aligning optical axes?

[Andrew C]

Simple question (I think). When first attaching a new OTA to a GEQ mount, how do people ensure that the optical axis of the OTA is aligned with the RA rotation axis of the mount, or do they just hope for the best? And if they need to collimate the mirrors later, doesn't that necessitate subsequent re-aligning of the two axes as well?

[Wavytone]

Andrew,

With a scope pointed at the pole (-90 degrees) the misalignment between the RA axis and what you see in the scope is the sum of five mechanical misalignment errors:

a) the perpendicularity (or lack of it) between the RA and dec axis;
b) the perpendicularity (or lack of it) between the dec axis and the and OTA, at the dovetail or OTA,
c) the parallelism (or lack of it) between the OTA and the actual optical axis as seen in the eyepiece, due to the net alignment errors of the optical components. This is simple enough for a refractor, but gets more complex if there are 2 or more mirrors involved (Newtonians, SCT, Maks or cassegrains).
d) the rotational error in the declination circle or encoder (ie does it really read -90 degrees when the OTA is in the same plane as the RA axis ?)
e) the net effects of flexure in the mount and the OTA (things bend and sag, more than you may realise, even in small mounts).

Determining all of these errors is non-trivial, and each should be determined separately. There is an old optical lab method used to check all of these for theodolites, it can be applied for telescopes but it is a bit pointless as most telescope mounts have no provisions to adjust these.

These days instead the correction can be done mathematically, by pointing the telescope at bright stars all over the sky and measuring where the encoders think its pointing vs the actual star positions. After solving a heap of equations using least squares, a computer can figure out the corrections required to compensate for the above effects. Some - not all - of the better amateur scopes have this capability in their software.

[Andrew C]

Thanks for this. So it sounds like typically 'hope for the best' unless one has this better software (I suspect my Celstron Nexstar software is not in that category, though I may be wrong).

The background to my question is that I use the scope primarily for long exposure video astro, and smearing or star trails sometimes start to become evident for exposures over say a minute.

In the absence of a polar scope on my CG5 mount I use the camera view to polar align. I can get the true SCP centred to a high degree of accuracy (a few arcminutes) but of course with this method the effect of any misalignment between the RA and optical axes remains. It would be really nice if there were an elegant way of reducing said misalignment, short of what I do once in a while whenever I change something significant, which is swinging the scope manually around the RA axis and carefully setting the dec -90 point and shimming the OTA height respectively to maintain the target centering, which is a pain because the two actions interact.

Some of those other factors you mention are beyond my control, short of buying a new mount. The scope is only a 6" newtonian (6kg all up), so I suspect flexure is not much of an issue in my case.

I just don't really know how good (or bad) my scope/ mount combination is. Sometimes after all this palaver the smearing is better, sometimes it is not. Periodic error may also be a factor.

Cheers,

Andrew

[bojan]

Andrew,
if polar alignment is OK (you should use drift alignment method), there should be no problems with tracking (apart from component caused by PE, of course).
Perpendicularity will only affect positional accuracy (if you are using GoTo)- and this is sorted out by software as Wavytone mentioned and explained earlier

[naskies (Dave)]

Originally Posted by Andrew C

The background to my question is that I use the scope primarily for long exposure video astro, and smearing or star trails sometimes start to become evident for exposures over say a minute.

In the absence of a polar scope on my CG5 mount I use the camera view to polar align. I can get the true SCP centred to a high degree of accuracy (a few arcminutes) but of course with this method the effect of any misalignment between the RA and optical axes remains.

This sounds more like a polar alignment issue? Out of curiosity, how do you know that your polar alignment is within a few arc mins?

As bojan mentioned, drift alignment is a good way of getting very accurately aligned. If you have a computer connected to the mount, you can use software such as AlignMaster to do it more quickly.

[Andrew C]

Polar alignment of the scope (I stress, not the mount) is achieved by centering the SCP on the centre of the video monitor screen connected to the camera, by adjusting the mount alt and az screws. I locate the SCP in the star field shown in for example the more detailed FOV2 diagram at http://www.users.on.net/~pbl/art_sh_polar_alignment.htm.

The field looks much the same as shown in the diagram, although my field is somewhat smaller and of course rectangular.

For example, using the two stars either side of the SCP it is not hard to do that to within 3 or 4 arcmin (the radius of the circle in the diagram is about 22 arcmin).

[bojan]

Andrew, you have to use drift method if you want to have adequate alignment.