g__day
21-10-2007, 11:42 AM
Ever had one of those nights when everything was going so well, until...
I was trying to perfect my polar alignment on the pier mounted Vixen Atlux. Of course my house (two story) is to the East, blocking view lower than 30 degrees, and my observatory roof doesn't let me see straight up (whoopsie on my first design) - so that's just to make things more challenging.
I started at 9pm and finished at 4:30am. To cut a long story short had I finished at 12:30am I would have been a much happier boy with alot less work to do!
My starting point was a mount that is say within 1/2 a degree of perfectly level and I believe its aligned within 1/2 a degree off the SCP (according to MaxPoint). My goal was to do a 400 second unguided shot on a 9.25" SCT and get close to perfectly round stars - no trails whatsoever.
So at the beginning of the run I note that my Canon 400D when it snaps the moon as a 6" x 4" shot well the Moon almost completely fills the length-wise frame. S I estimate that 6" or 150 mm equates to 30 minutes of arc. My star trails for a 400 second (6 and 2/3 minute) shot (chosen randomly) started as about 5-6 mm. By just after midnight star trails where down to I estimate just under 1 mm - that would have been a perfect place to stop and start again fresh.
Instead I pushed it too far and too clumsily and managed after much effort to undo most of the good work I'd achieved! I gave up when I realised that hovering helicopter was actually Venus - time to go to bed!
My beginners mistake was of course to correct for movement in both Alt and Az (measured as movement from the Canon's central viewfinder focus point). Oh yes that's exactly what I did!
And of course as I started losing alignment I kept digging the hole deeper! I started with Archenar when it cleared the house - until it was almost vertical - not bad alignment. Then I continued with Sirius and managed to undo most of my good work.
Here's the question - do anyone else do daft things like this or is it only me? When your short circuit starts to fail do you tough it out or continue to dope it through until its clear your an idiot?
Well - I might try again tonight (the right way). Does anyone have a shot of what a long duration, unguided shot of a star on a long focal length tube looks like? Or can you mention what perfect alignment should look like?
I presume it will show no trials but maybe slightly blooby or football like stars if PEC isn't enabled. With PEC turned on I expect the stars will look closer to textbook points of light?
Here's four shots showing 1) size of the Moon in a frame and the rest are 400 second, unguided shots of Achernar 2) starting alignment 2) almost right alignment and 3) heading backwards oops!
PS
Given that 5 mm drift in a frame where the full 150 mm is about 30 arc minutes (width of the moon) it follows that 5 mm / 150 mm equals 1 / 30 of 30 arc minutes. So 1 mm of drift in my frame is 1 arc minute of drift. If I get 5 arc minutes drift in a 400 second shot this equates to 1 arc minute every 80 seconds - how far off the SCP am I - to any budding mathematicians?
PPS
Anyone local ever want to visit and help me really get this thing spot on - I'd be eternally grateful!
I was trying to perfect my polar alignment on the pier mounted Vixen Atlux. Of course my house (two story) is to the East, blocking view lower than 30 degrees, and my observatory roof doesn't let me see straight up (whoopsie on my first design) - so that's just to make things more challenging.
I started at 9pm and finished at 4:30am. To cut a long story short had I finished at 12:30am I would have been a much happier boy with alot less work to do!
My starting point was a mount that is say within 1/2 a degree of perfectly level and I believe its aligned within 1/2 a degree off the SCP (according to MaxPoint). My goal was to do a 400 second unguided shot on a 9.25" SCT and get close to perfectly round stars - no trails whatsoever.
So at the beginning of the run I note that my Canon 400D when it snaps the moon as a 6" x 4" shot well the Moon almost completely fills the length-wise frame. S I estimate that 6" or 150 mm equates to 30 minutes of arc. My star trails for a 400 second (6 and 2/3 minute) shot (chosen randomly) started as about 5-6 mm. By just after midnight star trails where down to I estimate just under 1 mm - that would have been a perfect place to stop and start again fresh.
Instead I pushed it too far and too clumsily and managed after much effort to undo most of the good work I'd achieved! I gave up when I realised that hovering helicopter was actually Venus - time to go to bed!
My beginners mistake was of course to correct for movement in both Alt and Az (measured as movement from the Canon's central viewfinder focus point). Oh yes that's exactly what I did!
And of course as I started losing alignment I kept digging the hole deeper! I started with Archenar when it cleared the house - until it was almost vertical - not bad alignment. Then I continued with Sirius and managed to undo most of my good work.
Here's the question - do anyone else do daft things like this or is it only me? When your short circuit starts to fail do you tough it out or continue to dope it through until its clear your an idiot?
Well - I might try again tonight (the right way). Does anyone have a shot of what a long duration, unguided shot of a star on a long focal length tube looks like? Or can you mention what perfect alignment should look like?
I presume it will show no trials but maybe slightly blooby or football like stars if PEC isn't enabled. With PEC turned on I expect the stars will look closer to textbook points of light?
Here's four shots showing 1) size of the Moon in a frame and the rest are 400 second, unguided shots of Achernar 2) starting alignment 2) almost right alignment and 3) heading backwards oops!
PS
Given that 5 mm drift in a frame where the full 150 mm is about 30 arc minutes (width of the moon) it follows that 5 mm / 150 mm equals 1 / 30 of 30 arc minutes. So 1 mm of drift in my frame is 1 arc minute of drift. If I get 5 arc minutes drift in a 400 second shot this equates to 1 arc minute every 80 seconds - how far off the SCP am I - to any budding mathematicians?
PPS
Anyone local ever want to visit and help me really get this thing spot on - I'd be eternally grateful!