Changes to my obsy

Now that I finally came round to the fact that my current telescope mount (Celestron CGEM) simply is not up to the task of handling my large and heavy Orion Optics VX12 Newtonian, I took this telescope off and replaced it with the much more manageable Astro-Tech 8″ Ritchey Chretien astrograph. Suddenly there looks to be more room to manoeuvre around in my Obsy. So on Wednesday (4/5/16) night I decided to drift alignment to improve on my Polar alignment using PHD2 guiding and its drift align tool.

As you can see in the picture below, the 8″ RC scope does not make my obsy look like a tiny closet unlike my 12″ Newtonian, although the dew shield does make it look larger than it really is. Now as I really want to use that 12″ Newtonian for imaging, I am waiting to upgrade my mount to a Skywatcher EQ8. I used the Celestron Star Sense Autoalign to align my mount to the sky last night, and at the end of my imaging session I put the mount into Hibernate and powered off. So when I next use the mount, I can simply power back up and carry on from where I left off.

Now changed to my Astro-Tech 8" RC telescope
Now changed to my Astro-Tech 8″ RC telescope with dew shield on

Before I did some imaging, I decided that my polar alignment really needed to be readjusted. Hence I used my ZWO 60mm Guide scope with 280mm focal length, paired with the QHY5L-II mono camera for guiding. Pointing towards the sky in the West first, I initially did the adjustments to the Altitude and found that it was off by over 20 arcminutes (due to some fiddling I had done the night before). Once I had improved on this, I moved over to pointing at the Meridien and did the Azimuth adjustments. As you can see from the graph below, I don’t think I can improve on the polar alignment in Azimuth with a pretty straight line.

Drift Align for azimuth done with PHD2 drift align tool - as good as it gets
Drift Align for azimuth done with PHD2 drift align tool – as good as it gets (-0.00′ error)

With the polar alignment seemingly sorted, I decided to test various guiding paramentes on PHD2 for 2 minute subs to see if I could find settings which would work for my setup. I found that I had to keep RA aggressiveness at 70, increase Hysterisis to 20 and reduce minimum motion to 0.05, using 1.0 sec exposures to get as round stars as I can with my setup. As you can see from the curves below the periodic error in blue does swing somewhere between +/- 2″, although with longer guiding it sometimes exceeds the 2″ mark. But the RA Oscillation looks about right at 0.51 (ideally around 0.5). I did manage to capture data set for M51 which I will process when the skies are not suitable for imaging.

Guiding using PHD2 with these settings allows for round stars from subs
Guiding using PHD2 with these settings allows for round stars from subs

I did find that at the end of the imaging for each colour set, that the declination had drifted off the screen. This tells me that the adjustment of the Azimuth settings may have affected the polar alignment for Altitude as well. Hence I did another drift align for the Altitude. This time round the initial start off point was not as bad as previously and I managed to get it to as good as I can get.

Altitude polar alignment results after second round of PHD2 drift align
Altitude polar alignment results after second round of PHD2 drift align – as good as I can get it

What did  I achieve on Wednesday?

  1. Successfully align the mount and calibrate the 8″RC to the StarSense Autoalign.
  2. Successfully adjust polar alignment to as good as I can get it.
  3. Successfully find the right guiding settings for my setup to get round stars.
  4. Imaged in LRGB the Whirlpool Galaxy M51 with my setup.

Bearing in mind that my imaging setup with 8″RC and Astro-Physics CCDT67 running at 0.538x telecompression, this works out as about 860mm focal length. So when paired with the 2.2 micron pixels of the ZWO ASI178MM-cooled, this means that I am imaging at 0.53″/pixel which is way beyond the seeing limit of our skies here in UK. So I am rather pleased to have even managed to capture what I did below, using the Celestron CGEM which some people have criticised on different astronomy forums as unguidable.

Single 3 minute sub with PHD2 guding
Single 3 minute sub with PHD2 guding

So it did not matter that I did not get much sleep, having gone to bed after 4 am. There is a sense of achievement that I am managing to do much more with astroimaging that I thought I could, and am learning different techniques to improve my imaging. Great!

Boon