Barn door errors?

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maceemiller
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Barn door errors?

#1

Post by maceemiller »

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
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Re: Barn door errors?

#2

Post by maceemiller »

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"
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Re: Barn door errors?

#3

Post by OzEclipse »

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
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Re: Barn door errors?

#4

Post by maceemiller »

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 ...
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Re: Barn door errors?

#5

Post by maceemiller »

@OzEclipse ive found this page and wow.....this is a goldmine of info!

http://davetrott.com/inventions/double- ... oor-drive/
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Re: Barn door errors?

#6

Post by Star Dad »

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.
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Re: Barn door errors?

#7

Post by Thefatkitty »

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... :lol:

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

Hope this helps, and all the best,
Image
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Re: Barn door errors?

#8

Post by OzEclipse »

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:
Screen Shot 2020-09-24 at 9.31.01 am.png
Trott Type 4 Error Plot.jpg
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
Amateur astronomer since 1978
Astronomical interests : astrophotography, visual observing, nightscape photography, solar eclipse chasing
asteroidal occultations, nightscape astrophotography workshops
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Re: Barn door errors?

#9

Post by maceemiller »

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 :)
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Re: Barn door errors?

#10

Post by maceemiller »

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

I'm struggling to find any?
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Re: Barn door errors?

#11

Post by maceemiller »

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?
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Re: Barn door errors?

#12

Post by OzEclipse »

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.
10%22 Trott TYPE 4.jpg
cheers
Joe
Amateur astronomer since 1978
Astronomical interests : astrophotography, visual observing, nightscape photography, solar eclipse chasing
asteroidal occultations, nightscape astrophotography workshops
Bortle 1-2 skies, 149 E, 35 S
web site : http://joe-cali.com/
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Re: Barn door errors?

#13

Post by OzEclipse »

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 -
Type 4 error.jpg
Amateur astronomer since 1978
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asteroidal occultations, nightscape astrophotography workshops
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web site : http://joe-cali.com/
SCOPES - ATM 18" Dob, Vixen VC200L, ATM 6"f7, ED80
MOUNTS- EM-200, iEQ45, Push dobsonian with Nexus DSC
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Re: Barn door errors?

#14

Post by maceemiller »

@OzEclipse ... 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.
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Re: Barn door errors?

#15

Post by JayTee »

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

Cheers,
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Re: Barn door errors?

#16

Post by maceemiller »

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?
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Re: Barn door errors?

#17

Post by Arsene37 »

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

Image
Either β = 2.300025, or b or c have another value ?
Nevertheless, this job is greatly incentive !
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Re: Barn door errors?

#18

Post by 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?
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Re: Barn door errors?

#19

Post by OzEclipse »

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
Amateur astronomer since 1978
Astronomical interests : astrophotography, visual observing, nightscape photography, solar eclipse chasing
asteroidal occultations, nightscape astrophotography workshops
Bortle 1-2 skies, 149 E, 35 S
web site : http://joe-cali.com/
SCOPES - ATM 18" Dob, Vixen VC200L, ATM 6"f7, ED80
MOUNTS- EM-200, iEQ45, Push dobsonian with Nexus DSC
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Re: Barn door errors?

#20

Post by OzEclipse »

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

Image
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.
10%22-Type4-error.png
Apologies

Joe
Amateur astronomer since 1978
Astronomical interests : astrophotography, visual observing, nightscape photography, solar eclipse chasing
asteroidal occultations, nightscape astrophotography workshops
Bortle 1-2 skies, 149 E, 35 S
web site : http://joe-cali.com/
SCOPES - ATM 18" Dob, Vixen VC200L, ATM 6"f7, ED80
MOUNTS- EM-200, iEQ45, Push dobsonian with Nexus DSC
CAMERAS : Pentax K1, K5, K01 / VIDEO CAMS : TacosBD, Lihmsec
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