OTA Alignment

[nightster]

For imaging, Do I need to align my OTA to the Polar Scope after PA is complete?  Or Should the Pole star be approximately the center of a test image taken after PA?

Thanks,

Jeff

[jambouk]

If you are using an equatorial mount, the RA axis of the mount needs to be polar aligned (in essence pointing at Polaris); and this can be done with a polar scope if you have one.

It doesn't matter really if the OTA, in the home position, doesn't show polaris. It is the mount which needs to be polar aligned, not the OTA.

If you have goto, when you do a star alignment, you are educating the handset/mount as to where the OTA is pointing.

Tracking accuracy is mostly down to how well polar aligned your mount is.

Not sure that helps.

James

[IanL]

My main OTA is about a degree and a half out.  This causes a problem on the first goto after completing polar alignment, since the target star is usually outside the field of view of the camera.  I don't have a finder so getting the first alignment star in to the field of view used to be rather hit and miss.  I started using AstroTortilla to do plate solving and this dealt with the issue nicely, since AT doesn't really care where you are pointing and will build the alignment model as you go along.

If you want a simpler solution you just need to figure out how far off Polaris your main OTA is once you have a good polar alignment and in which direction.  You might try centring it by unlocking the Dec axis clutch and slightly rotating it one way or the other and then re-locking.  If that works then you just call that your new 'home' position.

If you need to move the OTA in the other direction (i.e. move Polaris up or down in the field of view when the scope is parked in the home position, then it's a bit more tricky as you need to raise one end of the OTA relative to the dovetail and lower the other.  Some (long) dovetails have cone error adjusting screw holes (but maybe not screws) at the four corners of the bar.  You just put long screws through them so that the tips push on to the OTA - by slightly slackening the main screws holding the tube rings (or 'foot' for some scopes) and then screwing in one pair of adjusting screws and loosening the other, you can make the OTA point higher or lower relative to the dovetail.

If you have a short dovetail with no cone error screws, then you need to put shims (thin bits of metal with holes in) between the tube ring and the dovetail.  You can make them from small bits of aluminium drinks cans with holes drilled in for example.  You just shim the end that you need to raise to adjust the pointing.

As James says, if your first alignment star appears in the field of view then don't worry about it (or if you have a finder aligned to the main OTA, you can just look though that and use the handset to get the star in to the camera's field of view before centring it).

[nightster]

Thats the info I was looking for, thanks. For us beginners, the overlap on these procedures can be confusing. Maybe someone could confirm my understanding of the following statements: PA aligns the axis of the mount to your pole star. Star alignment calibrates your GOTO for accurate targeting. Star alignment will not correct for poor PA. Alignmaster uses star alignment to correct PA. Astrotortilla uses plate solving to correct star alignment.

I was reading about cone error which spawned the original question about OTA alignment.

[wuthton]

Thats the info I was looking for, thanks. For us beginners, the overlap on these procedures can be confusing. Maybe someone could confirm my understanding of the following statements: PA aligns the axis of the mount to your pole star. Star alignment calibrates your GOTO for accurate targeting. Star alignment will not correct for poor PA. Alignmaster uses star alignment to correct PA. Astrotortilla uses plate solving to correct star alignment.

I was reading about cone error which spawned the original question about OTA alignment.

I think you've got it bang on.

I doubt you need to worry about cone error.

[jambouk]

Cone error may impact on your goto accuracy, but for short unguided tracking it won't pose you tracking problems - is my understanding.

[nightster]

Will cone error affect guided long exposures?

[jambouk]

I know nothing about guiding, but i think if you have significant cone error, the arc the OTA follows will be different from the arc the RA axis follows, but how guiding comes i to this, i'm not sure, maybe it will introduce slight field rotation... You need to ask someone else. Ask in the imaging section as i may be talking rubbish.

Good luck.

James

[LeeB]

as I frequently rotate my newt during a session I collimate the optical center line to the ota, so rotation causes no field change , any cone error induced by the mount is eliminated beforehand by spacers.

[IanL]

I know nothing about guiding, but i think if you have significant cone error, the arc the OTA follows will be different from the arc the RA axis follows, but how guiding comes i to this, i'm not sure, maybe it will introduce slight field rotation... You need to ask someone else. Ask in the imaging section as i may be talking rubbish.

Good luck.

James

Cone error has no effect on tracking accuracy and does not cause field rotation.  The only thing that matters for tracking is whether the RA axis is aligned with the Earth's axis of rotation or not.

If you have cone error in the 'left/right' direction (as viewed through the OTA when it is parked in the home position and pointing to polaris) that is just the same motion as rotating the Dec axis.  Cone error in the 'up/down' direction is similar, it just means the scope ends up pointing too far East or West, which you fix by creating a star alignment model.

Once the scope is tracking, the OTA will describe a circle following a line of declination across the sky.  At the most extreme example, you could try to mount the OTA so that it is pointing out along the Dec Axis instead of at 90 degrees to it.  The OTA would track along the celestial equator in that case (but you wouldn't be able to point it to higher or lower declinations!)