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"Compression" rings and differential flex

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#1 Midnight Dan

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Posted 04 January 2015 - 10:41 AM

The reason I put "Compression" in quotes is because it is a complete misnomer.  It gives the impression that they tighten evenly around the eyepiece or camera, holding it from all sides.  In fact, there are only two areas of contact and the item being held is free to move a bit from side to side unless you really tighten the thumbscrew excessively.  These rings do a great job of protecting the item from the tip of the thumbscrew, but don't do a lot for holding it securely.  

 

The following diagram illustrates the problem.  The gaps between parts have been exaggerated to make it more clear what's going on:

 

ScreenShot2015-01-04at90944AM_zps0a7a166

 

I noticed this because of differential flex introduced at my guide camera, a QHY5L-II, which is the size of a small 1.25" eyepiece.  Because of its diminutive size, it gets put into a 1.25:-to-2" adapter, all of which seem to have the compression ring and a single thumbscrew.  This is also an issue with 2" focusers, most of which use the same single-thumbscrew system.  

 

The solution, is to add a second thumbscrew at about 120° from the first to provide a 3-point holding system.  If you want centering capability, a third thumbscrew would help, but I didn't think that was necessary.  Another solution is to use centering clamps like the Baader click-lock, but those are expensive.

ScreenShot2015-01-04at93359AM_zps7d12e81

 

Thumbscrews can be had cheaply from ScopeStuff but, while these are serviceable, they look pretty clunky compared to the original thumbscrews provided with most adapters and focusers.  I found a better source at Agena Astro for a very reasonable price compared to what I could find elsewhere on the web:

http://agenaastro.co...-screw-set.html

 

To add a second thumbscrew, you'll first need to remove the compression ring.  This can be easily done by using a small screwdriver to rotate the ring until one end of it is near the thumbscrew.  Then screw the thumbscrew in to push the end of the ring in until you can see a gap behind it.  Insert the small screwdriver into the gap and pull up on the ring with your finger while sliding the screwdriver around the ring to pull it out of the gap.

 

You'll also need a 4mm x 0.7 tap.  Do not mistakenly get the 4mm x 0.75 tap like I did! :foreheadslap: It won't work with the thubscrews!  Drilling and tapping can probably be done by hand, but I'd highly recommend a drill press if you have access to one.  Keeping the hole straight while you're drilling is critical to getting the end of it to be more or less centered in the compression ring groove.  Precise location of the hole isn't too important - I just eyeballed the 120° spacing, as well as the alignment with the ring slot.

 

Be sure to de-bur the ends of the hole before tapping.  For the inner end, I used a small drill bit that was a little larger than the hole, and just turned it against the opening with my fingers.  And be sure to clean the part of all metal particles when done.  I cleaned mine with an air gun and wiped them down with alcohol.

 

I've done this with all my 1.25" to 2" adapters.  Once you have the tap and the drill press set up, doing a bunch of them is easy.  I also plan to do this with the 2" focusers on my imaging scopes, but that will be a lot more effort to disassemble the focusers to remove the drawtube for drilling.

 

Adapters_zps4786d645.jpg

 

-Dan


Edited by Midnight Dan, 04 January 2015 - 05:44 PM.


#2 brettecantwell

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Posted 04 January 2015 - 11:43 AM

Ive started to have issues as well on my setup. I have an ES ED80 and it has 3 screws and the compression ring. My FF/DSLR(with battery grip) has got some heft to it and i have really crank down on it to get it secure and still keep the field flattener flat with the drawtube. The nose piece of the FF has slight slop in the drawtube which allows it to get pulled out of square if you are not careful. It would be nice if things would just be a bit tighter fits but with many companies making parts i can see where this could end up causing issues. I also found a better screw was a big help. The synta finder base set screws are a good screw that worked for me, they have a large knurled head that wont hurt your fingers when clamping down.


Edited by brettecantwell, 04 January 2015 - 12:12 PM.


#3 tim57064

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Posted 04 January 2015 - 12:29 PM

MD ,Thanks. I think this should be a sticky. :waytogo:  :bow:



#4 ManicSponge

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Posted 04 January 2015 - 12:39 PM

Great post, Dan. I just went through this with my William Optics 102. It came with a third screw hole, that was plugged with a set screw, for some odd reason. A quick trip to the hardware store, and it now has three contact points. I had significant slop between the Flat6A FF/FR and the drawtube on my new scope. I wrapped HVAC aluminum duct tape around the FF, and now it is a snug slip fit in the drawtube. I chose this tape because it is a consistent thickness, and does not compress. (It varies by manufacturer, so take your calipers to the hardware store) Now that it is wrapped, I carefully hold the camera perfectly square against the drawtube flange, and tighten each screw an even amount, until they are all tight. The tape needs to be at both the front and back of the FF, or the tension screws would tilt it, when tightened.

I did not do any imaging without this little mod, but it's kind of a no-brainer that it must help. I could see the stock two screw system tilt the FF when I tightened the screws. There is no such movement with three, and the tape as a shim.

Regards, Kyle

 

Attached Thumbnails

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Edited by ManicSponge, 04 January 2015 - 12:40 PM.


#5 Jon Rista

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Posted 04 January 2015 - 01:23 PM

Thanks, Dan! This definitely belongs in the "Best Threads" topic. Good stuff.

#6 nitegeezer

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Posted 04 January 2015 - 01:45 PM

I just looked and it is already in the "Best Threads"!   :whistle: 



#7 Midnight Dan

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Posted 19 January 2015 - 11:32 AM

Just an update on this.  While adding an extra thumbscrew to the 1.25" to 2" adapters is helpful for use with a small guide camera, the 2" focusers found on most scopes are arguably a more important issue.  A heavier camera is often attached at the end of a string of other items like extension tubes, filters, and field flattener.  While all of these items tend to have solid connections such as a T-thread, the primary point of attachment is often a 2" focus tube with a "compression" ring.

 

Over the weekend, I addressed this issue as well.  I started with my EON 72mm ED refractor and removed the focuser assembly.  I wanted to remove the tube completely from the rest of the assembly so I didn't get metal chips into anything.  To do so, I had to completely disassemble the crayford focuser.  Big mistake.  It took a lot of time to figure out exactly how to get everything apart, including exposing hidden set screws under the rubber focus-knob rings.  But more of an issue was getting the focuser properly adjusted during reassembly.  Getting everything correctly aligned, and with the proper tension, took a lot of time doing trial-and-error (mostly error) adjustments.  I never did get it to be as smooth as it was originally, but I got it pretty close.

 

My next effort was on my SV110 refractor.  Removing the focuser assembly was easy, but I decided not to disassemble the focuser and remove the draw tube.  Instead, I turned the focus knobs so that the draw tube was extended as much as possible.  I then wrapped a 1-gallon sized ziplock baggie around it to cover all of the assembly except the end of the draw tube which I planned to drill and tap.  I used Scotch tape to seal the baggie around the drawtube.  Finally, I stuffed multiple wadded up paper towels into the drawtube to keep chips from falling down in.  This worked perfectly, keep everything clean, and allowed me to maintain the precise adjustment of the focuser mechanism.

 

The result is excellent!  Prior to this, I had to really crank on the focuser thumbscrew to try to hold things in place.  I was always concerned that I was going to strip the threads, especially since the draw tubes are made of aluminum.  Now I can apply a MUCH lighter force on the two thumbscrews to achieve a substantially more rigid connection.

 

Not surprising, the one scope I own that is specifically designed to be an astrograph, the ATRC6, already had two thumbscrews on its focuser's compression ring.

 

-Dan



#8 emflocater

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Posted 18 February 2016 - 10:11 PM

What if your focuser does not have a compression ring but only 2 screws 120 degrees apart. Do you recommend a 3rd screw or would the 2 screws be just fine to hold a guide camera that is 10 oz. in weight and square shape that sits right up to the focuser as opposed to the round long guiders that hang out several inches.

Cheers

Don



#9 Chucke

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Posted 18 February 2016 - 11:44 PM

2 screws are better than 3.  Ideally they should be at 90 degrees instead of 120.  On my tubes I use 2 rows with 2 screws each.  It keeps everything nice and snug.



#10 james7ca

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Posted 18 February 2016 - 11:57 PM

I think the "safety" undercut that is on many 2" or 1.25" eyepiece tube adapters is just as big of a problem since it seems that the edge of the undercut often falls right where the screws or compression rings tighten against the adapter. It would be nice is someone made adapters that didn't have the undercut, so you'd have a smooth and straight wall for the compression ring to tighten against. Of course, you'd have to be careful about keeping the screws tight, since without the safely undercut you'd have a greater chance of having the camera slip out of the drawtube. However, you could always use a tether line to make sure that the camera couldn't move very far from the drawtube.

 

Fortunately, all of the scopes I now use for astrophotography came with three thumbscrews to hold any adapters to the drawtube. Interestingly, the Tele Vue NP scopes come with two different focuser end ring adapters, one to hold traditional 2" eyepiece tubes that has four sets of thumbscrews and another for imaging applications that uses three thumbscrews. Why they used four screws to secure one of the adapters and three for the other is something that I've never fully understood but it's kind of a hassle since you have to move the screws to different holes when you change the style of the adapter. Tele Vue just says that they have found that three lock screws (rather than four) are better to hold the imaging adapter flat to the focuser end ring.



#11 Midnight Dan

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Posted 19 February 2016 - 09:32 AM

What if your focuser does not have a compression ring but only 2 screws 120 degrees apart. Do you recommend a 3rd screw or would the 2 screws be just fine to hold a guide camera that is 10 oz. in weight and square shape that sits right up to the focuser as opposed to the round long guiders that hang out several inches.

Cheers

Don

 

Two screws is definitely better than 3.  With 2 screws, the side that has no screw is pressed agains the inside wall of the tube along it's entire length.  This helps to keep the camera straight regardless of the orientation of the rig.  A 3rd screw would essentially allow the camera to be suspended in the space in the tube, being held in place only at the plane defined by the 3 screws.  Other points along the length of the camera nosepiece could be free to move a bit as the mount shifts orientation during the night.

 

In my opinion, a 120° separation is better than 90° because you end up with 3 equally spaced points of pressure.  In fact, in the scopes I have which already have 2 screws from the factory, they are spaced at 120°.

 

-Dan




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