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Secondary Mirror Milk Jug Washers

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#1 Abhat

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Posted 26 December 2013 - 10:38 AM

I have been reading a lot about this famous idea originated by Jason D. I wanted to understand how exactly it works and what benefits does it have.

The reason I am asking this last night I was trying to adjust the tilt of secondary to get primary centered by looking at the clips. After substantial frustration and tightening the secondary collimation screws I finally got primary centered inside secondary but heard a cracking sound and not sure if I cracked the secondary's housing by over tightening. I was wondering if having plastic washers can help in this process.

#2 rdandrea

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Posted 26 December 2013 - 10:52 AM

I realize that I'm answering a different question than what you actually asked, but the screws on most secondaries work in concert with each other. Typically, if you want to tighten one screw, you need to loosen the others. If you didn't do that, that might explain your difficulties and any resulting cracking sounds.

#3 howard929

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Posted 26 December 2013 - 11:59 AM

IME adjusting those screws is like a balancing act. Too tight risks damage to the screws and/or the holder, too loose and the secondary might move during normal transport.

I was finding that tightening the screws tended to also rotate the secondary. Not good. I tried milk jug washers but wasn't satisfied with the result since tightening the screws just rutted out the washers. In the end, I removed the screws, rounded the ends which made them less likely to grab the holder and turn it when tightening the screws and just snug then down nice and tight. You'll get a feel for that. YMMV.

#4 mayidunk

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Posted 26 December 2013 - 01:26 PM

I use two of these washers, along with a set of Bob's Knobs in the secondary of a 10", f/3.9 astrograph that I use for visual. While I have yet to do a star test with the washers in place, while collimating the optics in the house using Vic Menard's collimating tools, I was able to close the optical system using the autocollimator eyepiece, and it remained closed when attempting to tilt the secondary using slight pressure.

As for the screws punching holes through the washers, I had previously smoothed the ends of my Bob's Knobs screws using a file, making sure that there were no rough edges that could mar the anodized disk that they were bearing on before installing the washers. Since I don't crank these screws down anywhere nearly as tight as some do, I anticipate having no issues with them punching through the washers any time soon, if at all. However, I do suspect that, for a short time anyway, I will have to keep tweaking the collimation as the washers compress at the point where the screws contact it, kind of a break-in period. However, once those areas of compression have stabilized, I suspect holding collimation will no longer be a problem, and that by only using firm, finger tight pressure, those compression areas will hold up without breaking down.

Time should tell.

#5 David Castillo

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Posted 26 December 2013 - 02:49 PM

I want to just add that you don't have to use milk jug material. Any relatively soft plastic plastic material works. I have used tops from margarine and yogurt tubs -they work the same way. Also, I wold like to add that the milk jug modification idea was first written up by Tom Hole about 12 years ago on the Yahoo Skyquest(XT10) users group. I don't really think you could credit Jason D. for the idea, but I'm thankful that he did pass the idea along to the Forum.
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#6 Abhat

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Posted 26 December 2013 - 03:56 PM

Thank you everybody for your replies and suggestions!

#7 ManicSponge

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Posted 26 December 2013 - 04:50 PM

Yes, to all of the above. I used a disc cut from the top of a blue plastic tote, that I had laying around. I hit the tips of the knurled head screws from Opticsmart with my disc sander, and then a final slight rounding with 600 grit sandpaper. I also applied a tiny amount of anti-seize to the threads. Seems to be very smooth now. The stock screws dug into the original steel washer very quickly. I haven't been using this scope for very long, but I suspect the plastic washer will make a big difference.
Hope this helps a little. Regards, Kyle

#8 Jason D

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Posted 26 December 2013 - 04:58 PM

I have been reading a lot about this famous idea originated by Jason D. I wanted to understand how exactly it works and what benefits does it have.

The reason I am asking this last night I was trying to adjust the tilt of secondary to get primary centered by looking at the clips. After substantial frustration and tightening the secondary collimation screws I finally got primary centered inside secondary but heard a cracking sound and not sure if I cracked the secondary's housing by over tightening. I was wondering if having plastic washers can help in this process.


Using one or two milk jug washers should be helpful. What you get from the milk jug washer is the elasticity. Those tiny bumps on the surface will add more elasticity and facilitate rotation. The milk jug washers will allow you to fine tune the secondary mirror by tightening one of the set screws without the need to loosen the other two.

Posted Image

If you feel uncomfortable removing the secondary mirror then you can cut the washers as shown below then slide them in.

Posted Image

Jason

#9 Jason D

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Posted 26 December 2013 - 05:25 PM

Also, I wold like to add that the milk jug modification idea was first written up by Tom Hole about 12 years ago on the Yahoo Skyquest(XT10) users group. I don't really think you could credit Jason D. for the idea, but I'm thankful that he did pass the idea along to the Forum.
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Dave


Dave, I came up with the milk just washer idea for the secondary mirror and I have been promoting it in this forum and other forums for many years with positive feedback.
But your statement got me curious. I googled it and came up with the following post by Tom:
http://tech.groups.y...s/message/14546
Was this the post you were referring to? It is a 10 year old post. According to Tom, he added the washers because his set screws became too short when he replaced the central bolt with a longer one and he needed to fill-in the gap between the stalk and the set screws. Tom was solving a different problem. That is the only post I found by Tom referencing jug washer within the context of the secondary mirror.
It is unfair to discredit me for the idea in a public forum.
Jason

#10 David Castillo

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Posted 26 December 2013 - 07:25 PM

Jason- I'm sorry, I didn't mean to offend you or discredit you. It was 10 years ago and details get blurry, but to those of us at that time found that the modification also stopped the screws from galling the plastic secondary holder. This was also discussed on Yahoo's [long lost] "Harden users group" that morphed into the "Chinese telescope users group" that died out around '03. He posted the hack previously on his website,Tomhole.com, but it has gone by the wayside. If you want an apology from me on this forum, listen up, because my intention was not to discredit you, but point out Tom's modification. So, to you, and anyone who fells slighted, I'm sorry. Now, let's pause a minute as I pull my foot out of my mouth. :vomit:
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Dave

#11 Jason D

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Posted 26 December 2013 - 09:47 PM

No problem, Dave. All forgotten. Peace :)

#12 ManicSponge

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Posted 26 December 2013 - 11:40 PM

JasonD, thanks for the images. In all the surfing I have done, I had not seen a visual of the washer mod. Are the dual washers intended to allow a little slippage between the two? Excellent idea, on splitting the washers. As a brand new telescope owner, I wasn't thrilled when the secondary mirror had to be removed. I did learn a little, realigning it, though!
The plastic mod has shown a marked improvement over the steel washer, in the the last four collimations I have done. Just a "feel" thing that is difficult to articulate in one dimension. Thanks for the idea! Regards, Kyle

#13 SteveG

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Posted 27 December 2013 - 12:44 PM

Here's a pic of my old ones. I've replaced them with black Kydex (similar material).

Attached Thumbnails

  • 6275327-Milk Jug Washers.jpg


#14 WyattDavis

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Posted 22 February 2015 - 11:31 AM

Along the lines of the milk jug modification, I had substantial frustration with the lack of accuracy/stability with the metal-to-metal contact between the collimation screws on my XT10 and the back side of the secondary housing. Especially when trying to use an autocollimator, I couldn't get the level of accuracy and stability for good secondary alignment. I put in Bob's Knobs and then put vinyl screw-end caps over the screw ends. It seems to have the same effect and makes achieving a stable secondary alignment much easier.



#15 howard929

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Posted 22 February 2015 - 01:36 PM

That's because you used screws w/ vinyl ends (those sound like a nice product BTW) and milk jug washers help to fix a hidden problem of those sharp screw ends grinding into and rotating the secondary housing while making tilt adjustments. Ouch.


Edited by howard929, 22 February 2015 - 01:37 PM.


#16 precaud

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Posted 22 February 2015 - 02:59 PM

Before going the washer route, examine the ends of the collimation screws. They should be square to the length and flat or slightly convex (domed). Many screws have sloppily cup-shaped ends that make for very imprecise, unpredictable adjustment and make grooves in the metal, which only makes things worse. I have found plastic washers to be unnecessary after doing this.



#17 acochran

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Posted 22 February 2015 - 03:42 PM

On my Astrosystems secondary mirror holder, the 4 collimating screws dig into the holder material, probably from me tightening the screws too much. So I found some thin metal signs at a hardware store with adhesive on their back. I cut small metal squares from the sign and stuck them on the holder, under the 4 screws. On top of that, I put 2 milk jug washers. So screws hit milk jug, then metal.

Andy


Edited by acochran, 22 February 2015 - 03:43 PM.


#18 Jason D

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Posted 22 February 2015 - 05:04 PM

The milk jug washer is not only meant to protect the secondary mirror stalk from the adjustment screws otherwise a metallic washer would have been good enough.

The main benefit of the milk jug washer is to provide little elasticity to make fine adjustments -- something a hard object like a metallic washer or even a hard plastic will not provide. That is why I suggest using 1 or 2 washers to provide different levels of elasticity.

The elasticity of the milk jug washer seems about to be the right amount.

In addition, it is easy to install. If the scope owner does not want to remove the secondary mirror then he/she can make a radial cut and slide the washer in as I have shown in post # 8 in this thread.

Jason



#19 SteveG

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Posted 22 February 2015 - 06:57 PM

The milk jug washer is not only meant to protect the secondary mirror stalk from the adjustment screws otherwise a metallic washer would have been good enough.

The main benefit of the milk jug washer is to provide little elasticity to make fine adjustments -- something a hard object like a metallic washer or even a hard plastic will not provide. That is why I suggest using 1 or 2 washers to provide different levels of elasticity.

The elasticity of the milk jug washer seems about to be the right amount.

In addition, it is easy to install. If the scope owner does not want to remove the secondary mirror then he/she can make a radial cut and slide the washer in as I have shown in post # 8 in this thread.

Jason

Yes, this is it and it works very well. I used black Kydex from Scopestuff, and have 2 of them stacked between the seconday holder and the screws.

 

As John mentioned above, look at the ends of those screws. I filed each of mine down, to round off and eliminate those rough edges.



#20 choran

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Posted 22 February 2015 - 07:04 PM

I haven't used milk jug or other washers yet, but I did find that carefully rounding and fine polishing the screw tips to a mirror shine helped a lot.



#21 Starman1

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Posted 22 February 2015 - 07:14 PM

I've used a relatively thick polyethylene washer (just one) and while it was harder than the plastic in a milk carton, it still was soft enough for the screws to dimple it.

And I found the aluminum of the top of the secondary holder slides against it very well.  So, one washer or two, the concept seems to be:

--have a sliding surface to allow rotation of the secondary

--have an intermediary material that allows the screws to press on it and create dimples without doing that to the secondary holder itself, which restricts easy rotation.

I also prefer thumbscrews (Bob's Knobs style is fine) because they can be adequately snugged but not tightened too much.


Edited by Starman1, 22 February 2015 - 07:15 PM.


#22 Jason D

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Posted 22 February 2015 - 07:39 PM

I am using Bob's Knobs which are great and frankly a must have.

But I did not file the tips of the set screws because there is no need when using the milk jug washers. Sure they dip into the washer but who cares since the washer will prevent them from getting to the secondary stalk and the stalk will still rotate freely.

I have adjusted my secondary mirror more then a dozen would do in a life time -- all as part of experimenting with collimation tools (still do). I have not replaced my milk jug washers since I installed them around 7 years ago.

Jason



#23 Jason D

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Posted 22 February 2015 - 08:01 PM

 

The milk jug washer is not only meant to protect the secondary mirror stalk from the adjustment screws otherwise a metallic washer would have been good enough.

The main benefit of the milk jug washer is to provide little elasticity to make fine adjustments -- something a hard object like a metallic washer or even a hard plastic will not provide. That is why I suggest using 1 or 2 washers to provide different levels of elasticity.

The elasticity of the milk jug washer seems about to be the right amount.

In addition, it is easy to install. If the scope owner does not want to remove the secondary mirror then he/she can make a radial cut and slide the washer in as I have shown in post # 8 in this thread.

Jason

Yes, this is it and it works very well. I used black Kydex from Scopestuff, and have 2 of them stacked between the seconday holder and the screws.

 

 

 

 

Hi Steve, I did not know Scopestuff carries them. Did you mean Opticsmart?

http://www.cloudynig...asher-question/

Jason



#24 beatlejuice

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Posted 22 February 2015 - 08:56 PM

I got the thumbscrews and  washers from Opticsmart.   Tweaking the secondary tilt is so much easier since.    

 

Eric



#25 howard929

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Posted 23 February 2015 - 07:24 AM

It's been mentioned that installing milk jug washers is easy to do, which it is but it's not the easiest or fastest way to rectify this problem.

 

The process of getting the washers between the screws and the mirror holder even if the washers are split entails backing all three tilt screws away from the mirror holder at the same time. Placing milk jug washers results in the secondary mirror being loose and flopping around. Once they are in, the mirror would need to be rounded and the tilt screws would need to be re-positioned properly. The enjoyment of that process is very much YMMV.

 

OTOH, most of that could be avoided with the screws referenced in post #14 if they were removed and replaced one at a time.   


Edited by howard929, 23 February 2015 - 07:28 AM.



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