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Off axis auto guiding on comets


  1. My equipment and software to do off axis guiding on comets
  2. Setup of PHD2 Beta
  3. Setup of CdC (Skychart) Betaversion
  4. Start comet guiding in PHD2
  5. Overview of Astroserver Screen
  6. Test on comet C/2014 W2 Pan-STARRS

I take no responsibility or liability for what are written here, you use the information at your own risk!

6: Test on comet C/2014 W2 Pan-STARRS

It's not very often there is a clear sky and a comet in my limited view from the balcony. But now in April 2016 the comet C/2014 W2 Pan-STARRS is there and tonight the sky is clear.

Here you can read more about the comet:

Software in action during comet guiding:

Tracking on comet C/2014 W2 Pan-STARRS (Click on the image and you get a full resolution image in a new window)

Here is all the programs that I have running to catch the comet, they are all updated to the latest beta version of today (April 2016).

Program Version
CdC (Skychart) skychart-3.11-3261-windows
PHD2 phd2-2.6.1dev6-installer
APT APT_3.072 full version

The idea is to indirect track on weak comets to get longer exposure times. Weal comets that are not visible on the sub images, but later when stacking them together (without align) the comet will maybe be visibly. This comet is of magnitude 14 and is not very weak, it will be visible even on the sub images.

Did I succeed?

C/2014 W2 not aligned

Did I succeed ? No, something went wrong, you see the comet in the center. All these 45 sixty seconds images are stacked on top of each other, no align as it shouldn't be necessary when guiding (indirect) on the comet.

A little bit strange because what I can see all settings are correct. Notice that the comet track is longer than the star tracks. Did it correct in wrong direction?

I have had problem earlier to get the information in EQMOD to be correct. If the telescope does a meridian flip the camera will be upside down and the other programs must know this to be able to set the sign of move to the right direction. In this case it should be "Pier side West, telescope pointing East", when I now look at the screen dump I see that it hasn't been updated as it should, EQMOD say "Pier side East, telescope pointing West". Could that be all that is wrong? It worked with the earlier EQMOD versions when I tested on asteroids.

Because this comet was not very weak I can align on the comet core and then it look like this:

Comet C/2014 W2 Pan-STARRS aligned

Now the comet is easy to see, it's in the center as a point object. The first image taken 20:30 and the last 21:27 UT. The photo session lasted 57 minutes. Pixel scale about 2"/pix, image size 800x800 pixel. Polar align precision is about 8' last time I did a check. Camera's long axis oriented along RA axis. More info about the comet: Panstarrs C/2014 W2.

It must have been something wrong with the comet relative speed data, maybe just the pier side data that caused that, here is the data from CdC, comet database is updated today:

Comet relative speed data

The dRA speed differ from the data sent by CdC to PHD2, but it should. CdC correct for Declination, COS(a). The direction on comets move is north and west (up and right in this photo). The dots on the yellow line correspond to one day. The big rectangle is my 2x3 degree field from full frame camera and the small rectangle is my off-axis guide camera's fov.

According to NASA it should be like this, rate is arc seconds per hour:

Date UT Hour Minute RA DEC dRA*cosD dDEC
2016-Apr-09 21 00 20 09 +69 19 -24.9787 23.54706

It correspond to what CdC send to PHD2.

Here is the link to NASA if you want to have your own table:

I did some more test where I manually reversed the comet relative speed parameter, first only dDEC.

Test with dDEC parameter reversed comet guiding

First test, only dDEC reversed.

Test with dRA and dDEC parameter reversed comet guiding

Next test I reversed both dRA and dDEC:

Now it looks much better, the bad thing I couldn't do equal numbers of sub frames. And I also have a 8' bad polar align. Maybe just by luck this looks better.

I will come back when I have found out what's wrong.

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