9 replies to this topic

[starcruiser]

Years ago I often read here the following rules of thumb concerning astrophotography….

 

1/2 rule - “You should not exceed more than 1/2 the rated weight capacity of your mount": So if your mount's max rating is 30 pounds, forget mounting that C11 or even the C9.25. Your true max is a C8 or a 4” Refractor.

 

2x rule - “Your mount should cost twice as much as the OTA you intend to mount on it”:  Another way of saying is that if your mount costs $1000, you should only put a $500 OTA on it -which roughly translates into an 80mm ED refractor. You got a fancy $2000 OTA? - expect to spend $4000 for a proper mount if you want good results.

 

Are these rules still valid today? Haven’t the new mounts improved to the point that the rated weight is the true max? And everything is so expensive these days, do I still need to spend 2x the OTA?

 

 

 

[Don W]

Rules? We don’t need no steenkin’ rules!

 

The first “suggestion” I only agree with partially. For imaging it’s probably not a bad idea to follow. 
 

The 2nd “suggestion” I think also applies to imaging, but many people are getting great results with inexpensive mounts

[luxo II]

There’s another:  the diameter of the worm wheels on each axis should be not less than ½ the aperture of the scope for good mechanical stiffness and guiding . 
 

this is still valid imho.

[Astrojensen]

Years ago I often read here the following rules of thumb concerning astrophotography….

 

1/2 rule - “You should not exceed more than 1/2 the rated weight capacity of your mount": So if your mount's max rating is 30 pounds, forget mounting that C11 or even the C9.25. Your true max is a C8 or a 4” Refractor.

As a rule of thumb, it isn't wrong. Much depends on the length and bulk of the telescope, though, as well as factors such as wind, etc. A very strong pier or tripod will also allow you to go closer to the max load.

 

 

 

2x rule - “Your mount should cost twice as much as the OTA you intend to mount on it”:  Another way of saying is that if your mount costs $1000, you should only put a $500 OTA on it -which roughly translates into an 80mm ED refractor. You got a fancy $2000 OTA? - expect to spend $4000 for a proper mount if you want good results.

This is just nonsense. Today, you can get small, very expensive OTAs that can be easily carried by small, cheap mounts. For example, 80mm refractors today can cost from $400 to $4000, but their mount requirements are generally the same. 

 

 

Clear skies!

Thomas, Denmark

[Astrojensen]

There’s another:  the diameter of the worm wheels on each axis should be not less than ½ the aperture of the scope for good mechanical stiffness and guiding . 
 

this is still valid imho.

This is a much more useful rule of thumb. 

 

 

Clear skies!

Thomas, Denmark

[Sacred Heart]

Years ago I often read here the following rules of thumb concerning astrophotography….

 

1/2 rule - “You should not exceed more than 1/2 the rated weight capacity of your mount": So if your mount's max rating is 30 pounds, forget mounting that C11 or even the C9.25. Your true max is a C8 or a 4” Refractor.

 

2x rule - “Your mount should cost twice as much as the OTA you intend to mount on it”:  Another way of saying is that if your mount costs $1000, you should only put a $500 OTA on it -which roughly translates into an 80mm ED refractor. You got a fancy $2000 OTA? - expect to spend $4000 for a proper mount if you want good results.

 

Are these rules still valid today? Haven’t the new mounts improved to the point that the rated weight is the true max? And everything is so expensive these days, do I still need to spend 2x the OTA?

The 1/2 rule depends on the mount used, generally the less expensive mounts, $800 - $2000. Me, what I do on a Losmandy, CEM70, class and above, I still do not go over 80% because I'm portable and for variables like wind. Strainwaves, as I approach 70% of the no counter weight spec I would put the CW shaft and CW on. This helps when slewing from east side of meridian to west side.  I also would weigh down or tie down the tripod. Strainwave mounts can handle off balance loads, the tripods cannot.

 

Joe

[luxo II]

2x rule - “Your mount should cost twice as much as the OTA you intend to mount on it”:  Another way of saying is that if your mount costs $1000, you should only put a $500 OTA on it -which roughly translates into an 80mm ED refractor. You got a fancy $2000 OTA? - expect to spend $4000 for a proper mount if you want good results.

Careful what you wish for - vendors might use it to price their gear accordingly.

 

By that rule Skywatcher should quadruple the cost of the EQ6-R mount - I have seen one carrying a 10" RC and outperform a Paramount, in both GOTO and guiding performance.

 

As for the strain wave mounts, it suggests they're overpriced and worth about half what most are currently paying.

Edited by luxo II, 24 June 2025 - 07:08 PM.

[WadeH237]

I have never been a fan of the "1/2 Rule".

 

The problem is that weight of the scope is simply not sufficient by itself to characterize the forces required to work with it.  In particular, the moment arm of the scope has a significant effect on the behavior of the mount.  To calculate the moment arm, you look at the weight multiplied by the distance from the axis squared.  The distance of the center of mass of the scope from each axis is more important than the weight.

 

Another problem is that mounts have a level of tracking accuracy that is independent of the weight of the scope.  If your scope tracks with, for example, 1 arc second accuracy, an image scale of less than 1 arc second per pixel is going to look pretty bad.  But the same mount with a scope and camera that yield 2.5 arc seconds per pixel, you'd get pretty good results, even if the second scope is heavier.

 

And finally, there is no standard at all for rating "acceptability" of a particular payload.  The advertised payload is 100% a marketing number, not an engineering number.  If two manufacturers produced identical mounts, they could very well market them with very different numbers.

 

For understanding a mount's capability pre-purchase, I would put far more stock into the reports of skilled users, than any marketing material.  These days, it's pretty easy to find reports of your exact scope on the mount you are considering.

[luxo II]

In particular, the moment arm ... has a significant effect on the behavior of the mount. 

And some mounts are pretty awful in that respect ... Losmandy in particular. Pretty machining, but lots of room for reducing the moment arm especially on the G8, which is ridiculous..

Edited by luxo II, 24 June 2025 - 09:00 PM.

[Sacred Heart]

Years ago I often read here the following rules of thumb concerning astrophotography….

 

1/2 rule - “You should not exceed more than 1/2 the rated weight capacity of your mount": So if your mount's max rating is 30 pounds, forget mounting that C11 or even the C9.25. Your true max is a C8 or a 4” Refractor.

 

2x rule - “Your mount should cost twice as much as the OTA you intend to mount on it”:  Another way of saying is that if your mount costs $1000, you should only put a $500 OTA on it -which roughly translates into an 80mm ED refractor. You got a fancy $2000 OTA? - expect to spend $4000 for a proper mount if you want good results.

 

Are these rules still valid today? Haven’t the new mounts improved to the point that the rated weight is the true max? And everything is so expensive these days, do I still need to spend 2x the OTA?

To me these are the rules I follow and why.

 

The mount / tripod is King in astronomy and not just imaging.  Payload, physical size of the telescope, focal length of telescope all affect tracking / pointing performance of the mount.  

 

Payload is just heavy.  Physical size, diameter / length, goes to having a wind sail and or moment arm.  Focal length goes to FOV, the closer an object appears sometimes the harder it is to find and if imaging - image scale. The tracking error should be less than your image scale.

 

Telescope is second, for the reasons listed above.

 

Camera / pixel size and focal length of telescope go to image scale.

 

Everything points back to the mount tripod.  This is a ground up operation.  

 

As for what the mount costs, or should I say you pay for,  that is up to you. 

 

Weight on the mount, that 50% rule, for a good imaging mount - me personally, I do not go above 80% of the rated payload. Does not matter who makes it.  Reason, I am portable so I use that 20% for variables like wind.  

 

Strainwaves are a different animal, if I am at 70% of the mounts rating before counter weights, I will use the counter weights to help with meridian flips. Also, I will weigh the tripod down or tie it down, it will be secured to the ground. I do not want it to fall over.

 

Just me,   Joe