Polar alignment and pointing issues with my PME

[gregbradley]

Originally Posted by frolinmod

The old Tpoint for Windows aligns you with the real pole, which is not what you want to be aligned with. You want to be aligned with the refracted pole. Tpoint Add On for TheSkyX aligns you to the refracted pole.

Isn't that the other way round?

The Earth rotates around the actual pole not the refracted pole. So aligning to a refracted pole would be less accurate not more accurate.

Greg.

[frolinmod]

I'll amend my statement about which is better. For instance, he following website says that it's better to align on the refracted pole at mid to high latitudes and better to align on the true pole at low latitudes:

http://canburytech.net/DriftAlign/DriftAlign_3.html

And wouldn't you know it, I myself am right on the borderline between the two where one can just about flip a coin as to which is better.

[Bassnut (Fred)]

Originally Posted by frolinmod

The old Tpoint for Windows aligns you with the real pole. Tpoint Add On for TheSkyX aligns you to the refracted pole.

Guys, TheSkyX is where all the new features are going. It's constantly getting better and better. It's reaching critical mass and the pace is just increasing. If you're still using TheSky6, you're quickly getting left behind.

Understand that if you're extremely comfortable with TheSky6, you might find TheSkyX to confusing or even a bit of a turn-off at first, but that will change after six months or so of use, then you'll like it way more than you ever did TheSky6.

You're all smart guys, you can handle change.

Oh upgrade god . Once you have a PME singing to spec, what possible advantage would there be in the pain, expense and time involved in upgrading anything (incl to MKS-5000 electronics in another post). Unlike other gear, once the obs PC just works on win XP and sky 6 and everything else, its time to quit mucking around and just get on with imaging .

A stable OBs PC is a valuable thing, its job is well defined and measurable so unless it actually improves image quality, fiddling can only make things worse.........or the same.

OK, ease of setup/calibration improves with upgrades, but if you have learnt an efficient proceedure already, and need to do it very occassionally, why bother with changes?.

[gregbradley]

Good point Fred.

A stable system that works every time is actually an achievement and a goal in itself. Its easy to take it for granted and think the grass is greener on the other side of the upgrade!

Greg.

[Paul Haese]

Update

Well finally got some clear sky and I wanted to report back a few things.

First the use of pinpoint and Tpoint mapper is superb. Of the 846 points I mapped last night only two failed. That was because it was trying to look through the pitch of the roll off roof. My polar alignment is now pretty good. Using 50 points on the east in one sector I got the PA to 11 seconds in altitude and 15 seconds in azimuth.

However, that is where the fun stopped. I found slap in the dec drive too. Not a lot but enough to be a problem. It seems that if I tighten up the balance knob and then turn it back one tick as per the manual the entire drive has about a millimeter or two of movement. I think the spring in the spring plunger assembly has lost some of its tension. I checked the screws on the spring plunger, and then set these back to factory defaults. So now to hunt down a new spring. The drive still works and I will now need another clear night to test to see if this is the last issue. As sort of confirmation I noted that movement on the scatter plot is in the same direction as this apparent slap. That being; points plotted in north and south hemispheres have similar separation angles but each hemisphere has a large separation angle. Is my thinking correct here?

Its been fun so far. Once I get this nutted I will have a huge sense of satisfaction that some just don't get with a PME.

[frolinmod]

Paul, 846 points shows quite some dedication. Are you still using TS6 or have you upgraded to TSX? If you're still using TS6, then could you please go into Tpoint and export your data to a text file and PM that to me. I'd like to run it through the TSX supermodel, look at the resulting model, look at the graphs, etc.

[gregbradley]

With that sort of accuracy in the model how did you determine there was slop in dec?

Isn't it usual to have a certain amount of play in a mount?

Greg.

[Paul Haese]

Originally Posted by frolinmod

Paul, 846 points shows quite some dedication. Are you still using TS6 or have you upgraded to TSX? If you're still using TS6, then could you please go into Tpoint and export your data to a text file and PM that to me. I'd like to run it through the TSX supermodel, look at the resulting model, look at the graphs, etc.

Ernie, I think you have mis-understood me. I did not do 846 in one go, it was several runs of successive models. The longest of which is a 192 point run, which you could take a look at it you like. I am still using TS6 though with the GSC1.1 catalogue.


Greg, I once again just grabbed the mount and found the slop. I am sure it is not supposed to be there. Like I said though, I need to test my theory and see if that has made any difference. I have got to contact software bisque to get another plunger spring too.

[gregbradley]

I thought it was best to do the model in one go rather than over successive nights.

Not sure why - I suppose the object is not at the same angle each night and you no longer have apples with apples.

Greg.

[Bassnut (Fred)]

Originally Posted by Paul Haese

Greg, I once again just grabbed the mount and found the slop. I am sure it is not supposed to be there..

Your right, I just grabbed mine to check, no slop, at all.

[Paul Haese]

Originally Posted by gregbradley

I thought it was best to do the model in one go rather than over successive nights.

Not sure why - I suppose the object is not at the same angle each night and you no longer have apples with apples.

Greg.

Greg, I just wanted to demonstrate that of the 846 points I mapped on about 6 runs each one solved except two points and that was because it could not see stars. A much larger model could be done but that will not help until I have solved the slop issues in the mount. Once this is done and it points better on the entire sky then I will do a larger model and perhaps a super model in one night. The main issue of course is getting the mount to point around 60 seconds without new terms first instead of on an all sky model of pointing at 159 seconds, despite a model of 50 points on the east side (narrow section of sky) showing pointing of 15 seconds. Then when I started on a whole sky model the pointing went to pieces.

Originally Posted by Bassnut

Your right, I just grabbed mine to check, no slop, at all.

Thanks Fred for the confirmation. I take it you are referring to both axis having no slop? That would be my understanding of what you are saying here.

[Bassnut (Fred)]

Originally Posted by Paul Haese

I take it you are referring to both axis having no slop? That would be my understanding of what you are saying here.

Yes

[Paul Haese]

No need to hide Fred. I will fix it. By the time I am done I will be one of the resident experts with PME's . Well at least I reckon there are not too many people who have pulled these things apart in Oz. Besides I have to look at the positive side here. There is always something good to take out of a situation.

[gary]

Originally Posted by frolinmod

I'll amend my statement about which is better. For instance, he following website says that it's better to align on the refracted pole at mid to high latitudes and better to align on the true pole at low latitudes

Hi Ernie,

Unfortunately there is no magical point in the sky to which one can align the polar
axis of an equatorial scope and then achieve zero field rotation for all possible observing
positions in the sky.

At best, there is a different optimal point at which to align the polar axis for each
possible declination and hour angle the scope can be pointed at and for each possible
value of the local temperature, local air pressure and wavelength of light at
which one is observing. The latitude and elevation of the observatory are also
factors.

The details are complex.

However ...

The version of TPOINT Paul has is fine and has been used to successfully align the
world's largest and most expensive telescopes for over 30 years. A table appears
on page 29 of the user manual that has estimates for the difference between the
true and refracted pole for various latitudes and different observatory altitudes.
The values in the table are in arc seconds and one should, in the southern
hemisphere, simply add the appropriate value to the value for ME. This then
gives the position of the refracted pole. In any case, the refracted pole is always
lifted above the true pole.

As a compromise, at Paul's mid latitude location he should be aiming for the
refracted pole especially for observations within a few hours either side of the
meridian. At your equatorial station, go for the true pole.

Best Regards

Gary Kopff
Mt Kuring-Gai

[Paul Haese]

Gary does Tpoint put you at the refracted pole via the corrections it gives you?

[gregbradley]

So should we be allowing a bit extra above to adjust our mounts to get to the true pole as opposed to refracted pole? Or is it not worth it as the differences are so fine that it is lost in PE anyway?

Greg.

[gary]

Originally Posted by Paul Haese

Gary does Tpoint put you at the refracted pole via the corrections it gives you?

Hi Paul,

The actual numerical value for the ME term that TPOINT provides is for the true pole.

However, adjusting ME for the refracted pole is made trivial by use of a lookup table.
The following box is a table straight out of the TPOINT manual.

  lat     0m  250m   500m  1000m  1500m  2000m  2500m  3000m

   85     5     5      5      4      4      4      4      4
   80    10    10      9      9      8      8      8      7
   75    15    15     14     14     13     12     12     11
   70    20    20     19     18     17     17     16     15
   65    26    25     25     24     22     21     20     19
   60    32    31     31     29     28     26     25     24
   55    39    38     37     35     34     32     30     29
   50    47    46     45     42     40     38     36     35
   45    56    54     53     50     48     46     43     41
   40    66    65     63     60     57     54     52     49
   35    80    78     76     72     68     65     62     59
   30    96    94     92     87     83     79     75     71
   25   119   116    113    108    102     98     93     88
   20   152   148    145    138    131    125    118    113
   15   205   200    195    185    176    168    160    152
   10   305   298    290    276    263    250    238    227

  deg     0ft 820ft 1640ft 3280ft 4920ft 6560ft 8200ft 9840ft

Look up your nearest latitude in the left-hand column. For example, Melbourne
is approximately -37 S so the row for 35 will be the closest. One can always
interpolate a little. Then go across the columns to one that best matches the
elevation above sea level of your observatory. For example, for an observer
at -35 S and 250m above sea level, the table gives a value of 78.
That value is in arc seconds.

For observers in the southern hemisphere, if you want to set the polar axis of
the mount to the refracted pole, then aim for ME (in arc seconds) plus the
appropriate value from the table.

(Observers in the northern hemisphere will subtract the value in the table from ME).

The actual optimal point to which one aligns the polar axis depends on what
declination and hour angle you plan on observing.

Best Regards

Gary

[rally]

Guys this is all in the manual.

Tpoint's Polar Alignment Report corrections relate to the True Pole.

Use that to get it down to a level where any further adjustments on your system simply make it seesaw back and forth and then turn on Protrack and you will be good to go.

To get a good PA you really don't need many points at all, 10-20 is plenty, the extra mapped points are needed for Tpoint to correct for other terms of mechanical error and for Protrack.

PE has nothing to do with the polar alignment and pointing corrections - PE is usually much less of an error than the refracted pole to true pole error ie an arc second or two for PE verses an arc minute or two for atmospheric refraction shifting the optical polar alignment.

Good Guiding will take care of all of this in any event without Protrack

[gary]

Originally Posted by gregbradley

So should we be allowing a bit extra above to adjust our mounts to get to the true pole as opposed to refracted pole? Or is it not worth it as the differences are so fine that it is lost in PE anyway?

Greg.

Hi Greg,

The effect or refraction is to make things appear slightly higher in the sky than
they actually are.

Therefore the refracted pole is just above the true pole.

In Sydney, the refracted pole is approximately 90 arc seconds above the true
pole.

A simple thought experiment can help one visualize this.

Imagine for one moment that there actually was a star right at the South
Celestial Pole (wouldn't that be convenient ).

Now that star, because of refraction, will appear slightly higher than it actually is.
if you were to set the polar axis to the true pole, that is just below the star, and then
take a long time exposure, the star trail would be an arc. However, if, you were
to align the polar axis to the refracted pole, then when you took the exposure the star
would essentially be a point.

Unfortunately there is a range of optimal points to adjust the elevation axis of the
scope for the infinite number of HA/Dec positions in the sky.

For practical purposes at these latitudes, a compromise of somewhere between
the true and refracted pole would be typical.

At the end of the day, one is trying to minimize the amount of field rotation
and if the mount does not provide a variable tracking rate facility but only
a fixed sidereal tracking rate, one is also trying to keep any residual between
the optimal instantaneous tracking rate and the fixed sidereal rate of the mount
to a minimum.

For example, for a star at the zenith, its true and refracted positions are the same.
However, it turns out that if the mount's axis is set to the refracted rather than the
true pole, the instantaneous tracking rate at the zenith becomes the classical
sidereal rate of 15 arc seconds per second.

So one can also make compromises between field rotation and tracking.

With regards these two specific problems, if software controlled variable
tracking rates are available, for most enthusiasts, trying to minimize field
rotation for the exposure time they use will be their primary concern.

Whether making an adjustment to your elevation axis to point above the
true pole will help you will depend upon your specific particulars, demands
and expectations. But not taking it into consideration can certainly impact
upon the quality of the results from some amateur rigs.

At the end of the day, it is important to remember that equatorial mounts can't
provide a perfect solution to the reality of us observing from the bottom of the
atmosphere. At best they are an engineering compromise. There is no magic
place in the sky to which their polar axis can be aligned to provide zero
field rotation and a uniform tracking rate.

Another common misconception is that a drift test is a gold standard way to
provide a mythical perfect alignment. At best, a drift test will give one solution
specifically derived from those two particular points used in the test, which may
be, for practical purposes sufficient, but a different drift test will give a slightly
different result, which is not surprising, as there is no one correct result that
is applicable across the whole sky.

For some, just the knowledge that there is no perfect solution will free them up
from the impossible task of trying to find one, which will then allow them
to focus more attention on aspects of their setup where there really is room for
improvement

[rally]

I think Gary meant to say polar refraction is around 90 arc secs not arc minutes !