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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.




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