Getting the correct spacing for a focal reducer

[JamesF]

I clearly don't have the distance to the sensor quite right for the focal reducer (TSRED279) that I have on my Photoline 72.  I suspect I need a little more distance between the two given that the stars are a bit stretched away from the centre of the image towards the edges of the frame.  Unfortunately TS don't specify the exact distance required, so it's all trial and error.

Is there any neat way to test this and set everything up in daylight, or perhaps when the sky is overcast?  I'm all for avoiding faffing with kit in the dark, especially when clear sky seems to be such a rare occurrence.

I'm thinking that I might be able to use an artificial star imaged away from the centre of the frame to test the star shape whilst adding or removing spacers.  Is that likely to work?  Is there a better/easier way?

James

[michael8554]

Daytime on an artificial star gave me an approximate setting I further tweaked on stars. 

Focus with the artificial star in the middle of frame and then move to near a corner. 

Michael 

[david_taurus83]

Should it not be around 65mm?

[JamesF]

"Around" 63mm according to TS, but I'm as close as I can reasonably manage to 63mm and I'm not getting round stars, so clearly a better way is needed to set it up.

James

[spillage]

Probably will not help but I have always found that getting close to the stated distance and then add or remove about 5mm check and then half it until it starts looking better and then do a little fine tuning. I wasted hours on my mpcc adding and removing minute spacers so decided that the bull in a china shop style works better for me.