Mesu mount. Maths help needed for tracking

[Barv]

Hi

Thanks for your help Guys with my last post. Thanks to you all I have now been able to get into the Servocat settings. I now need to INCREASE the tracking speed. For all you maths genius's out there here is one for you:

My mount is losing 0.68 degrees (or 40.8 arc mins) in 4 minutes 43 seconds. The current gear ratios setting in the Servocat are 6000. I know I need to reduce this figure but am unsure how much? Is there enough here to go on or do all you clever chaps need any more information?

Kind regards

Harvey

[ronin]

I suspect the question is if it is losing 0.68 degrees (or 40.8 arc mins) in 4 minutes 43 seconds, what should it be slewing over in 4 minutes 43 seconds.

That way I think you can work out what the rate is and what the rate should be.

The information says it is losing 0.68 degrees but is that 0.68 in a movement of 5 degrees or 10 degrees or what.

Need to know what the rate is either now or what it should be.

Need to know either the rate now or what the rate should be.

[Joe12345]

I've done some sums and I think the gear ratio should be reduced by 10.122, although that was done in 3 mins so don't blame me if it's wrong! If the gear ratio is easy to change then you should give it a try, although if you need to buy stuff or spend lots of time on it then probably leave it to someone else. I don't know anything about eq mouts gear ratios, I was just lured in by the word maths!

[Joe12345]

Sorry, made a mistake!

I've done some more calculations (probably wrong) and found that you should reduce the gear ratio to 5886.66666667. Please don't do anything until someone else confirms my answer!

Sorry if this turns out to be wrong again!

[Scooot]

I don't know how to work this out but am curious. In 4 mins 43 seconds the earth would spin about 1.17°. So if the tracking is short by 0.68° doesn't it mean the mount has only turned about half a degree. In other words it's less than half the correct speed. I'm sure I'm completely wrong but curious to know why?

[steppenwolf]

I don't know how to work this out but am curious. In 4 mins 43 seconds the earth would spin about 1.17°. So if the tracking is short by 0.68° doesn't it mean the mount has only turned about half a degree. In other words it's less than half the correct speed. I'm sure I'm completely wrong but curious to know why?

That's about the way I see this too! I make correct tracking 900 arcseconds in 1 minute so 4245 arcseconds in 4m 43s. The error is 2448 arcseconds! Unless I have lost the plot, the gear ratio should be 3460.070671.

However, maths never was my strong point ....

[Scooot]

That's about the way I see this too! I make correct tracking 900 arcseconds in 1 minute so 4245 arcseconds in 4m 43s. The error is 2448 arcseconds! Unless I have lost the plot, the gear ratio should be 3460.070671.

However, maths never was my strong point ....

Thanks for that, it doesn't seem likely to me that the mount would be delivered that far out so I still doubt my logic

if the fov of the eyepiece was say 1° and it took 4mins to cross it whilst tracking, would it be correct to say the error was 1° over 4 minutes I wonder?

[steppenwolf]

Thanks for that, it doesn't seem likely to me that the mount would be delivered that far out so I still doubt my logic

I can't believe for one moment that the mount was supplied that far out either but based on the data supplied, unless my maths is out by several magnitudes, the error does seem extreme!

[ollypenrice]

Ask Lucas?

Olly

[steppenwolf]

Ask Lucas?

A very approachable man in my experience!

[michael8554]

Barv, how did you measure this figure of 0.68 degrees (or 40.8 arc mins) in 4 minutes 43 seconds ?

Michael

[Barv]

Barv, how did you measure this figure of 0.68 degrees (or 40.8 arc mins) in 4 minutes 43 seconds ?

Michael

Michael.

I used a 38mm Skywatcher Panaview eyepiece, calculated the true FOV at the scopes focal length of 3905mm which is 0.68 degrees, Then timed how long it took Altair to cross the width of the eyepiece.

I am in discussions with Lucas Mesu now so will keep you all posted on progress. I purchased the mount as an ex-demo mount and I am fairly certain that the settings have been played with. As everyone mentions, Lucas is very helpful.

Regards

Harvey

[Pompey Monkey]

Michael.

I used a 38mm Skywatcher Panaview eyepiece, calculated the true FOV at the scopes focal length of 3905mm which is 0.68 degrees, Then timed how long it took Altair to cross the width of the eyepiece.

I am in discussions with Lucas Mesu now so will keep you all posted on progress. I purchased the mount as an ex-demo mount and I am fairly certain that the settings have been played with. As everyone mentions, Lucas is very helpful.

Regards

Harvey

Although I can't help you solve the gearing problems (sorry about that), I can suggest two possible sources of error in your measurement:

1. Assuming that the calculated FOV of the eyepiece is correct, it is possible that you timed Altair moving across a chord of the FOV rather than across the whole diameter (probably a minor error).

2. Altair is at 8^o 52' declination, meaning that it's angular movement across your FOV is actually cos(8^o 52') times the sidereal rate. OK, just over a 1% error, but if you are attempting absolute accuracy, then this is another factor to consider. 

But I agree with the others, a discrepancy of 0.7 degrees in less than 5 minutes is a gross error! Good luck sorting it out.

[michael8554]

Another point Scooot made, that it's tracking at about half the correct rate - is there a half-sidereal speed setting in play here?

Michael

[Steve Ward]

I know this might sound silly , but is the mount polar-aligned ... ?

Surely this might be a more rational explanation for a target to drift across a FOV than a Mesu mount tracking incorrectly ... ?

And does the mount have alternative tracking rates such as Solar / Lunar that might have been inadvertently triggered.

Just a thought ... 

[Barv]

Hi Fellas and thanks for all of your help.

The problem has now been sorted thanks to Lucas Mesu. (Luckily he is extremely responsive and helpful)!

I purchased the mount as an ex demo unit from Teleskop Service. The problem with it was that both the azimuth encoder resolution and the gear ratio had been changed by someone prior to my acquiring the mount. Really frustrating! The drive rate is now very good but I will look at getting it more accurate when I can put my ccd camera on. Now I can concentrate on my polar alignment etc etc.

Thanks once again Guys

Harvey

[JamesF]

I assume you're using your C14 Edge for this.  In which case, unless you have measured the focal length with the eyepiece in question then it may not be what you think it is.  SCTs only have the stated focal length with the primary mirror in one location and whilst it's possible that the Edge OTAs correct for that I don't believe it to be the case.

Personally I think I'd use a camera to verify the focal length, then use it to calculate drift using the image scale.

James

[Barv]

I assume you're using your C14 Edge for this.  In which case, unless you have measured the focal length with the eyepiece in question then it may not be what you think it is.  SCTs only have the stated focal length with the primary mirror in one location and whilst it's possible that the Edge OTAs correct for that I don't believe it to be the case.

Personally I think I'd use a camera to verify the focal length, then use it to calculate drift using the image scale.

whilst I am sure you are right James can you explain the physics behind this. Surely if the primary mirror is moved up or down the tube the focus distance changes directly in relation to this. Doesn't it just mean that the eyepiece just focuses in a different place or am I missing something?

Regards Harvey

[JamesF]

whilst I am sure you are right James can you explain the physics behind this. Surely if the primary mirror is moved up or down the tube the focus distance changes directly in relation to this. Doesn't it just mean that the eyepiece just focuses in a different place or am I missing something?

You're missing something

What you suggest would apply to a newt, for example, because it actually only has one optical component -- the primary.  Move that up the tube 20mm and the focal plane moves out of the focuser 20mm.  The secondary doesn't count as an optical component because it is flat.

In the case of an SCT however the secondary is not flat.  It has a hyperbolic surface.  Where there are two or more such optical components in the optical train the effective focal length is actually a function of the focal lengths of each component and the distances between them.  Change the distance between them and the focal plane does indeed move, but the effective focal length changes too.  The change in focal length can be quite significant.  Though a Mak uses a different combination of forms for its mirrors it also behaves the same way.

The upshot of this is that there's only one position for the primary where it is separated from the secondary by the correct distance to give the focal length stated by the manufacturer.  Either side of that position the focal length will be greater or less than the stated figure.  Because we don't generally have enough information to work out what the focal length is (how do you measure the separation of the mirrors when the OTA is in use, for example?) it's often easiest to capture an image of something you know the size of and work backwards to the focal length.

You can demonstrate this if you, say, put a camera on the visual back, get the focus correct and take an image of the Moon.  Then put a 50mm spacer between the camera and visual back, refocus and take another image.  The focal length C14 might make this a bit awkward, so perhaps just sticking to a specific crater would work.  There should be a noticeable difference in scale between the two images.  I've done this with my 127 Mak and whilst I can comfortably fit the entire lunar disc on my DSLR when it is directly on the visual back, with a 50mm extension the image won't even come close to fitting the frame.

James

[Barv]

You're missing something

What you suggest would apply to a newt, for example, because it actually only has one optical component -- the primary.  Move that up the tube 20mm and the focal plane moves out of the focuser 20mm.  The secondary doesn't count as an optical component because it is flat.

In the case of an SCT however the secondary is not flat.  It has a hyperbolic surface.  Where there are two or more such optical components in the optical train the effective focal length is actually a function of the focal lengths of each component and the distances between them.  Change the distance between them and the focal plane does indeed move, but the effective focal length changes too.  The change in focal length can be quite significant.  Though a Mak uses a different combination of forms for its mirrors it also behaves the same way.

The upshot of this is that there's only one position for the primary where it is separated from the secondary by the correct distance to give the focal length stated by the manufacturer.  Either side of that position the focal length will be greater or less than the stated figure.  Because we don't generally have enough information to work out what the focal length is (how do you measure the separation of the mirrors when the OTA is in use, for example?) it's often easiest to capture an image of something you know the size of and work backwards to the focal length.

You can demonstrate this if you, say, put a camera on the visual back, get the focus correct and take an image of the Moon.  Then put a 50mm spacer between the camera and visual back, refocus and take another image.  The focal length C14 might make this a bit awkward, so perhaps just sticking to a specific crater would work.  There should be a noticeable difference in scale between the two images.  I've done this with my 127 Mak and whilst I can comfortably fit the entire lunar disc on my DSLR when it is directly on the visual back, with a 50mm extension the image won't even come close to fitting the frame.

James

Now I see. Thanks so much for the explanation James. I did know that the secondary was hyperbolic and thinking about it this makes sense. The corrector plate also has minimal effects on focal length and are there are also lenses in the baffle tubes on the Edge HD, (although I think you are correct and that they are only there to produce the flat imaging field).

Kind regards

Harvey