A focused approach

In my last post on astrophotography I wrote about planning for dark skies and about my plans to build an observatory. Well, finances haven’t permitted for the observatory – this year – so this month I opted to get my existing telescope and mount working at their theoretical best.

This mostly boiled down to:

  • Improving the ability to achieve and hold accurate focus
  • Getting the mount running as smoothly and accurately as possible
  • Making small improvements to the way I set things up
  • Getting the optics as precisely collimated as possible

If I do all those things then the limiting factors should be the intrinsic limits of the kit I have and the environment, and I should be able to produce some great pictures with all that! So I started off, knowing the focuser was mechanically weak and a real problem in terms of operating the scope, by replacing the focuser.

Focusers and motors

This started out as a “undo 4 bolts, replace 4 bolts” project and turned into a bit more work. It also required me to remove the secondary mirror for the first time, which meant tooling up to collimate that properly – my laser isn’t enough to set the position completely.

The new, on the left, and the old. Note the chunkier construction, much bigger bearings, and the larger motors. The new one’s also larger, and internally baffled to help cut down on stray light.

The holes in the tube didn’t quite fit the new plate, so I had to drill new holes – I measured very carefully, several times, with different measurement approaches (not wishing to recreate the Hubble problem of relying on a single instrument). The position isn’t critical but it makes life easier if it’s in the right place.

Nervous drilling

The focuser I went for was a Baader Steeltrack Diamond. To summarise the choice, there’s only a few major groups of manufacturers – Moonlite and friends sit at the “fine for visual, okay for imaging” end of things with traditional Crayford designs. Then you’ve got people like Baader and JTW who are a bit more serious about focuser slop and rigidity. Then there’s Feather-Touch. FT appear to be held in messianic regard by literally everyone, which I can only assume is for good reason. They’re also two to three times the price. Which rules them out. Baader’s Diamond NT focuser appeared to be very well regarded mechanically, and having bought a number of parts from them I knew they were of good quality.

It didn’t disappoint – it’s very well made, and manual movement of the focuser when it arrived was buttery. I popped the fine focus knob off and prepared it for the addition of the focus motor

If you’ve not done imaging before you might think that motorising a focuser is a bit excessive – and indeed when I started out I just focused manually once at the start of the evening and then left the camera to it. But to get the most out of a telescope, frequent or constant refocusing is needed, to compensate for contraction of the telescope and optics due to temperature change. It’s also useful to be able to let the computer focus for you to achieve the most precise focus.

Again, there are many options here. I opted for a lower cost option which was fairly well reviewed, the Primaluce Lab Sesto Senso focus motor. This despite it missing a key feature, temperature compensation. This feature automatically moves the motor based on a temperature reading, rather than having the computer do it for you. However, most software supports doing this. Sadly, KStars/Ekos does not – yet.

The new focuser and motor installed on the tube

Spot the difference

After installing the focuser and motor I had to re-install the secondary and collimate it – this was actually pretty straightforward. However I also wanted to replace the centre spot on my telescope with a “hotspot” to make barlow laser and autocollimator checks easier, so the primary mirror came out too. Both got a very gentle soak and rinse with no agitation, and then the old primary spot was removed with some isopropyl alcohol.

The old spot and mirror in its cell
The mirror, spotless!
Spotting the mirror using a Cats Eye template, weighted down with cotton wool. There was a lot of careful staring at this before I affixed the spot.
The completed install.

After this there was just a lot of very time consuming adjustment to get everything set up as well as possible. This mostly just involved staring down cheshire eyepieces and then moving things very slowly with an allen key until it all looked like it should.

A quick barlowed laser check as part of reassembly, looking down the tube – you can see the reflection of the centre trefoil in the middle, which is actually a reflection off a piece of paper in the bottom of the barlow in the focuser.

I still need to add an autocollimator to my toolbox, but the Catseye ones are quite dear, so that’s a “next month” purchase. That will however be the last tool I need to add there, I think!

Mount problems

I had been seeing issues with my tracking the last few attempts I made to set up, so wanted to verify my mount was mechanically sound. This mostly involved adjusting the worm carrier blocks – large metal blocks which form both part of the housing and the mechanism by which the worm meshing can be adjusted. This, again, involved a lot of slackening off one thing, tightening another, then rotating the whole axis through 360 degrees to make sure nothing bound or stuck.

Dismantling an axis driver to check everything is okay – the worm carrier block is the lower bit of metal, where the big gear sits. Behind this is the worm gear shaft.

After a lot of measurement, trying to work out what was going on, I realised it was the obvious thing – polar alignment. My Polemaster – a camera that sits on the mount to do a polar alignment – wasn’t getting good enough results, and that was all I was using. I used a method called drift alignment and improved from ~15 arcminutes accuracy down to about 2 arcminutes. This has radically improved my guiding, which is now down at around 1 arcsecond – where it should be! The adjustment knobs on the EQ6-R Pro are the limiting factor now – it’s just not possible to get the alignment much better.

Balancing the mount more carefully has helped, too, and I’ve rotated the telescope in its tube so the focuser points at the RA axis. This means that as the axis rotates the weight distribution remains constant. It also means I can’t really use the telescope for visual observation, but I’ve not done that in a long while!

I’ve also added some Celestron anti-vibration pads to the tripod. While a cubic metre or two of concrete would be better, these should help isolate vibration from the ground and also help with oscillation in the tripod itself as a result of mount movement.

To help minimise the number of cables coming off the mount I’ve also put my INDI server on the tube itself by mounting a Raspberry Pi, 12V-5V step-down, and USB hub. This also helps to counterbalance the focuser around the Dec axis. There’s now only three cables to the mount – 12V, Ethernet, and the mount control cable.

The other major upgrades I’ve made lately have been on guidescope mounting – I now have some very solid aluminium guidescope brackets that a colleague at work milled for me. This does appear to have solved the differential flexure problem. I still want to upgrade the camera and explore off-axis guiding, but it’s a great improvement.

Worth it?

It’s too early to say, really, but the indication is that probably, together, this has all produced a much improved system for astrophotography for not much (in AP terms) money. This image of M101, the Pinwheel galaxy, I produced last night with less than 2 hours of light:

Precise focus has helped massively, though temperature compensation and per-filter focus offset automation would be very welcome additions to Ekos – it might even be enough to push me back to Sequence Generator Pro, though I’m very much enjoying the INDI/Linux approach so far (bugs that require me to completely shut down KStars mid-session aside). The mount guiding is definitely a big upgrade over where it was – I think I had broadly been getting lucky with this over winter, though I suspect the colder atmosphere might’ve helped the Polemaster.

All in all it’s a good step forward – now I just need some really cold clear skies!