[APML] Drift alignment - a common misunderstanding

[Reinhard Ilk <ilkr@EUnet.at>]

Ulrich and Steve,

I followed up a recent thread where another very popular misunderstanding
was reiterated: Go as low as possible to adjust the polar axis inclination.
The complete OPPOSITE is true (except for extremes, say > 80 deg). 60 to 70
degrees declination is best. Due to the differential air refraction you
would mess up your polar axis inclination adjustment, if you go as low as
20 to 30 degrees declination.

Ulrich, it is obvious that you CANNOT mess up your east/west alignment by
adjusting you polar axis inclination - you shouldn't be able to change the
azimuth position unless you have a really lousy mount. (I have a Vixen GPDX
only, but I never experienced this.)

In addition, it is important to mention that the sensitivity towards
misalignment is INDEPENDENT from the declination (for inclination
adjustment only, not for azimuth!). I have quantified a targeted drift
alignment procedure described by Gerald Rhemann earlier in the mid 90ies
which I communicated back in 1999. It tells you exactly not only where to
move your RA axis to, but also the distance how far to go in order to get a
precision landing with only a single pair of adjustments.

Although I encouraged all users to spread it out as far as posible, the
procedure of drift alignment seems still to be misunderstood. I am going to
post a how-to-article on my website with download options for the Excel
spreadsheet.

My procedure had only a single challenge where it needed to be modified.
This was when I was helping a list member who was living at 1° latitude.
Obviously, there are different procedures to follow in tropical regions.

Reinhard



> Ullrich,
> 
> I've been just getting up to speed on my new G11 as well and
> am finally pretty comfortable with drift alignment.  I use the
> polar scope Losmandy sells and am usually quite close to start
> with.  There's a good application note on the Losmandy site by
> Dave Kodama (or his web page) on using it.
> 
> I find the directions to do drift alignment confusing because
> they refer to north/south drift of stars and mount re-adjustments
> but often don't specify relative to what (center of reticle eyepiece,
> mount...?) so you end up guessing what they mean.  So I thought
> I'd share what I do:
> 
> I just point to a star near the meridian intersection of the
> celestial equator, align my reticle eyepiece axes for north/south
> and east west and start watching for drift.  With my set up I'm
> looking through a diagonal so north/south is correct and east/west
> is mirror image reversed.  You may have to reverse the direction
> of correction depending on your setup.  If the star drifts up
> from the center on the north direction axis (ignore east/west
> drift) I adjust the east/west position of the mount to make the
> star move to the right of center in the eyepiece along the east/west
> axis.  How much is enough is dependent on the field of view,
> focal length you are tracking with etc. so you have to experiment.
> This didn't take long once I knew I was going in the right direction.
> A couple of partial turns on the east/west mount adjustment
> knob usually does it but you need to check between each adjustment
> for a few minutes.
> 
> Then point at a star along the equator as close as is reasonable
> to the eastern horizon (much above 20 to 30 deg and you will
> mess up the first east/west adjustment).  Align the eyepiece
> axes again and notice direction for north/south and east/west.
> Then again center and watch the north/south drift only.  As
> the star drifts up or down the north/south axis you just adjust
> the altitude mount adjustment knob to make the star go the opposite
> direction.  This is again just a partial small turn of the knob
> and you need to experiment till you get the feel of what's enough.
> A lot of folks go back and recheck the first alignment to see
> if it needs fine adjustment but I usually don't find this necessary.
> I usually let the star drift several minutes before each adjustment
> to make sure I have a sense of the direction and distance of
> drift.
> 
> The main thing is just being patient and practicing for a while.
> Devoting an evening to just this some time reduced my frustration
> level considerably.  Hope this helps.
> 
> Steve Lindsey
> 
>>--- Original Message ---
>
>...Who can tell me EXACTLY what I have to do
>>when I observe a certain drift (North or South), and especially
>how MUCH I
>>should move my mount by?
>>
>>Ulrich    >>www.analemma.de<<

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