TheSkySearchers

hi all.

I've been using my barn door trap for a while now yet im having an issue with it.

I get the tripod levelled and polar aligned (green laser at Polaris) and start it up.
What I'm getting though is a few spot on images (over 20 to 30 second exposures) then an image or 2 with really elongeted stars then back to spot on images.
This happens randomly so I end up chucking as many images as I'm keeping.

What could be a possible cause for this? I use APT to create a plan and I just sit there on my chair so I don't think its movement vibration.

I'm going to build a new one soon anyway yet im interested to understand why this is happening.

Thankyou

Carl

Sorry to double post yet ive just found this on the net.....possible issue?

"The biggest challenge I found was making the nut/gear rotate freely on the threaded rod. Slight resistance during each part of the rotation meant a cyclic ‘wobble’ of the camera, causing very long, single exposures (esp. also with a long lens) to come out blurred"

Hi Carl,

What design are you using? The curved threaded rod design has problems. Have you looked at the Trott Double arm design? Much more accurate and quite simple to make. No rod bending required!
Joe

OzEclipse wrote: ↑Wed Sep 23, 2020 1:06 pm Hi Carl,

What design are you using? The curved threaded rod design has problems. Have you looked at the Trott Double arm design? Much more accurate and quite simple to make. No rod bending required!
Joe

Hi.

If im honest I've never heard of the Trott Double design but will be looking!

Mine is a curved bolt trap ...

[mention]OzEclipse[/mention] ive found this page and wow.....this is a goldmine of info!

http://davetrott.com/inventions/double- ... oor-drive/

I use a single threaded and curved brass rod. I've not had a problem with one minute exposures. If you look at the graph on davetrott.com you'll notice that his time is in minutes - and that under 10 minutes the two designs are esstianally the same for error. I have yet to be in a dark enough spot to use anything more than 3 minutes before noise takes over (Bortle 4 skies). Maybe I got lucky in bending the rod, but I've played with the gear running up and down the rod and it is as free as a bird... no snags at all.

Hi Carl, interesting post. I'm wondering if there are a few "flat spots" in your curved rod? I have the same type of tracker; what I did was bend a 3" 1/4 stainless rod to the specs, and cut the best part out of it. My skies are either cloudy or Bortle 6/7, plus it's manual (me) driven, so 20 secs is usually the most I go for an image. I can't think of why else this would happen for you, unless there is a flat spot where the curve isn't exact.

Joe is so right; the double-arm will go for much longer and is probably even simpler to make. So glad I found that out after I built my curved-rod tracker...

I'd be curious to see a pic of your tracker; this is mine:

Hope this helps, and all the best,

I remember reading the Trott article in Sky and telescope when it was first published in 1988 and all the followup correspondence. In those days I even conceived a simple modification of the double arm type 4 that ran with near zero error for 4 hrs. If you can build a design that lets the boards start at a -ve 30 degree angle, pass through parallel then extend to +30 deg. But a double board design is not complex and gives you 2 hrs of near perfect tracking. The argument that you live in light pollution and only do short subs is a false one. Living in light pollution, you want to stack many short subs. Being able to collect and stack 2 or more hours of short subs is a distinct advantage over having to rewind.

I originally designed it as a tangent arm drive that could be attached to the polar axis of my home made equatorial mount and provided 5 hrs of excellent tracking- most of the duration of a typical night's observing/photography. It was a complex construction and I never invested the time to build it.


I have an excel spread sheet that can iteratively optimise the construction parameters and plot the accumulated error.
For these type 4 construction parameters: I don't make it widely available because it isn't very slick. It requires a bit of understanding to drive it and make it work. But if you want to give me an approximate scale of the device you want to build, I can give you a set of build parameters. A set of build parameters is given above and an error plot. If built with those parameters but as atypical type 4 starting with parallel boards, the low error period will "only" be 120 mins - still very impressive.

One day I will probably build one of these just for kicks.

Regards

Joe

Very interested in the double arm barn door. Also, thinking about making my own alt-az base.

Cant seem to find any build designs for a double arm..

Been watching some presentations on YouTube about mounts and traps.....feeling quite inspired and definitely learnt some new stuff

Has anyone access to build plans for the souble arm tracker?

I'm struggling to find any?

Ok, I've been reading some stuff and watched Daves video a few times but for the life of me I cant figure out 2 things.

1, how to attach the bolt to the motor and 2, the way its mounted on the wooden block with a hinge?

I presume the motor moves on the hinge but I dont know why.

Sorry to sound dumb but I just can't wrap my brain round it.
The rest of the build seems simple, just the motor/rod/hinge thing is pickling me?

maceemiller wrote: ↑Fri Sep 25, 2020 8:44 am Ok, I've been reading some stuff and watched Daves video a few times but for the life of me I cant figure out 2 things.

1, how to attach the bolt to the motor and 2, the way its mounted on the wooden block with a hinge?

I presume the motor moves on the hinge but I dont know why.

Sorry to sound dumb but I just can't wrap my brain round it.
The rest of the build seems simple, just the motor/rod/hinge thing is pickling me?

how to attach the bolt to the motor
You will need a coupling. Either the thread needs to be machined off leaving a cylindrical machined surface to which you attach the motor coupling, or you make a motor coupling with a female thread at one end.

the way its mounted on the wooden block with a hinge?
I presume the motor moves on the hinge but I don't know why.

The error model is based on and is smaller when the screw geometry what is referred to as an isosceles drive. The drive screw, base board and hinge board must always maintain an isosceles triangular geometry. The motor is connected to a hinged board and the nut has to be on a pivoting mount as well.

How to build?
http://davetrott.com/inventions/double- ... oor-drive/

About halfway down the page is a reprint of the original 1988 S&T article that describes how to construct a type 4: -
The dimensions published in Trott's design were chosen to make the drive screw able to be driven by a 1rpm synchronous motor-common motor in those days. Stepper drivers were large expensive and had to be built from discrete components. These days, get a microstepper drive - either an easydriver, or a stepstick and drive it with an oscillator card. Both cards could almost be squeezed into a matchbox. Get a decent sized jiffy box and you can put a battery snap in there as well. You can add another high speed oscillator and a reversing switch to rapid rewind the barndoor.

I have prepared this drawing based on the parameters in my post. cheers
Joe

maceemiller wrote: ↑Fri Sep 25, 2020 8:44 am Ok, I've been reading some stuff and watched Daves video a few times but for the life of me I cant figure out 2 things.

1, how to attach the bolt to the motor and 2, the way its mounted on the wooden block with a hinge?

I presume the motor moves on the hinge but I dont know why.

Sorry to sound dumb but I just can't wrap my brain round it.
The rest of the build seems simple, just the motor/rod/hinge thing is pickling me?

I should have mentioned that the error graph I published is for the bisymmetric Trott 4 modification where the boards start at -30 degrees and travel through to +30. The design I drew below where the boards start parallel, and the error is like this & still excellent -

[mention]OzEclipse[/mention] ... gulp!

Thankyou so much for your replies yet now I realise I'm WAY out of this league.....

I cant understand half or this, never mind build it!

Looks like I'll be reverting back to the 500 rule on a static tripod.

Think my astrophotography limit has been reached.

If you are using an APS-C sized sensor then the 500 rule becomes the 300 rule.

Cheers,
JT

JayTee wrote: ↑Sun Sep 27, 2020 12:19 am If you are using an APS-C sized sensor then the 500 rule becomes the 300 rule.

Cheers,
JT

Oh...thats odd as ive been using the 500 rule with no trailing?

Hello !
Trying to put the Barndoors into mathematical graphs, I note that there is some bizarrerie in

OzEclipse wrote: ↑Wed Sep 23, 2020 11:38 pm…
…

Either β = 2.300025, or b or c have another value ?
Nevertheless, this job is greatly incentive !
Arsène

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?

maceemiller wrote: ↑Sun Sep 27, 2020 7:25 am 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

Arsene37 wrote: ↑Sun Sep 27, 2020 7:04 am Hello !
Trying to put the Barndoors into mathematical graphs, I note that there is some bizarrerie in

OzEclipse wrote: ↑Wed Sep 23, 2020 11:38 pm…
…

Either β = 2.300025, or b or c have another value ?
Nevertheless, this job is greatly incentive !
Arsène

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