102mm f7 Refractor - 2 M6 bolts enough?

[Dave-P]

Hello, just picked up a 2nd hand Altair 102mm f7 refractor with the CNC tube rings that are quite common on these. Looking to attach to Vixen dovetail, but the only way this would seem to work would be a single M6 bolt on each tube ring through the dovetail (rings can take 5 M6 bolts each, but spacing not helpful). Is 2 M6 bolts enough, or should I be looking at some sort of plate between rings and dovetail? Thanks.

[CraigT82]

One M6 on each ring is fine, that’s how it comes on the Starfield optics version with its supplied rings and dovetail. 

[Dave-P]

Thanks!

[Kobayashi Maru]

28 minutes ago, Dave-P said:

Hello, just picked up a 2nd hand Altair 102mm f7 refractor with the CNC tube rings that are quite common on these. Looking to attach to Vixen dovetail, but the only way this would seem to work would be a single M6 bolt on each tube ring through the dovetail (rings can take 5 M6 bolts each, but spacing not helpful). Is 2 M6 bolts enough, or should I be looking at some sort of plate between rings and dovetail? Thanks.

An old engineer (my father) tells me a 1/4 inch bolt will hold about 1 tonne in tension. While there will be a little shear force in your setup I suspect you should be good.

[Dave-P]

28 minutes ago, Kobayashi Maru said:

An old engineer (my father) tells me a 1/4 inch bolt will hold about 1 tonne in tension. While there will be a little shear force in your setup I suspect you should be good.

It's not the bolts that worry me, more the fact that they're only going 7 or 8mm into soft aluminium

[Franklin]

I had the same worry when I had the f/11 version of your scope, my solution was to get a Vixen plate with an extra wide top and with that I could secure the rings with two bolts on each side. Expensive but rock solid.

 

[Dave-P]

11 minutes ago, Franklin said:

I had the same worry when I had the f/11 version of your scope, my solution was to get a Vixen plate with an extra wide top and with that I could secure the rings with two bolts on each side. Expensive but rock solid.

 

Can you remember where you picked that up? Struggling to know what I'm searching for...

[Kobayashi Maru]

49 minutes ago, Dave-P said:

It's not the bolts that worry me, more the fact that they're only going 7 or 8mm into soft aluminium

The bolts would still have to pull through 7-8mm of aluminium. Even with the leverage of a long scope that would require a huge amount of force. The scopes tube would deform and collapse well before the bolts pull out. Just did a quick google search. There are calculations you could do but the first result seems close enough. https://www.eng-tips.com/viewthread.cfm?qid=292990

TLDR - 5/16th bolt with 5/8th engagement has a pull out resistance of about 6.8 tonne. The plate itself would fail well before you reach that i imagine.

[Moonlit Knight]

I use exactly the same scope as you and the same bolts, but with a Losmandy. They are absolutely sound, you have nothing to worry about, your scope isn’t going to come crashing down anywhere. I have had no issues in 3 years 

[Dave-P]

I know that, logically, 2 bolts should be fine. Think it's the lack of redundancy in the system that bothers me, if one bolt failed it would rapidly become "not fine". With 4 bolts, one could fail and fineness would still prevail 

[Cosmic Geoff]

From an engineering point of view, no need to worry unless you strip threads in soft alloy, by over-tightening. A M6 bolt is not going to snap under the weight of a 102mm refractor.

[Franklin]

14 hours ago, Dave-P said:

Can you remember where you picked that up? Struggling to know what I'm searching for..

The problem I had with the Ascent 102ED f/11 was, not only a much longer, heavier scope but the supplied rings attached via one bolt each at the center of each ring and at that point the ring is at its thinnest. My worry was that the bolts were only secured to the rings by just a couple of threads and this is why I opted for the wider plate which enabled me to bolt through at the sides where the rings were much thicker, allowing much more length of the bolt for fixing. The scope was absolutely rock solid after this upgrade over the stock plate. The one I used in the picture came from TS Optics but I've seen William Optics supply them as well.

[Dave-P]

29 minutes ago, Franklin said:

The problem I had with the Ascent 102ED f/11 was, not only a much longer, heavier scope but the supplied rings attached via one bolt each at the center of each ring and at that point the ring is at its thinnest. My worry was that the bolts were only secured to the rings by just a couple of threads and this is why I opted for the wider plate which enabled me to bolt through at the sides where the rings were much thicker, allowing much more length of the bolt for fixing. The scope was absolutely rock solid after this upgrade over the stock plate. The one I used in the picture came from TS Optics but I've seen William Optics supply them as well.

That's exactly what worries me, the rings are so thin, and I'll have to hacksaw the bolts back to avoid scratching the tube, so there may only be 5 or 6mm holding. Think I'll use a dovetail plus 2 bolts to get first light, but I'll order up a wide plate dovetail as a "forever fix" today. Thanks everyone.

[Franklin]

1 hour ago, Dave-P said:

the rings are so thin,

These kind of scopes are made to a price, when I started out in the 1980's buying a 4" Apochromat was the stuff of dreams. A decent 4" achromat was over £1k back in those days and a lot of people would build their own telescopes using a mirror kit or even grinding their own mirror. The young newbies of today don't realize how lucky they are.

[Kobayashi Maru]

On 04/03/2023 at 04:28, Franklin said:

These kind of scopes are made to a price, when I started out in the 1980's buying a 4" Apochromat was the stuff of dreams. A decent 4" achromat was over £1k back in those days and a lot of people would build their own telescopes using a mirror kit or even grinding their own mirror. The young newbies of today don't realize how lucky they are.

Grinding your own mirror!? Luxury!!! When I was young we had to make our telescopes out of used drinking straws and the bottom of coke bottles which we ground with our teeth! Try telling that to the young ones today and they won’t believe you!

Edited March 4, 2023 by Kobayashi Maru

[Dave-P]

1 hour ago, Kobayashi Maru said:

Grinding your own mirror!? Luxury!!! When I was young we had to make our telescopes out of used drinking straws and the bottom of coke bottles which we ground with our teeth! Try telling that to the young ones today and they won’t believe you!

You had teeth? We had to spend months on end spitting gravel at the bottom of coke bottles to try and get a decent figure on them. And that was before they'd invented stars, we'd spend night after night staring into the inky blackness, waiting for the big bang to happen ...

[Kobayashi Maru]

1 hour ago, Dave-P said:

You had teeth? We had to spend months on end spitting gravel at the bottom of coke bottles to try and get a decent figure on them. And that was before they'd invented stars, we'd spend night after night staring into the inky blackness, waiting for the big bang to happen ...

Welllll……when I say teeth I mean those we picked up after our farther had beaten us to death the day before for complaining about the lack of stars. Inky blackness….We used to dream of inky blackness! All we had was clouds! And we couldn’t even them due to the death beating of the day before…of course things were tougher back then.

Edited March 5, 2023 by Kobayashi Maru

[Dave-P]

9 hours ago, Kobayashi Maru said:

Welllll……when I say teeth I mean those we picked up after our farther had beaten us to death the day before for complaining about the lack of stars. Inky blackness….We used to dream of inky blackness! All we had was clouds! And we couldn’t even them due to the death beating of the day before…of course things were tougher back then.

You had a farther? I had to differentiate myself from my siblings by a rudimentary process of cellular division, floundering around in the primordial soup....

[900SL]

I'm a Structural Engineer

A 10 kg scope at 100mm offset to centerline from base exerts a moment of 10 N-m

A 40mm wide vixen rail will result in a force of  250N per bolt  (25 kg)

The safe working load of a standard 6mm bolt is around 4000N (400 kg), conservative rating assuming mild steel bolt. Yield is around 700kg

 

Thread engagement in aluminium should be around 1.5 x bolt diameter or say 10 mm, and I would reduce allowable load by 50% for 'Chinese'

You still probably have a factor of safety of at least 10. Just make sure you have sufficient thread engagement and that the bolt fits well, not undersize

 

[900SL]

On 03/03/2023 at 15:08, Franklin said:

The problem I had with the Ascent 102ED f/11 was, not only a much longer, heavier scope but the supplied rings attached via one bolt each at the center of each ring and at that point the ring is at its thinnest. My worry was that the bolts were only secured to the rings by just a couple of threads and this is why I opted for the wider plate which enabled me to bolt through at the sides where the rings were much thicker, allowing much more length of the bolt for fixing. The scope was absolutely rock solid after this upgrade over the stock plate. The one I used in the picture came from TS Optics but I've seen William Optics supply them as well.

I have the WO rail. Pretty sturdy, but check that things like bolt sizes and cross centers match.

[900SL]

Oh, and whilst I am on, why don't retailers include clear dimensions with things like adapters and plates and brackets?

The number of times I've cancelled a prospective order because there is no drawing showing bolt holes and setting out dimensions is legion. How can I tell if it will suit my current arrangements?

[John]

I used to use the longer version of the William Optics DT bar with my 130mm F/9.2 triplet. I attached it to the tube rings using 4 M6 bolts. It was pretty stable even with that long 9.4kg scope on board. I've since moved to a Losmandy bar though for even more stability. I would think that a 102mm F/7 would be very stable on the WO bar.

 

[Dave-P]

8 hours ago, 900SL said:

Oh, and whilst I am on, why don't retailers include clear dimensions with things like adapters and plates and brackets?

The number of times I've cancelled a prospective order because there is no drawing showing bolt holes and setting out dimensions is legion. How can I tell if it will suit my current arrangements?

Fully agree, the lack of dimensions on a lot of these expensive CNC-milled objects is criminal. Think I've found a TS Optics plate that will do the business (and has dimensions for bolt holes). Just need to double check that my telrad base will fit between the rings on it. Currently only seeing the long version of the the WO Vixen plate, which would cause dew-shield issues. Be a shame to put it straight on the chop-saw to hack it down a bit.

[Carbon Brush]

Not exactly what you are after, but maybe useful.

I had questions about the standard dimension for a Losmandy style dovetail.
Basically I had different size dovetails from different manufacturers.
Which part do I file? Who is building to the agreed standard?
ADM were the only company to reply to my enquiry. No prizes for guessing the name on all my Losmandy size parts👍

 

DUP7.pdf

Edited March 12, 2023 by Carbon Brush