astrophotography and alt-azimuth mounts?

[bcsman]

here is an idea ive had rattling around my head for a couple of days,might have already been mentioned or even been invented and already utilised by thousands of imagers.

here goes.

i was thinking about some kind of motorised eyepiece holder that counteracts the image rotation in scopes on alt-azimuth mounts,is there already something like this on the market or is it even possible to engineer something like this?

anyway its just an idea i have,might be stupid or even a eureks moment of mine

cheers

[riklaunim]

Field derotators are available from for example Meade. Usually it's easier to user a wedge for the alt/az mount.

[Zakalwe]

Optec do a field rotator (clicky)...the Pyxis. It retails at $925...dunno how much value it has over just buying an eq mount?

[ollypenrice]

Well, I think it is the professional norm and means you can have a very stable mount and no house sized counterweight...

It is intriguing that they have not caught on in amateur imaging. How would you guide them because the rotator, the az and the alt would all need corrections, no?

Olly

[FraserClarke]

Well, I think it is the professional norm and means you can have a very stable mount and no house sized counterweight...

It is intriguing that they have not caught on in amateur imaging. How would you guide them because the rotator, the az and the alt would all need corrections, no?

Olly

The rotator tends to run in open-loop (i.e. you calculate what the rotation rate needs to be, and set it running). If you get the rotation rate wrong, your guide star will drag the image about in X and Y, blurring your image. However, the accuracy you need for the rotator is much less than you need for a telescope drive, so it's relatively easy in practice.

If you have two guide stars, you can simultaneous solve for position and rotation, and then you can also 'guide the rotator' as it were...

I think the reason they are not popular for amateur set-ups is that for an Alt-Az telescope you need to drive three axes (alt, az and rotation), whereas for a (well set up) equatorial, you only need to drive one. Probably not a massive challenge these days, but 3x as many bits to go wrong...

Even on equatorials though, people use rotators to change the angle of the camera on the sky -- very useful particularly if you have an on-axis guider with a small field of view, and you can rotate it about to find a guide star.

[Dangerous-Dave]

Well, I think it is the professional norm and means you can have a very stable mount and no house sized counterweight...

It is intriguing that they have not caught on in amateur imaging. How would you guide them because the rotator, the az and the alt would all need corrections, no?

Olly

Cost. A field rotator is more or less a fixed cost regardless of the size of telescope, whereas the cost of building an equatorial mount varies from very cheap at EQ1 level to astronomical (groan) for a professional scope. Imagine the volume metal that must have gone into the horseshoe mount of the Hale 200".

IIRC the 16" Meade LX comes on an alt-az mount with field de-rotator. You should get one for the gite.