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Bob's Knobs do not turn (or turn with great difficulty)

Collimation SCT
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#1 Bob Campbell

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Posted 08 May 2022 - 02:22 AM

Greetings All

 

Just picked up a used c6 with Bob's Knobs. As a bit of background, previously I  had 3 sct's and have successfully collimated all of them. However, none of them used Bob's Knobs.

 

The initial EAA images from this used c6 I got on CN were pretty good, but when I defocused a centered star at high magnification, the scope was severely out of collimation. This in a way was a great thing, because I knew that if my images looked this good out of collimation, then once I nailed it, they should be great.

 

I tried to turn the bob's knobs, and 2 of the 3 were jammed solid, only one was able to turn, and of course this was not the right one to fix anything regarding collimation.

 

 

I assume a/the previous owner put them in incorrectly, thus the difficulty in turning them. I looked at the knobs under a light, and saw some of the finish had been taken off the heads like someone took a pair of pliers to them (ugh!).

 

 

Has anyone else run into something like this, and has some advice on how to proceed? I have several ideas on how to address this, but would appreciate if someone could weigh in if they dealt with this before or has any other insights.

 

Thanks for looking,

 

Bob

 

 

 

 



#2 DLuders

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Posted 08 May 2022 - 03:13 AM

If you use a Hair Drier on the area surrounding the knobs, the gentle heat will expand the metal A BIT to possibly allow the two stuck knobs to be removed.  The technique works on automotive parts, and it often works on other parts too.  The metal surrounding stuck fasteners expands in relation to the (cold) bolt, allowing some slack between the two.

 

Using lubricants (like WD-40, Master Blaster, etc.) is not recommended because the fluids may seep onto optical surfaces. However, they could be used as a last resort if used SPARINGLY.  


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#3 quilty

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Posted 08 May 2022 - 03:40 AM

are you sure to have found the correct gearing?


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#4 pyrasanth

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Posted 08 May 2022 - 04:32 AM

Obviously be very careful with a hair dryer. Remove the mirror assembly (i presume its a faststar type assembly- not sure on the C6 so may not be possible-ignore advice then around protecting the mirror as it will be under the corrector plate) and drop a couple of drops of light oil into the screw hole but don't flood as you don't want oil on the scondary mirror. This can help ease galled threads however if the screws have been cross threaded then only risky brute force will be able to release them. This invarably damages the threaded holes (probably damaged anyway) so you may need to retap new threads however that task is not that difficult. There are tools designed to remove stuck threads however great care will be needed around the mirror- so protect it well even it it means making some form of hard shell around the mirror to protect it whilst you get the screws out- especially if your forced to use mechanical tools.


Edited by pyrasanth, 08 May 2022 - 04:38 AM.

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#5 Traveler

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Posted 08 May 2022 - 05:21 AM

Did you bought the proper type? 

http://www.bobsknobs.../page26/C8.html

 

As you can see, there are several types (a-f)


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#6 Bob Campbell

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Posted 08 May 2022 - 09:47 AM

If you use a Hair Drier on the area surrounding the knobs, the gentle heat will expand the metal A BIT to possibly allow the two stuck knobs to be removed.  The technique works on automotive parts, and it often works on other parts too.  The metal surrounding stuck fasteners expands in relation to the (cold) bolt, allowing some slack between the two.

 

Using lubricants (like WD-40, Master Blaster, etc.) is not recommended because the fluids may seep onto optical surfaces. However, they could be used as a last resort if used SPARINGLY.  

Thanks for the hair dryer tip. I'll give it a try. Yes, no lubricants whatsoever!



#7 Bob Campbell

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Posted 08 May 2022 - 09:48 AM

are you sure to have found the correct gearing?

If you mean were they threaded on correctly originally, I have no idea since I did not install them. My guess is they were crossthreaded.


Edited by Bob Campbell, 08 May 2022 - 09:57 AM.


#8 Bob Campbell

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Posted 08 May 2022 - 09:52 AM

Obviously be very careful with a hair dryer. Remove the mirror assembly (i presume its a faststar type assembly- not sure on the C6 so may not be possible-ignore advice then around protecting the mirror as it will be under the corrector plate) and drop a couple of drops of light oil into the screw hole but don't flood as you don't want oil on the scondary mirror. This can help ease galled threads however if the screws have been cross threaded then only risky brute force will be able to release them. This invarably damages the threaded holes (probably damaged anyway) so you may need to retap new threads however that task is not that difficult. There are tools designed to remove stuck threads however great care will be needed around the mirror- so protect it well even it it means making some form of hard shell around the mirror to protect it whilst you get the screws out- especially if your forced to use mechanical tools.

Yes, I think all celestron modern scts are fastar. Yes I was planning to remove the mirror assembly to work on it. Sure hope I do not have to retap the threads frown.gif.  Thanks!



#9 Bob Campbell

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Posted 08 May 2022 - 09:54 AM

Did you bought the proper type? 

http://www.bobsknobs.../page26/C8.html

 

As you can see, there are several types (a-f)

I would have to ask the person I bought it from. I did not install them, they arrived with the OTA. Thanks.
 



#10 Xeroid

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Posted 08 May 2022 - 10:45 AM

My C6 does have a removable secondary so after your succesful repair procedure, you may wish to inspect the secondary and its postion.  My C6 uses double side tape and if collimation screws are over-tighten, its pssoble to dislodge the scondary from it mounting plate.


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#11 Bob Campbell

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Posted 08 May 2022 - 12:14 PM

My C6 does have a removable secondary so after your succesful repair procedure, you may wish to inspect the secondary and its postion.  My C6 uses double side tape and if collimation screws are over-tighten, its pssoble to dislodge the scondary from it mounting plate.

Great suggestion. I will take note. Thanks

 

 

PS: "Of course, I talk to myself. Sometimes I need expert advice" I love this in your description!


Edited by Bob Campbell, 08 May 2022 - 12:19 PM.


#12 Old Speckled Hen

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Posted 08 May 2022 - 12:53 PM

Hideous and needless situation to find oneself in.

 

I hope you can get it sorted and it turns out they are not cross threaded and are the correct kind.

 

If they have been cross threaded IN it really does beggar belief.

 

Bob Knobs 101

 

" Also, since telescope manufacturers often re-design their hardware during production, check that the knob

threads match the factory screw threads before attempting to install the knobs. 

 

The center tang in the housing fits into the corresponding central depression on the backing plate, forming a pivot point.

When the three collimation screws are tightened at the factory, the secondary housing flexes outward
slightly, thus maintaining tension between the housing and the backing plate and keeping the
secondary mirror assembly firmly in place.

 

If a factory screw is removed without releasing this tension, the backing plate can pivot away from the
housing at that screw location, preventing the knob threads from reaching it. Furthermore, the factory screw that was
just removed may no longer reach the backing plate either, and even if it does reach, the risk of cross-threading is increased from misalignment."

 

"If the knob threads won’t reach the secondary backing plate, first pivot the telescope on its mount so that its
secondary end is pointing downward at about a 45 degree angle with the hole at the lower (close to six o’clock)
position.

Loosen each remaining screw a maximum of one turn. Do not remove either of the remaining screws. The
goal is to allow the secondary backing plate to pivot slightly toward the hole so you can install the knob. By placing
the hole below the other two factory screws, gravity will assist you.

Pushing on the other two factory screws will
pivot the backing plate further, if necessary.

 

Now thread the knob into the hole two to three full turns. Remove the
second factory screw, push on the knob and remaining factory screw if necessary, and thread a knob two to three
turns into the backing plate.

Now remove the third factory screw, push on the other two knobs if necessary, and
thread the third knob two to three turns into the backing plate.

 

In stubborn cases you may need to adjust the telescope on its mount for additional gravity assistance.

If you cannot get a knob to reach the backing plate after loosening each remaining factory screw one turn, do not
loosen these screws further or you risk releasing the secondary mirror assembly. Instead, obtain a screw at least one
inch (25 mm) long with the same threads as the factory screws.

 

You can find this at your local hardware store, or
contact us and we will send you such a screw without charge.

 

Insert the long screw into the hole and screw it two to three turns into the secondary backing plate.

This will hold the secondary in place while you remove a second  factory screw and install a knob.

You can manipulate the long screw and remaining factory screw until the knob
threads reach the backing plate.

 

Install the second knob in place of the remaining factory screw, then install the third knob in place of the long screw.

 

Now the knobs will probably be very loose.

 

Tighten each knob one-half turn at a time, moving from knob to knob,
until each knob is finger tight and the entire assembly is secure. Proceed with coarse collimation, followed by star
collimation, as outlined in our instructions."

 

 

As always experience is gained the second after you needed it.


Edited by Old Speckled Hen, 08 May 2022 - 12:59 PM.

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#13 Bob Campbell

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Posted 08 May 2022 - 01:44 PM

Hideous and needless situation to find oneself in.

 

I hope you can get it sorted and it turns out they are not cross threaded and are the correct kind.

 

If they have been cross threaded IN it really does beggar belief.

 

Bob Knobs 101

 

" Also, since telescope manufacturers often re-design their hardware during production, check that the knob

threads match the factory screw threads before attempting to install the knobs. 

 

The center tang in the housing fits into the corresponding central depression on the backing plate, forming a pivot point.

When the three collimation screws are tightened at the factory, the secondary housing flexes outward
slightly, thus maintaining tension between the housing and the backing plate and keeping the
secondary mirror assembly firmly in place.

 

If a factory screw is removed without releasing this tension, the backing plate can pivot away from the
housing at that screw location, preventing the knob threads from reaching it. Furthermore, the factory screw that was
just removed may no longer reach the backing plate either, and even if it does reach, the risk of cross-threading is increased from misalignment."

 

"If the knob threads won’t reach the secondary backing plate, first pivot the telescope on its mount so that its
secondary end is pointing downward at about a 45 degree angle with the hole at the lower (close to six o’clock)
position.

Loosen each remaining screw a maximum of one turn. Do not remove either of the remaining screws. The
goal is to allow the secondary backing plate to pivot slightly toward the hole so you can install the knob. By placing
the hole below the other two factory screws, gravity will assist you.

Pushing on the other two factory screws will
pivot the backing plate further, if necessary.

 

Now thread the knob into the hole two to three full turns. Remove the
second factory screw, push on the knob and remaining factory screw if necessary, and thread a knob two to three
turns into the backing plate.

Now remove the third factory screw, push on the other two knobs if necessary, and
thread the third knob two to three turns into the backing plate.

 

In stubborn cases you may need to adjust the telescope on its mount for additional gravity assistance.

If you cannot get a knob to reach the backing plate after loosening each remaining factory screw one turn, do not
loosen these screws further or you risk releasing the secondary mirror assembly. Instead, obtain a screw at least one
inch (25 mm) long with the same threads as the factory screws.

 

You can find this at your local hardware store, or
contact us and we will send you such a screw without charge.

 

Insert the long screw into the hole and screw it two to three turns into the secondary backing plate.

This will hold the secondary in place while you remove a second  factory screw and install a knob.

You can manipulate the long screw and remaining factory screw until the knob
threads reach the backing plate.

 

Install the second knob in place of the remaining factory screw, then install the third knob in place of the long screw.

 

Now the knobs will probably be very loose.

 

Tighten each knob one-half turn at a time, moving from knob to knob,
until each knob is finger tight and the entire assembly is secure. Proceed with coarse collimation, followed by star
collimation, as outlined in our instructions."

 

 

As always experience is gained the second after you needed it.

Hen

 

Ironically, the seller did send me the Bob's Knobs instruction sheet, which suggests that they were installed during the seller's ownership of it.

 

Hoping that I can sort it out, because the images the scope put up with bad collimation suggests the optics are in fact excellent.

 

CS,

 

Bob




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