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Retap threads on a brand new SCT

Catadioptric Celestron Equipment SCT
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#1 bamantooth

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Posted 12 June 2021 - 11:31 AM

I am thrilled that I got my Celestron Edge HD 9.25 SCT this week after a long wait.

 

I am trying to install the Celestron dovetail bar on the top of the scope.

Learned that the back two screws are #8-32, and the front screw on the corrector plate is #10-24.

The dovetail bar kit includes mounting screws that are longer than the screws that come in the SCT.

 

However, the #10-24 kit screw won't fit into the threaded hole.

 

visual inspection of the threaded hole shows that the threads seem to be in good shape.  I don't see shavings.  there is a touch of black paint near the top but not really enough to cause issues.  And the OEM screw that came with the scope goes in without resistance.

 

I inspected the OEM screw that came with the OTA.  It is black anodized and I noticed that the 'external' thread outsize edges are not currently anodized - as though they were scraped off from forcing into too narrow of a thread.  I can picture if the tap was worn down from use and now not cutting the full thread depth.

 

I used a bit more finger force to get the kit screw about 1/4 a turn into the hole then backed it out.  This removed some of the anodized surface from the outside of the first thread of the ket screw so that it looks like the OEM screw.  See attached image - short = OEM, long = kit screw.

 

I am putting the dovetail bar on to carry other optics and can imagine using it as somewhat of a grip when mounting the OTA or getting it out of its storage container - so that screw will carry a load and failure of the threads would be BAD.

 

here were my thoughts on ways to proceed

Option 1A) force the screw in and strip the threads like the OEM screw [not really an option, in my opinion];

Option 1B) file the screw threads a little to make them fit better [seems like a bad idea].

Option 2)
I could go buy a tap and clean it up... but 1) have to buy the tap kit, 2) this needs to be a through hole, so I can't do it in place, I would need to remove the front corrector plate assembly of the SCT. taking apart the corrector plate assembly of a brand new OTA to do this and reassembling and re-aligning everything seems like a lot could go wrong - especially on a scope fresh out of the box.
I am a scientist who has designed, machined, and assembled 10 different scanning probe microscopes... so I should have the skills to do it.  Just haven't done optics of this scale yet.

 

Option 3) get other options/ideas.

 

I also have this question in to HighPoint Sci. Technical support.  Seems like overkill to return a scope over one tapped hole - would rather find a fix.

 

thanks for any ideas

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#2 Taosmath

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Posted 12 June 2021 - 11:50 AM

Are you CERTAIN that both the kit parts and the Edge parts are the same system?  Many telescope parts (made in China) use metric screws and M5 is close to 10-32 & M4 is close to 8-32.

 

You might take the screws on a trip to your local hardware store to verify.


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#3 Couder

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Posted 12 June 2021 - 11:51 AM

My thoughts are to get (if you don't have) a bottoming tap and simply re-tap the hole. It is probably anodizing or paint in the threads, the tap will clear it out. I don't know if the hole goes all the way through or not, so I would use a toothpick and see how deep the hole is. Then put a piece of tape on the tap barely shorter then the depth of the hole.


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#4 Stevegeo

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Posted 12 June 2021 - 12:04 PM

As a machinist myself..welder fabricator as well ..have you tried another screw? 

As some may not be aware a lot of hardware comes from China. And some are not known for quality standard. The end may just be buggered. 

   the short will go in as it's has mated to the thread . The longer bolt the end may have the first thread with a Dinger lead edge .my term for this .

Find another bolt to try. Find a nut that fits the short one easily .then try it on the long one .

If it fits the short. But not the long ..it's the bolt .

Mind you ..these bolts could be only a few thousanths off and not fit .something only a very experienced eye could see .

 Try another bolt that will screw into a nut that also fits the short bolt .

Unless you buggered up the threads in the OTA.  Then you may have to use a finishing tap.

Btw unless you're doing a different size.or total rethread. The corrector plate will not be needed to be removed . The amount of material just to clean a thread that size is ANT poop .. 

Stevegeo


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#5 Chris Johnson

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Posted 12 June 2021 - 12:44 PM

When I deforked my nexstar gps C-11, I encountered the same problem with the new CGE dovetail. I suggest you try a 10-32 screw for the front. That was the correct size screw for my C-11 front screws.


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#6 Lola Bruce

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Posted 12 June 2021 - 01:05 PM

Make sure the hole to be taped is down and the optical assembly is on it's side when you do this. You do not want tap crumbs and such on your optics. then vacumn the hole


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#7 bamantooth

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Posted 12 June 2021 - 01:38 PM

Thanks for all the ideas.

 

I have attempted to fit: 10-32, M5-0.8, M4-07, and 8-32 into the hole. The M4/8-32 are obviously too small.

 

Visually the 10-32 thread is a much smaller spacing.  If I hold a 10-24 screw against the OEM screw the threads fit together really well.

 

Using some digital calipers my #10-24 screws have a diameter of ~0.185 to 0.190 in at the thread OD, the OEM screw that seems to have been 'shaved' has an OD of ~0.179 in

 

I have tried 3 other 10-24 screws to see if it is a mfg variability thing, they all have the same 'fit' issue.

 

My machining experience has been more on through holes.  I will look into a bottoming tap - that seems like a really good place to start. and avoids dis-assembly


Edited by bamantooth, 12 June 2021 - 01:51 PM.

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#8 tturtle

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Posted 12 June 2021 - 02:21 PM

If I follow you correctly this problem is on the single screw for the corrector housing on the front of the scope. It is actually very easy to remove the corrector plate and fix this.  I removed the corrector a couple of months ago on my C925 to do the first cleaning after 8 years and was surprised at how simple it was. If you take the corrector off you can redrill, retap and put in a real bolt.  Forget about the Mickey Mouse screw Celestron uses - put in a real fine threaded bolt so you know it will hold. You can buy a single tap of the size you want either at your home improvement store or Ebay.  Buy a quality tap and you will have it fixed in 30 minutes.


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#9 Old Speckled Hen

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Posted 12 June 2021 - 02:23 PM

Sorry to be such a dullard here, but are the new [longer, almost twice the length ish] screws going to be in either the light path OR the range of movement of the primary mirror? You have to be pretty cautious about SCT's and screws...


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#10 bamantooth

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Posted 12 June 2021 - 02:42 PM

Sorry to be such a dullard here, but are the new [longer, almost twice the length ish] screws going to be in either the light path OR the range of movement of the primary mirror? You have to be pretty cautious about SCT's and screws...

the OEM short screw sits against the OTA housing (black band that holds the corrector plate).  I am trying to attach a dove tail bar that has a thickness of 0.25 in from screw surface to OTA interface, original screw was ~0.5 in.  The OEM screw was so short that only a very few of the threads would engage.  The new screw is 0.75 inch, so I don't think it will protrude into the OTA.


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#11 Migwan

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Posted 12 June 2021 - 06:54 PM

If the threads on the 10-24 fit into the OEM threads across the entire length, then it's a match.   I'd double check that.  You know, measure twice cut once.

 

That said, the pitch on the starting thread of the fastener on the right doesn't look correct on the right side.  Just compare it to the the thread above it.  It looks a wee bit fat to me.  File past that or use a different one.

 

As far as filing the threads down, that might be a little iffy.   A wire wheel would do a better job of taking the annodization off, if you happen to have one.  

 

Ultimately it's not all that uncommon to have to lightly force a new (different) fastener into a threaded hole in pot metal .   Trick is, not to overdo and go back and forth slowly.   Don't be stronger that the pot metal threads.   And of course, be sure its the same size

 

Good luck


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#12 Ironrust

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Posted 13 June 2021 - 09:24 AM

I got a brand new edge HD 8 back in December. One of the accessory mounting screw holes had a similar issue, I did not feel comfortable tapping it my self so I sent it in for a warranty claim. I got it back and the the hole was re-tapped properly and useable. I am surprised to hear someone else had a tapping issue as well, I assumed that I had just gotten very unlucky.
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#13 Old Speckled Hen

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Posted 13 June 2021 - 10:30 AM

Hmm, you shouldn't have to do stuff like this in the first place.

 

Of course there will be those who say "return it under warranty" and that is their opinion. Me, I don't think that is an acceptable path.

 

If everyone who had issues with new scopes etc, however minor, returned them for FULL refund, they would get the message pretty quick.

 

As it is they know their quality is, shall we say, floating down the river like a sewage discharge, but they are quite happy to produce shoddy goods and cut corners whilst basically ripping the $$$$ out of your wallets.

 

Chipped correctors and secondaries, paint that falls off the inside of tubes, correctors that are glued in place with sealant [never mind the centering screws], mechanical issues with OTA construction, it's not a good look.


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#14 Lola Bruce

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Posted 13 June 2021 - 01:48 PM

Don't pull a strong vacuum on the scope, you don't want to crack the corrector.



#15 teashea

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Posted 13 June 2021 - 07:08 PM

I am thrilled that I got my Celestron Edge HD 9.25 SCT this week after a long wait.

 

I am trying to install the Celestron dovetail bar on the top of the scope.

Learned that the back two screws are #8-32, and the front screw on the corrector plate is #10-24.

The dovetail bar kit includes mounting screws that are longer than the screws that come in the SCT.

 

However, the #10-24 kit screw won't fit into the threaded hole.

 

visual inspection of the threaded hole shows that the threads seem to be in good shape.  I don't see shavings.  there is a touch of black paint near the top but not really enough to cause issues.  And the OEM screw that came with the scope goes in without resistance.

 

I inspected the OEM screw that came with the OTA.  It is black anodized and I noticed that the 'external' thread outsize edges are not currently anodized - as though they were scraped off from forcing into too narrow of a thread.  I can picture if the tap was worn down from use and now not cutting the full thread depth.

 

I used a bit more finger force to get the kit screw about 1/4 a turn into the hole then backed it out.  This removed some of the anodized surface from the outside of the first thread of the ket screw so that it looks like the OEM screw.  See attached image - short = OEM, long = kit screw.

 

I am putting the dovetail bar on to carry other optics and can imagine using it as somewhat of a grip when mounting the OTA or getting it out of its storage container - so that screw will carry a load and failure of the threads would be BAD.

 

here were my thoughts on ways to proceed

Option 1A) force the screw in and strip the threads like the OEM screw [not really an option, in my opinion];

Option 1B) file the screw threads a little to make them fit better [seems like a bad idea].

Option 2)
I could go buy a tap and clean it up... but 1) have to buy the tap kit, 2) this needs to be a through hole, so I can't do it in place, I would need to remove the front corrector plate assembly of the SCT. taking apart the corrector plate assembly of a brand new OTA to do this and reassembling and re-aligning everything seems like a lot could go wrong - especially on a scope fresh out of the box.
I am a scientist who has designed, machined, and assembled 10 different scanning probe microscopes... so I should have the skills to do it.  Just haven't done optics of this scale yet.

 

Option 3) get other options/ideas.

 

I also have this question in to HighPoint Sci. Technical support.  Seems like overkill to return a scope over one tapped hole - would rather find a fix.

 

thanks for any ideas

I think that you know the answer - do it right. 



#16 teashea

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Posted 13 June 2021 - 07:11 PM

Hmm, you shouldn't have to do stuff like this in the first place.

 

Of course there will be those who say "return it under warranty" and that is their opinion. Me, I don't think that is an acceptable path.

 

If everyone who had issues with new scopes etc, however minor, returned them for FULL refund, they would get the message pretty quick.

 

As it is they know their quality is, shall we say, floating down the river like a sewage discharge, but they are quite happy to produce shoddy goods and cut corners whilst basically ripping the $$$$ out of your wallets.

 

Chipped correctors and secondaries, paint that falls off the inside of tubes, correctors that are glued in place with sealant [never mind the centering screws], mechanical issues with OTA construction, it's not a good look.

amen




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