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Coma corrector backfocus Fast Newtonians · Paul Ecclestone-Brown · ... · 6 · 236 · 1

WeAreAllStardust 1.20
Paul Ecclestone-Brown
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Hi

I am working with an F4 200m TS Photon with a skywatcher f4 aplanatic coma corrector, using an altair 269c for galaxy season and an ASI294mm for nebula season- so both effectively 4/3 sensors with approx 22mm diagonals.  I have been working through a range of issues like accurate collimation, pinched optics and decent guiding and balance of my rig together with installation of a ring over the mirror clips.  I am finally getting some almost decent results.  the final  stage is back focus distance so that the final smaller diffraction spikes on brighter stars aren't spread away from the centre of the image

I read that the stated backfocus distance is 55mm but ither people claim about 53mm.  I guess that the optimum distance varies from setup to setup as the sensor distance in any camera has a manufacturing tolerance of 0.5mm....  Is there a diagram anywhere for newts and coma correctors that is similar to this one for refractors and field flatteners? - or is the field flattening element of the cc roughly equivalent to the field flattening element in a refractor reducer/flattener and so could this diagram be used for cc's?

Thanks 

Paul
Move-Flattener-Outwards-or-Inwards-Spacing-Star-Pattern.jpg
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dave1968 2.81
Dave B
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Give Alacant a shout on Stargazerslounge he’s quite up to date on backfocus for comacorrectors .
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WeAreAllStardust 1.20
Paul Ecclestone-Brown
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Dave B:
Give Alacant a shout on Stargazerslounge he’s quite up to date on backfocus for comacorrectors .

Thanks for the tip Dave
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CCDMike 5.02
CCDMike
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Hi Paul,

You can use the same diagram as the stars show the same pattern if too close/far from the CC.
Usually some of the other "failures" you've mentionend come together. So you did well to fix them one after another.

CS
Mike
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DalePenkala 19.38
Dale Penkala
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I use the same CC for all my f/5 newts and the 55mm bf works out for me pretty good. I second Mikes comment about the diagram, it will work as its the shape/direction of the stars your looking to distinguish which way to adjust the spacing.

Dale
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alacant 0.00
alacant
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HiFor 800mm focal length, the GPU should be at 53mm. For 1000mm, 55mm.
HTH
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jheppell 3.21
Jacob Heppell
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From my experience you can't use the same diagram for coma correctors that you use for field flatteners. They do different things. CC's correct for coma, flatteners correct for field curvature. When I used the skywatcher F4 CC, I just set it to 55mm for my 1000mm F4 newt as recommended and it was all good but I would still have tilt issues because I was using a 2" compression ring (I use a televue  paracorr II now with a threaded connection and no tilt). For your 800mm F4, the recommended backfocus is 53.6mm for a SW F4 CC but give or take 0.5 to 1mm and you'll be fine. One time I deliberately added 2mm to the backspacing (57mm), which only gave slightly fuzzier stars everywhere otherwise it was hard to tell.
For fast newts, excellent collimation will be your primary consideration (I use the catseye cheshire and autocollimator with a hotspot on the mirror). I also found tilt to be another strong consideration. When using a 2" compression ring, it was hard not to induce tilt when locking it in, which would show up as radially and/or tangentially elongated stars around the edges. If you can get collimation and tilt down pat maybe then errors from a not-quite perfect backspacing will show up.
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