 2020 - 04:16 PM.


#5 Vic Menard

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Posted 27 October 2020 - 05:59 PM

@Vic: Out of curiosity, what are the consequences of not being able to center the secondary like before? Light being trimmed when moving focus away from the secondary?

If the secondary mirror is reasonably oversized, then yes, the light profile would be unbalanced. But if the secondary mirror is optimized to reduce obstruction, you could end up using more of the edge of the secondary mirror even at high magnification (it's better to use the center which is often better corrected)--reducing resolution, or if it's really undersized, you may even lose full illumination in the center of the field of view (effectively reducing your aperture). 


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#6 SteveG

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Posted 28 October 2020 - 03:28 PM

Hi sorry for the response delay. While collimating the best I could to further illustrate the issue the compression springs of the secondary mirror screws started to come loose and fall out. Based on your responses I decided to take the assembly apart and to look for a solution right away.

Here are some pictures of what happened and to further illustrate the issue:

 

 
 
 

 

I found some screenshots of when I first examined the problem when I bought the scope in 2016, these clearly show the offset of the secondary from the tube center:

 

 
 
 

 

I tried to find longer screws to push the mirror further down as the screw that holds it is long enough, only the set screws are to short. The threads seem to be very specific and even a specialised supplier I went to didn't recognize the threads.


Another solution without drilling into the tube to shift the spider or focuser was to find a thicker spacer to fit between the mirror and the set screws. I did find some stainless steel ones online which did almost match the diameter of the original. The only thing missing was a hole of 5mm in the center, went to buy an HSS Cobalt drill bit to be able to drill through this stuff.

It turned out to be extremely hard to drill through, after half an hour of drilling I was barely one mm trough.

 

 

 

 

I eventually gave up on this solution and decided to give 3D printing a try. The local observatory printed a spacer in PLA with a thickness of 1cm:

 

 

 

 

It seems to hold collimation and solved the centering issue, here is the view through the concenter eyepiece:

 

 
 
 

 

 
 

I'm almost there I think, the primary could be centered a bit better. smile.gif

 

@Vic: Out of curiosity, what are the consequences of not being able to center the secondary like before? Light being trimmed when moving focus away from the secondary?

 

Best regards,

Michael

And now your secondary silhouette finally shows a proper offset (pic #2).




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