New EXOS-2/PMC-8, need advice on tracking [1 Attachment]

David Pickett

Dear r,

Others with more experience than I will
undoubtedly have more insights, but it seems to
me that you have more than one problem going on
here. I wonder whether everything is tightly
mounted. There should obviously be no play
anywhere and the mount must be solidly planted.
(I know all this is obvious, but it is worth
checking.) It would help to know where you are
(100v is used over a wide area, though from the
temperature, somewhere northish!) and what time
this happened. What is your mount actually
standing on? Are you anywhere near any possible
vibration from motors or traffic?

One thing I notice is that the stars at the top
left of the photo 004 are ovals pointing top
left/bottom right, whereas those at the bottom
left are ovals pointing top rightish/bottom
leftish. There is a similar disparity on the
right hand side. A similar difference between
left and right can be seen in photo 008. I think
this is a coma effect. Do you have a field flattener?

I would try a longer exposure (several minutes)
with a polarising filter or something to cut down
the light and avoid over exposure and see what
trails you get then. Also, after polar
alignment, a drift alignment to check it. But
your exposure times are so short that I think something else is going on.

I hope this helps. How accurate was the GoTo?


At 20:45 30-11-17, r_hoskin@... [ESPMC-Eight] wrote:

Last night, the clouds finally parted for a
while, and I was able to take scope and mount
out for a test run. My sole objective was to
figure out an acceptable unguided exposure
length for future work. I wanted to know the
point at which tracking errors accumulated and
my round stars turned into potatoes.

Potatoes happened a lot earlier than I expected it to, but I gathered enough data to see that
there isn’t just one thing going on – there is
clearly more than one commponent to the overall
tracking error. I not only have potatoes, I
have bidirectional seagulls, and sine waves as
well! I’m looking for good advice on what to do about that.

Equipment and software:

EXOS-2/PMC-8 on 110v, SV 102EDT @ f6.1, Nikon
D5300 with t-adapter and a 110v power adapter,
Live View focus, Polaroid intervalometer,
Parallax USB-to-Serial adapter, Lenovo x230
laptop on battery, Win7 Pro, Ascom w/Explore’s
driver, Cartes du Ciel. Latitude and longitude
set manually with values from google earth.


Did a rough (Polaris near eyepiece crosshairs)
polar alignment. Did a two-star alignment with
CdC. Slewed to M42 and started the
intervalometer on its first set of
exposures. While I did not intend to touch the
scope while imaging, I found that at 32F/0C, it
was too cold for the intervalometer and its
display was unresponsive. I had to unplug it
from the camera and take it in the house between
shots to reset it. So there may be some image
shift from that. One bit of serendipity from
this is that I ended up accidentally triggering
a short 10-second shot that’s one of the nicer ones in the collection.

A selection of compressed jpegs of my images are
in the attached zip. You’ll have to zoom
waaay in, to see some of the effects. I’ve
concentrated on the line of 3 stars with CA, at
8 o’clock from the nebula, and the smaller stars around them.

DSC_0184_004 is the 10-second image. If I zoom
straight in the three stars are a bit rough, but
I’d guess that’s the effects of overexposure
and seeing. The smaller, surrounding stars seem sharp.

DSC_177_005 is a 30-second image. Potato shapes
starting to develop, and the small surrounding stars are becoming oblong.

DSC_185_007 and DSC_186_008 are both 60-second
images, and the smaller stars are now
seagulls. In DSC_185_007, they fly left, but in DSC_186_008, they fly right!

At this point, clouds are moving in and starting to obscure the shots.

DSC_0187_009 is a 120-second shot. The jpeg’s
lost detail from the original, but if you look
closely, you see that some seagulls are now becoming “W�’s.

DSC_0189_010 is a 192-second shot. Clouds are almost completely obscuring it, but if you drill
in, you see that the seagulls are now sine waves.