Barn door errors?

[maceemiller]

Looking further in to my tracker design and its random errors,
I think I've come up with a potential problem that I overlooked.

The design I built was taken off the Internet, measurements and all.
I've been looking at this calculator and its sort of hit me in the face!

https://blarg.co.uk/astronomy/barn-door ... calculator

By not having the bolt and hinge at the right distance, could this explain why some exposures work and many do not?

Carl
It is more likely to be that the curved bolt is not perfectly circular so that the radius changes from correct to incorrect on different parts of the bolt.

Joe

Quite possibly! Also the flimsy tripod its on May not help.
I've a solid tripod now, the type used on building sites to set out levels for houses so will use that and build a new tracker.

Thankyou

[Arsene37]

Hi !

Hi Arsene,
Thank you for picking this error up. I had not used the spread sheet for some years. Last time I used it, I was playing around with a "construction error factor" into the spreadsheet to investigate the effect of imperfect build dimensions on accumulation error. I have changed the ratio to the ideal of 2.186.

Apologies
Joe

Many thanks for the reply ! No need of apologies, I was looking for concrete numbers to put in the equations. When I am ready, I may post the results if anyone is interested !
Clear skies !
Arsène

[OzEclipse]

[mention]Arsene37[/mention]

The bisymmetric modification I devised is produced by placing a spacer between the baseboard and the hinges so that the boards can travel between a positive and a negative angle. As long as the displacement angle remains between -30 and +30 degrees, the fantastic accuracy remains valid. I originally devised this so that I could build it as a tangent arm drive for the polar axis of an equatorial telescope. But the construction became very complex and I never proceeded. I have always been tempted to build a bisymmetric barndoor just to prove the principle.

If you live in the northern hemisphere, consider building it with the hinge to the left so that the boards open as time progresses. Otherwise, the boards can bind up when they reach the end of travel and you need to include a microswitch to cut the power as the boards close up.

Joe

[SkyHiker]

You can also use a straight screw and get perfect tracking if you use an Arduino to drive, and compensate for the trigonometric deviations digitally by programming compensation for it. The one caveat is that you need to start at a fixed position, and perhaps the travel time is more limited. Otherwise such barndoor are the simplest to build. I built two of those. The small motor was on the top board driving a screw in a T nut. It can't get any simpler, mechanically, I think.

[maceemiller]

Think I've found the problem.....

I've built a new tracker which im very happy with and used the same cogs from from my old one.
One thing I failed to notice before is when I drilled through the large cog to insert a nut I was slightly off centre....

After watching the cogs turn I see that the large cog moves towards the small cog on every rotation, causing the small drive gear to become under slight pressure and you can hear the motors sound change.....like its trying to work harder.

This actually seems like the culprit for my problem

[SkyHiker]

Think I've found the problem.....

I've built a new tracker which im very happy with and used the same cogs from from my old one.
One thing I failed to notice before is when I drilled through the large cog to insert a nut I was slightly off centre....

After watching the cogs turn I see that the large cog moves towards the small cog on every rotation, causing the small drive gear to become under slight pressure and you can hear the motors sound change.....like its trying to work harder.

This actually seems like the culprit for my problem

Mechanical problems can be avoided by keeping the design simple. With control by an Arduino we can compensate for the geometric imperfection and avoid the need for bent rods, double sliders etcetera.

There are very light weight stepper motors with built in gears, so all you need to get perfect tracking is the motor, a screw and a T nut. I attached the motor to a piece of threaded rod with an epoxy putty blob - simply press it in there. Here's the idea, it worked well for me. Some day I may build one with a longer arm more a la Astrotrac, that's a pretty cool barndoor design.

[maceemiller]

[mention]SkyHiker[/mention]

I love the simple build....perfect, yet the Ardingly side of it scares and confuses me.....

[maceemiller]

Hi all.

Well, my "upgraded" barn door is no better than my original.
I've tried and tried and unfortunately I'm so frustrated that I'm going to call it a day.

[JayTee]

I hear you and completely understand.

This same frustration led me in a different direction documented by this post.
viewtopic.php?f=24&t=3129

Cheers,
JT

[ARock]

[mention]maceemiller[/mention] If I recall correctly, you have an EQ2 mount. I don't remember if you have a RA drive with it. Have you tried just mounting the camera and lens on the EQ2 mount (without the scope) with the RA drive? With a short focal length lens, thats a simpler option to try.

[maceemiller]

@maceemiller If I recall correctly, you have an EQ2 mount. I don't remember if you have a RA drive with it. Have you tried just mounting the camera and lens on the EQ2 mount (without the scope) with the RA drive? With a short focal length lens, thats a simpler option to try.

Hi.

It is an EQ2 mount and I havnt a drive anymore. I have the tracker mounted above the ALT knob.
I don't use my telescope, just my DSLR.

One thing I noticed from last nights attempt was there wasn't so much star trailing but what I can only describe as morse code type stars.....a star, then the same star millimetres to the left connected by a thin line....

I'm guessing this is vibration somewhere? Its not me moving as I'm sat and APT controls my camera shots.

Could be the bent rod, or the gears....I have no idea atm and seriously thinking of not bothering anymore.

I may try a straight rod tracker

[maceemiller]

I hear you and completely understand.

This same frustration led me in a different direction documented by this post.
viewtopic.php?f=24&t=3129

Cheers,
JT

Hi.

Just had a quick scan of your topic and looks very interesting.

In the post above ive tried to explain what images im seeing and possible problems

[ARock]

What lens are you using with the DSLR?

A $40 Celestron RA drive should get you about 1 min shots with a 55mm lens. If you have a DC motor with a speed controller you could use that too instead.

And if you want to get into Stepper/Arduino projects, the simplest one would be a stepper connected to the RA drive of the EQ mount. The Arduino code is about 10-20 lines and you can probably find it online and change 1 number (the speed the motor should run for your mount) in it. You do need a multimeter to set the driver voltage.

The camera on the EQ mount will be quite stable if you can get the electronics working.

[maceemiller]

What lens are you using with the DSLR?

A $40 Celestron RA drive should get you about 1 min shots with a 55mm lens. If you have a DC motor with a speed controller you could use that too instead.

And if you want to get into Stepper/Arduino projects, the simplest one would be a stepper connected to the RA drive of the EQ mount. The Arduino code is about 10-20 lines and you can probably find it online and change 1 number (the speed the motor should run for your mount) in it. You do need a multimeter to set the driver voltage.

The camera on the EQ mount will be quite stable if you can get the electronics working.

I've a 80-200mm canon lens. I have a 3rpm motor with a 20 cog gear on it driving a 60 cog gear on the rod.
Just been out to try again and still no good.

I'm that deflated I will stop trying as its gone from bad to worse.

[maceemiller]

Hi all.

Please excuse my "im giving up" last reply......its pathetic.
I was very frustrated at the time but now, couple of days after I'm back on track

I'm going to build a single straight rod tracker.
I understand that tangent error is a problem and I've seen the cam or whatever its called that you make to let the nut slide over to keep the rate the same yet has anyone got solid advice on how to calculate this?

I'll be using an M6 threaded rod.

Thankyou in advance and again, my apologies for my last post.

Carl

[yobbo89]

i'm always watching this thread, a barndoor and a dob platform tracker for me has alway been on the mind well before i got a eq mount ,maybe sometime in the future i will make a project

[JayTee]

Rob,

[mention]SkyHiker[/mention] (Henk) has already done this. You should probably pm him.

JT

[maceemiller]

Hi all

So, heres something else I've found out which may be helpful to anyone else trying to build a barn door.

I bought a cheap 3RPM synchronous motor online so I thought id put a 20 toothed gear on that and the nut on a 60 toothed gear (60÷20=3......3÷3=1), bringing the speed down to 1RPM.....perfect!

......except.....

I'm currently building a straight rod tracker so I thought id look at the motor properly and what I found is its more like 3.75RPM!

I had a broken Celestron motor that I took the circuit board out of and using that I now have the speed as near to 1RPM as I can get using the potentiometer. Its still pretty much cog perfect (I put a tiny bit of paint on one of the gears teeth and marked the bottom board with a line so I have visual markers) at 4 minutes.

Just be aware then that while a motor may state a certain RPM, definitely test it!!