A barn door tracker - astrophotography on the cheap.

[Ian Robinson]

OK , you don't have a GEM or Fork which is RA driven , but you still want to try your hand at astrophotography and you've done the star trails thing ....

Not everyone has the readies to buy a nice GEM , so what are the options :

1) build your own GEM or fork and get hold of a secondhand RA wormwheel set and a motor to drive it (hard to find useful secondhand RA drive systems)

2) build a barn door tracker , might be easier to get parts.

Here's how to do it : http://www.astronomyboy.com/barndoor/

[johnno]

Hi Ian,

Many thanks for that interesting link.

Looks like a good Project.

Regards.
John

[Geoff45]

Here's a pic of one I built. It worked quite well---see the pic of Orion. Slight star trailing, but that was probably poor polar alignment, rather than any fault of the BD. However, after a few goes you soon get the urge for deeper, higher res images.

[Ian Robinson]

Just refound this " A Better Barn Door" at http://www.beaconhilltelescopes.cwc.net/ , you need to drill down to find the link.

Get a load of the claim tracking accuracy for the properly polar aligned 2 arm barn door tracker http://www.astunit.com/tonkinsastro/atm/projects/sc-graph.gif. (< 10 arcsecs error in 60mins tracking .... and no PEC u-processor !!!).

What a great project perhaps for high school science class maybe ?

When I have a chance - this is one project I want to have a go at.

[Bassnut]

< 10 arcsecs error in 60mins tracking .... and no PEC u-processor !!

60 mins? and 10 arcsecs error tracking on a barn door?, nuh, dont believe it. Excellent for planetary tho.

[Ian Robinson]

Yeh .... I was surprized by that too, but the mathematics (trig calculations) used to get that look correct ....

Got to get hold of a copy of Sky and Telescope, Apr 1989, to find out more though , if I can't find a copy in my boxes full of old S&T and Astronomy Mags which have I have stored in my garage.

[leon]

Although mine is not a Barn Door tracker the principle is the same, well sort of, I built mine using a curved bolt arrangement with a formula of thread pitch, per inch dia of the curved rod and a one rev per minute motor, plus some other specs added on, it also works a treat.

Leon :thumbsup:

[Ian Robinson]

That looks pretty nifty Leon.

(Based on the Palomar Telescope ??).

[seeker372011]

that's a very nifty bit of kit there leon

[leon]

Thanks guys, yes it works exceptionally well, but really good for short focal length wide fields, where one can hide a few sins of error :lol:

Leon :thumbsup:

[thorsdad]

Nice looking tracker leon:).

Do you have any pics you've taken with it:shrug:.

Looking at making something like the double barn door myself. Don't really fancy paying a grand or two on a sturdy GEM mount:doh:.

Cheers,:thumbsup:

Rob.

[leon]

Rob I haven't used this in ages since I advanced in this hobby with the great set up I have now, I will have a look to see if I can round up a couple of images, but, hold your breath. :lol:

Leon :thumbsup:

[bojan]

This is theoretical performance for double arm Scotch mount, and it does not take into account gears (that means, PE is not accounted for), only arms.
But, since mounts like that are used for wide angle photos only, PE does not really matter.

[MrB]

I worked out a very simple modification to the very simple tangent barndoor tracker to double its exposure-time for same tracking error.
Eg, a standard tangent tracker has a similar amount of error as a 60min two-arm after only 10-12 mins of exposure.
With my mod, 20-25mins for same error as 60min two-arm exposure.
And much easier to build.
Got a photo here somewhere but can't find it.

Edit: Still no photo, but an edited version of the graph Ian posted might give an idea of what I did.
Using the graph for figures, the double arm has ~8 arc-sec(+- combined) error at 60min exposure, and ~3 arc sec error for exposures <50min.
The modded tangent with ~8 arcsec error will give approx 25min exposure and at around 3arc sec error(not shown on graph) will give approx 20min exposure.

Edit2: Found the photo. Can you spot the mod? Ignore the rough Alt-Az mount.
This barndoor was made about 5 or 6 years ago(Before I had a decent set of tools!), the mod seems so obvious and so simple(one piece of wood!) so I'm surprised I haven't seen someone else do similar. :shrug:

[middy]

It is a simple and effective mod. Well done. I achieve the same effect on my barndoor by running my motor slightly faster than required. This way the mount starts off too fast until halfway when it is moving at the correct angular rate. Then at the end of the movement it is running too slow.

[stephenb]

Ian, thanks for posting this and to all the replies, ta :thumbsup:. Like Rob, I've been contemplating a barn door project as an excuse to hibernate in the workshop over Winter. I've been meaning to make one for years.

I assume a simple single door would suffice for widefield tasks? I understand the theory of increasing it's accuracy with double doors etc. but as a bit of a starter project, no reason why a single wouldn't suffice?

I've had this bookmarked for some time: http://www.tucsonastronomy.org/barndoor.htm only because the calculations are already worked out for me :lol:

Any thoughts on a cheap but workable motor/gearbox?

Stephen

*Edit - On second thoughts, a double doesn't looks so complicated....