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Off axis auto guiding on comets

Off axis auto guiding on comets

  1. My choice of 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

1: My choice of equipment and software to do off axis guiding on comets

Latest update: 2016-01-21

Comets and asteroids are sometime fast moving objects relative to stars and then it's tricky to take sharp photos of them.

You can reprogram your motors in the mount to have speed adjusted for the comet, but then you are limited to the precision of your mount. You can also have a separate telescope and auto guide on the comet core and it works if you are lucky, sometime comet core are so diffuse that it's not possibly.

When I visited the observatory at La Palma 1997 I come in contact with the people that was collecting data of the comet Hale Bopp. They did something that I noticed, they follow a star with the guide camera and did some trick to follow the comet. I don't now any details more than that the guide camera was analog and with an intensifier. The guidecamera was also free moveable relative the telescope's main camera by stepper motors.

Now 17 years later when I have my own goto mount (EQ6) I wanted to do something similar as an amateur astronomer and in Windows environment. By luck I found that the program I'm already use, PHD2 had a beta version that could do exactly that.

How it works? PHD2 guide on a star and let the software calculate how much the speed differ from the star and offset the guiding speed online. Then it's possibly to even have a off axis guider. The last is what I'm going to talk about here.

The hardware you need to do it the same way as I do it. I have a EQ6 Synscan controlled by EQMOD, but I believe most computer controlled goto mount will do. And you also need a guide camera, I have the QHY5 and it's connected to the main telescope through an off-axis adapter.

Here is a list of what software I use and where you can download them, all of them are free.

There are maybe other software that can do this too. Frank wrote to me and told me that his program MetaGuide can do this years ago: Bliss/MetaGuide/

I don't have any experience from that software, but would be interesting to do a test in future.

I assumes you are already familiar how to use a guide camera that is computer controlled before continue with this. And you must know how to setup PHD2 for auto guiding, otherwise it will be too complicated. See the PHD2 manual, PHD2 is under development so function will be added and changed from time to time.

In its simplest form you only need the guide camera and PHD2, but then you loose some functions.

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2: Setup of PHD2 Beta

PHD2 (Push Here Dummy) has a really nice support implemented for comet guiding. Before starting the comet auto guiding enter the guide telescope's focal length and guide cameras pixel size (you find it under the brain symbol). Then do a calibration of PHD2 and then let it guide on a star nearby the comet. You must have all that working before proceed. Don't forget to enable the server in PHD2!

PHD2 setup comet tracking 1

Under tools in PHD2 you have a choose of comet tracking, click on it. A new menu popup like the one down here.

Setup comet tracking PHD2 2

You can calculate the comets speed relative stars and enter it here. If you are used with it NASA has a page to assist you: horizons.cgi?s_type=1#top
I will not describe here how to do it in that way, I have a better solution.

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3: Setup of CdC (Skychart) Beta version

Now, with CdC (Skychart Beta version) up and running you can transfer the needed data from that software.

Skychart setup for comet tracking 1

Under setup you have "manage tool box setup" it's a new function in CdC.

Skychart setup for comet tracking 2

Here you see how I have it setup. What you need is the "PHD Guiding interface". If it's not there, right click on an empty line and choose it.

Skychart setup for comet tracking 3

Some simple steps to find the comet guide rates.

  1. Now you will have this tool under some F key, in my case the F3 button.
  2. Push F3 (or your F function key) to get the menu up.
  3. PHD host and port are normally all ready there.
  4. Choose a comet or asteroid you want to track and guide on. When you left click on the object you will get the relative speed data under Lock Shift menu.
  5. Click on the Connect button and CdC will connect to PHD2 and to other equipment that run over ASCOM.
  6. The click on "Set rates" and data will be transferred to PHD2.
Skychart, drift data avible 4

How it will look in CdC with the comet drift speed. You must also have the latest orbit data for your comet or asteroid, you can download it in the setup menu of CdC.

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4: Start comet guiding in PHD2

PHD2 Comet tracking start 1

If you are already guiding on a star you just have to enable comet tracking.

PHD2 Comet tracking enabled 2

What will happens now is that PHD2 will start moving the star it's locked on at the same speed as the comet move relative the stars. You must have space around the star on the guide chip so it doesn't hit the borders. How long you can guide depends on the guide cameras FOV and the relative speed of the comet. For more far away comets I can guide for hours.


I noticed a problem when I did a meridian flip of the telescope, it cause the rDEC rate to be of wrong sign, in EQMOD there is setting to let EQMOD send over which side of pier the telescope is. But there is little bit confusing about this. It's very important to have the latest version of EQMOD, v1.28m or later.

Here is two pages of information about the ASCOM interface an how to handle pier side information (thanks Patrick for the links): neo/groups/EQMOD/ conversations/ topics/42570 Help/Developer/ html/ P_ASCOM_ DeviceInterface_ ITelescopeV3_SideOfPier.htm

I will give more instructions later when I collected more experience. It's only when pointing to East (northern hemisphere) with the telescope the sign of rDEC get wrong, You can also change it manually in the PHD2 comet tracking window if you don't succeed to setup EQMOD correct.

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5: Overview of Astroserver Screen

Here is an overview of the screen with the software I have running under a comet tracking session.

Tracking on comet

One very interesting aspect off this is, you can guide on very weak moving object that guide camera and main camera could not see. But you can see it afterwards after stacking all the sub images.

You can find more information on the PHD2 site how to use comet tracking: man-dev/Tools.htm#Comet_Tracking

This is my first try to guide on a comet with this technique: Comet Catalina

Could it be easier and more handy?

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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:

Comet C/2014 W2 Pan-STARRS

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 fullversion

The idea is to indirect track on weak 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

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. 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 oriented along RA axis.

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: horizons.cgi?s_type=1#top

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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