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!

When the skies are bright, plan for darkness

Gotten a bit quiet here, hasn’t it? Well, here in the UK, it’s wonderfully sunny and bright. We don’t get proper darkness, and the planets are in an awful position, so imaging deep-space objects is a bit of a non-starter, or at least challenging. We’ve also had a run of crap weather, just to drive the point home.

I’ve been using the time instead to plan out my next astro-related project (though I may well push the execution out to 2020, just to make sure I have the cash to get it done right) – a fully automated roll-off roof observatory. The logic behind this is simple – my next “improvements” to my imaging system that I can make are:

  1. Upgrade the camera – already got a pretty good camera, so this means something quite high-end (>£3-4k), and would just get me more sensitivity/bigger pixels/larger field of view
  2. Upgrade the telescope – already got a decent Newtonian so a meaningful upgrade means either a high-end Newtonian/R-C astrograph (£3-4k) or a decent large-aperture apochromatic refractor (£4-5k)
  3. Add a second telescope – to do planetary imaging I could add a SCT or long-focal-length scope of some other sort, but the planets will be too low for the next couple of years for serious imaging, and it’d still be £2-3k of investment
  4. Update my telescope’s other parts (focuser, focus controller) and invest in tools (collimation, etc) – more reasonable investment (£1-2k) but just gets me slightly better images – this is my favourite option if I don’t do the observatory this year
  5. Build an automated observatory – easily doubles the number of images I can capture with my existing kit, thus acting as a massive force multiplier for my previous investments – but £4-5k at least!

So the biggest “bang for buck” is definitely the observatory, but only if it is fully automated. I’ve lost track of the number of nights where the sky was beautiful and clear, the clouds nowhere to be seen, ground and ambient temperatures low enough to make seeing incredibly clear – and I’ve been packing away the telescope at midnight because I’ve got work tomorrow, despite the further 7 or 8 hours of imaging I could have. And then there’s all the “well, it might be good enough, but…” nights – nights where the forecast says it won’t be good enough, but you might get lucky; often this involves going out frequently to stare at the sky, setting up if I feel optimistic, and usually being disappointed – but often not.

With a fully automated and remotely driven set-up the setup time is nil, as is the tear-down time. With the scope set up permanently, with the camera and other components mounted, there’s much more scope (no pun intended) for tweaking and tuning in advance of an imaging night, and fine tuning on cold-but-cloudy nights that just isn’t possible when you’re stripping the whole thing down each night. Being able to work in the dry and the day has a lot of appeal.

System-wise, full automation is pretty simple – you need a box with relays to drive motors and read sensors, a proper cloud/rain sensor (hard-wired to the relay box, so if any computers fail there’s a pretty dumb box responsible for shutting the roof when it rains), and a system capable of automating the selection of targets (what’s good tonight?), acquisition of images (frame the target, autofocus, guide and image), and the observatory start-up/shut-down. I’m most of the way here – I need the relay box and auto-focuser. The rest is already ready – I’ve been using INDI/Ekos/KStars for a while which can do all of this. The main INDI instance for the observatory will run on a 1U server in the observatory, with an INDI server on a Raspberry Pi 4 strapped to the telescope doing actual image acquisition and telescope equipment control. This makes the pier-to-desk cables simple – 12V for power, USB for the mount, and an Ethernet cable for the rest, with just 12V and Ethernet onto the telescope itself.

Making a plan

So, the objectives of this build are:

  • Full automation – but at a minimum, a roll-off roof which can open and close under all circumstances for safety – so I can program the observatory to image opportunistically
  • Imaging-stable pier, with room to expand – just the one pier, but room to set up a second non-isolated pier for a small solar/planetary telescope (isolation is less critical for these applications)
  • “Warm” room with enough room for a server rack, desk, chair and a little storage – somewhere I can sit while setting up
  • Good visibility down to ~30 degrees everywhere
  • Strong enough to resist opportunistic forced entry and 100mph wind when closed

Beyond this – it’s basically a shed! So I’ve started by getting a bunch of books on shed design and construction and reading them. My day job at the moment is (mostly) telling people how to properly build a fibre optic network, so I know a reasonable amount about concrete, aggregates, rebar, admixtures and slab design. Making a good solid observatory is mostly about mass, just like in acoustic isolation design, and I’ll be using almost an entire ready-mix concrete truck worth of C40 low-moisture concrete to pour the base slab and the (isolated) pier. The framing and design of walls, floors and doors is all fairly simple, though benefits from careful planning to make sure all the services will work and the structure remains rot-and-rat free for a few decades.

Some basic renders of the general layout – working floor-up. Note the duct from pier to warm room to allow for cables to reach the telescope safely

The tricky bit is the roll-off roof – I need to keep this building rodent-proof and ideally near-airtight to aid in humidity/temperature control. I will use forced, filtered airflow for cooling with a positive pressure maintained to minimise dust ingress. Active cooling with the roof shut will help cool-down times and avoid any kit getting too hot in summer. This means the roof needs to seal well onto the frame when shut. I also need to be able to shut the roof at any time – that means any internal rafters need to be minimal or non-existent, so the telescope doesn’t have to be “pointed down” to let the roof pass. This means when the mount fails or is unsure of its position the roof can still shut safely to keep the rain out. The roof needs to roll back enough to give good visibility, so the whole thing has to roll onto rails that extend beyond the back of the warm room. To further improve visibility and keep rain off the rails, some of the side walls will be mounted on the roof so the walls “lower” as the roof rolls off. There’s a lot of complexity in this (and it has to be something I can build), so this is taking some time to work out.

I’ve started designing in detail in Autodesk Fusion 360 – while I’ve used Sketchup for this sort of thing in the past, Fusion 360 in Direct Modelling (non-parametric mode) is about as user-friendly and can produce much prettier outputs as well as decent engineering drawings.

An early rendering of the pier and shuttering for the initial concrete pour
An early drawing with some detail/section views to show the base layout and design – the deep, chunky base should help isolate the pier from surface vibrations/movement, and the really deep and heavy pier root should by virtue of being heavy do the rest

I’ve also reconstructed my current telescope and mount with photogrammetry so I can build a digital model and check the motion all works – I haven’t gotten around to tidying up the mesh into some simpler models, but it’s a great reference for getting the dimensions and motion right.

Location, location, location

The other question is where to put this – I dithered quite a bit and in the end took a lot of level photos around the garden at twilight with a Ricoh Theta S 360 degree camera at roughly my telescope’s aperture height. With the moon visible in each photo and knowing where and when I took the photos, I could align the photos to north with a fairly simple Python script which spat out a nice set of data for horizon plotting.

Plotting horizons straight out of images. Probably should release the code for this…

It turns out there’s only a few places where I don’t enjoy visibility to 30 degrees pretty much everywhere, so I decided to plug the panorama for my favoured location into Stellarium – this turns out to just involve having a panorama with a transparent sky and a small .ini file to set north properly.

My observatory’s home, Stellarium-ready
… and loaded into Stellarium, so I can see how things will look – spending all the time with Photoshop’s background eraser to get the trees properly semitransparent makes a big impact on the visuals of this (though in summer they’re somewhat more opaque!)

The chosen location makes power and network connectivity simple enough – with 25 metres of mains cable and single-mode fibre I can connect to proper mains and Ethernet, only one switch hop away from my storage arrays.

Security is a concern – that field is adjacent to a footpath, though set back from the road, and there have been break-ins in the area. Other than making the building fairly secure against “opportunistic” crooks – reinforcing the door, lack of windows, and a solid lock – there’s not a lot that can be done. PIR sensors externally won’t work due to the abundant wildlife, so a combination of internal sensors and an alarm to make a racket if someone does force the door or climb in through the open roof will have to do. CCTV around the perimeter might work but could work just as well as an attractant as a deterrent, and wildlife would probably again make alarming impossible. I’m also planning on using a worm geared or lead screw based roof mechanism, which should be very hard to force open.

Making plans

I took the view early on in this that I wanted to build this myself. I’m still not 100% sure about this, but I think it’s a reasonable project and something I should be able to do! I am budgeting for some help, though, and will have to hire kit in regardless – a mini digger for the groundwork, compactor to pack down aggregates, concrete vibrator to settle concrete in the forms, etc.

I also need planning permission. I started with a footprint that wouldn’t normally need it, so long as the building isn’t tall – but I’m in a conservation area, which means “permitted development” doesn’t really apply. I’m not concerned about getting planning permission – it’s a small building in an otherwise empty field (except for a shed we’re going to remove) and will blend in just fine. Having to go through planning permission also means I can relax around some of the limits that I’d otherwise be avoiding.

Working through the material costs there’s easily £2k, maybe £3k of materials – labour would be another £1-2k atop that if not more. That’s quite an investment, and I’m really keen to make sure that everything about this is right – giving up power to a third party feels risky. It may be that when I get the design done I sit down with some local builders that I trust and see what they say.

The first step remains the plan and design, which is taking time – but I think time invested here is time well spent. I may not start until later in the year, or even early next year – one more winter without it wouldn’t be the end of the world. It’s going to be a fun project if I can get the plan right!

MORE DOMES

Fans of domes will be wondering why I haven’t just dropped £3k on a nice big Pulsar/insert-vendor-here dome. The answer is simple:

  • It’s not £3k, it’s £7k by the time you’ve automated it
  • It’s impossible to insulate the roof nicely – you end up slapping neoprene sheets up with glue just to stop condensation build-up raining on your scope
  • They’re relatively small and uncomfortable to work in unless you get big ones which are even more money
  • They only allow for a single telescope
  • They’re definitely harder to get through conservation area planning permission committees

I’ve looked at a few other dome designs and while there’s some good contenders they all have similar problems. I did consider making a “clever” geodesic dome – something I could build pretty cheaply but which would still have decent wind resistance – but automation remains the problem. Ground-level domes (where the whole structure rotates, rather than using a rotating section on a cylinder) make the construction simpler, but the bearing and rotation mechanism have to cope with increased gravity load and all of the wind loading. Cylinder-style observatories have similar problems.

The round/dodecahedral designs of these structures also make literally everything harder. Want to bolt a light to a wall? It’s not flat, so if you want it level/flat you now get to make a bracket… weatherproofing, insulation, and more all get more complicated. Having four flat walls which never move makes life simple – mounting insulation, cable entry glands, coolers, dehumidifiers, fans/filters, lights, shelves, etc is all so much simpler.

So – no dome here for now.

And another thing…

While we’re building a light-shielded box in a quiet location with power and networking, what else could we do? I’m also going to include infrastructure to support a small ground-level dish and motors for radioastronomy, as well as some mounts for meteor spotting cameras, an all-sky camera, and a weather station. I won’t have all this on day one, but putting a little extra concrete in now is way easier than doing it again later, and it means I can put in cable ducts to make wiring it up simpler. The cost of the pads, etc is tiny and turns those future projects from a pain into something much simpler.

Adventures in Differential Flexure

How’s that for a thrilling title? But this topic really does encapsulate a lot of what I love about astrophotography, despite the substantial annoyance it’s caused me lately…

Long exposure of M51 in Hydrogen Alpha – 900s

My quest for really nice photos of galaxies has, inevitably, driven me towards narrowband imaging, which can help bring out detail in galaxies and minimise light pollution. I bought a hydrogen alpha filter not long ago – a filter that removes all of the light except from a hydrogen emission line, a deep red narrow band of light. This filter has the unfortunate side effect of reducing the total amount of light hitting the sensor, meaning that long exposures are really required to drive the signal far above the noise floor. In the single frame above, the huge glow from the right is amplifier glow – an issue with the camera that grows worse the longer my exposures. Typically, this gets removed by taking dozens of dark frames with a lens cap on and subtracting the fixed amplifier glow from the frames, a process called calibration. The end result is fairly clean – but what about these unfortunate stars?

Oblong stars are a problem – they show that the telescope failed to accurately track the target for the entire period. Each pixel in this image (and you can see pixels here, in the hot pixels that appear as noise in the close-up) equates to 0.5″ of sky (0.5 arc-seconds). This is about two to four times my seeing limit (the amount of wobble introduced by the atmosphere) on a really good night, meaning I’m over-sampling nicely (Nyquist says we should be oversampling 2x to resolve all details). My stars are oblong by a huge amount – 6-8″, if not more!

My guide system – the PHD2 software package, an ASI120MC camera and a 60mm guidescope – reported no worse than 0.5″ tracking all night, meaning I should’ve seen perfectly round stars. So what went wrong?

The most likely culprit is a slightly loose screw on my guidescope’s guiding rings, which I found after being pointed at a thing called “differential flexure” by a fantastic chap on the Stargazer’s Lounge forums (more on that later). But this is merely a quite extreme example of a real problem that can occur, and a nice insight into the tolerances and required precision of astronomical telescopes for high-resolution imaging. As I’m aiming for 0.5″ pixel accuracy, but practically won’t get better seeing than 1-2″, my guiding needs to be fairly good. The mount, with excellent guiding, is mechanically capable of 0.6-0.7″ accuracy; this is actually really great, especially for a fairly low-cost mount (<£1200). You can easily pay upwards of £10,000 for a mount, and not get much better performance.

Without guiding though it’s not terribly capable – mechanical tolerances aren’t perfect in a cheap mount, and periodic error from the rotation of worm gears creeps in. While you can program the mount to correct for this it won’t be perfect. So we have to guide the mount. While the imaging camera takes long, 5-10 minute exposures, the guiding camera takes short 3-5 second exposures and feeds software (in my case, PHD2) which tracks a star’s centre over time, using the changes in that centre to generate a correction impulse which is sent to the mount’s control software (in my case, INDI and the EQmod driver). This gets us down to the required stability over time.

My Primaluce Lab 60mm guidescope and ASI120MC guide camera on the “bench”, in PLL 80mm guidescope rings on ADM dovetails

The reason why my long exposures sucked, despite all this, is simple – my guide camera was not always changing its orientation as the imaging camera was. That is to say, when the mount moved a little bit, or failed to move, while the imaging camera was affected the guiding camera was not. This is called differential flexure – the difference in movement between two optical systems. Fundamentally, this is because my guidescope is a completely separate optical system to my main telescope – if it doesn’t move when my main scope does, the guiding system doesn’t know to correct! The inverse applies, too – maybe the guidescope moves and overcorrects for an imaging system that hasn’t moved at all.

With a refractor telescope, if you just secure your guidescope really well to the main telescope, all is (generally) well. That is the only practical potential source of error, outside of focuser wobble. In a Newtonian such as the one I use, though, there’s plenty of other sources. At the end of a Newtonian telescope is a large mirror – 200mm across, in my case. This is supported by a mirror cell – pinching the mirror can cause huge deviation (dozens or hundreds of nanometers, which is unacceptable), so just clamping it up isn’t practical. This means that as the telescope moves the mirror can move a little bit – not much, but enough to move the image slightly on the sensor. While moving the mount isn’t an ideal way to fix this movement – better mirror cells reduce this movement – it’s better than doing nothing at all. The secondary mirror has similar problems. The tube itself can also expand or contract, being quite large – carbon fibre tubes minimise this but are expensive. Refractors have, broadly, all their lenses securely held in place without issue and so don’t suffer these problems.

And so the answer seems to be a solution called “Off Axis Guiding”. In this system, rather than using a separate guide scope, you use a small prism inserted in the optical train (after the focuser but before the camera) to “tap” a bit of the light off – usually the sensor is a rectangle in a circular light path meaning this is pretty easy to achieve without any impact to the light that the sensor receives. This light is bounced into a camera mounted at 90 degrees to the optical train, which performs the guiding function. There are issues with this approach – you have a narrower (and hard to move) field of view, and you need a more sensitive guide camera to find stars – but the resolution is naturally far higher (0.7″ rather than 2.5″) due to the longer focal length and so the potential accuracy of guide corrections improves. But more importantly, your guiding light shares fate with the imaging light – you use the same mirrors, tube, and so on. If your imaging light shifts, so does the guiding light, optically entwined.

The off-axis guiding route is appealing, but complex. I’ll undoubtedly explore it – I want to improve my guide camera regardless, and the OAG prism is “only” £110 or thereabouts. The guide camera is the brunt of the cost – weighing in at around £500-700 for a quality high-sensitivity guide camera.

But in the immediate future my budgets don’t allow for either of these solutions and so I’ve done what I can to minimise the flexure of the guidescope relative to the main telescope. This has focused on the screws used to hold the guidescope in place – they’re really poorly machined, along with the threads in the guidescope rings, and the plastic tips can lead to flexure.

Before and after – plastic-tipped screws

I’ve cut the tips almost back to the metal to minimise the amount of movement in compression, and used Loctite to secure two of the three screws in each ring. The coarse focus tube and helical focuser on the Primaluce guide scope also have some grub screws which I’ve adjusted – this has helped considerably in reducing the ability for the camera to move.

Hopefully that’ll help for now! I’m also going to ask a friend with access to CNC machines about machining some more solid tube rings for the guidescope; that would radically improve things, and shouldn’t cost much. However, practically the OAG route is going to be favourite for a Newtonian setup – so that’s going to be the best route in the long run.

Despite all this I managed a pretty good stab at M51, the Whirlpool Galaxy. I wasn’t suffering from differential flexure so much on these exposures – it’s probably a case of the pointing of the scope being different and so not hitting the same issue. I had two good nights of really good seeing, and captured a few hours of light. This image does well to highlight the benefits of the Newtonian setup – with a 1000mm focal length with a fast focal ratio, paired with my high-resolution camera, I can achieve some great detail in a short period of time.

M51, imaged over two nights at the end of March
Detail, showing some slightly overzealous deconvolution of stars and some interesting features

Alongside my telescope debugging, I’m working on developing my observatory plans into a detailed, budgeted design – more on that later. I’ve also been tinkering with some CCDinspector-inspired Python scripts to analyse star sharpness across a large number of images and in doing so highlight any potential issues with the optical train or telescope in terms of flatness, tilt, and so on. So far this tinkering hasn’t lead anywhere interesting, which either suggests my setup is near perfect (which I’m sure it isn’t) or I’m missing something – more tinkering to be done!

Map of sharpness across 50 or so luminance frames, showing a broadly even distribution and no systemic sharpness deviance