GOTO accuracy and collimation in newts

[kalasinman]

Sometimes we learn from our missteps. I have a GSO 150/750 newt. GOTOs have been spot on until last night when they weren't on the chip, not even find-able. Yesterday I decided to check collimation, as it had been awhile and I made some changes. It was bright sunshine (first mistake) so I threw a towel over my head and carried on. It seemed of a bit so I went through the process. I realized late last night that I had rotated the secondary way off somehow. I reasoned that the effect would be seen in RA GOTO error and started searching afield with RA . Sure enough, that brought targets right in. For those who are beginners like me and have newts, this might be a source of GOTO inaccuracy, other than PA or the other usual suspects. I wonder now if after everything else is correct and GOTO off in RA, if minor tweaks to secondary rotation might be a way to get therm right.-Jack

[Atreta]

jack, but wouldn't it get it out of collimation? i mean the final minor tweaks.

never tried to do it with my little newt yet.

thanks for the post

[kalasinman]

I'm not sure, but a small change in mirror rotation may or may not show up in collimation procedure, but would clearly show up in GOTO accuracy. My point being that there may already be a minor error in collimation that is causing a small shift in GOTO centering. Not as bad as my case. Lets say the difference between a GOTO being off center and needing adjustment for framing, and that perfect GOTO. There is more sweet spot as the FR gets larger. Seems to be some tolerance at F/5 as in my case, perhaps not at F/4.

[brantuk]

Collimation and goto accuracy are entirely independent of each other. However, you will find that image positioning on the camera chip (or the view through the eyepiece) is dependent on having a well collimated and focused optical system.

What does affect goto accuracy is polar alignment - which has nothing to do with the scope and everything to do with the mount. If you have an accurately polar aligned mount, and well aligned optical system, with properly star aligned electronic goto, then altering the optical collimation will screw up your pictures (and view), but the mount will remain accurate when told to goto objects in the sky. You (or the camera) would just be looking at objects differently cos the optics got messed up.

Now if you messed up the optics and then did the star alignment using the messed up optics - you will get inconsistency between the polar alignment and the star alignment if you subsequently collimated the scope. Goto is merely electronics, but it does have to be tuned together  with a physically polar aligned mount and soundly aligned optics in the right order - it's a prerequisite.

[kalasinman]

Collimation and goto accuracy are entirely independent of each other. However, you will find that image positioning on the camera chip (or the view through the eyepiece) is dependent on having a well collimated and focused optical system.

What does affect goto accuracy is polar alignment - which has nothing to do with the scope and everything to do with the mount. If you have an accurately polar aligned mount, and well aligned optical system, with properly star aligned electronic goto, then altering the optical collimation will screw up your pictures (and view), but the mount will remain accurate when told to goto objects in the sky. You (or the camera) would just be looking at objects differently cos the optics got messed up.

Now if you messed up the optics and then did the star alignment using the messed up optics - you will get inconsistency between the polar alignment and the star alignment if you subsequently collimated the scope. Goto is merely electronics, but it does have to be tuned together  with a physically polar aligned mount and soundly aligned optics in the right order - it's a prerequisite.

We're talking semantics here. In my world an accurate GOTO is defined by have the intended target appearing in the center of the camera chip. I certainly agree that an accurate PA is a prerequisite. In this case my PA was near perfect, but the object was off the chip, hence my post.

Here and elsewhere I have seen complaints by beginners that GOTO is off. In the same sense as my posting, accurate GOTO is very dependent on secondary mirror rotation.

I have seen comments here and elsewhere by experienced folks responding to beginner questions about collimation that in most cases close is plenty good.

I believe this is an instance where this is not the case. Just trying to provide another line of inquiry for those wanting to improve GOTO accuracy.

In the end, if the target isn't in the center, where the mount is pointed matters little.