Barndoor Images - Need Help!

[davewaldo]

Hi everyone!

I finally got a chance to test my curved rod barndoor tracker I made this week. I had heaps of fun last night making 2-3 minutes exposures for the first time! Overall I'm happy with the results however I have two issues I can see.

1. As you can see for the shots I've attached, the stars are drifting downwards (this image is of the CRUX area and is near paralel to the horizon and looking SE). So I'm assuming this is an alignment issue however I'm not sure how I need to change it. The method I used to setup was as follows:

I used a compass to orientate the barndoor south, corrected for magnetic deviation and made sure the base was level. This would ensure the altitude angle was correct for my latitude. I then made several 2-3 min exposures.

This was the best one. I'm guessing to fix this alignment I need to angle more to the left or right.

Issue #2 is a bit more confusing...

Not only are the stars drifting but they also appear as wiggly lines. This tells me my tracking speed is not constant. However I have tried both motorised and manual tracking with similar results. So I'm thinking that maybe as i turn the nut on the threaded rod it is not advancing at a constant speed. As though it is warped or something. I think this because if I do a 2 minute exposure I will get two peak wiggles, 3 min exposure 3 peak wiggles etc.

What do you guys think?

Cheers

Dave.

[huckabuck]

hey dave, what lenses did you use?

[davewaldo]

Hi,

The setup I used was a canon 20D @ ISO 800. Canon 50mm 1.8 @ f2.8 for 3 mins. In camera NR, no darks, flats or multiple exposures.

I'm not convinced I've got my focus just right yet but that will come....

Here are some shots of my barndoor. To motorize it I found an old car CD player which had 12V motors and gear as well as the roller you can see which actually drives the CD which is glued to the nut on the rod. I then have a control box which controls the speed of the motor. It works quite well (in theory).

Cheers

Dave.

[huckabuck]

hey dave,
i'm no expert but manually guided wide-field photos are among my favorite pastimes. i would suggest that you drop the f2.8 down one or two steps. i generally get better results around f4 to f 5.6 for a 50-55mm lense. especially with an exposure over 1 minute. also i think you are correct in assuming your alignment is off a bit. and probably one or two things are happening to cause the jumping around problem. either the bolt is warped as you say or there are some burrs om the threads that need to be filed down. but i suspect your motor is pulsing just a bit or it is introducing vibrations into the whole set-up. try a manual guideing session a couple times. and check out my eq (barndoor) tracker. maybe get a few pointers.

http://www.iceinspace.com.au/index.php?id=63,373,0,0,1,0