Barn door tracker

[Ags]

I recently built a polar scope for my NexStar 4SE mount (bit of wood with RDF ) but sadly it proved that my little old 4SE mount doesn't track very well in RA. After relatively precise polar alignment, I just got relatively precise east-west star trails.

So I would like to try building a barn door tracker. Is it realistic to aim for exposures of several minutes at 100mm with something like Gary Seronik's design?

http://www.garyseronik.com/?q=node/52

I can get about 1 minute at 100mm with my 4SE so if I build something I want to improve on that significantly.

Where could I buy parts? I am in Holland but I will buy in the UK as this project is beyond my Dutch language skills. I don't know what a 'blind well nut' is in English, letalone translating it to dutch!

[Ags]

I didn't realise this US site delivers internationally:

https://sdp-si.com/eStore/Catalog

[Ags]

I bought the bits today :-)

I spent 9 euros at the hardware store, for all the parts needed for a manually driven barn door... :-)

Then I went to the camera shop and spent 100 euros on a Vanguard SBH-100 ball head... :-(

I am going to try a manual barn door first so I can experiment with the design. It's a straight-bolt type barn door so it would suffer from tangent error, but I have written an android app which is basically a variable speed clock, counteracting tangent error.

Once I'm happy with the mechanical design, I will convert my barn door clock app to a stepper motor controller. It's probably hubris, but I have dreams of wiring in a finder guider and feeding that info to the stepper motor too!

[happy-kat]

You might like this thread.

http://stargazerslounge.com/topic/210166-barn-door-tracker/

[Ags]

Thanks for the link... To be honest my tangent error correction software was fun to write but hardly necessary. The error from manual driving of the bolt will swamp tangent error unless you track for about an hour. It will be more relevant when I hook up the stepper motor.

Where the software will be useful will be that I can adjust the reference clock to arbitrary times, like 59 seconds. Given my carpentry skills, this is essential :-)

[Ags]

The deed is done! I think it looks pretty good :-) ... Since taking the photos I realised the barn door is upside down, and I had forgotten to attach the RDF - both fixed now!

[happy-kat]

Looks like you get to try it tonight as well :-)

Have you put a support wedge under it for your latitude?

Your camera mount looks much better substantial than mine, in case mine proves too weedy do you have a link to the one you got please?

[Ags]

It is an AZ3. It has slo-mo controls so it should make polar aligning a breeze.

[happy-kat]

I asked the question on my own thread as I did not understand why some barn doors had a support piece.

http://stargazerslounge.com/topic/207904-busy-reading-but-missing-the-explanation/?fromsearch=1

[Ags]

Polar aligning was not a breeze - I need to tighten something in the AZ3... it kept jumping about instead of panning smoothly into alignment.

[Ags]

I tested the mount with a 50/1.8 lens. I got very nearly round stars at 1 minute after a bit of trying - round enough to fix in post. What is blocking me from using the 50 at home is the LP gradients, so this means my mount will work for camping trips at least with the 50mm. Next time I'll push it up to 2 mins or try out the 100mm.

I found it rather difficult pointing the camera with the ball head, never used one before.

Thinking about it, I might get rid of the bit of wood that gives clearance for the Alt slow-motion rod. I think there is too much leverage on a rather thin bit of wood. it's probably the cause of the imperfect stars.

[happy-kat]

I think I twigged it, you are using the control on the az3 mount to turn the barn door as opposed to having a handle and rod on the barn door directly?

[Ags]

Not quite... The AZ3 is used to tilt the barn door hinge to point to the pole. Then I twiddle the bolt (you can see my handle for the bolt in the first image) and this rotates the top piece of wood around the hinge. As the hinge is aligned with the pole, the camera is now tracking the sky!.

[happy-kat]

Ta :-) was not sure if that was your turn handle as it was small so thought you were doing it differently.

[Ags]

Yes, I might need to make the handle a bit longer. But the first change will be to attach it to the mount more firmly.

[Ags]

Thinking about it, I might get rid of the bit of wood that gives clearance for the Alt slow-motion rod. I think there is too much leverage on a rather thin bit of wood. it's probably the cause of the imperfect stars.

I fixed it I think... I added a couple of protruding screws to bear down on another part of the AZ3. It is a lot more solid now... No wiggle at all.

[happy-kat]

Look forward to seeing your resulting images

[Ags]

I did some more test shots tonight - 50mm @ 1, 2, 3, and 5 minutes.

I still get a slight elongation of the stars... I think it must be a combination of tracking error on my part and the slight wiggle that must be introduced by touching the bolt - the base board is much more firm, but the RA board must wiggle slightly whenever I turn the bolt. There is also the slight error introduced by the fact I turn the bolt in 1/8 increments (1 turn is a quarter degree, which is 1/100th of my FOV, which means 1 turn is 40 pixels - so an eight of a turn is 5 pixels - that alone is enough to account for the elongated stars...  )

The error does not get larger with longer exposures, so my maths and carpentry are correct and maybe i can squeeze more from the system with better technique / a lighter touch.

But even so, the stars are round enough to fix with a bit of GIMP magic.

Here are my five minute stars:

[AstroAdam]

Interesting to see how you fare with this.  I tried to build one years ago, but never had any luck myself.  Got frustrated and gave up in the end....

[JamesF]

That looks like an excellent little project, Agnes.

Do you now know what a blind well nut is, too?

James

[Ags]

Now i need to plan some trips to dark skies so i can use the tracker for real. Stamina is the main obstacle - hopefully I could stretch to six or eight 5 minute subs at a dark site. My barn door tracker clock app has a Rest function so it can alert you after a two-minute rest period is up (when you want to take a break, you just dial two turns ahead quickly and it beeps when you have to carry on nudging).

By the way, I had fun doing proper polar alignment tonight. i.e. aligning on Polaris and inching a couple of moons back to Kochab. I have never been so polar aligned in my entire life!

Earlier in the day I aligned my polar RDF with my hinge using a technique of my own invention. I placed the tracker open 90 degrees on straight-edge table so that the hinge is in line with the edge of the table. I stuck a bit of card on the far edge of the table so that it stuck up, and then slid the tracker forward and marked the location  of the RDF on the card. Then I slid the tracker back to the other end of the table (keeping it folded around the edge so it was aligned) and then adjusted the red dot to the mark on the card.

Do you now know what a blind well nut is, too?

No, in the end I used a thingamajiggetybob instead. No idea what it is called: a cylindrical thing with wood-screw thread on the outside and bolt-thread on the inside. A dream part, exactly what I wanted, but then i did not have a drill bit large enough for it so I had to file out the hole, then getting the thing screwed in straight was a real challenge...

[JamesF]

I think a well nut is a "deep" nut often in a flanged casing, that is designed to fit into a hole in (say) a piece of wood so that the flange stops it passing all the way through.

A blind one will presumably not have a hole all the way through ("blind" being an engineering term for a hole that doesn't go all the way through a piece of material), so it would look something like a top hat with a thread up the inside.

James

[happy-kat]

Interesting.

So you do not continuously turn the handle but do it in a series of short turns accepting some pixel trailing.

[Ags]

Actually, I am following the advice of a link you posted :

http://www.philharrington.net/scotch.htm

...which advises 90 degrees every 15 seconds, which I doubled because:

1. He probably means full-frame so my effective focal length is 80mm

2. I think he means film and DSLRs have a higher resolution

3. I did the math and 10 pixel trails seemed unacceptable

I will try a "continuous drive" experiment, but that sounds difficult and also there is the disadvantage that the mount is continually being touched.Also, I don't know if I can turn to an accuracy much higher than 1/8.

EDIT: Anyway, I am having a long-term senior moment here... I actually turn the bolt 16 times a minute.   Obviously I am having trouble counting past eight! So my turning is introducing a 2.5 pixel error.

[Mr TamiyaCowboy]

small 'ish DC motor , a small gearbox ( wallmart / maplins ).

a couple electrical transistors and likes a potentimeter and a small 9v battery

knock up a cheap mans RA drive motor ( sold n astro shops for EQ-1 mounts).

couple the drives output shaft to your barn door rod. turn on and fine tune your poteniometer 

to adjust speed motor turns and the speed your tracker runs at.