Coma correction and off-axis guiding

It’s November, which means we’re well into the winter season for astrophotography, with early starts and long nights. So far, I think I’ve had two nights – the weather has been utterly absymal and I lost a few nights to not having everything ready to go.

This season I thought I’d treat my Newtonian (in an upgradeable fashion) to a new coma corrector and some guiding tweaks. All Newtonian telescopes suffer from coma – a phenomenon which causes the light field to curve near the edges, resulting in “comet-like” blurry stars in the corner of images. A coma corrector does what it says on the tin and fixes this, producing a flat light field for your eyepiece or sensor.

As with any bit of glass between you and the sky, though, there’s potential to make things worse. I bought a coma corrector when I was starting out with imaging, since it was clear I’d need something – but I bought a cheap one, a Baader MPCC Mk3. And while it does indeed largely “fix” the coma, it also introduced some other distortions to the field, which limited the sharpness I could achieve.

So this year, impressed by a second-hand eyepiece I picked up (a 17mm DeLite) I coughed up for a TeleVue Paracorr 2, which is widely regarded as the best coma corrector mortals can lay their hands on for a reasonable sum (for some definitions of reasonable).

I also wanted to have a crack at replacing my guidescope with an off-axis guiding solution – as I’ve written about in prior blogs, this should produce more accurate guide corrections without any differential flexure or mirror slop. To this end I picked up a second hand ASI174MM Mini guide camera, and a ZWO off-axis guider. The off-axis guider uses a prism to bounce otherwise-unused light from the edge of the visual field into the guide camera.

Adapters, adapters, adapters

Of course, having a new shiny thing means integrating it with your other shiny things. In my optical train I had my MPCC, an adjustable spacer, the filter wheel, and then the camera. Between the back of the coma corrector and the sensor there has to be exactly 55mm of space (1mm of path is added by the glass filter) to get the focal plane to converge properly.

And of course then there’s the threads. T2 is the main one I use for the imaging system – being a small sensor as it is – but M48 is also used. What isn’t used is what TeleVue supply by default which is a bizzare 2.4″ imperial thread, because the Americans have yet to be civilised in this regard. So, after a quick stop at Teleskop Express to pick up a special TeleVue to T2 adapter, we were sorted.

But then it gets complicated – we have to get the right back-focus from the Paracorr’s top lens surface to the sensor, now adding in the off-axis guider. I ended up with a smorgasbord of adapters and spacers and padding shims, and a bunch of diagrams. As it turns out I was hideously overthinking this and ZWO simply standardised all their depths so you can do it all with the kit they supply between the camera, filter wheel and OAG.

I’ve actually ended up shimming things a little to get closer to a mechanically correct fit for the Paracorr lens, but still might do some other adjustments to get the Paracorr-to-sensor distance spot on and reduce vignetting for the guide camera.

So this is what it all looks like, assembled…

Thoughts on the Paracorr

The overall initial impression, though, is very good. Mechanically it’s a very well made thing and my only complaint (though I understand why they haven’t done this) is the lack of a safety stop on the outside of the tube. Optically, it looks superb in comparison to the MPCC. I’ve only shot a few frames so far but even unguided they’re looking great, with sharp stars to the edge (some polar-alignment drift aside) and good detail in the middle. This is an unguided 120s exposure of M42.

However, the lack of a good manual hurt me. The documentation for the Paracorr is incredibly sparse for imaging use. The adapter has no mechanical drawings in terms of relation to the top lens surface, meaning you get to guess at what the offsets end up looking like. The focuser positioning took me longer than I’ll admit to get set right, too.

So now it’s all set up, it’s looking great – though the weather’s back to abysmal – but TeleVue could do with writing another two sides of A4 on how to make best use of it all.

The OAG I’ve gotten parfocal, but haven’t yet had much experience with – and after taking off my guidescope I forgot to rebalance the mount, so a quick guiding test was really struggling mechanically. I’ll do a further post on that in due course.

As with last year I’m still running everything on a Raspberry Pi 4 mounted onto the telescope directly. This basically works very well with Ethernet to the scope – I’ve got a couple of Reolink IP cameras for monitoring mount motion remotely so I can run it all night from inside the house.

About the only thing I still need to nail down is dew heaters. I’ve now not got a guidescope – the main reason I got one in the first place – and the Paracorr was a bit tricky to get a heater strip around. In its final position I can now refit one and avoid dew on the front optic – a common issue for me. Secondary and primary heaters I can hopefully avoid!

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