Max. sub length with polar align inaccuracy

[raymo]

Does anyone know if there exist any tables or formulae that will tell you the
max sub length with round stars with a lens of Xmm focal length at Y degrees
declination with Z secs/mins/degrees inaccuracy in Polar alignment?
Even a simplified table giving figures that I could extrapolate from would be
great.
raymo

[glend]

Guiding can cover a multitude of sins.

[raymo]

Guiding my little nano tracker would be problematic Glen.
raymo

[Icearcher]

Great idea Raymo,

The first question might be how do you measure how far off PA you are?

[kens]

Is this guided or unguided? Unguided there is declination drift where the rate varies with the cosine of the hour angle. Guided you have field rotation which you can get from http://celestialwonders.com/tools/rotationMaxErrorCalc.html

[billdan]

Hi Raymo,

There probably are some formulas out there somewhere but it would be complicated trigonometry (lots of Sin and Cos equations).

You have to take into account your home latitude, the DEC of your target, the hour angle of the target and refraction of the atmosphere based on the elevation from the ground your target is. Its a moving target so the drift error is always changing.

I think your best bet is to rely on trial and error, make up a table that shows this part of the sky I can go 2mins unguided, this part I can only do 30secs unguided etc.

Bill

[raymo]

You work backwards Chris. If such a table existed, the max exposure I was getting with any given f/l lens at a particular declination with round stars, would tell me how far off I was,[by using the table] and whether further attempts to improve the alignment were sorely needed, or whether I was already close enough that any improvement would be minimal.
Even a table that gave figures for say 0-30-60 degrees declination and say
1-15-30minutes and 1 and 2 degrees inaccuracy would be of great benefit.
The figures would only need to be given for one lens length, say 50mm,
and one could easily extrapolate from that.

Unguided Ken.

Good idea Bill. There are tables of max exposures at different decs, with different f/l lenses, but are
for a stationary camera, not a tracking one.
raymo

[kens]

If unguided then the average declination drift is the PA error per 6 hours. Since it is non-linear, the maximum rate is about 1.5x the average. The average is probably close enough for most purposes.
You'll need to also take into account your pixel size which combined with FL gives you the pixel size in arc-seconds.
You want the drift to result in no more than about 20% elongation for stars to look round.
At some point, as you improve your PA, periodic error is going to become dominant.

[raymo]

Thanks Ken, some food for thought.
raymo