Perfect alignment

I’ve been having a really troubling time with my new AZ-EQ6 equatorial mount.  I say new, but I bought it about a year ago – second hand – and it has puzzled and infuriated me ever since.

First up, the previous owner had snipped the power supply cable and wired it to a mains adapter.  No problem there, so I undid his modification and wired it back to a “cigar lighter” plug for use with a battery pack.  Despite all the care in the world, I somehow got it wired to the wrong polarity and promptly blew the mother board as soon as I switched it on.

Embarrassing.  Took it to the “repair and service division” of one of the UK’s best known retailers, who smiled sympathetically, wagged a finger, tut-tutted and said they would check it out.  Days turned into weeks, weeks turned into months, gentle reminders turned into an increasingly despairing string of phone calls and emails, and eventually they returned it, fixed.  Their apology for the shameful delay was not to charge me.  Fair enough.  We have since made up – you can’t stay angry all your life now, can you?

There’s no doubt that the AZ-EQ6 is a fabulous mount.  Some evolutionary changes and some revolutionary changes make it so much better than the standard EQ6, so my expectations were really high.  On my old EQ3 mount, I could polar align by eye and track with a 300mm telephoto lens for up to 8 minutes without star trails.  It took a lot of practice, but I could do it time after time.  Here’s Comet C/2011 L4 (PANSTARRS) from May 2013 at those settings:

PANSTARRS approaching Cepheus 300mm f/5.6, ISO 800, 8 min. Single 8-minute frame.

PANSTARRS approaching Cepheus
300mm f/5.6, ISO 800, 8 min.
Single 8-minute frame.

The AZ-EQ6 is a heavy beast, and I use it out of the back of the car.  Incidentally, it lives in my basement, so each astro-session starts with lugging the boxes up the stairs and out the front door to the car.  Practice makes perfect, however, and I can set it up in the dark in about five minutes, then run through the alignment routine and we’re good to go.

Except we weren’t good to go.  It just didn’t work very well.  The “advanced polar alignment” routine, using feedback from the star-aligned telescope to adjust the polar alignment of the mount, kept producing crazy results.  Crazy adjustments, endless frustrations.  I resorted to drift alignment, convinced that I must be getting something wrong but baffled as to what that might be.

Then we hit rock bottom.  Drift aligning produced near-perfect levels of polar alignment, but the stars still trailed for anything over 60 seconds of exposure, even at a relatively modest 300mm focal length telephoto lens.  After several sessions it occurred to me that the drift was entirely in RA, and the mount drive was therefore running slightly fast.  There was no drift at all in Dec.  I sent an email to SkyWatcher to ask them how to fix it.

The moment I sent that email, it came to my attention that there were a couple of “firmware updates” on the Skywatcher website: one firmware update for the motor drives, one for the handset.  The list of “fixes” for these updates read like a copy of my list of queries.  Maybe, just maybe, I wasn’t doing anything wrong at all.  Could it really be that the mount was inherently faulty?

I downloaded the updates and reflashed the motor drive and handset according to the instructions.  The polar alignment feedback routine is now slightly different, so I took the first opportunity to test it out last night.  After dodging clouds to complete two iterations of the alignment routine, I watched the sky cloud over leaving just one star visible: Vega on the western horizon.  Nothing else to do but take photos of Vega.  Here’s the result – first a couple of frames through the 300mm telephoto lens, spaced 7 minutes apart:

Vega at 20:19 300mm telephoto.

Vega at 20:19
300mm telephoto.  Field of view 4.5º x 3.0º

Vega at 20:26 300mm telephoto.

Vega at 20:26
300mm telephoto.  Field of view 4.5º x 3.0º

That looks good, but it’s not the position of Vega that caught my eye, as that’s impossible to tell without layering the frames together.  It’s the stars at the edge of the frame.  I had become so accustomed to seeing them drift off between frames.

Next, a single five-minute frame:

Vega for 5 minutes. 300mm telephoto.

Vega for 5 minutes.
300mm telephoto. Field of view 4.5º x 3.0º

The image is poor quality because the clouds kept coming and going, but the important thing is that it appears not to be drifting.  On the other hand, I could do that with the old EQ3:  the AZ-EQ6 was supposed to do better than this.

Okay, last test before the clouds completely finish the evening.  Prime focus through the Altair Wave 115/805.  That’s a focal length of 805mm.  I leave the shutter open for six minutes while I start to pack stuff away:

Vega for 6 minutes. 805mm prime focus.  Field of view 1.7º x 1.1º 1.4 arc seconds per pixel

Vega for 6 minutes.
805mm prime focus. Field of view 1.7º x 1.1º
1.4 arc seconds per pixel

A bit fuzzy from the cloud, but it looks pretty solid to me.

That’s six minutes, unguided.  Happy days.  That’s what I believed I had bought in the AZ-EQ6, but had begun to think I’d made a mistake.  Next clear sky, I’ll see how far it can go!

One thought on “Perfect alignment

  1. Pingback: Theoretical alignment accuracy | simon in the lakes

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