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Old 06-03-2018, 03:20 PM
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Stonius (Markus)
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Anyone factor tracking correction into a Barn Door mount?

Hi there,

I notice there is a lot of talk about removing tangential error in these mounts by mechanical means, using curved rods or type 4 systems with 3 arms, etc.

My understanding is that the accuracy required in manufacturing is the undoing of the gains they provide. EG, the curved rod system requires a gear with the attendant PE and backlash, and aparrently the three arm system requires manufacturing tolerances smaller than a CNC can provide according to one site that studied the tolerances and resultant errors from that design.

The simplest design is a motor that drives a straight rod with a bendy connector, surely? I know that people have done straight rods with mathematical compensation for the Tan error, but has anyone then done an analysis of the inevitable tracking error in something like PhD? If there is a constant repeatable error, then surely the curve can be used to compensate for the error (again in software)?

Is anyone doing this?

Best

Markus
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Old 06-03-2018, 07:35 PM
sharkbite
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My only questions would be...

Do you need to?

At what focal length and exposure time would tracking correction become necessary?
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Old 06-03-2018, 08:05 PM
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Merlin66 (Ken)
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The Trott double arm comes pretty close.....

Bear in mind these are designed primarily for relatively short focus lenses...
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Old 07-03-2018, 07:22 AM
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Quote:
Originally Posted by sharkbite View Post
My only questions would be...

Do you need to?

At what focal length and exposure time would tracking correction become necessary?
Good point. I guess now we're getting into signal to noise ratios and sky glow questions that I'm still figuring the anwers to.

Basically I'd be using a 5D Mk2 or 4 at a dark sky site. Available focal lengths range from 24-400mm. Neither camera is modded but thats not out of the question in future.

I'm mostly visual, but Id like to have some subs ticking away in the background to play with while I observe other objects.

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Markus
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Old 07-03-2018, 09:19 AM
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kinda -

You could of course write some algorithm to change the speed of the
motor to correct for the change in angular velocity.

I actually have no clue myself,
but it seems to me if the tracker has an angular velocity
of 15 degrees per hour (or 1 degree every 4 minutes)
and one uses a wide angle like 24mm or so,
you might not need tracking correction for exposures of a few minutes....
Would there be a noticeable difference in tracking between a
straight and bent rod?

Anybody out there know more about this?

The need for correction would go up with the focal length,
and length of exposure - but at what point does it become critical?
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Old 07-03-2018, 11:20 AM
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Hi Markus. I had to redo a camera tracking mount I built to something a bit more sturdy. I came up with this after borrowing some ideas. It's a peg and slot drive set up which pushes a contact surface with a gradual curve. This then turns the axis pointing to the SCP
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As you can see from the diagram, (I must stress this is an example only), the curved surface meets up with the ball bearing.
13.4mm of travel from the drive system will equal 13.4mm or 10 degrees of circumference. It's not perfect and I only have about 15 degrees on my set up. It's a trial and error in progress. hope this helps. Cheers!
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Old 07-03-2018, 01:06 PM
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Wow, astro-nutt! I commend you on your engineering skill! that is unfortunately where I lack, which is why I was hoping to compensate with software. I don't trust myself to biuld something that complicated without introducing a ton of errors into the bargain!

Nice work! What sort of accuracy have you managed to achieve with that rig?

Markus
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Old 07-03-2018, 02:11 PM
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Hi Markus. Thanks for the kind words. I haven't done a field test on this set up with camera's attached, but I have tuned it to 1 degree per 4 minutes via voltage control. it'll have first light this Sat at the ASV Messier Night. Fingers Crossed!
I'm not tech savvy but would it be possible to gradually slow the drive speed over time to compensate? Say, reduce the drive rpm over a time period, eg after 5 sec, slow down 5 rpm, 10 sec slow 7 rpm, after 15 sec slow 10 rpm. Just as an example. Hope this helps. Cheers!
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Old 07-03-2018, 03:09 PM
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Quote:
Originally Posted by astro_nutt View Post
... it'll have first light this Sat at the ASV Messier Night. Fingers Crossed!
Dang! Wish I could make it. I'll likely be up earlier in the week but I can't make the party itself.

Quote:
Originally Posted by astro_nutt View Post
I'm not tech savvy but would it be possible to gradually slow the drive speed over time to compensate?
Yes, that's the intention, but I'd be hoping to measure the error after it's theoretically perfect and incorporate that error curve into correcting whatever mistakes I may have made in construction. here won't be anything I can do about slop, but repeatable errors can surely be compensated out of existance. But as another smart cookie said on here; how long do I want the subs to be?
Well, as long as possible - I don't want to be restricted by the mount. It would be nice to shoot at 400mm if I wanted (assuming a cracking good Polar alignment).

Cheers

Markus
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Old 07-03-2018, 08:03 PM
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By all means mess with it as a simple DIY experiment, but its really only suitable for wide-angle shots with short lenses, and its the sort of thing I'd make with my son to learn from, in his teens.

Simple geometry - the linear error at the focal plane equals the linear error of whatever pushes the barn door, multiplied by the ratio of the lens focal length divided by the length of the barn door. Lets say I'm using a 15mm lens, and the barn door is 15cm long, so the ratio is 0.1. Also lets suppose the camera sensor pixel spacing is 3 microns (typical for APSC or m4/3).

Also suppose we would like to keep errors in the image below, say 3 pixels for nice star images without trails or blobs. 3 pixels is about 10 microns.

This means the tracking mechanism must be accurate to 10 x 10 = 100 microns, or 0.1mm. Not impossible for a homemade mechanism made by someone with a lot of skill, but highly improbable relying on a bent metal strip guesstimated by eyeball. To that add whatever errors are in the screw thread that drives it.

Now suppose you want to put a 300mm telephoto lens on. The ratio of lens/barn door is now 2, twenty times greater than previous. It means that for the same tolerance in the image of 3 pixels, the accuracy of the tracking mechanism must be 20 times better, 5 microns. This is way beyond what can be done without a machine shop and for that kind of precision you'd find it easier to buy an equatorial mount.

There's an electronic issue, too, which will be familiar to anyone who has dabbled in building a DIY drive. If you are relying on a simple resistor-capacitor network and CMOS oscillator (eg a 7555) to govern the frequency of the signal to the motor, you will find this is not particularly stable and random variations occur - bad enough to be visible rather like "periodic errors" in gears except that its not periodic, and can't be predicted - and really annoying.

The only real solution that really works is to use timing regulated by a quartz crystal clock, wth the frequency divided down to whatever you need one way or another (a microcontroller is a good way). Which is what is in most telescope control handsets.

Last edited by Wavytone; 07-03-2018 at 08:40 PM.
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Old 08-03-2018, 06:55 AM
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In Dave Trott's original article in February 1988 Sky & Telescope, p213, he shows the comparative drive error between the single and double arm barndoor designs.
For the single arm after 40 mins, the error is roughly 40 arc sec whereas for the double arm it's about 5 arc sec. I think this is a very good result.

IMHO I think polar error in set up would probably be the greatest potential error risk.
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Old 08-03-2018, 09:17 AM
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Quote:
Originally Posted by Merlin66 View Post
In Dave Trott's original article in February 1988 Sky & Telescope, p213, he shows the comparative drive error between the single and double arm barndoor designs.
For the single arm after 40 mins, the error is roughly 40 arc sec whereas for the double arm it's about 5 arc sec. I think this is a very good result.

IMHO I think polar error in set up would probably be the greatest potential error risk.
I think the polar error is a big issue too, but not insurmountable.

While I don't dispute the theoretical accuracy of the Trott mount for tracking, what I'm not sure about is the ability to construct one within the tolerances required to achieve that level of accuracy.

It was after I read this post on Cloudy Nights that I began to wonder. Essentially the post outlines an analysis of how an error of 2.5mm in certain dimensions will introduce enough error to make it equivalent to a basic type 1 two arm barn door mount.

On the other hand, how much tolerance does a type 1 have if you were 2.5mm out? No-one analysed that. I'm pretty sure that I could get it within 1mm, which is still better than a type 1, I guess.
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Old 08-03-2018, 10:55 AM
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There are a couple of articles on the Trott double arms showing 4 different designs. The Trott Type 4 has much smaller errors 2" over 2 hrs. The errors in Trott's article are based on starting with the arms parallel and pushing apart. If you space the boards and start with a negative angle, the accurate tracking time is doubled.

The tolerances for certain components are very important. Most important, the pivoting nut and drive thread must both pass at right angles through the pivoting axis. This could be given to a commercial machining company to make for you. Other dimensions are less critical eg hinge spacings, if a little less than perfect, can be compensated by a variable motor speed controller which as already mentioned needs to be accurate.

Polar alignment is especially important because you can't make dec corrections for this type of drive without significant modifications. A good quality polar finder scope would be essential.

Joe
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Old 08-03-2018, 08:16 PM
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Here are some photos of my Trott barn-door mount I made back in the 90's - used it in France and Canada for a short period.
From memory it worked OK.
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