[avandonk]

The RH200 has a ten position FLI filter wheel and Atlas focuser. The Atlas focuser's resolution is 85nm per step or 100 steps is 8.5 micron.

Did a bit of testing last night. I used a Bahtinov mask to focus all my different filters.

The temperature of the optical train started at 17.8C and fell to 17.3C over the course of the measurements.

This is a table of the results.

FILTER___ Absolute Steps__ Relative Steps____ Actual differential distance in Micron
RED -----------39600__________ -200_____________ - 17
GREEN ____39800 _____________0_______________ 0
BLUE ______39800_____________ 0 _______________0
LUM _______39800 ____________0_______________ 0
HA ________39900 __________100 _______________8.5
OIII________ 40600__________ 800 ______________68
SII _________40200__________ 400 ______________34
CONT ______39400__________ -400_____________ -34
NII ________40200___________ 400 ______________34




Astrodon filters have a Silica substrate and are 3mm plus or minus 0.025 mm in thickness. This 0.050 mm maximum thickness difference is actally 1.5x0.050 = 0.075mm or 75 micron in optical path difference as the refractive index of the silica is 1.5. As you can see from the measurements they are much better than the quoted maximum especially the LRGB set. The RH200 is of course not totally free of CA as it has lenses and is most probably contributing to slightly different focal positions at the IR and blue ends of the spectrum without any filter thickness difference.

I have wrapped six dew heater straps onto the RH200 and Atlas focuser. A PID (proportional integral differential) controller holds the temperature of the optic train within 0.2 C of a set temperature. This means focus should not change with ambient temperature.

Bert