Patrice Boyer wrote:
Hi everybody,
i was on holydays for a few days but unfortunately the weather was awful
onthe french riviera, quite unusual here. Anyway, i succeeded to image
between the clouds on friday night with a big moon high in the sky.
i have received my astrodon lrgb filters for my "I" chip, they are sold
to be parfocal, i noticed this was not really true as r,g and l are
very near for the focus the blue one needs some focus correction, did
you notice the same thing or is it a defect of my filter ?
Here is a classic M81 6X20mins for lum unbinned and 20 m each color
unbinned too, ST2K at F/D5 with lumicon focal reducer mounted on meade
8" SCT, autoguiding with cam, mount is an old but great working vixen
SPDX with skysensor 2000PC.
The full process of the pic includes a ddp stretching.

Merry Christmas for you.

Patrice BOYER


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Not a bad shot for all that moonlight. You even picked up a bit of
Holmberg IX out of the moonlight. See
http://www.usenet-replayer.com/cgi/c...567447.10.jpeg
for my no moonlight version.

I've no focus shift with my filters. But I do get a shift due to
temperature change. Your shot covers 3 hours of time. If blue was last
it could be the temperature changed enough to be the cause. My 14" SCT
needs constant refocusing due to temperature. I used to check it every
sub until I invested in a temperature compensating focuser. Now it
makes the adjustment for me. 1 degree C is enough for me to need to
refocus. For instance the temperature dropped 4C during my shot above.
That would be a 160 count difference on my focuser. 4000 counts
equals one inch, 25.4mm, so that would be 0.04", 1.0mm, change in focus,
a very noticeable amount. At F/5 such a change would be far worse as
the depth of focus is half what it is at f/10. How a compressor giving
F/5 changes this I don't know. It may also cut the focus shift from
temperature in half keeping things constant. I just don't have any idea
having not used one -- They are mostly worthless to me as my CCD already
captures virtually the full usable field of view of my scope.

If it is temperature then you shouldn't see a change when rapidly going
from one filter to the next, only over time and it should change all of
them.

I don't use a compressor. If that has some chromatic aberration in it
it could cause the blue focus to be different as well I would think. It
may not show on the Luminosity frame as it could be lost in the
correctly focused red green and longer blue light. Filters can't
compensate for this as it varies from scope to scope.

Is the blue filter seated the same in your filter wheel. The distance
from the filter to the scope may change focus as well.

In short there's a lot of places to look besides the filters themselves.

Rick
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