Help with secondary mirror fix

[Pixies]

I'm afraid I've made a bit of a boo-boo!

It looks like I've been a bit cack-handed with the screwdriver when trying to adjust the secondary mirror of my new Bresser 8" dob. The mirror holder is made of cheese (apparently), and I've managed to dig the screws into it! (see pic below)

It has always 'turned' as I tightened down the screws, and now I've really dug into it. I didn't think I was using a lot of force, but it's hard to tell using a screwdriver. I noticed some tiny swarf lying on the inside of the tube (it was horizontal) stopped and panicked! Eventually saw what was happening by taking some photos. Anyway - I have learnt my lesson the hard way. Now I have to fix it.

I intend to remove the secondary, add a steel washer (countersinking for the new 3 machine screws with rounded ends). I'll also make sure the face of the holder is smooth and add a plastic (milk carton) washer to the surface between metal washer and secondary holder.

However, I'm paranoid that I don't do any (further) damage - esp to the mirror itself. Can you recommend a safe way to protect the mirror? Is it worthwhile wrapping it in a freezer bag, say, or a clean lint-free cloth with elastic bands, etc. Any suggestions on this or other considerations very welcome.

[fozzybear]

 @Pixies Can you recommend a safe way to protect the mirror

To protect the mirror wrap it in an accident bandage. the one with cotton wool layer then lint as wrapping  or raid the wife's makeup stuff, and cover in "makeup remover pads" same stuff just don't scratch the secondary 

[johninderby]

Never known anyone to damage one before as you don’t need a lot of pressure to lock things in place and it’s not something that needs adjustment very often.. 

Would suggest fitting thumbscrews as that way you won’t be overtightening the screws.,

Edited May 20, 2020 by johninderby

[johninderby]

Just realised after looking at my 10” Bresser that you seem to be missing the metal plate that the screws tighten against. Did you remove the secondary at any time?

[Pixies]

Nope. This was as it came. I've only tried to adjust it, not remove it yet.

I know there are some differences with the latest models, mainly the fittings. How long have you had yours?

 

 

[johninderby]

About nine months. Wonder if the 8” is different to the 10” and doesn’t have the metal plate?

 

Edited May 20, 2020 by johninderby

[Ricochet]

The 8" doesn't have the metal plate. Why they are different I don't know. From memory the y shaped part of the secondary holder has three recesses that the screws press into. I wonder if perhaps the swarf is from where the recesses were originally drilled out, rather than the screws cutting into the holder. 

[johninderby]

Perhaps with the 10” having a bigger secondary they thought the plate was needed? 

In the closeup pic of the 8” seconday it looks a lot like grease around the ends of the screws. Think the seconday support is cast metal, probably aluminium so unlikely the screws would dig into it. 

 

[Pixies]

Fingers crossed. I'll email FLO and see what they say.

If the holder has 3 recesses, that would explain why the holder rotates back to the same (slightly off) position when I tension the screws. Seems a strange design.

[johninderby]

These look about right.

https://www.amazon.co.uk/Mudguard-Penny-Plated-Washers-Screws/dp/B07H6HZWCB/ref=sr_1_1?dchild=1&keywords=M6+x+50mm+Mudguard%2FPenny+Large+washers&qid=1590013438&s=diy&sr=1-1

[Pixies]

1 minute ago, johninderby said:

These look about right.

https://www.amazon.co.uk/Mudguard-Penny-Plated-Washers-Screws/dp/B07H6HZWCB/ref=sr_1_1?dchild=1&keywords=M6+x+50mm+Mudguard%2FPenny+Large+washers&qid=1590013438&s=diy&sr=1-1

Thanks John. I've already ordered some stainless steel ones (same size). Wanted black, but that was a minimum order of 100!

[johninderby]

Just for interest found a pic of a GSO secondary holder.

[faulksy]

i made a aluminium disc and put a ptfe washer in between 😛

[Pixies]

9 hours ago, johninderby said:

Just for interest found a pic of a GSO secondary holder.

OK - that looks like it (although I haven't removed mine yet).

How the flip does that work? I can't see how you could rotate the holder?

[Pixies]

I'm saying that , because from that picture, they look like threaded holes. However, look at  this one below.

I need to take it out to get a good look at it.

[Rob]

Seems unusual that Bresser would cut corners on this. I'm sure it should of had a metal plate. Well I hope all works out well for you

Rob

[johninderby]

Here’s the ultimate solution. ££££££ 😁

https://www.teleskop-express.de/shop/product_info.php/info/p4133_TS-Optics-Adjustable-Holder-for-Newtonian-secondary-mirrors-from-50-mm-diameter.html

[Pixies]

3 minutes ago, johninderby said:

Here’s the ultimate solution. ££££££ 😁

https://www.teleskop-express.de/shop/product_info.php/info/p4133_TS-Optics-Adjustable-Holder-for-Newtonian-secondary-mirrors-from-50-mm-diameter.html

Thanks - but I'm still a little away from the need for that, yet.

[johninderby]

Lots of “ultimate” upgrade parts available for any dob but gets into silly money. 🙀🙀🙀

[Ricochet]

14 hours ago, johninderby said:

Perhaps with the 10” having a bigger secondary they thought the plate was needed?

You also mention M5 screws, perhaps those would foul on the recesses. 

13 hours ago, Pixies said:

Fingers crossed. I'll email FLO and see what they say.

If the holder has 3 recesses, that would explain why the holder rotates back to the same (slightly off) position when I tension the screws. Seems a strange design.

Are you sure you need to rotate the holder? The spider arms are in line with the focuser, so the recesses should ensure that the secondary is orientated towards the focuser properly. I suppose if the focuser axis isn't aligned with the spider arms you might need to rotate the secondary. I actually had that sort of issue with mine as the tube isn't quite circular. I shimmed the focuser with a couple of washers under the bottom screw fixings to get a better alignment. 

[johninderby]

The 10” uses M5 adjusting screws and has a metal plate. Din’t know what size screws are used on the 8”.

Edited May 21, 2020 by johninderby

[Pixies]

46 minutes ago, Ricochet said:

You also mention M5 screws, perhaps those would foul on the recesses. 

Are you sure you need to rotate the holder? The spider arms are in line with the focuser, so the recesses should ensure that the secondary is orientated towards the focuser properly. I suppose if the focuser axis isn't aligned with the spider arms you might need to rotate the secondary. I actually had that sort of issue with mine as the tube isn't quite circular. I shimmed the focuser with a couple of washers under the bottom screw fixings to get a better alignment. 

If I can get it to stay aligned with the arms, it's OK. But usually, as I tighten it, it rotates slightly. I can usually get it at the correct rotation, but when I then try to fine-tune its alignment with the primary, it's pops back to the wrong position. It took a while to work out what was going on, as it has screw heads, so I can't watch through the focusser as I adjust. 

[Pixies]

37 minutes ago, johninderby said:

The 10” uses M5 adjusting screws and has a metal plate. Din’t know what size screws are used on the 8”.

M4 (30mm) on the 8" Not sure what the centre screw is yet, though.

[Pixies]

All fixed and feeling quite chuffed, now.

After checking  that removing the secondary wouldn't invalidate my warranty, I duly did and found the state of the holder (pic below). No recesses cut for the screws apart from the ones that the screws cut themselves!

I'm still waiting on new adjusting screws, but I have managed to get a suitable washer (M6 x 50mm for the record - as John has also suggested). I decided to additionally try a milk-carton plastic washer to assist with reducing friction. I was a little worried that it might mean too little friction and I would have to really crank the bolts tight to keep it from rotating and was prepared to remove it if necessary. Initially I had it on the screw-side, but they cut through it immediately. Then I tried it on the holder side, and it seemed fine (pic below).

I put it all back together and did a quick line-of-sight adjustment. It feels soooooo much better now. Perhaps because I'm more confident in it now, and less worried I'm going to break something. I can tighten the adjuster screws enough to make the mirror very solid, without having to crank it up. I'll have some new hex-headed stainless machine screws arriving shortly and I'll see how they go. I won't bother with Bob's Knobs just yet, as hopefully this will now be solid enough to mean it won't need adjusting very often.

I'll do a proper collimation tomorrow. I'm feeling much more confident now. Probably because I've now seen how everything works. Primary mirror next then! 😬

 

Edited May 22, 2020 by Pixies

[John]

That does look better.

With the milk carton washer modification, I think the optimum benefits are gained from having the collimation screw tips bearing directly onto the washer / washers. This PDF file from long time SGL member @Moonshane explains:

Collimation of Newtonian Telescopes-1.pdf

And discussed here: