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Originally Posted by g__day
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* fit
coeff change value sigma
5 ME +0.431 -127.57 95.333
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Hi Matthew,
A negative value of ME for the Southern Hemisphere means the RA axis must be
raised, in this case by 127 arc seconds for the true pole position. You can ignore the
minus sign in the verbose report. The refracted pole in Sydney is about another
86 arc seconds above the true pole. Typically you will want to be aiming somewhere
between the true and refracted pole. The specific point depends where you are
imaging in the sky. (Come to Lostock at the end of this month, attend my presentation
and I will be discussing this topic more
).
Notice, however, that the sigma value for ME is 95 arc seconds. Sigma is
its standard deviation. A value of -127 +/-95 is saying that the value of -127
is in the noise.
Your MA term looks "real" enough. I would adjust it in Az and then repeat a star pointing run.
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* fit
coeff change value sigma
1 IH -0.572 -86.76 92.540
2 ID -1.039 +285.83 151.450
3 NP -0.001 +22.54 130.689
4 CH +0.668 +88.18 93.320
5 ME +0.431 -127.57 95.333
6 MA -0.604 +232.40 93.818
7 TF +0.168 -41.84 18.084
8 FO +0.415 +59.55 200.598
Sky RMS = 52.72
Popn SD = 56.02
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A pointing performance of 56 arc minutes RMS is a pretty good result for a mount of
this type.
Firstly, drop the FO (Fork Flexure) term. Based on the above, it is not
apparent you have any. Notice the value is +59 but the standard deviation is
200? That is telling you that it is more noise than signal and that it will
not be making a meaningful contribution to the model and quite possibly "stealing"
a little from your other terms, in particular a little from MA and NP.
FO models fork flexure on fork mounts. On your GEM, DAF which is the
Declination Axis Flexure, is the one you should investigate.
Keep the TF (Tube Flexure) term for now.
However, the signal to noise ratios for NP and CH aren't good. The two terms have
very similar signatures and when you sample stars, you need to get a good spread
of stars at differing altitudes in order to help distinguish them. Having said that,
the report shows the two aren't highly correlated but I recommend you see what
happens if you keep one and drop the other. Experiment by first seeing what happens
when you drop the NP term. Untick the NP term or you can type at the command line
reset
lose np
fit
See if the signal to noise ratio of CH then improves. If not and in any case,
also experiment by dropping CH but including NP. Untick the CH term and tick the NP term
or you can type at the command line -
reset
lose ch
use np
fit
It might well be you only need one or the other. CH is the more likely of the two
and especially so for a GEM.
After experimenting with CH and NP, try the suggest term dialog and add one or more additional recommended
terms if they have good signal to noise ratios and the RMS/PSD values drop as well.
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So not nearly as good pointing under Tpoint RMS 52.7 arc second - versus +/- 16 arc seconds for MaxPoint the other day...
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Given the standard deviations in some of the above terms, you can expect some
to and fro in the RMS values.
Based on the above, your FO is not worth keeping and even more so because
you have a GEM. Check what happens when DAF is added. The RMS
is likely to drop some more when you add some additional terms. DCEC, DCES,
HCEC and HCES are always worth checking as these often account for various
runout errors. Looking at the above, I would try DCEC first. Depending upon gearing
ratios, higher order harmonic terms may also assist.