A scorching hot night. Camera won't cool below -25C. Half moon. Horrid seeing. Thin high haze. But we'd just replaced the Hederick focuser (after 5 years of noble service), and wanted to do a quick holiday snap to see if the system was working.
NGC 288 is a smaller, sparsely furnished, mag 8.1, very lonely globular cluster near the south galactic pole, and at the edge of the binocular field if you're looking for NGC 253.
Easily resolvable right to the core. Its main redeeming feature as an image is that there are two very distant spiral galaxies shining through, one at the top, one at the bottom. There seems to be a whole cluster of distant red ellipticals shining through the left-hand edge.
Due to the ghastly conditions, there's probably not much point giving a link to the original, but we thought it was a different diamond to snap on a hot later-than-August night.
Aspen CG16M on 20" Planewave. RGB each 3 subs at 30min. Field about half a degree.
(PS: PlaneWave shipped the new focuser very promptly by ordinary post. Removing the old one and reinstalling was a breeze, albeit 2 metres up a ladder with shaking knees. No shims to mess around with. Just accurate machining.)
That's nice resolution and colour MnT. It still looks quite faint at 30 minutes subs. It must be very faint.
I have often wondered what you thought of using a big scope like that with a fork mount. How much flex do you get with the forks? I am thinking that one day I might travel down the same path with at least a 16" scope.
That's nice resolution and colour MnT. It still looks quite faint at 30 minutes subs. It must be very faint.
I have often wondered what you thought of using a big scope like that with a fork mount. How much flex do you get with the forks? I am thinking that one day I might travel down the same path with at least a 16" scope.
Thanks, Paul.
Having a fork mount at all is great - no meridian flip, and it means you can photograph through the very best part of the sky without anything funny happening. The forks are huge heavy aluminium castings. My pointing model says the droop is only 0.01 degrees. (Note to self: redo pointing model for summer). On the other hand, we feel that to be honest, the motor drive for our particular MI-750 fork as delivered left a lot to be desired. We suspect that despite asking for a southern hemisphere version (Australia), we actually got a Northern hemisphere version (Austria?), because the main RA worm gear would creep up the face of the gear and then drop down repeatedly, causing utterly shocking tracking. We added some extra ball bearings and a pressure plate to hold the gear down, not against the worm (it's spring loaded - that's fine) but in the other direction, parallel to the axis of the earth). We made the same mods for the dec drive, and that hugely improved the Dec backlash. We also changed the servo gearmotors to ones with three times higher gearing, and that also greatly improved tracking at the expense of slow maximum slew. The RA clutch also had some missing bolts. Mathis talked us through replacing them and that was fine. Having made those mods, we're delighted. The mount was only $18K, and having made the mods, we think it was good value for money.
But if we were ever to buy another instrument (unlikely), we might think about an alt-az fork mount and a field rotator. We'd never try putting a really big scope on a German mount.
Thanks, Paul. Living proof that they're not all exactly the same.
Quote:
Originally Posted by gregbradley
Very nice Mike. Did you apply any sharpening? What was wrong with the old focuser?
Greg.
Cheers, Greg. Gentle deconvolution applied. FWHM went from 2.2" after stacking to 1.9". The old focuser has been groaning under the weight of the Aspen CG16M, an Apogee 10 position filter wheel, my home-made off-axis guider, and an Apogee cooled guide camera. Two things went wrong. Firstly, perhaps with some mixture of the weight and 5 years of freezing winter nights and baking summer days, the linear bearings had developed rather a lot of slop. Secondly, somewhere in the servo-motor gearbox something, perhaps bearings, were starting to grab, resulting in the focuser missing its requested position by up to 0.2mm. The 20" focus sweet spot is huge and forgiving, and that on its own wasn't catastrophic, but it was getting rapidly worse and was likely to seize one night during an unattended auto-focus run. The replacement motor gets within 1 micron of where it's supposed to be every time. Also PlaneWave have improved the focuser housing somewhat, which might help keep grit and dust out of the mechanics.
Quote:
Originally Posted by strongmanmike
Great test image guys, love the little faint galaxies visible in the cluster. Looks like you had a satellite in one of the red subs?
I agree about a fork mount too, if I ever get a big scope I would really like it to be on a fork mount
Mike
Thanks, Mike. Yes, satellite. With only 3 frames in each colour, hard to do good data rejection, so it is still partially visible.
