Finally a feasible way to asses mount performance "in house"?

[vlaiv]

I was not sure if I should post this in mount section or here.

It has to do with assessing mount performance - how well it tracks, without the need for wasting clear skies or always having doubt if seeing is responsible for measured mount roughness (although with multi star guiding this is now much less of issue).

I personally came up with it for the need of testing how smooth 3d printed reduction gears work combined with stepper motor - to get the idea of positional accuracy, backlash and any sort of periodic or non periodic error in tracking.

Initial idea was to simply strap the laser pointer on top of axis and monitor what the laser point projected on white surface far away from motor is doing (maybe use millimeter grid paper or something like that or record with camera and analyze footage).

However, as you will soon see - this does not really fit into "in house" criteria.

One degree of arc is roughly 1 meter at 57.3 meters of distance. This further means that one minute of arc is 16.667 mm at that distance and of course, one arc second is 1/60th of that - which is ~0.3mm. Not really something you can easily measure - at least when light dot projected on the paper is in game.

Let alone the fact that "in house" distances need to be at least x10 smaller and everything is scaled down x10 - so movement of one arc second would be 0.03mm - now that is 30 microns movement at 6 meters.

I played with all sorts of different ideas in my head of how could we amplify everything. Maybe using mirrors to create larger distance by bouncing light ray several times off the mirrors - but every imperfection in mirror surface would be amplified as well - so we would need optical flats of high quality and way to align them properly - too complex for simple "in house" device.

If only there was a way that we could amplify light angles easily

Well, that was the question I asked myself just before light bulb moment (sometimes its worth just asking the right kind of question).

We have and often use (at least when weather allows) devices that are great at amplifying light angles

So here is what I came up with (still need to test it, but I think it will work as I expect).

Shine a laser thru the front objective of a telescope with the eyepiece at the other end in straight thru configuration (or even use a prism if we want 90 degree bend for some reason). Telescope should do what telescope does - it should amplify incoming light ray of collimated light (it needs to be focused at infinity - but we can easily tweak the focus to get smallest dot on the projection screen - no need to "prefocus" on stars or anything like that).

Depending on focal length of telescope and eyepiece used - we can have significant angle amplification. Most of things will happen near optical axis so we don't need wide angle eyepieces - in fact we want as low distortion as possible. We can change eyepieces change magnification of the effect so we can measure different behavior - for backlash and positional accuracy we can use x200 for example to get down to arc second resolution, but for tracking we need arc minute resolution as sidereal is ~15 arc second per second - so we might want to have enough of screen to capture few minutes of tracking and that would mount to say 200x15 = 3000 arc seconds - so we need less magnification for that.

In any case - if we have 6 meters distance to projection screen and use x200 - angle of laser beam won't be 1 arc second but 200 arc seconds instead, so deflection won't be 0.03mm but x200 larger - or 6mm - now that is easily measurable with millimeter grid paper.

However at that magnification we would need 3000 x 6mm = 18 meters of screen for few minutes of tracking - clearly not good idea, but we can drop magnification for that purpose to say x20 or even less - depending on what we have at disposal (maybe even use finder that is x7 magnification for this purpose).

We can even create setup that amplifies just x2 - x3 by combining two eyepieces - one would have "telescope" role and other would be regular eyepiece. 32mm plossl and 12mm plossl (or 17mm one, I'm just listing ones I have on me )  could give interesting combinations.

In fact - If I pair 9-27 zoom with 32mm plossl - I can get range of magnifications for this.

Anyway, all that is left to do is to try it out (I might just do that now as I have laser and finder on the desk with me).

What do you think about the concept?

[vlaiv]

And of course - it works

Tested with green laser pointer and SkyWatcher 50x8 finder (or whatever its magnification is x7, x8 or x9 or somewhere in between).

[vlaiv]

Just occurred to me that I don't need to use less magnification - I can just project at closer distance

 

[Adreneline]

Interesting read Vlad.

I am having problems with erratic RA tracking of my CEM25-EC mount and rigged up one of these to try to access tracking accuracy:

It worked reasonable well for short duration runs.

Sadly the mount is still defective

Adrian

[vlaiv]

1 minute ago, Adreneline said:

I am having problems with erratic RA tracking of my CEM25-EC mount and rigged up one of these to try to access tracking accuracy:

That is also neat idea.

How did you rig everything up? Did you use some sort of lever and if so - how long was it and how did you ensure against flex?

[Adreneline]

Just now, vlaiv said:

That is also neat idea.

How did you rig everything up? Did you use some sort of lever and if so - how long was it and how did you ensure against flex?

Thank you.

It was difficult making it rigid but I was able to show that the RA axis would either stop completely or it would stutter during movement.

Here are a couple of videos of it in use - both short and for a lot of the time nothing appears to be happening!

 

 

 

I measured (best I could) the radial displacement  and so was able to calculate the actual movement and compare with expected for a given duration.

Adrian

[vlaiv]

@Adreneline

Above idea of yours is actually much better way to asses tracking / periodic error as it gives better resolution for that sort of purpose.

Let's say we have 50cm aluminum bar that is as wide as regular vixen dovetail (about 44mm) and say 10mm high or something like that. It should not flex much if at all.

At 50cm we have x12 less resolution than at 6 meters, and from above calculation 1 arc second is 0.03mm at 6 meters, so it will be 0.03 / 12 = 0.0025 at 50cm. That is about 1/4 of what instrument can read - but let's say we read every second - so we have movement of 15 arc seconds - so that is 0.0025 * 15 = 0.0375

and that should easily be visible at the gauge.

Since we have 10mm of total motion - that is about 266s - not bad at all.

 

[Adreneline]

1 minute ago, vlaiv said:

not bad at all

It worked well enough to show there was a problem but I still have no idea whether the problem is in the encoder, the RA board, the Main board or the handset and short of sending a blank cheque to iOptron I am no further forward.

In terms of measuring mount tracking performance in either axis I am sure it could work very well providing you can (a) set up a rigid mounting system for both mount and gauge, and (b) measure accurately the radial displacement from the RA axis centre.

For long term tracking errors with an EC type mount I used a much cruder approach - I set the mount off tracking (let's say) IC1396 and after 'n' hours I re-centred on the chosen target and looked to see how many degrees the mount moved - which varied from almost nothing to 30-40 degrees!!

Adrian

[vlaiv]

14 minutes ago, Adreneline said:

For long term tracking errors with an EC type mount I used a much cruder approach - I set the mount off tracking (let's say) IC1396 and after 'n' hours I re-centred on the chosen target and looked to see how many degrees the mount moved - which varied from almost nothing to 30-40 degrees!!

That is really down to time keeping rather than anything else and it is sign of poor implementation in microcontroller.

You can use two approaches - you can say:

perform n ticks in next m seconds - where ticks are stepper micro steps or whatever, or you can say

it is now exactly that much time - I should be on tick m but my counter says that I'm n - I need to perform m-n ticks to move mount forward.

First approach accumulates error over time while second does not and that small error can turn into 30-40 degrees which is really half an hour two hours of time difference which is fraction of micro second over many many steps taken.

In any case - not down to mechanical issues nor is important for mount performance for imaging (especially if guided). I'm more concerned with errors that are "real time" - say how much mount deviates peak to peak over few minutes - as those values impact on how well can mount be guided out.

 

Edited November 21, 2023 by vlaiv
divide with two or multiply with two? :D

[Adreneline]

1 minute ago, vlaiv said:

I'm more concerned with errors that are "real time" - say how much mount deviates peak to peak over few minutes - as those values impact on how well can mount be guided out.

I do not guide my EC mount - over the past four years it has performed faultlessly but it suddenly started tracking badly in RA and I was just trying to find out what was going on. I don't wish to derail your thread with my mount problems - suffice to say something is wrong in the electronics.

Good luck with your continued experiments.

[vlaiv]

Just now, Adreneline said:

I do not guide my EC mount - over the past four years it has performed faultlessly but it suddenly started tracking badly in RA and I was just trying to find out what was going on. I don't wish to derail your thread with my mount problems - suffice to say something is wrong in the electronics.

Good luck with your continued experiments.

Good point, mount with encoders should have no such issues and yep, above clearly showed the issue with yours.