# Would this work ?

## Recommended Posts

Imagine you have an alt-az mount with goto.

Not much good for astrophotography though due to field rotation

If you were to put an eyepiece in which rotates at the sidereal rate, would this not remove field rotation when doing astrophotography ? :scratch:

Just hypothesising of course....

Maybe someone could bodge up a battery powered 2" to 1.25" adapter which does this...

##### Share on other sites

You can buy a field derotator and Meade used to sell one although i'm not sure if they still do? but expensive.

Jeff.

##### Share on other sites

The rate that the camera would have to rotate will vary according to the declination which will be one reason why they are expensive.

Any way you CAN do astrophotography with an alt-az mount. The following was taken through my alt-az Nextstar 9.25. You are limited to how long an exposure you can take. The following was 18 x 30s exposures with an unmodified 40D.

##### Share on other sites

Nice picture!

I was just daydreaming on the train, trying to come up with a way to make my millions.. How silly of me to think it would be that simple..

Still pondering......

Wayne M

##### Share on other sites

http://www.stargazing.net/yizen/fieldrotation.html

This link gives you all the gen you'd need to design the rate of field rotation into your scope; depends on your latitude, the declination and hour angle of the object ( that's simpifying the formulae given - by dividing by zero ( the sum total of my knowledge!))

To Quote:

From vector analysis we have r_hat . E_hat = || r_hat || || E_hat || cos[G] = cos[G], where G is the angle between the two unit vectors from the RA-Dec and alt-azimuth frame fields. We want to find out what the rate of change of this angle G is when our mount tracks in RA, so we now differentiate both sides with respect to R. (d/dR) (r_hat . E_hat) = -Sin [G] dG/dR. By using the relation sin^2 + cos^2 = 1 again, we therefore get an expression for dG/dR. But we already have the expressions for r_hat and E_hat, so all I needed to do was to plug them into Mathematica. Furthermore, what we really want is dG/dt, the rate of rotation with time, so we do dG/dt = dG/dR x dR/dt, where dR/dt is simply the rate of rotation of the earth, which is 2 pi (i.e. 360 degrees) divided by one sidereal day (23.93446965 x 60 x 60 seconds).

dG/dt =

where the units is in radians per second, R is our pseudo-RA coordinate, D is our pseudo-Dec coordinate, and L is our observer's latitude. To convert D to the usual declination d, note that for D = 0 degrees, d = +90 degrees; D = +90 degrees, d = 0; D = +180 degrees, d = -90 degrees.)

##### Share on other sites

OTOH, you could buy an EQ mount.

##### Share on other sites

If you get the targeting right, you can get up to 2 minute exposures with an AltAz mount. I've done it. To get that long you need to be targeting something low down in the east or west. As the target moves higher in the sky, or you get closer to North or South the faster field rotation affects the images and the shorter the subs you can gather. This was before I worked out a method for flats and couldn't process out all the Vignetting. This was 8 x 120s exposures using a NexStar SLT

## Create an account

Register a new account