Jump to content

SkySurveyBanner.jpg.21855908fce40597655603b6c9af720d.jpg

New (experimental) PHD2 guide algorithm


kens

Recommended Posts

A new guide algorithm has been added to PHD2 2.6.5 dev3 to see how it compares with the older algorithms. The new algorithm is called Z-filter and is a discrete frequency domain filter. Specifically it implements what is known as a Bessel filter.

Like all the other algorithms it uses a low pass filter to attenuate the influence of  short period, seeing induced movements whilst responding strongly to long period, mount induced deviations. The difference is that it is actually a family of filters that can be tuned to suit the guide exposure being used. Plus the filter design gives a flat response across the long period pass band then rolls off sharply at shorter periods.
The use case for this algorithm is guiding with short guide exposures and/or poor seeing.

The guiding parameters for the Z-filter are currently selected via a drop down list which selects the two parameters of Order and Corner.
Order determines how sharply the response rolls off so a higher order ignores the deviations from seeing more than a lower order filter. Order 4 is recommended as a starting point. You might select a  lower order if your mount has some relatively short period error that you want to correct for.
Corner determines the period where the roll off starts. Multiplying the guide exposure by the corner parameter gives the period. A 2s guide exposure with a corner of 8 has the same response as a1s guide exposure with a corner of 16 or a 4s exposure with a corner of 4. A recommended start point is a corner value that results in a period of 4 to 8 seconds.

If you are currently using a guide exposure of 2s you might want to try 1s instead with a corner of 4 or 8. Note that with a shorter guide exposure the guide graph starts to look a bit ugly as the seeing becomes more visible. What matters is the images you get.
I'm keen to see guide logs where the algorithm gets used

Link to comment
Share on other sites

16 minutes ago, kens said:

using a guide exposure of 2s

Hi and thanks for the heads-up.

I'm currently using PPEC with a fixed period of 122s for my eq6. What values would you suggest for a guide exposure of 2s?

TIA and clear skies.

Link to comment
Share on other sites

1 hour ago, alacant said:

Hi and thanks for the heads-up.

I'm currently using PPEC with a fixed period of 122s for my eq6. What values would you suggest for a guide exposure of 2s?

TIA and clear skies.

Start with order 4 and corner 4. But do try 1s exposures also with the same settings as this would help with the 10s tooth engagement period of the EQ6. This does need a slightly brighter guide star. If it doesn't work optimally then increase the corner to 8. This has the same response as using a longer exposure but retains the advantages of shorter exposures.

By the way we've also added a new option under the graph Settings menu for "Corrections to scale".  This shows the corrections at the same scale as the graph (currently it autoscales). This lets you see just how big the corrections are compared to the guide star deviations, This is important when using short exposures as the seeing is more noticeable and you can be fooled into thinking that the guiding is at fault. When you can see how small the guide corrections are you can be more confident that you are not chasing the seeing. This option works for all algorithms.

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.