Mach1 and checking PASILL4 results


Pierre Henrotay
 

Hello all,

I am running a Mach1 and experience some problems with my polar
alignment scope (PASILL4). I'd appreciate your ideas here.

My goal is to use for alignment the PASILL4 only and reach a polar
alignment within 5 arcmin at worst, hopefully 2 arcmin.
As far as I heard the PASILL4 is very capable of this.

The very few clear nights we just had here (I live 50°N, 5° E)
allowed me to make some tests.

I spent a long time to:
- center the reticle in day light
this looks OK: centering an object and rotating the housing does not
show a significant displacement of the target object wrt to the
centre of the reticle
- make the scope orthogonal (indeed makes a big difference when
gotoing, definitely worth the effort !)
- train PEM (impressive low error, great !) using PemPro; PEM is
active

There is a very (very very) tiny amount of play when the polar scope
is tightly screwed in the polar axis. Not expecting this to be a
problem.

Last night I did a polar alignment using first the polar scope for
rough alignment, then I used PoleAlignMax (I am familiar with this
and checked in the past that I have consistent results with PemPro);
resulting alignment error was under 1 arcminute in both directions.

I verified that goto was accurate and I took 4x4 min unguided
exposures of M51 that show no significant drift (resolution: 2
arcsec/pixel). Excellent alignment thus.

Then I checked what PASILL4 was showing; the 3 reference stars were
visible (of of them barely as expected) but obviously
when rotating the polar scope, the best match showed Polaris
approximately with a perpendicular offset from its gap:
it was near the base of the engraved arrow head pointing at the gap
where Polaris should be.
Or, expressed differently, when rotating the polar scope to bring
Polaris in its gap,
the two other stars were far from their radial lines (30 degrees or
so for delta UMi).

I then redid a polar aligment just carefully using the PASILL4.
Then rerun PoleAlignMax to derive how big the misalignment was.
The result: a bit more than 10 arcminutes both in azimuth and in
altitude, so clearly way beyond my expectations.
And too big to be explained by refraction effects.
I took a series of images of M51 again (4x4minutes unguided) and
indeed there was an evident drift.

This morning I rechecked the optical centering of the reticle: it is
still OK, nothing changed.

One thing I thought of was that the polarscope might be not aligned
within the axis once screwed in but I would be very very surprising.
Is there a way to check this ?
Maybe I am doing wrong or assuming something incorrectly but I
cannot see what is the trouble. I'd welcome your ideas and hints.
The mount is properly balanced, the telescope is rigidly attached,
the imaging combo is not very heavy (NP127 + DSLR).

Regards,
Pierre

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