DIY mounts

[Guest peepshow]

It was this article and the rather  attractive error figures of type 3 that persuaded me to go

through the barn door to the stars

 

http://education.jlab.org/tracker/index.html

Edited March 15, 2014 by peepshow

[Werisit]

Aargh. I didn't want to do that!

<iframe src="https://www.flickr.com/photos/119738074@N05/13494894655/in/photostream/player/" width="500" height="375" frameborder="0" allowfullscreen webkitallowfullscreen mozallowfullscreen oallowfullscreen msallowfullscreen></iframe>

[+RonC]

Flickr has changed, I got caught out earlier too!

You have to press the 'Bobbin' shaped icon before copying!

 

Ron

Edited March 29, 2014 by Ron Clarke

[Werisit]

Thanks Ron. That was supposed to be.

 

Arrgh. Didn't want to do that!

 

 

Turned the ends of the screw shaft and got a nice fit in the pillow block bearings. Cut the thread holding the die in the lathe chuck, great, nice and square, then thought I'd save time by reversing the die off. BIG mistake! So I've ordered a new shaft and will start again when I've finished kicking myself! 

[Werisit]

Yep, thanks Ron. I'd never have worked that out.

 

So after that disaster I thought I should get back on the horse and crack on with something else. So here is the base plate with one side machined, still in the lathe.

 

 

 

 

And these are the bits I've done so far partially assembled. I still need to cut the recess for the lower bearing in the base plate.

 

 

I'm still thinking about how to mount the camera on top of this.

[Guest peepshow]

Nice to see ballraces in new design, Alan.

 

  Don't forget that if you ever have two of them on a RA or DEC shaft to preload them. 

I  lock one on my shaft then  force the other one along the shaft and then lock it to the shaft.

 

In this way the balls in the ballrace are hard against the race walls so nil axial/radial movement.

 

Nice lathe you have there......you should see my clapt out iron works.

[Werisit]

Hi Richard,

Yep, the plan is to load the bearings up a bit against on another. They are a bit big really but they were free  . My original plan was for a compact version but I realised that the larger it is the more accurate it will be.

How are you getting on with your new Barn Door?

[Guest peepshow]

Here's a few pics of it.  Trouble is I need clear skies to set PA very accurately and get the two adjustments correct.

These last two require many 30 second time exposures to get it right.

 

First pic shows the laser PA now adjusted to within 1 minute of arc.

  It is mounted straight onto the main camera platform shaft (12mm silver steel running in pre loaded bearings) so is precisely in line with it.  Ali tube housing the laser has adjustments to get laser beam excactly 45 mins of arc off centre.

 

Next pic the gimble mounted drive nut all in ball races. It is essentric mounted for slight adjustment to the drive dimensions.  End stop relay is blue thingy on right

 

 

 

Last pic shows the top adjusment to get PA correct.  The bottom end stands on a single

ball bearing.  Four bolts at 90 degree to adjust for N/S and E/W on PA

Edited March 30, 2014 by peepshow

[Werisit]

Hi Richard.

It looks like you are pretty much there with the build. Let's hope for clear skies so you can get aligned and start imaging. Looking forward to seeing some results. You are a good way ahead of me. I've not started my control box yet and still have a fair way to go with the mechanics. 

I had to refer back to the diagrams I downloaded off the net (below) and I think I understand what I'm looking at in your photo's. 

 

 

[Guest peepshow]

Yes that's the one Alan.  I have shimmed the 2.000" dimension and that is fixed permanently. 

The 12.928 and the 13.519  are fully adjustable in my tracker.

It is these two dims that are the key to good and prolonged tracking.

So far a single 15 minute exposure of stars shows no hinge wobble at all.

[Werisit]

Fifteen minutes is impressive Richard. What focal length are you using? You must have a good dark sky. A 15 min exposure from my back garden would produce a white frame, even with a CLS clip light pollution filter.

[Guest peepshow]

Well normally 15 minutes exposure here too would result in a whitish image.  But this was a hinge wobble test

only  so the camera was set at a low 100 ISA and at f5.6  The lens was a 200mm. 

 

There was some star trailing but no wobble.

Trailing because  the tracker is not yet adjusted due to the weather............

 

You may have seen my other thread where I am getting a deep red filter to try and image a few stars in

the daytime when the skies seem a little better at times.

Edited April 1, 2014 by peepshow

[Werisit]

Daylight stars would be interesting. I'd like to try some daytime observing sometime. I did manage to spot Venus some weeks back but only because it was very close to a crescent moon. 

[Werisit]

16 minutes at 200mm, 2 mins at 800mm. I'd be more than happy with that!

[Guest peepshow]

My next idea......I have taped a small  'push to on' push switch onto my binoculars.

This controls my laser PA.  So I find Polaris in the bins,  (when these b***** couds roll by  ),

and push the switch momentarily to see where the beam end lies. 

Make adjustment and try again.

 

In this way the laser is only on for very short periods and , of course, is never

pointing elsewhere where aircraft might be.

[Werisit]

I thought I'd post a sketch of my project to give a better idea of how it works. 

Plus my second attempt at the screw which thankfully I didn't mess up this time. At 40tpi x 3/8 the thread is quite delicate. I've mounted it in the pillow block bearings and attached the flexible coupling to show the complete assembly. Bearings and coupling were from     http://www.motionco.co.uk/     they do all sorts of interesting stuff for these sorts of project.

 

 

 

[stash]

Jesus that's a fine thread  thats bl***y brillant work - I am very impressed 

[Guest peepshow]

That's a mighty fine piece of engineering that you are putting together there , Alan, and thanks for the link.

 

Thinking about it all.........

 

That camera pulley will never have to travel more than 180 degrees, if that, from E to W horizons. 

So if it were me I would firmly anchor the drive cable to it  at the half way point  to stop any possible slip............... 

 

...........I don't know your sizes, but If your RA pulley was, say, 12 " in dia,

 then even less than 2 thou radial error represents an angular change of 1 minute of arc on the RA,

which would be too great for AP with longer focus lenses.  

 

Even a second of arc error on RA would only need a movement of approx 1/3 of 1/10000"  on a 12 "dia  pulley. 

 

But will the clouds ever clear for you try it all out and for me to do more testing?

[Guest peepshow]

Last night I took a very long 30 minute exposure using a Tamron 70-300mm zoom lens set at 200mm  and f4.5 aimed at the Celestial Equator again.  ISO 100  to prevent LP.

 

This focal length and exposure time are quite a severe test for a poor ol' barn door.

 

You can see some star trailing but I hope to clear this with further fine adjustments to the barn door measurements.

The PA must be getting pretty good too to get this far, although I still want to have another go at PA again sometime.

 

Next test, 1 HOUR exposure ?   That'll make the barn door creak on its hinges.

 

Edited April 9, 2014 by peepshow

[Werisit]

Thanks for the kind comments guys.

Yes I will anchor the cable where the two ends meet on the pulley to prevent slipping. All the calculations and bits are at work but from memory the pulley is about 3 1/2" diameter. The screw will allow about 5 hours run time which should be plenty for my needs. I decided to keep it fairly compact as, although I do a bit from the back garden  I need be able to take it to darker sites and get away from the Milton Keynes light pollution. If I can get 5 mins with a 500mm lens I'll be very happy. From your calculations it looks like I might need to polish the surface of the pulley! I suppose I have the option of making some larger pulley discs if I want more accuracy. The base, hub and screw were the tricky bits and wouldn't need to be changed.

 

Now a 30 minute exposure! That's just showing off! Next an hour??? So it's working very well then. The AP folks in MK are using guiding and doing around 10 minute exposures. 30 minutes tracked is some achievement! How long can the barn door run for in total?

Weather. Humph. We have had some clear skies down here but very unpredictable. The forecasts generally say cloudy but it seems to be worth going out to check. Trouble is, it makes it impossible to arrange any group gatherings.

[Guest peepshow]

Yes Alan, that 3 1/2 inch pulley will need to be very accurate as my error calcs can be multiplies by 3.5 when I

assumed a 12 inch dia. 

Your 5 minutes with a 500mm lens would be good.  I hope you can achieve that eventually.

 

My barn door will run for about 90 minutes.

 

 

I hour?  ..........I spoke too soon.

 

Last night I tried 4 x 15 minute shots and they all had a weird angled trail.  

 I always take a single non tracking shot to act as a  benchmark in order to see the

exact angle the stars would move in on my images if trailing at all.

 

In all my 4 shots last night the trailing was way out from that angle.  About 30 degrees out.

 

Something is slowly and constantly moving or slipping during those exposures.

  It might be my mount, but I haven't changed anything since my good images.

 

 So the 1 hour shot will have to wait until I've sorted all  this out.

 

DIY, AP is a hard task master, but ain't it just blooming fun? 

[Werisit]

You're right there Richard. It's as much getting round all the problems these things throw at us as the final nice picture that we can show around.

I think the trails will be a different angle when tracking compared to stationary. It seems as though the speed and PA have changed. 

If I stack my images without dark frames any hot pixels come out as diagonal streaks. They also wiggle a bit due to the periodic error. If your lines are not wiggly then you are getting close. 

[Guest peepshow]

I maybe misunderstanding you, Alan, but if one takes an exposure, without any tracking, then the streak made by stars across the final  image will be at exactly the same angle in the image  when subsequently tracking either too fast or too slow, 

PROVIDING,  the PA is correct.

 

So tracking fast or slow cannot change any angle in the image as the camera is on the same pivoting system throughout.

 

In my case the PA is very good.  If it wasn't then I would not have been able to take 30 minute exposures.

 

As I said, my weird 'streak' is some 30-45 degrees (!!) off the star track angle, so it certainly is not PA.

It's some extraneous movement which I am investigating.

 

BTW, it is only over a distance of about 6 pixels on the Canon 450d with 200mm lens so I am in the

thou or so movement territory. 

 

So just a few thou movement in the camera mounting per hour would cause this to happen.

[Doc]

How have you got your camera mounted?

 

Is it via a ball mount, if so could it be moving very slightly 30-45° to the PA

 

Most likely completely wrong here 

[Guest peepshow]

Mick,thanks for suggestion but I have certainly checked the obvious things. 

 

It's the un obvious that's gonna be the real corker.

 

Camera firmly mounted at present in a sturdy pan and tilt head.  All bolted down and screwed up tight.

Over an hour the very tiny movement causing this is at a constant speed in same direction too.

 

Maybe the sensor in the 450d is slightly slipping? 

I do have an old 350 so might try that if all else fails.

 

Maybe the ground around here is slightly heaving?     Ooooops.