Hi,

Using Focusmax, can anyone describe what the procedure is to create the filter offsets that are necessary to use non-parfocal filters?

Thanks,

-jim

Hi Jim,

One way to do this would be the following:

1. Establish your clear filter as the "reference" and base all the offsets off of it. It will have a "0" offset in CCDAP3. Do this by letting FocusMax focus the clear filter near the zenith and do this 5 to 7 times. At the finish of each focus run, write down the focus count. When all runs are done, throw out the highest count and the lowest count and average the rest. This becomes the "reference" count.

2. Now repeat this procedure for all your filters. If you can use the same star, all the better, if not, try and pick a star very close to the same mag and spectral value to keep the focus runs consistent. Collect the average count for each filter. It is also best to try and do this when the temperature is not changing so the focus counts won't be effected by that change.

3. Using your clear filter reference count, the difference between the two counts is your offset for that filter. If the filter count is greater than your reference, the offset entered in CCDAP3 is positive, if the filter count is less than the your reference, it will be entered as a negative number.

Example :

Clear filter average count = 2300

Red filter average count = 2436

difference = 136

Red filter > offset, enter = +136

This makes it very easy to always focus with your clear filter and then let CCDAP3 move the focuser to the offset for all the other filters.

HTH .....

Frank,

Thanks. That sounds pretty straightforward. I'll give it a try the next clear night that affords me the opportunity.

-jim