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Alignment query


phattanglo
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Can anyone tell me if everything is supposed to be aligned when a mount is Polar aligned ?

By this I mean if the Polarscope was aligned correctly should you have Polaris more or less central in the eyepiece and the finderscope also.

The reason I ask, is that to achieve this I have to use the dovetail adjusting screws to align the OTA to Polaris and then I seem to have insufficient adjustment on my finder mount to also align it to Polaris

Am I in error trying to achieve this degree of alignment.

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I would align the OTA and the finderscope on a distant object first. You should then align the polarscope to the "pole". It should have a reticle for placing Polaris in the right place which will depend on your local siderial time. This will require you to rotate the polarscope. Then you should use the altitude and azimuth adjustment on your mount to get the polarscope to point at Polaris. Andrew s

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Polaris is almost 1° from the pole so if your mount is polar aligned and your scope is "perfectly aligned with the mount" you won't see Polaris in the field of view! (Unless you have a fov of more than 2° which is unlikely through a scope!).

Polar align the mount as a seperate issue then, using a bright star, align the finder with the telescope.

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So actually you shouldn't expect to be able to see Polaris or any distant object in the Polarscope, eyepiece and finder all at the same time.

This being the case, what are the 2 small adjusting screws for on each end of the dovetail ? :eek:

This is probably a silly question but if your RA axis is correctly polar aligned, how would you locate Polaris centrally to view it, wouldn't it be permanently 1° off axis ?

Edited by phattanglo
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Polaris is not in the centre of the fov of the polarscope. It follows the "ring" drawn on the reticle which is approximately 1° in "diameter" - you set the polarscope scales to the date and time and Polaris is then aligned inside the tiny ring drawn on the polarscope circle.

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You are aligning the scope and mount to the polar axis of the earth not to polaris.

Polaris just happens to be a convenient star get within 1.2 deg. But it is then 1.2 deg off of the correct alignment.

Suspect that the confusion is Polar alignmnet and use of Polaris. Sounds the same but not really.

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So if the dovetail is parallel to the RA axis and you are correctly polar aligned then Polaris is just as viewable as any other object would be.

Is cone error something to be concerned with if your not imaging

Edited by phattanglo
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Indication would be one of two things

(1) When you do a one or two star align the telescope cant find an object that you subsequently slew to.

(2) Inaccurate tracking because the mount is polar aligned but the telescope is not execatly parallel to the RA motion of the mount. Negligible to an observer more critical to an imager.

Correcting cone error is a nightmare from what I can see - I have read the instructions at least 30 times in my own manuals and I still cant claim to REALLY understand it. Astroimagers might be happy with it because its a kind of drift alignment.

If you do a three star alignment in the GoTo the cone arror is cancelled out - I am happy to leave it at that :eek:

By the way not all dovetails have the adjusting screws. Dovetails on the base of SCT and Maks dont have them for instance.

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"By the way not all dovetails have the adjusting screws. Dovetails on the base of SCT and Maks dont have them for instance. "

presumably, the dovetails are parallel to the optical axis from the factory.. solong as the dec head is orthogonal to the RA, no further adjustment needed.

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I'm sure that others more experienced will correct me if I'm wrong, but cone error only really affects slewing accuracy in the Goto system, and as you say, a 3-point alignment procedure will take care of the offset from true polar alignment (2 degrees of freedom) and cone error (the other degree of freedom that makes up the 3). The critical thing for imaging is accurate polar alignment so that as the equatorial mount tracks, there is no rotation of the image in the view (or on the imagine chip) - cone error on its own cannot cause this field rotation.

I'd better duck now as the corrections start to fly in!

Terry

Indication would be one of two things

(1) When you do a one or two star align the telescope cant find an object that you subsequently slew to.

(2) Inaccurate tracking because the mount is polar aligned but the telescope is not execatly parallel to the RA motion of the mount. Negligible to an observer more critical to an imager.

Correcting cone error is a nightmare from what I can see - I have read the instructions at least 30 times in my own manuals and I still cant claim to REALLY understand it. Astroimagers might be happy with it because its a kind of drift alignment.

If you do a three star alignment in the GoTo the cone arror is cancelled out - I am happy to leave it at that :eek:

By the way not all dovetails have the adjusting screws. Dovetails on the base of SCT and Maks dont have them for instance.

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