Questions about improving unguided imaging.

[mg]

Hi.
I have been taking unguided subs of 100 seconds recently.
I have had a throw out rate of less than 50%, ie: over half the pics have round stars and are keepers.

I'm fairly happy but would like to increase the good sub strike rate and I have a few questions regarding this.


If the good subs have nice round stars, can I assume polar alignment isn't contributing to the bad subs? I figure bad PA would ruin all the subs.
Does that then mean my movement errors are probably caused by periodic error / sloppy focuser / wind etc etc, rather than polar alignment?
What have you folks found to be common causes of bad unguided subs and where should I be looking to eliminate movement?
I'd like to keep the exposure time as long as I can and keep tweaking for better tracking rather than shorten the subs.

Mount is AZ-EQ6 GT
612mm focal length refractor
Canon 600D

Thanks.

[rogerco]

I think you would need to look at when the bad ones were. If they were all at the end you could consider focusor slip, or temperature movement. Is poor shots being cause by dew build up on lens? Is the camera adapter tight and not moving over time. From my limited experience with a similar setup these are some of the issues I had.

How are you triggering the camera? are you using mirror lock up with a pause to dampen mirror vibration?
Good luck

[multiweb]

100s is not bad at all at 600mm FL for any budget mount. I don't know the worm cycle for an EQ6 but it is unlikely to be Polar alignment. May be PE but more likely wind or stiction or other mechanical issues. To do long unguided subs you need a high end mount with small PE. You're better off starting to look into auto-guiding to avoid heading into a world of pain. That's the easiest and cheapest way out .

[RobF]

Yep, those mounts have worm periods of 6-8 minutes. You might get one or two 100 sec exposures in a flat part of the worm, but there will likely be a quarter of the worm cycle that heads off with significant periodic error, than the next 1/4 taken up heading back the other way.

Both my EQ mounts have periods where a 2-3 min exposure unguided is possible, but no way you get away with it for the rest of the cycle! ;)

Marc's right - best to go to "dark side" of autoguiding :thumbsup:
You may have to purchase the odd bits and pieces here and there as you get further into astrophotography though :D

[mg]

Thanks for the help. Will definitely investigate all your suggestions.

Periodic error makes the most sense because I seem to sometimes get pairs of subs with the first one trailing in one direction followed by the next in the other direction.

Will have a go at using the PEC function in the mount, but I can definitely see money being exchanged for autoguiding equipment in the near future.

Thanks for the quick answers!

[Lee]

I'm sure I'd throw more than that if I tried unguided with my G-11, and I put an ovision worm in it.... Guiding is really too simple now to not do it....