Probably Stating The Obvious

[mick pinner]

after reading Mikes thread about his new scope and congratulations by the way, and then Ezy's thread regarding his NGT 10" and guidscope congratulations again they were both refering to their concerns regarding weight and l started to think, and this is where l am probably stating the obvious to most but the newbies may get something out of it, weight to a point is not the overriding issue it is where it is placed that becomes the problem.
l am not sure how the manufacturers come to their ratings and exactly what parrameters they use to get there but lets assume they calculate it along a line that runs parallel to the dovetail and say 125mm above it, a 250mm scope at f/l 1250 maybe under the load limit along that axis but adding say an ED 80 guidscope the axis of the guidscope may be 350mm from the balance line and this will, due to the cantilever effect make the ED 80 in effect weigh twice as much as it really does.
A long focal length Newtonian weighing the same as a short focal length scope will in fact present more problems when it is on the mount because you are moving the weight further fron the balance point, the dew shield on my Meade weighs around 600 grams but it takes 3 times that ammount of weight to balance it.
What weighs say 12kg in your hand could have the effect of weighing twice that when it is loaded because of the motors having to drag the weight through a greater arc than it was designed for.

[Starkler (Geoff)]

I had the same thoughts Mick, and was contemplating the idea of mounting the ed80 off the side of the dovetail to bring it closer to the axis of rotation.

The issues then are that you need a 3D balancing system and it becomes a lot more complex along with the possibility of things banging into the tripod legs.

As for Mike going from an eq platform to an eq6, I would have thought the platform would be more stable Perhaps the difficulty is in getting accurate polar alignment. Just random ramblings on my part, I might be totally wrong and i hope for Mikes sake that I am.

[Harb]

I might also add that we have a stedicam system, the sort you see at the football on the side line or on a movie set etc.....
Now I have to tell you that balance is the biggest factor with this system and if balanced , quite large heavy cameras can be moved with this slightest effort.
If it is out of balance the smallest amount the effort required is multiplied by huge amounts.
My point is the mounts carrying capacity is in relationship to the direct force on the centre line of the rotating surfaces, and asumes that the weight is balanced with precision.
It the weight is not balanced, it is not the mount that is dealing with the load, but the gear train in the mounts drive system.....this is bad.
So to sum up, the size of the scope does not dictate the size of the mount, its total weight does.
The size of the OTA only effects the torque loading against the gears in say the wind or in an unbalanced situation.

So who was it that said "give me a lever long enough , and I can lift the earth" , well same thing appies here , I recon I could break an EQ6 with an eyepiece it it was out of balance far enough

cheers

[74tuc]

Hi All,

I am just going through the work of adding an autoguider to my mount and am trying to overcome some of the problems you are discussing.

For a GEM placing any weight off axis will require a counterbalancing weight that is relatively easily calculated (approximately!!). The two methods (side and top mounting) cause different balance problems.

If you mount the guide 'scope on top of the main 'scope then for balance on the vertical axis (as an approximation) the following holds:

if Wm is the weight of the main 'scope
Wg id the weight of the guide 'scope
Wc is the weight of the counter weights

and

Dg is the distance of the Guide 'scope (C of G) to pivot point
Dm is the distance of the main 'scope (C of G) to the pivot point
Dw is the distance of the counterweights to the pivot point.

Then the following relations must hold for the system to balance:

1. NO guide 'scope:

Wm x Dm = Wc1 x Dw1

2. If you add the guide scope on top:

( Wm X Dn ) + (Wg x Dg) = Wc2 x Dw1 ... (new counter weight at original distance)

or

( Wm X Dn ) + (Wg x Dg) = Wc1 x Dw2 ... (Original counterweight at a different distance)

To illustrate the points above I guess some realistic weights and distances and do the calculations.

If Wm = 12 Kg (300mm tube dia), Dm = 0.4M, Dc1 = 0.8M then Wc1 = 6Kg

If a 3Kg guide 'scope + rings is put on top at 0.7M from the pivot point then

1. For the counterweight to be in the same place (above) the new counterweight is :

12 x 0.4 + 3 x 0.7 = Wc2 x 0.8

or Wc2 = 8.62Kg.

or For the same counterweight:

12 x 0.4 + 3 x 0.7 = 6 x Dc2

or Dc2 = 1.15M

In the first case adding an ED80 will increase the total weight by 5.62Kg

or

Increase the weight by 3 Kg (plus add length to weight bar) but the bar is almost 0.4M longer.


To discuss side by side mounting and the issues of differential flexure and the sizing of guide 'scopes, If one is interested, I could discuss it offline.

Regards,

Jerry.

[iceman (Mike)]

Interesting, thanks Jerry.

I'll be putting a camera + filterwheel + powermate hanging off the end of the focuser, so i'll have to think carefully about the balance aspects due to the long tube of the 12" newt.

[Striker (Tony)]

Ok I'm lost...I wish I listened to my Maths teacher in those algerbra classes.

But I believe you Jerry what ever all this means.....lol

[RB (Andrew)]

Here's a link I found ages ago that explains it with examples/diagrams.

[ving (David)]

hmm... most of the calculations go over my head... so lets see, the total weight hanging off the end has less relevance than where it is place. so if mike for instance wanted to put a guide scope on ih 12" he would be better off placing it on the side or underneath so that the extra weight is closer to the axis of rotation therefore needing less torque?
what if he were to place his camera, filter and indeed fovuser under the scope closer to the axis of rotation? would that require less torque from teh motor and therefore make weight less of an issue?

[74tuc]

Hi All,

Just pointing out that even adding a small extra load on the mount can end up putting upto twice the weight of the added pay load. Sorry about the detail!

Out of balance loads will wear the gears much faster and make backlash problems even worse.

Putting the main 'scope off axis with the guide scope to the side will mean making a heavy mounting plate to carry the guide 'scope. Adjustment must be made to rebalance the outfit quite accurately. You will find that the main scope will be moved about 50 mm off centre to balance the guide scope.

To reduce weight one suggestion would be to reduce the size of the guide scope - generally a 150mm to 300mm focal length (depending on the guiding CCD) guide scope will guide to better than 1 pixel. If you have a fancy CCD (which I don't) like an Sbig ST4 you may get away with a 50mm to 100mm focal length guide scope. Guide scope diameter need only be 60 to 80mm (max).

Kind Regards,

Jerry.

[74tuc]

To continue from my previous post I found making the guide scope modification was "more complicated then I thought". I discovered a couple of problems (lucky I did not build anything!!).

Placing stuff under the main scope needs a bit of thought as there has to be clear movement for 360 degree round the declination axis. When going through zenith will anything knock the tripod?

Is the guide scope far enough from the main scope to give about a total adjustment angle in dec of about 20 degrees (10 degrees off centre).

Is the mounting plate rigid enough? Pointing to zenith may increase local imbalance loads and cause the guider mounting plate to flex so much so that the guiding will be off.

Suggestion: 6081 aluminium plate is ideal for this application.

Jerry.

[EzyStyles (Eric)]

i learnt something out of this post. very informative guys. thanks.