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secondary mirror washer question

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#1 BDS316

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Posted 11 May 2013 - 01:20 PM

I have been using a hardware store metal fender washer between the adjusting screws and the secondary mirror holder in my XT8 Dob. At NEAF I purchased a pair of the plastic(?) washers. Can the plastic washers be placed between the metal washer and the mirror holder so that the screws still make contact with the metal washer to prevent divots in the softer plastic washers and the mirror holder? Or is there a better way?

Thanks

#2 Jason D

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Posted 11 May 2013 - 01:46 PM

Many years ago I came up with the idea of using Milk Jug washers for the secondary mirror instead of the commonly used steel washer and it works great as reported by all those who tried it. I recommend forgetting about the steel washer and using Milk Jug washer. They work better than plastic washers because Milk Jug washers have some elasticity to them. Elasticity will allow you to make that final fine adjustment my tightening one set screw without the need to untighten the other two. Try it. It is an easy setup. If you do not want to remove your secondary mirror, you can always cut the washers diagonally then slide them in. Try one then two stacked washers and see which setup works better for you.
Jason

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#3 Jason D

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Posted 11 May 2013 - 01:48 PM

Use the following idea to slide the washers in then try them. If you like the idea, then remove the secondary and install uncut washers.

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#4 DavidOpticsmart

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Posted 11 May 2013 - 03:50 PM

Bryan, I assume you bought the washers at our booth at NEAF? In any case, Jason was indeed the originator of the Milk Jug Washer for secondary mirrors, which was a very simple, yet elegant solution to a minor design weakness in this class of scopes (if I'm not mistaken, both GSO and Synta scopes can benefit from this "tweak"). We simply created a commercial version of "milk jug washers" for all those lactose-intolerant astronomers out there. :lol:

Milk jugs are made from plastic, in most cases High-Density Polyethylene (HDPE). And HDPE does work very well, as Jason said, because of the natural elasticity of the plastic (also because of its coefficient of friction which keeps the collimating screws from getting "stuck" in one spot so easily). We simply used a slightly higher grade of polyethylene called Ulta-High-Molecular-Weight Polyethylene (UHMW). As compared to HDPE, UHMW has slightly higher impact strength and abrasion resistance, and slightly lower coefficient of friction. It's kind of like milk jug plastic on steroids. :cool:

Anyway, we leave the metal washer in place, and put the two UHMW washers between the metal washer and the tips of the collimation screws. But you could certainly remove the metal washer altogether if you want to.

#5 Jason D

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Posted 11 May 2013 - 04:34 PM

Hello David,
I was unaware that plastic washers specifically for secondary mirrors were available commercially. I assumed these were hard plastic. But if yours are made of HDPE with reasonable elasticity then I think this is great. Best of luck.
Jason

#6 BDS316

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Posted 11 May 2013 - 05:17 PM

Bryan, I assume you bought the washers at our booth at NEAF? In any case, Jason was indeed the originator of the Milk Jug Washer for secondary mirrors, which was a very simple, yet elegant solution to a minor design weakness in this class of scopes (if I'm not mistaken, both GSO and Synta scopes can benefit from this "tweak"). We simply created a commercial version of "milk jug washers" for all those lactose-intolerant astronomers out there. :lol:

Milk jugs are made from plastic, in most cases High-Density Polyethylene (HDPE). And HDPE does work very well, as Jason said, because of the natural elasticity of the plastic (also because of its coefficient of friction which keeps the collimating screws from getting "stuck" in one spot so easily). We simply used a slightly higher grade of polyethylene called Ulta-High-Molecular-Weight Polyethylene (UHMW). As compared to HDPE, UHMW has slightly higher impact strength and abrasion resistance, and slightly lower coefficient of friction. It's kind of like milk jug plastic on steroids. :cool:

Anyway, we leave the metal washer in place, and put the two UHMW washers between the metal washer and the tips of the collimation screws. But you could certainly remove the metal washer altogether if you want to.


Yes I bought them from you. Thank you. I also bought a light shade for my XT8.

What would happen if I reversed the order and put the two UMHW washer between the metal washer and the mirror holder?

#7 nevy

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Posted 11 May 2013 - 06:35 PM

Hello Jason , did you round off the ends of the collimation screws?

#8 Jason D

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Posted 11 May 2013 - 07:11 PM

Hello Jason , did you round off the ends of the collimation screws?


No I did not. I do use Bob's knobs and frankly I do not know if these come with rounded off ends. I have been using the same washers for years without issues. The screws did dent the first washer but so far did not break through it.

#9 nevy

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Posted 11 May 2013 - 07:33 PM

Ok thanks Jason.

#10 DavidOpticsmart

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Posted 11 May 2013 - 07:52 PM

Yes I bought them from you. Thank you. I also bought a light shade for my XT8.


Thanks Bryan!




What would happen if I reversed the order and put the two UMHW washer between the metal washer and the mirror holder?


That wouldn't solve the problem that the washers are intended to fix. The reason that Jason came up with the washer idea to begin with is that when the collimation screws are used directly against the metal washer (as the scope comes from the factory), they tend to make little "divots" in the metal, after which it becomes difficult to make very small, precise collimation adjustments because the screws will tend to want to fall back into those same divots. When you put the plastic washers BETWEEN the screws and the metal washer, it will be easier to make those small, precise adjustments because of the abrasion resistance and the slickness of the washers.

#11 DavidOpticsmart

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Posted 11 May 2013 - 09:15 PM

Hello David,
I was unaware that plastic washers specifically for secondary mirrors were available commercially. I assumed these were hard plastic. But if yours are made of HDPE with reasonable elasticity then I think this is great. Best of luck.
Jason


Thanks for the best wishes Jason, and thanks for being the brains behind the product! :bow: I won't be retiring early from selling plastic washers, but we might make enough profit that I can afford to buy you a drink if we ever meet up. :grin:

#12 Pinbout

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Posted 11 May 2013 - 09:23 PM

I have my washers as you. But I have a spring on the ctr stalk bolt like 1800destiny so the collimation bolts can never dent the metal washer.


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