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# Maximum photoLoad on a Mount: a stupid calculation

14 replies to this topic

### #1 Nicola

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Posted 17 January 2013 - 09:30 AM

Dear All,

I'd like to share this with everyone of you, I am pretty sure you already talked about it but would like to understand if my thoughts are right.
Let's start with these statements:
- Manufacturers do provide normally the maximum load capacity of a mount.
- I do know that a certain pair scope/mount is working fine 8reference scope/mount)

Therefore, if I take a Scope/Mount combination and I calculate the moment of inertia for this setup, I can think that all the Scope/Mount which have a moment of inertia lower or equal the reference Scope/Mount and the weight does not exceed the one specified by the manufacturer, I am fine.

The moment of inertia for a bar with a central fulcrum is barely:
I=m*l^2/12
where m is the mass (well weight in our case) of the scope and l is its physical length.

From this formula, for my mount (Tak EM200) I get this table for some scopes:

```Scope		Weight	Length	Inertia
FSQ85		3.9	0.645	0.135208125
VC200L		6	0.62	0.1922
GSO8		6.2	0.58	0.173806667
Edge HD800	6.35	0.432	0.0987552
FSQ106		7	0.675	0.26578125
RH200		8.4	0.5	0.175
TEC140		8.6	0.98	0.688286667
Edge HD925	9.53	0.559	0.248161994
AG10		12	0.81	0.6561
ODK10		12	0.475	0.225625
Edge HD1100	13	0.61	0.403108333
IKHAROS10	15	0.64	0.512
```

The TEC140 is proofed to be working fine with the Em200 for photographic purposes, so I take it as reference.
Am I reaching a wrong result?
From this result for instance I can put an Edge HD1100 on my mount, but not for instance an AstroTech RC 10" (which would have an inertia higher then the one of the TEC140, not listed here).

What do you think about it?

Thanks a lot for your valuable feedback

### #2 orlyandico

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Posted 17 January 2013 - 10:16 AM

The inertia thing works fine. But you are not taking into account the focal length of the scope.

For example, the TEC 140 has one of the biggest moments there of 0.69

However it is only 1000mm in focal length.

Now if you look at the C11 EDGE, its inertia is "only" 0.40 - but as per Jerry Lodriguss, the C11 EDGE is extremely overloading the EM200.

I would submit that you have to scale the inertia vs the focal length. Say our focal length is in meters and the TEC 140 is 1 meter. So your "normalized" moment is 0.69 * 1 = 0.69

While the C11 EDGE is 0.4 * 2.8 = 1.12 which is almost double the TEC 140 and thus overloading the EM200.

### #3 jrcrilly

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Posted 17 January 2013 - 11:34 AM

The inertia thing works fine. But you are not taking into account the focal length of the scope.

I don't believe that focal length can be scaled into a load rating; a mount is either suitable for a given focal length or it is not. A mount operating beyond its limits and thus losing performance will reveal the degradation in performance more quickly at longer focal lengths, of course - but the degradation is there.

If an EM-200 can't handle a C11, I wonder what can? You never seem to see them on AP-900 or ME mounts. I know folks use them on G-11 mounts, and an EM-200 can definitely outperform those at a given load (both would lose performance beyond 45 pounds or so, but the Tak would retain the edge).

### #4 orlyandico

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Posted 17 January 2013 - 12:04 PM

Jerry is using a C11 on an EM200 and has taken some good photos with it. But I notice that even today, his best images are taken with his old GM100. He has said several times on these forums and on his website that he considers the C11 undermounted on the EM200.

Also I don't believe a mount is suitable for a given FL or not. I can easily get consistent round stars at 2350mm on my Mach1. I would not dream of mounting a 16" f4 reflector on it, even though the reflector is only 1600mm.

It's the focal length and mass (or inertia, which is a refinement of just mass).

### #5 Jon Isaacs

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Posted 17 January 2013 - 12:24 PM

Nicola:

Some interesting work. A couple of thoughts:

- When I have thought about this problem, it seems like you also have to consider the diameter of the tube because it results in the center of mass being offset further from the central bearing, exactly how this figures in, I am not sure.

- To be more exact, the moment of inertia needs to be integrated along the length because the mass distribution is not uniform.

- Shorter focal lengths have smaller image scales so that might need to be considered.

- Regarding SCTs, I have long wondered if the less than robust mirror mounting methods make them more susceptible to vibration and stability issues. Even with a mirror lock, the primary mirror, which is very fast, is not securely held the same way a Newtonian mirror or refractor is.

Jon

### #6 Nicola

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Posted 18 January 2013 - 09:43 AM

Thanks for all your comments. Based on them I lookd to the web for a formula that better depicts our situation. I didn't fin one, but I founf a website (unfortunately in Italian, but Google Translate may be your friend in this case) that provides calculation for an empty tube rotating around the fulcrum. It is of course far from a telescope, but at least in this case takes in consideration that a tube has got a diameter and a thickness.
Therefore I recalculated the data above and where I found no data regarding tube thickness and external diameter I assumed: 1mm thickness for all scopes and 1cm difference between lens diameter and external diameter.It comes out something like this:

```Scope		Weight	Length	Inertia	    Corrected Inertia
FSQ85		3.9	0.645	0.135208125	0.103696
VC200L		6	0.62	0.1922	        0.220022
GSO8		6.2	0.58	0.173806667	0.270491
Edge HD800	6.35	0.432	0.0987552	0.13443
FSQ106		7	0.675	0.26578125	0.279236
RH200		8.4	0.5	0.175	        0.297866
TEC140		8.6	0.98	0.688286667	0.818032
Edge HD925	9.53	0.559	0.248161994	0.319085
AG10		12	0.81	0.6561	        0.790203
ODK10		12	0.475	0.225625	0.359728
Edge HD1100	13	0.61	0.403108333	0.537894
IKHAROS10	15	0.64	0.512	        0.753654
```

My comment: except for the FSQ85 where the inertia seems lower then the previous calculation, the new result reflects the fact that telescopes have a diameter, therefore all inertias are increased in value.
But now what puzzles me is the comparison between TEC140 and EDG HD11 from the calculations it would seem that the inertia of EDGE HD11 is lower then the TEC one. If I'd consider the Edge HD1100 a limit, then the TEC140 is definitely out of reach, but I see some people using this combo for taking pictures and they don't seem to complain.
So, after all, probably the focal length IS an important factor to provide to the formula a "rule of thumb" for understanding the limits...

What do you think about it?
Nicola

### #7 orlyandico

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Posted 18 January 2013 - 09:49 AM

given two tubes of equal inertia (hence both will wiggle by the same amount in arc-seconds given a disturbance), the one with longer focal length will definitely show more trailing due to the larger image scale.

if a wiggle is 1" on two tubes, the one with 0.5" / pixel will show 2 pixels of wiggle, while the one with 2" / pixel will only show 0.5 pixels of wiggle.

hence my suggestion of scaling the inertia by the focal length.

### #8 Midnight Dan

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Posted 18 January 2013 - 11:13 AM

Moment of inertia alone is not enough to determine whether a mount can handle a scope or not. It is just one factor.

The raw weight of the scope and associated gear is also a factor. Even if the load is balanced, a large load is still riding on the bearings and will perform differently than a smaller load.

The focal length is certainly a factor. If you double the focal length, you double the effect of any tracking or periodic errors in your images. Depending on the size of the errors, the pixel size, and the focal length of the scope, some errors may be invisible at shorter focal lengths. Even if not invisible, they can be reduced to the point where they are negligible by using a short enough focal length.

-Dan

### #9 Nicola

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Posted 18 January 2013 - 11:59 AM

Thanks again!
So, what if we weight the result by multiplying the inverse of the pixel size in arcseconds?
Something like this, using a KAF8300 sensor, would give:

```Scope		Weighted Corrected Inertia
FSQ85		0.103696*0.4=0.0414784
VC200L		0.220022*1.61=0.35423542
GSO8		0.270491*1.44=0.38950704
Edge HD800	0.13443*1.8=0.241974
FSQ106		0.279236*0.45=0.1256562
RH200		0.297866*0.54=0.16084764
TEC140		0.818032*0.8=0.6544256
Edge HD925	0.319085*2.11=0.67326935
AG10		0.790203*0.85=0.67167255
ODK10		0.359728*1.53=0.54903985
Edge HD1100	0.537894*2.51=1.35218287
IKHAROS10	0.753654*1.8=1.3565772
```

For better reading this I just multiply the number by 100 and round the value. I get:

```Scope		Weighted Corrected Inertia
FSQ85		4
VC200L		35
GSO8		39
Edge HD800	24
FSQ106		13
RH200		16
TEC140		65
Edge HD925	67
AG10		67
ODK10		55
Edge HD1100	135
IKHAROS10	136
```

Interesting: the TEC140 seems to have the half of the load then the EdgeHD 11.
I could think then that configuration with Edge HD11 is the maximum and the one withTEC140 is working just fine, therefore anything with a value around the one of TEC would perform the same.

Other comments?

### #10 orlyandico

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Posted 18 January 2013 - 01:46 PM

personally i would not gamble with such an ad hoc, empirical formula when planning to spend money on a mount... i'd look at the typical loads everybody else is putting on the mount, and follow that.

i.e. no more than a C9 for an EM200.

### #11 Nicola

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Posted 18 January 2013 - 02:17 PM

My problem is the opposite. I want to stick with that mount and find the largest scope for galaxies that I can put on without hassle.

### #12 orlyandico

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Posted 18 January 2013 - 09:10 PM

Nicola, I was in your position a couple months ago because I wanted to know if an EM200 was good enough for my long FL scope.

Based on Jerry's experiences on his blog, I decided not to go with the EM200.

For reference, my scope is a C9.25 which is 2350mm and 22lb bare. A C11 is 2800mm and 27lb bare. Since the C11 is right at the EM200's limit (or a bit over), the 9.25 would probably be OK - but I wasn't comfortable with such a small margin.

Personally I think your VC200L should be plenty decent for galaxies... it is much below the weight limit of the EM200 which means it would not be too sensitive to balance, etc.

### #13 Nicola

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Posted 21 January 2013 - 05:45 AM

Personally I think your VC200L should be plenty decent for galaxies... it is much below the weight limit of the EM200 which means it would not be too sensitive to balance, etc.

Sure, but you are not considering aperture fever

### #14 AstroGabe

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Posted 21 January 2013 - 10:04 PM

Another thing to consider is the moment of inertia about the RA axis in addition to the dec axis. This should account for the larger diameter folded mirror scopes like an SCT.

Gabe

### #15 Nicola

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Posted 31 January 2013 - 08:32 AM

Another thing to consider is the moment of inertia about the RA axis in addition to the dec axis. This should account for the larger diameter folded mirror scopes like an SCT.

Gabe

True, the formula I found takes into consideration the external tube diameter, in fact.

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