Max exposure time with Nexstar ?

[ianB]

Hi,
I will soon be using a webcam to do some basic astrophotography with my nexstar SE 6
and i was wondering what will be the max exposure time that i will be limited to as i will NOT be using a wedge.
( i intend to start with the moon first )

Thanks.

[Amaranthus]

After 20 sec you start to get noticeable field rotation on the SE alt-az mount

The moon needs a fraction of a second. If you are doing planetary cam work, then alt-az tracking is not really an issue, and you can take minutes of video.

[astro744]

See http://www.astropix.com/GADC/SAMPLE7/SAMPLE7.HTM

Note exposure time changes with declination of object for a given focal length and ISO.

[ianB]

Thank you both, i will check out that link now.
P.S when u say the moon needs a fraction of a second, what do u mean? To move and spoil the image or to capture the image ?
Sorry if the answer is obvious but i am totally new to this.

[astro744]

The exposure for the moon varies depending on the phase.

First quarter at f10 and ISO 200 use 1/125sec, 1/1250sec and 1/500sec.

If you are using a digital camera and not film then you have nothing to lose by trying multiple exposures. Also transfer the images to your PC during takes to check for focus.

Use the mirror lock up feature of your camera to reduce vibration and use a cable release (careful not to tug on the cable release when pressing the button). My Pentax only has mirror lock up if I set a 2 or 10 sec. delay which is good and because of the delay I could get away without a cable release as 10 sec is enough for vibrations to subside.

I do not have live view (and am in search for a right angle magnifying prism). If you have live view you can check for focus with it although checking on the PC gives you a bigger image.

Try different ISO to see what it does to grain/noise. You may be able to use 1/1000 and 1/2000 sec exposures at higher ISO. Do not use anything less than 1/125 and even this is bordering on showing image movement.
'
If you have video on your camera use it and stack frames in Registax or other stacking software. I do not have video in my DSLR and for that I use a separate video camera with a much smaller field of view.

[ianB]

Thank you.