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Coma corrector backfocus

Sky-Watcher F4 Aplanatic Coma Corrector ZWO ASI294MM Equipment Reflectors Coma
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Paul Ecclestone-Brown avatar
Paul Ecclestone-Brown avatar
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

andrea tasselli avatar
andrea tasselli avatar
The  answer is no. Coma correctors correct for coma only (and a bit of field curvature) and they already have a hard time doing that with a parabolic mirror. You need to look on how well the field is corrected in the actual practice. Your CC manufacturer should have listed the backfocus requirement. This should have given you the right initial ballpark placement. In my case is 56mm from the CC M48 flange. Adding filters would change that obviously so arm yourself with a set of spacers (m48 or m42) and optimize the final distance either way (reducing or increasing the CC flange to sensor distance).
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Paul Ecclestone-Brown avatar
Paul Ecclestone-Brown avatar
andrea tasselli:
The  answer is no. Coma correctors correct for coma only (and a bit of field curvature) and they already have a hard time doing that with a parabolic mirror. You need to look on how well the field is corrected in the actual practice. Your CC manufacturer should have listed the backfocus requirement. This should have given you the right initial ballpark placement. In my case is 56mm from the CC M48 flange. Adding filters would change that obviously so arm yourself with a set of spacers (m48 or m42) and optimize the final distance either way (reducing or increasing the CC flange to sensor distance).

Thanks Andrea - As I am using an Lpro on the Altair 269 - it looks like I should add about another 1mm to the quoted 55mm

Will try that....
andrea tasselli avatar
andrea tasselli avatar
It's closer to 0.7mm but do not get hung up on few tens of mm. It should probably work nearly as well too. I have a set of 0.2/0.1mm spacers on hand so I can fine fettle the final distance, if I want to.
Andy Wray avatar
Andy Wray avatar
The only issue you have with those two cameras is that one has a sensor back spacing of 6.5mm and the other 12.5mm.  That means you will need a distance from the CC to the imaging camera body of either 49.5mm or 43.5mm depending on the camera (assuming a 56mm back focus allowing for the filter).  

I've found that you can quite successfully get the back focus close enough using a set of digital calipers to measure this distance.
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