Takahashi FC100 classic reducer spacing

[yuklop]

Hi All,

Hoping an expert on here has some idea of the backspacing of the tak f5.9 reducer for the original FC100 (image attached). I had a go at trial an error last night and realised that was a fools errand. I'd guess the spacing is too large as I get comet shaped stars in the corners. Anyway, help greatly appreciated.

Is there way to determine correct back spacing in the daytime? It was cold last night!

Dan

 

Edited October 15, 2023 by yuklop

[yuklop]

So its seems this isn't a question without the answer anywhere online, so I'll post it here in case anyone else ever runs into the same issue.

I asked Takahashi and they said 83.5mm. Helpful folks they are. They also said that their new fc-35 reducer would work better with modern cmos cameras. I am sure it would, and I'm sure its superb, but its quite expensive, so that's one for another day. In the meantime some analysis on the old original fc-100 reducer.

It is a fantastic chunk of glass, with a 60mm clear diameter and weighing 366g.

I ran my own measurements and got the result below by using the relationship that focal length in mm = (206*pixel size in micron) divided by pixel scale in arcsec per pixel. Plate solving gives the pixel scale. The graph below shows the results and inserting 590mm as the desired focal length (for f5.9 reducer on 100mm scope), the calculated backspacing is 83.572mm. A pleasing if probably unnecessary confirmation.

The question that remains is whether or not it is any good vs a modern system. To my eyes a full frame looks great at all the tested spacings. Siril's aberration inspector takes a close look at the corners. They aren't bad. Best at a range around the optimum length. I compared it to a modern imaging rig using a ES80APO and a 0.8x reducer and it compared favourably under pixel peeping conditions with both setups showing similar amounts of distortion (the distortion with slight comet shapes pointing inwards to the centre). The tests were done with an Atik Horizon 4/3 sensor in both cases. I tested the FC-100 scope with no reducer and that has a slight distortion with the starts smeared perpendicular to an imaginary line pointing to the centre of the image (apols I forget the names of these distortions, perhaps someone can chip in below...). 

The final observation is that the 83.5mm backspacing is good compared to many refractor reducers at 55mm. The FC-100 back is M72, so a M72 to whatever you want (e.g. M54 or M42) is ideal for making most of that back spacing. With a 2'' nosepiece that I use into the Tak visual back that 83.5mm disappears quick, and doesn't leave space for my SX OAG and wheel. Best go buy yet another adapter I suppose.

Hope this is useful to someone one day!