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C8 image shift when focusing

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#1 Stevegeo

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Posted 14 November 2019 - 04:07 PM

I thought I would get opinions on something I noticed after setting up my C8 in my living room. Older c8 probably 80s vintage . 

Looking out my front window of my dome house I frequently site in my scope to a cell phone tower some 6 miles away .. just to check it out.

I noticed that as I focus in with a 9mm ( or any other eyepieces) there is a noticable image shift  left to right. 

Yes I have a right angle on it .. the focuser works smooth . 

Anything come to mind? Mirror shift ?     ..



#2 Tulloch

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Posted 14 November 2019 - 04:17 PM

Hi there, I assume you are using an SCT. This image shift is a consequence of the way the scope focuses. Have a look at this.

 

https://www.celestro...at-is-happening

 

Andrew



#3 WarmWeatherGuy

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Posted 14 November 2019 - 04:22 PM

Your focus knob is a threaded nut. There is a threaded bolt glued to the back of your primary mirror. When you turn the knob clockwise you're pulling the mirror towards you. When you turn the knob counter-clockwise you're pushing the mirror away from you. The attachment point of the bolt is away from the center. The center of the primary has a big hole in it and it slides along a cylindrical baffle that is welded to the back of your OTA and extends up towards the secondary mirror. There is necessarily some size difference between the hole in the mirror and the outer diameter of the baffle that it slides on. This is why you're advised to always make the last turn of the focus knob CCW. Usually the telescope is pointing up and gravity is pushing the mirror down. If your last turn of the focus knob was CW then over time gravity can pull the other side of the mirror down and you lose focus.


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#4 photoracer18

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Posted 14 November 2019 - 04:23 PM

That is mirror shift caused by the focus mechanism and the way the primary mirror is mounted. The amount varies depending on tolerances and the actual mechanism. In an SCT or moving mirror MCT you can ignore it if you put a focuser on the visual back. but not a cheap solution.



#5 Migwan

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Posted 14 November 2019 - 04:28 PM

Great link Andrew.   I had some slight, but noticeable shifting at higher mags on my C11.   I recently installed fans and had the primary mirror off.  So I stripped the OEM grease and applied specialty silicone based grease.  Really cut the shift down to where I don't even notice it, even at real high powers.  Last resolve, but I am thinking about doing the same with my older 2080, which has a lot of shift. 

 

jd



#6 Nicole Sharp

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Posted 14 November 2019 - 04:51 PM

Hi there, I assume you are using an SCT. This image shift is a consequence of the way the scope focuses. Have a look at this.

 

https://www.celestro...at-is-happening

 

Andrew

 

I've noticed this on my Omegon MightyMak 1000/90 Maksutov-Cassegrain as well.  Thanks for the link to the Celestron article explaining it.



#7 Stevegeo

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Posted 14 November 2019 - 05:00 PM

Thanks all who responded,

I understand the mechanics involved. And I know and seen a c8 apart to get how it works, I thought it was something simple like one side slightly out of sync with the other.

I was no aware of the cw and ccw dialing in... thanks for that tip.

 Great info....

Clear skies!



#8 fcathell

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Posted 14 November 2019 - 05:27 PM

This is pretty common on SCTs, particularly older ones, and less so on some Maks. Using the 9mm eyepiece can you estimate how much (%) of the total eyepiece field it seemed to move?  One thing to try is to run the focus from one end stop to the other several times.  This will move the mirror up and down on the central baffle/spindle and spread out the grease some. I would say under normal observing that if the object shift, say on Saturn, is about the same as the width of the ball of the planet, it is probably typical of an older SCT and, if it doesn't bother you, it's no big deal. The only fix is to remove the mirror/spindle assembly and remove the old grease and re-grease it.  It's not hard to do but it is not something I would recommend if you have never had one of these scopes apart.

 

Frank



#9 chubster4

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Posted 14 November 2019 - 05:30 PM

For visual observing, try a helical focusing adapter - fits in the diagonal like an eyepiece, and the eyepiece goes inside it. Turning it one way or the other permits extremely fine focus adjustments, with no image shift. One of the most useful accessories you can get. I looked around online just now, and found one on Orion's site. 



#10 chubster4

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Posted 14 November 2019 - 05:37 PM

I take that back...Orion says it's no longer available!  They are out there, though. I've had several over the years. The first one I had I made myself using the helical focusing mechanism out of an old camera lens. Made focusing at high powers with my C14 a breeze.



#11 Nicole Sharp

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Posted 14 November 2019 - 08:24 PM

My experience is when attempting to align a GOTO system at 370X magnification.  I use a Canon APS-C DSLR camera (with a 27-mm sensor) at prime focus in the 1000/90 Maksutov-Cassegrain (37X optical magnification), and then apply an additional 10X digital magnification to the liveview.  When I adjust the focus on a star at 370X, you can see the star jump position (in addition to the shakes from touching the focuser).  It's very minor though and more of a nuisance than a problem.


Edited by Nicole Sharp, 14 November 2019 - 08:26 PM.


#12 Eddgie

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Posted 15 November 2019 - 10:36 AM

At the age of your telescope, my guess would be that the baffle needs to be re-greased. 

 

As others have explained, the mirror on these scopes is attached to a carrier plate and this plate has a hole in it which the primary baffle runs through.  As you turn the focuser knob, the threaded rod pushes/pulls the mirror carrier along the baffle and it slides on a film of grease. This grease film takes up at least some of the gap between the mirror carrier and the baffle, and when it thins, the excess play causes excess tilt between the baffle and the carrier. 

 

Now at this age, I would consider re-greasing with a very high viscosity temperature stable grease, but prior to doing that, I would see if it is possible to re-distribute the grease that is already on the baffle.

 

The easiest way to do that is to place the scope on a horizontal surface.  Begin by turning the focuser knob fully counter-clockwise. Remove the focuser knob, remove the three screws that hold the focuser assembly in place, and with the screws removed, grasp the focuser assembly and pull it away form the tube as far as it will go.  Now, push and pull the focuser rod through its full range of movement maybe 10-20 times.   

 

If there is any grease left, this will force the mirror to travel outside of the normal focuser range over the area of the baffle where it has migrated to and re-distribute at least some of that grease back to the area of the baffle where the carrier normally travels when the scope is at infinity focus.

 

I would not use superlube or other thin grease compounds.  These are fantastic lubricants, but they will not provide a durable grease film between the surfaces of the carrier and baffle. The grease has to be high viscosity and ultra-low out gassing.  Probably something like NyeTorr 6200 which is a lab grease that is recommended for linear glides, where motion control (play) is important.  I am not saying this is the right grease to use, but Nye can probably give you a recommendation if you carefully describe the application requirements. 

 

Even with fresh grease, some movement might still be noticed during focusing, but anything more than an an couple of arc minutes probably means you need to re-grease.

 

A lot of greases outgass.  It is vital to choose one that is formualted to prevent that.  From the Nye 6200 description:

 

 

 

eliminating or minimizing
airborne molecular contamination or giving off vapors
that can fog optics in high-speed inspection systems
or even contaminate wafers.

Edited by Eddgie, 15 November 2019 - 10:39 AM.

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