Can AO8+de-rotator replaced the eq mode?

[joseph (Joseph)]

I have heard that someone use A08 + de-rotator to autoguide the star by alt-az mount. Is it really work? That will be save the time to set up the wedge. Do anyone have the experience to share with me? please post detail for me. http://www.cloudynights.com/ubbthrea...estionicon.gif
Thanks

Quote
" Although you asked the question in Stargazers Lounge, none of the people there actually answered your question. None of them used SBIG ST-4 (or LVI), one of them used PHD. Sorry to say that, no one confirmed that you can use the computerless autoguider with your alt-az mount.

Some of them used OAG (the system like the adaptive optics guider) while other suggested that you have to use a star exactly at the centre of the FOV.

Check SBIG web-site, it never claims that ST-4 works with Alt-az mount. Unlike PHD guiding which operates via ASCOM, ST-4 takes direct control of the motors and assume one is RA and the other is DEC. The software cannot make sense of something in other directions. If you use a field derotator with OAG, the relationship of the motors and the guide camera is always changing, it will be very problematic unless for very short period of time.

The adaptive optics autoguider can work behind the field de-rotator. The direction of the optical change and the guide camera remains constant. So this is what you need for you to autoguide an alt-az field de-rotated mount."

[Bassnut (Fred)]

You need an SBIG cam to operate the AO8 of course.

You could do what you describe, the AO8 orientation to the cam doesnt change, but mount bumps wouldnt work, so your polar alignment would have to be near perfect, and even then you may find exposure times limited by how long corrections were within the AO8 limit. You would also have to reset the AO8 after each exposure.

Another way might be to use an autorotator controled by automation software which adjusts mount guide bump orientation automatically.

This is all very complicated and tricky though, you would be better off with a wedge or external guide scope.