An arrangement for off-axis guiding of a Newtonian

[darditti]

I have been working for a short while on the problem of combining an f4.8 245mm Newtonian with a QHY8 camera, a Baader MPCC, and off-axis guiding, as some of you who have seen discussions on my recent images know.

I prefer off-axis guiding as, in the long term, it is much more accurate than using any guidescope. But most off-axis guiders introduce too much optical path length into the system to be accommodated in the back-focus of a Newt. Add in the Baader MPCC, which requires a fixed 55mm distance from the flange of the corrector to the chip for best results, and you have a problem of back-focus accommodation which few if any imagers with Newts seem to have solved, reverting generally to guidescope use or dual-chip cameras instead.

My solution (see pic below) has been to construct a Newtonian with the image plane, with the coma corrector fitted, at 19cm from the outside of the tube. I have fitted a JMI Next Generation low-profile motorised Newtonian Crayford focuser also to give maximum back-focus. Into the focuser I have inserted a Lumicon Newtonian OAG. This only consumes about 2cm of back-focus. Into this I have inserted the Baader MPCC (with an LP filter screwed into its nose, though this is not essential to the system). This is clamped with the three brass thumbscrews of the Lumicon OAG. I removed the T-threaded attachment which was originally clamped by these screws. The 2" barrel end of the OAG clamps into the 2" focuser. This is only held with a single compression-ring clamp, which is not very satisfactory. It needs improving. Note that I needed to turn the whole Lumicon unit around from the configuration in which it was supplied from the factory. I had to reverse the prism unit by unscrewing its Allen screws. The way round they supplied it makes no sense to me at all, though perhaps they had their reasons. Anyway, it works correctly as I have modified it.

Into the T-thread at the back of the MPCC is screwed a T to 2" ID adapter (as sold by Modern Astronomy). Into this I have inserted the nosepiece of the camera, and clamped it with the setscrew. Around the nosepiece I have clamped a 2" parfocalising ring, such that when the ring is against the face of the T to 2" adapter, the 55mm spacing is exactly achieved.

The guide camera (a DMK) also has a small parfocalising ring round its nosepiece, which fixes its focus correctly (or approximately) with respect to the main camera. This picture was taken after a night of imaging (Dec 6-7, note the frost) so everything is really focused correctly. You will note I STILL have about 4cm of spare back-focus left, which could be used for a filter wheel.

Things that are still unsatisfactory here are:

1) The clamp of the whole assembly to the JMI focuser - with only one screw, not good enough for all this weight. I intend to drill and tap some further holes for set-screws into the drawtube.

2) The seating of the MPCC/filter assembly into the OAG is not too secure like this. The better solution would be to utilise the 48mm thread on the front of the filter and MPCC and mate that to the T-thread attachment provided with the OAG. This requires a stepper ring which I am getting. Note that I have installed the filter on the camera side of the OAG, so the guide camera is using unfiltered light.

So, I have demonstrated that the QHY8 can be combined with the MPCC at the correct distance and with an OAG and with a filter on the correct side of the OAG, using commercially-available adaptors, not having had anything specially made. It would be better if a T-threaded adapter could be machined to the correct length to give the 55mm spacing with the QHY, and indeed Bern of Modern Astronomy is working on that. But in fact maybe what I have devised is better in a way, as it is adjustable for the specific telescope f ratio. Note also that most commercial Newtonians would need their primary mirrors moving some way up the tube to make this work - perhaps in the region of 10cm, depending on details.

David