Pointing File


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On the astronomy market nowadays one can find quite a few mounts with encoders on the axis, and most of them offer some way of modelling the characteristics of the system (that is, the combination of mount, scope, imaging train). ASA's software offers quite a few things that can be modelled and afterwards compensated for. Thanks to such models, the pointing accuracy can be improved greatly, to the point that no guiding is necessary anymore.

Pointing model types

Autoslew offers two distinct types of models, one being the global one, and a local version:

Normal model (global)

Covers the entire sky and is done either by hand or with Sequence. Number of stars can be freely choosen, but for best results one wants to get measurements on both sides of the pier.

Local model (MLPT)

Is done with Sequence and covers just a path on the sky needed to image a given object.

Understanding modelled variables

Maybe the most important thing to understand is that Autoslew tries to fit the pointing one has taken (either manually or with Sequence) with a formula which contains the following variables. It tries to reduce the residual error as much as possible, and tries to find the best values for all these variables to do so. Most of the time this works fine, but sometimes it also increases the error. Refer to the Advanced techniques further down for an in-depth explanation.

So back to the variables which Autoslew uses:

  • Polar alignment error: Corrects for bad polar alignment
  • Collimation error: Corrects for collimation changes due to position changes of the tube
  • Mount Angle error: Corrects for mechanical errors of the mount (like one axis not being perfectly perpendicular to another)
  • Tube Flex error (West/East): If repeatable it is modelled here
  • Swap pier flip constant:
  • Superfit: A new type of variable trying to catch all errors, requires a bigger pointing file (>45 stars)

Creating a model

One can build a model either by hand or with Sequence. Note that when using Sequence one needs a full license of DC3 PinPoint, otherwise one has to do it by hand.

Model by hand with Autoslew

To do a pointing file manually, you need to:

  • choose a starting star, center it in your eyepiece, and sync on it
  • click "Start pointing file" in the menu. Don't care about the message asking you to sync first, as it's already done
  • Name your pointing file
  • use the currently pointed star as the first star, slew to it (should do nothing) and click the ticked target button in the menu bar
  • then choose another star on the same side of the meridian
  • slew to it, center it, and click the ticked target button in the menu bar
  • check the error values in the bottom line of Autoslew, if they are too big, discard the star by clicking the "Delete last star" button in the pointing file submenu
  • repeat the last three steps with as many stars you want on this side of the meridian
  • then choose stars on  the other side of the meridian
  • take care that the errors in this case may be big, and you probably should not discard the star in this case (I don't know as I never use the other side)
  • once you have finished, click "Stop pointing file" in the pointing file submenu
  • then click the "Calculate configuration" button in the pointing file submenu
  • you will get a window where you choose  "Use now, save and use on next start"
  • then choose a star near zenith, slew to it, center it, sync on it
  • and click "Set new home position"

You're done.

Automatic Model with Sequence

Make sure you set up Sequence correctly before trying this.

Advanced techniques

Limits of a model

Of course a model can only represent repeatable errors. If your setup has flexure which changes depending from where you approach a position in the sky, or an unstable mount/pier, it won't help much. YOu should try to get your setup as stiff and repeatable as possible before attemping your luck with big pointing files. Usually, small poitning files, even with as little as 15 measurements, can yield good results. The precision is not limited by the mount, but by your whole setup.