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EDIT: Before entering here please note that I included an update in post #6 below which confirmed my original calculations were correct. My response to David in post # 4 was therefore not correct. Still worth pondering though as to why my measured results do not match the theory. With that disclaimer, please read on
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As we know, it is important when designing adapter spacing to get the set back distance correct and to take into account the optical thickness of the filters that are in the imaging train. I would have thought by now, after many years of imaging, that I would have this process down pat having quite successfully designed many adapters which have worked well. However, this seems to not be the case. Let me explain.
I've just spent a couple of months and many late nights trying to dial in the correct spacing and tilt for the Astromechanics adapter that I'm using to pair a Canon EF200mm f2.8 USM lens to my ASI6200 MM Pro CMOS camera.
See here for pictures of this configuration. My filters are the Astronomik 50mm LRGBHaOIISII set which have an optical thickness of 1mm. In my (questionable) wisdom I simply added 1mm to the set back distance and adjusted the spacing in the Astromechanics adapter for an additional 1mm. I then spent several nights trying to work out why I couldn't get a flat field until last night in a moment of monumental epiphany I realised that I had this back the front and I should have been subtracting 1mm from the adapter spacing. A quick reconfiguration of the adapter and, voila!, a nice flat field emerges from the depths. I don't know how many times that I have correctly and quite successfully configured similar set ups without any problems, but not this time. I just goes to prove that we should always test our underlying assumptions when we are designing our set up and particularly when problem solving issues with our configuration.
For those that may be interested, here are details of what I am doing.
The image train is:
Canon EF200mm f2.8 prime lens -> Astromechanics adapter -> ZWO 2" EFW -> ASI6200 MM Pro.
The Canon EOS lens requires a set back distance of 44mm. The sums to work this out with my image train are:
Astromechanics adapter: 10.5mm; Optical thickness of filter: 1.0mm; Mechanical thickness of filter wheel: 20mm; Sensor set back spacing in ASI6200: 12.5mm.
The above adds up to 44.0mm and all is good.
Last night I managed to execute a successful test run on a widefield image of the Orion/M42/Horsehead region.
Link here to images in my staging area on Astrobin. The image posted here is a singe Ha sub, 20 minute exposure, Gain 100. Only processing was a quick stretch, move black point and apply a high dynamic range process in Pixinsight. It is apparent that the exposure time is good and some shorter subs will be required to improve detail in the core of M42 (no surprises there!).
The lens has been stepped down to approximately f5.4 using a step down ring on the front of the lens. This helps to remove the diffraction spikes that come from the lens iris though there is obvious vignetting which I am sure will be dealt with in post processing and with flat frames. Pixel peepers will see some small evidence of star distortion around the edges of the frame but once again this can be dealt with in post processing or simply cropped out.
I am pleased with the way that the Astromechanics adapter is working as it provides direct access to aperture control and the lens focusing mechanism. I'm driving it all with SGP Pro which is working a treat.
Link to Astromechanics web site here.
So the thought for the day is to always test your assumptions.
In closing I do apologise for the fact that now I have got this working the clouds will probably roll back in again.
Clear skies,
Rodney