Guiding At Last! And a million more problems…

Well it’s been a while! Pretty much an entire month since my last post where I updated you about all my purchases…and my ever depleting wallet.

As it turns out, the weather didn’t play ball after all. It’s been constant cloud and rain for almost the entire month, not giving me any chance to try everything out. Not for more than an hour here or there at any rate.

One thing that the poor weather has given me chance to do though is fine-tune my mount. Last time I was getting some weird backlash errors in the declination axis – It was stopping me from properly calibrating with the guiding assistant in PHD2. It could not complete the ‘measuring backlash’ step, as when the mount reverse direction, it did not re-engage the gear before the assistant finished.

Fortunately though it was a simple fix. It did mean taking apart the motor compartment though and physically moving the motor back and forth. It only took 5 minutes, and another five minutes to run the dec-axis through 360º multiple times. It did take me three attempts to get no binding in the gears though, but I eventually got it right. I did also check the worm-gear before replacing the motor, but there seemed to be no play in this, so I left it as it was for now.

With that all done, I had no real way to test that everything was now sorted. Luckily, this past Monday night, there was a clear break in the endless cloud cover that allowed me to test out the hyper-tuned mount and to get a decent amount of exposure on a target in the process. Another bonus is that daylight savings are now over here in the UK, so sunset was 16:49 and Astronomical Darkness was 18:46, only giving me an hour window to get set up, polar aligned and start testing.

It felt so good to be back outside, I was practically buzzing! It was obvious that I was out of practice, as I had polar aligned before I made sure that the mount was level. An honest mistake I suppose.

For ease of setup and packdown, I moved location to the bottom of the ramp. This makes it easier to put the USB cable through a window, less distance to carry the scope in and out the house, and to get targets for longer as they drop quickly into the west at this time of year.

My original plan was to get M33 – The Triangulum Galaxy, however it wouldn’t be in view for another couple of hours from this new position. So, within N.I.N.A. I started scanning the Sky Atlas Database for a target that was near the meridian already, and would be in clear view all night. I thought I had decided on a few objects, but after checking them in Stellarium, they were quite dim with a magnitude of around 10 or less. That’s when I noticed a target that I had in mind for a while, but wanted to wait until I was guiding and using a coma corrector. Well both of those requirements were now fulfilled. So I had finally decided on my target for the evening: IC 5146 – The Cocoon Nebula.

The Cocoon Nebula is a bright (magnitude 7.2) reflection & emission nebula in the constellation Cygnus; because of the clocks going back an hour, it was now directly overhead while I was setting up. The physical nebula was quite small in N.I.N.A.’s Framing Assistant, but from previous research I know it is accompanied by a dark nebula, Barnard 168, that formed a dark dust lane that trailed westward behind the red cloud. This dust lane can be seen even in a single exposure, so is quite easily to frame the whole target. I framed the image with the Cocoon nebula towards the top right corner, with the dust lane trailing towards the bottom left, diagonally across the image.

Once the framing was chosen, the telescope slewed over to where it thought that target was position, using plate solving to hone-in on the target. Within two attempts to was on the target with an accuracy of 15 arc-seconds. At this, I started the imaging sequence.

Only after five 300-second exposures, the auto meridian flip feature in N.I.N.A. kicked in; stopping the sequence 10-minutes after the object had passed the meridian, flipped the scope, re-centred the image on the target, re-selected a new guide star, and re-started the imaging sequence. I didn’t have to touch a thing! At the time though I didn’t know this, so I watched the whole setup like a hawk!

As it turns out, whatever I did by moving the motor seemed to help with the backlash issue in the declination axis. It allowed the guiding assistant to finally finish calibrating by measuring backlash. I applied its recommendations and the guiding was working a treat, with an average total error of less than 1 arc-second!

PHD2 Guiding Graph

It’s not perfect, it’s a bit up-and-down, but for a first attempt it’s not half bad.

Single Exposure Within N.I.N.A.

This image shows a single exposure, along with the guiding graph including 3-pixel dithers – this translates to 12-pixel shift in my 600Da. The dithers do take a while to recover, so that’s something that I need to look at in the future.

In the end, I managed to get 42 300-second exposures before the clouds rolled in around half 11. I took 15 dark frames and 30 bias frames that night, taking me up to 01:40 in the morning. At that, I called it a night.

The following day, I took the 30 flat frames and stacked the best 37 light frames in Deep Sky Stacker as per usual, giving me a total exposure of 3h5m. In Photoshop, I ran through the usual stretching technics, and noticed along the top of the image a large blurred patch. It was not there in any of the individual frames, so must have been introduced by DSS. Another thing to look at…

To make the most of the the long and thin dust lane, I decided to crop it in a 16:9 aspect ratio. With the longer exposures that I’m now using, it looks like I need to readdress the coma corrector backspacing. It looks okay until you start to pixel-peep near the edges. Admittedly, I am a perfectionist, and so I won’t settle until it’s perfect across the entire field.

I’ve figured out that a solution to one problem brings another 2 more problems along with it! I need more clear skies to fix the multitude of niggling problems that now plagues my system. Looks like that’s not going to happen for a long while now, at least for the next fortnight.

Either way, I’m proud of this new image. It’s my first time using a guide scope, and so it can only get better from here!

Astrometry Annotated

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