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Is there a way to fix SCT Mirror shift

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#251 Kokatha man

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Posted 17 September 2015 - 09:18 PM

After carefully threading the two bolts in as you suggested, the focus knob could still turn 3-4 times in each direction before encountering any resistance. This really makes it impossible to get a quick fix since you would have to gauge where to put the bolts in so you could achieve correct resistance at 3-4 turns. I guess I could make custom bolts of variable length that would only thread into the second set of threads. It's a tricky endeavor since it depends on the focus position. Simplistically, I just need some bolts that are a little longer, like a half inch or so. I'm really not inclined to open my scope up since I don't trust myself to get into something like that.


Quite surprised to hear this Randy - tbh I never tried turning the primary knob once I'd put the bolts in for fear of doing damage: is this 3-4 turns either way or 3-4 turns clockwise & then back again..?

I ask this because of the way the primary adjuster thread is fixed into that particular arm of the mirror supports...

I didn't want to go back inside the scope either & why I chose JB Weld to cement the guide blocks onto the back of the scope "around" the rear thread-holes as in the pik I posted...I consider this approach a bit "sloppy" tbh but looking at it after cosmetics it looks pretty nice regardless...& I have no intention of selling the scope - although I don't think it would be any buyer's issues. ;)

On their effectiveness I can give the :waytogo: because they got their first light this morning...we collimated on Altair low in the West, swung halfway across the sky to lambda Aqu. & found imperceptible differences in star-pattern...another move to one of the small stars near Uranus later was similar although I want some better seeing to really qualify these results a bit better...
 
 

Here's a way to permanently fix mirror shift, mirror flop, mirror anything.


That would fix just about everything Rolo - & permanently!!! :)

#252 rolo

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Posted 17 September 2015 - 10:00 PM

While NASA spent $$$$$$$ and hundreds of hours developing a pen that would write in zero gravity, the Russians used a pencil... :scratchhead:



#253 RandyC

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Posted 17 September 2015 - 11:33 PM

Here, allow me

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#254 RandyC

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Posted 22 September 2015 - 08:06 AM

Kokatha, Would you be interested in making me a set of the Guide blocks and mirror stabilizing/locking rods as you have for the C14. I will happily pay you for them and shipping. It would be much appreciated. cheers, Randy



#255 RandyC

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Posted 22 September 2015 - 09:00 AM

Could I just ask one more time, is the upper left of this image compared to lower right, mirror flop. Or at least a similar primary alignment issue that can be fixed with this method? Pure coma and curvature issues would be uniform around the whole mirror, not just left to right. If i push the primary slightly on the left side, it would seem problem solved.

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  • C14 with A7s.jpg


#256 drmikevt

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Posted 22 September 2015 - 10:36 AM

Sometimes smearing on one side of the image is from the sensor not being perfectly perpendicular to the optical axis of the OTA.  The larger the sensor, the more obvious the issue will be.  There is a product, the Glatter Parallizer, that is made specifically to correct this - it is very well made device and works well, in my experience.


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#257 RandyC

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Posted 22 September 2015 - 10:45 AM

My sensor is in an A7s and screwed directly into the visual back. I've used the same setup on my C11 and it was fine. To cut to the chase, I think the problem is when Celestron ships, the mirror locks screws are over tightened, and it pulls the mirror inwards on that side. But there is no way to push it back out. Also the glatter parallizer is for 1.25" setup. 



#258 drmikevt

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Posted 22 September 2015 - 11:21 AM

Yes, I'm sorry - it won't work with the A7s.  I use smaller chip devices with nosepieces - I should have thought about that.  Either way, the issue seems to be being perpendicular to the optical axis.  

 

As an idea, if you can't get the mirror to cooperate, maybe you can switch to a Baader click lock visual back, get a nose piece for the camera, and then shim one side to create a situation where you were perpendicular again to the primary.  Just a thought - maybe a bad/clunky one, but a thought nonetheless.  Good luck!

 

Mike



#259 RandyC

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Posted 22 September 2015 - 11:42 AM

thanks, i'll try through my 80mm guidescope to be sure it's not the camera.



#260 RandyC

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Posted 22 September 2015 - 12:33 PM

Or even easier, rotate the camera upside down

#261 Kokatha man

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Posted 07 October 2015 - 07:31 PM

Kokatha, Would you be interested in making me a set of the Guide blocks and mirror stabilizing/locking rods as you have for the C14. I will happily pay you for them and shipping. It would be much appreciated. cheers, Randy

 

Sorry Randy, I missed this...been away a bit & tbh don't spend much time outside the Solar System Imaging & Processing Forum... :)

 

I think it would be a lot easier (for both of us!) if I sent you the drawings & you had someone local make them up: I mean, it is essentially 2 short sections (7/8" long) of 1&1/4" diameter aluminium rod stock, along with 4" lengths of 316 stainless steel of just enough sufficient diameter to tap 5mm male threads on one end for 5 or 10mm. 

 

Apologies for mixing inches & millimetres but I thought you'd be more familiar with inches - we don't use them anymore in Oz - but the threads in the Celestrons are 5mm so I had to give that in metric..! ;)

 

Very happy to correspond & send drawings & instructions...PM if interested - I'm sure someone local in your area would make these up very quickly & inexpensively: as mentioned I took the "cheat's way out" by JB Weld fixing them to the rear of the scope to avoid disassembly once more...but it is very secure & neat regardless, & much easier to put together! :)

 

Btw, I have also made up my own version of the "Tempest" cooling fan idea except I'm using 108cfm fans, one to pull air in via one of the intakes on my C14 & the other to push air out via the other vent opening...cannot believe how effective they are at getting the scope's primary down to ambient: I used ball-bearing fans for these fairly big grunter fans but don't run them whilst imaging, but that isn't necessary.

 

I used ABS to mount the fans which is a commonly-available plastic that can be moulded/conformed to profiles (like the C14's around the vents) with just the heat of a hair-dryer...comes in "hair-cell" variety surface on one side (the most common) & looks quite neat & complementary for the scope...here's a pik. :)

 

C14-modifications.jpg



#262 InlinePhil

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Posted 03 June 2016 - 10:50 AM

My Meade had a lot of slop between the mirror hole and the slider tube. This is a tolerance issue where glass expansion/contraction must be accounted for when deciding what diameter to make the hole.

 

As I said in the past tense, it had an issue but does no more. After reflocking the OTA, i had a lot of scraps left over. I cut out one continuous layer around the slider tube onto which the primary fit quite snugly.

 

The referenced RTV solution assumes that the mirror is centered which is an incorrect assumption. Adding a uniform layer of material more properly centers the mirror on the slider; adding RTV after centering is purely optional but RTV makes it difficult to remove the primary in the future for servicing.

 

Shift solved.



#263 drollere

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Posted 06 June 2016 - 10:47 AM

well, "perfect is the the enemy of the good." if you're only motive to "fix" something is because it's not "perfect", take a deep breath and think about all the other ways you could spend your time.



#264 Abbas

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Posted 07 January 2019 - 07:06 AM

Dear folks.
I have Meade LX200 ACF 16 f/10,
I want to ask about image shift and mirror flip issue in OTA.
Can any body give me a best solution in written, images or video on you tube?
I would like to share a small hystory about Meade 16 so called Flag ship.
Before this mirror flip issue Meade RA drive gear losen and lost its sidereal speed, hence I switched to ASA DDM 85 Premium mount. I installed the Meade 16 OTA using Paralex rings, but can not able to get arc sec accuracy due to mirror shift.
Sad!
Clear skies.


Abbas
Taqwa Observatory

#265 PETER DREW

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Posted 07 January 2019 - 11:15 AM

Just read this whole thread through for the first time. Interested because in the 70's and 80's I built and marketed 8" SCT's and 3.5" - 12" Maksutovs. Being aware of the image shift issue with Celestrons of the same era, a different method of primary mirror adjustment was called for. Nothing wrong with a focusing knob off axis provided that the effort to move the primary carriage is applied on axis. This was easily achieved by a hinged fork the tynes of which passed either side of the inner baffle tube. The tyne ends were closed off with a bar which the focus rod operated against. The two points of contact with the primary carriage were centrally on axis and provided by two single steel balls. At the time "telescope quality brass tubing was available, tolerances were such that successive diameters would slide in each other smoothly with no slop. The larger carriage tube had its inner diameter relieved such that just a quarter inch each end bore on the inner tube, the fit was carefully lapped and the lubricating grease was constrained within the clearance.A slot and pin ensured that the mirror carriage did not rotate. On the 8" SCT's and Maksutovs up to 8" aperture,a compression spring forward of the carriage tube, as per Questar,,provided the motive force. The larger Maksutovs had a tension spring arrangement behind.the carriage. I won't claim that the results were perfect but it was difficult to tell whether any minimal amount was image shift or the whole telescope moving.   .



#266 lwool

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Posted 20 March 2019 - 05:10 PM

I just installed an "o" ring holder on both the front and rear of the slider of my C8 leaving no clearance. These were square "O" rings that I stacked using three rings per stack. (McMaster Carr)

 

Since the clearance is removed, I found the lubricant had to be reduced in viscosity to a silicone oil rather than a grease.

 

If there were to be any changes made I would probably go from 60 durometer up to 80 or 90 durometer rings since there is still some "softness" in the focus that produces an image shift at 360 power. But, it is greatly reduced.

 

Upon inspection of the I.D. of the slider I found the witness marks of a hone and both the roundness and size consistency would indicate that this .006" total clearance was in fact put into the instrument by design.

 

We'll see how well this fix holds up over time, the lubricant being so thin migration could be an issue?  Time will tell.

 

Another concern would be that there is now no contact between the baffle tube and the slider except at the "o" rings which act as an effective insulator, thermal equilibrium may take longer without fans.  So, a fan was installed inside the tube to enhance exchange between the mirror and the tube.



#267 Arminovski

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Posted 21 March 2019 - 12:39 AM

While NASA spent $$$$$$$ and hundreds of hours developing a pen that would write in zero gravity, the Russians used a pencil... :scratchhead:


They did no such thing.

https://www.scientif...tion-nasa-spen/


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