383L+ with R200SS - coma correction

[DaveAus]

I wonder if anyone who is using the 383L+ mono with a Vixen R200SS telescope (+ coma corrector) can answer this. A few months ago I bought my 383L+ mono and, as weather conditions and other matters have allowed, I have been trying to get it work with my Vixen 8 inch f/4 R200SS astrograph using the Vixen coma corrector. I naturally use a filter wheel. At first severe coma was evident in test images but I have managed to reduce that by adding a 5mm spacer ring to the optical train. Now there is just a bit of coma in the four corners of a test image. My question is : Is anyone getting completely coma-free images from such a setup or am I expecting too much from the combination of f/4 + coma corrector + CCD size of 3,362 x 2,504 pixels?

[CCD-Freak]

You need 55mm spacing from the coma corrector to the CCD.  The R200ss with the Vixen coma corrector should cover the 383 quite well.  I use a 40mm extention tube with my 838L+OSC.

Hope this helps.

John
CCD-Freak

[DaveAus]

Thanks John.

I'm pretty close to the 55mm separation if not already there - it's hard to measure. If I'm out its probably by only 1 to 2 mm. I've tried a 3mm additional threaded spacer but due to the small depth of the female side it actually adds at least 4mm to 5mm and the result is worse. Without that spacer the corner stars are small streaks radial to the image centre. With it the corner stars are small mushrooms with the heads pointing inwards and the tails pointing to the corners. I've not yet found smaller spacers and I wondered if anyway it was worth pursuing the matter further if the format size of the camera is too large for the capabilities of the coma corrector - hence my question to see if anyone else has succeeded in getting completely coma free images with the 383L+ and the R200SS.