Fitting A Washer Behind Secondary Mirror Holder?

[Guest]

The Allen screws that tilt the secondary mirror on my 8" Dobsonian reflector, have left 'dimples' in the back of the holder.  I'm going to change them out for Bob's Knobs shortly and I'm wondering if I put a large penny washer between the spider part and the secondary, if this will prevent further scoring of the back of the holder?

Any thoughts?

Edited November 2, 2020 by Guest

[Vulisha]

That is good option. Take chrome washer to prevent rust and new dents, I don't think silver color will shine too much to worsen seeing. 

[DaveL59]

You can always black the washer edges to prevent any stray reflections but I see no reason why you can't, so long as there's enough play in the secondary holder to allow adjustments. Was thinking to do the same on the little NatGeo in fact but probably using a nylon washer for that. Doesn't much matter after all if the washer gets dimples so long as the secondary holder can rotate smoothly and not snag or shift as the bolts tighten up.

[Guest]

I was thinking of getting a stainless steel washer, so that it won't rust.  Regarding reflections off of it , I could also cover it with so matt black paint around the exposed edges, before I fit it.

Edited November 2, 2020 by Guest

[Seelive]

Sounds like something has been overtightend if the screwheads have deformed the back of the secondary holder.

[Guest]

6 minutes ago, Seelive said:

Sounds like something has been overtightend if the screwheads have deformed the back of the secondary holder.

No doubt that they have.  One was like that to begin with, the other two were done by me.🙄

[DaveL59]

depends on what the holder is made from too, the NatGeo is plastic so very easy to dig into with metal screws. Even ally will get marked over time with steel screws.

[Guest]

1 minute ago, DaveL59 said:

depends on what the holder is made from too, the NatGeo is plastic so very easy to dig into with metal screws. Even ally will get marked over time with steel screws.

I've no idea what SW make their's out of...

[DaveL59]

Probably some form of aluminium or other alloy but that'll be softer than the steel screws, hence will mark over time as the tip of the screw grinds into it, especially as the assembly tilts to an angle relative to the screw when it can cut in further/easier.

[Guest]

It was this video that gave me the idea...

https://youtu.be/zd-fl9SEYHw

[John]

I've added a large washer to the top of the boss of my secondary and also a couple of the milk bottle washers. Seems to be a good arrangement. The secondary adjustment screws press against the plastic washers and the large steel washer prevents pitting of the top surface of the aluminum secondary boss. I rarely need to adjust the position of my secondary now.

So it's this plus the large steel washer between the plastic disks and the secondary boss:

 

Edited November 2, 2020 by John

[Guest]

7 minutes ago, John said:

I've added a large washer to the top of the boss of my secondary and also a couple of the milk bottle washers. Seems to be a good arrangement. The secondary adjustment screws press against the nylon washer and the large steel washer prevents pitting of the top surface of the aluminum secondary boss.

So it's this plus the large steel washer between the plastic disks and the secondary boss:

 

So that's a steel washer, two plastic washers and finally another steel washer in a sandwich.  Is that right?

[John]

1 minute ago, merlin100 said:

So that's a steel washer, two plastic washers and finally another steel washer in a sandwich.  Is that right?

No. Steel washer and two plastic washers. The tips of the collimation screws press against the plastic washers as per Jason D's approach:

https://www.cloudynights.com/topic/446178-secondary-mirror-milk-jug-washers/?p=5775875

 

 

[Peter Drew]

I would do it slightly differently.   I would have the screws bearing directly on to the steel washer and the plastic disc/s between it and the top of the holder.  The adjustment screws, which should be rounded at the tips, will not dimple the steel washer and the plastic disc/s will allow the secondary holder to rotate smoothly by hand if necessary.  Screws bearing on plastic discs could lose their tension as the plastic deforms and alter the collimation.  Or whatever works for you!    🙂 

[John]

1 minute ago, Peter Drew said:

I would do it slightly differently.   I would have the screws bearing directly on to the steel washer and the plastic disc/s between it and the top of the holder.  The adjustment screws, which should be rounded at the tips, will not dimple the steel washer and the plastic disc/s will allow the secondary holder to rotate smoothly by hand if necessary.  Screws bearing on plastic discs could lose their tension as the plastic deforms and alter the collimation.  Or whatever works for you!    🙂 

I did wonder about this when I installed my system but it's worked well so I've stuck with it. 

 

[Peter Drew]

I did find what I suggested to work best for me, specially with large secondaries.    Any combination is a big improvement over the supplied arrangement.   🙂

[johninderby]

Wonder if anyone has tried these secondary holders from Telescope Express.? 🤔

[bomberbaz]

3 hours ago, johninderby said:

Wonder if anyone has tried these secondary holders from Telescope Express.? 🤔

Ain't gonna get any rotation on that once it's tightened up

[Pixies]

Hey - @merlin100

I did the same a while ago to my Bresser:

 

I have thumbscrews that fitted (M4x45mm) plus M8x50mm stainless steel washer. They could well be the same size. Perhaps someone with the same scope as yours can confirm.

If you want them, I'll send them to you. PM me if you're interested.

[Guest]

1 minute ago, Pixies said:

Hey - @merlin100

I did the same a while ago to my Bresser:

 

I have thumbscrews that fitted (M4x45mm) plus M8x50mm stainless steel washer. They could well be the same size. Perhaps someone with the same scope as yours can confirm.

If you want them, I'll send them to you. PM me if you're interested.

I've ordered a stainless steel M5x30mm washer, which will do the job.  I've fitted the Bob's Knobs to the secondary mirror holder, so I'm okay at the moment, thanks.

[johninderby]

For some reason the 8” Bresser doesn’t have a washer but the 10” Bresser does. 🤔

[Peter Drew]

4 hours ago, johninderby said:

Wonder if anyone has tried these secondary holders from Telescope Express.? 🤔

Looks to be a right pain to collimate!        🙂

[bomberbaz]

23 hours ago, Peter Drew said:

Looks to be a right pain to collimate!        🙂

yeah fiddly I agree peter but once it's bolted down you shouldn't have a lot of movement so recollimation will be infrequent you would hope!

[Alan64]

The factory(ies) overseas tightens those screws tighter than <insert here>, and for shipment, for the collimation to hold until it's delivered to the end-user.

My own base, Synta 150mm f/5 no less, and as a result the collimation was spot-on upon arrival.  But then, not too terribly long afterwards I took the telescope completely apart for renovating.  One of the things I did was to epoxy a nylon-washer for the three set-screws to bear against, arrowed at right...

I've also thought about a washer of iron, but stainless-steel might be even better.  Do sand/score where matte-black is to be applied, and inward to just before where the tips of the screws would make contact, all round.