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New EXOS-2/PMC-8, need advice on tracking


r_hoskin@...
 

Hi, David,

Thanks for the response!

I'm just outside of Windsor, Ontario.  The sunny south for Canada, but it's still Canada...  :-)

The mount appears solid. The EXOS-2 has rubber feet.  Not exactly sorbathane, but it plants firmly and does not slide or jitter.  The surface was a concrete paver walkway laid on a tamped base.  

You can't get away from roads, but the the area is generally quiet, more so in late evening.  Nearby traffic would be very intermittent while this was going on.

The SV 102EDT is a gem from the past, and a fine visual scope on a vixen shoe, nice and tight to the mount.  When I first decided to try astrophotography, I dropped Vic Maris a note and asked him what I should expect from it, in that application.  He warned that while there's no evident color or distortion in visual use, the camera would show chromatic aberration, and would require a field flattener as well.  So yes, it needs a flattener.

Rather than try to make the 102EDT into something it is not, I decided to press on with it as-is while I learn, and if this works out, see about an 80mm APO one day.  In the meantime, I can learn to stack, tune, crop, and get rid of blue doughnuts.  It'll build character!  :-)

I saw the distortion at the edges and put that down to the missing flattener.  That's why I looked to the middle of the image for my comparisons.  

I definitely think I have multiple things going on.  The sine waves are a giveaway.  Perhaps one thing per axis with a couple more on top, just for luck.  This will be one of those 'onion' problems - one layer at a time.  



David Pickett
 

At 22:43 30-11-17, r_hoskin@... [ESPMC-Eight] wrote:

I'm just outside of Windsor, Ontario. The sunny
south for Canada, but it's still Canada... :-)
I used to live in Port Huron, so I know exactly
what you mean. :) Windsor is about 42° N, so
Orion is about 6° lower than where I am (Vienna, Austria).

The mount appears solid. The EXOS-2 has rubber
feet. Not exactly sorbathane, but it plants
firmly and does not slide or jitter. The
surface was a concrete paver walkway laid on a tamped base.
But the kind of movement you are seeing, if
associated with the camera, could actually be
very small. And, given the shortness of the
exposures, it is likely to be due to this, if not
an unsteady mount. I would look again carefully
at the interface between camera and scope, particularly around the focusser.

Rather than try to make the 102EDT into
something it is not, I decided to press on with
it as-is while I learn, and if this works out,
see about an 80mm APO one day. In the meantime,
I can learn to stack, tune, crop, and get rid of
blue doughnuts. It'll build character! :-)
A good attitude!

I definitely think I have multiple things going
on. The sine waves are a giveaway.
I saw the D shapes, but not the sine waves. Can
you direct me to them? Are they sinewaves or
Lissajous Figures? If the latter, they would indicate movement in two axes.

Perhaps one thing per axis with a couple more on
top, just for luck. This will be one of those
'onion' problems - one layer at a time.
Quite so: you are in fact, perhaps trying to run before you can walk... :)

I would work at getting better polar alignment,
until you are certain that you have mastered it
-- it will help down the road. There is software
that can help with doing so and drift alignment
is free, if tedious. There is a lot of info about
this on www.cloudynights.com. This will take
several attempts, but I dont think it is responsible for your current problems.

The next time you have clear skies I recommend
you repeat your experiments closer to Polaris,
looking North -- a star pattern in Cassiopeia,
say. (Forget DSOs until you can guarantee
stationary stars!) By stopping down, you can
increase the exposure time. Try doing this
without the motors, i.e. with the PMC-8 turned
off once you have found what you want to look at,
to verify that you get the expected classical
drift patterns, and NOTHING else -- no wobbly
sine waves, seagulls, or even sparrows. When you
have achieved that, allow yourself to breathe and
do another run while moving around the mount to
see if that changes things. Try capturing shots
of parts of the sky that need different angles of
the camera. Then add the effect of the motors
and check that the mount tracks the stars without
gross wobbles. All these experiments will take
time, but you will have a solid basis for
diagnosis. With any luck you will become obsessed
with eliminating this problem, and at which point
a guide scope or similar will be necessary, which
will open another cna of worms.

All obvious stuff, I know, but by starting with
the easy things, you will probably find that this
is a very simple problem -- I hope so! Please
keep sending photos; this list needs something to
discuss in order to grow. I hope this is helpful.

Clear skies!

David


David Pickett
 

At 08:12 01-12-17, David Pickett yahoo@... [ESPMC-Eight] wrote:

Windsor is about 42° N, so
Orion is about 6° lower than where I am (Vienna, Austria).
Correction: I meant higher by 6°.

At culmination, Orion is higher the closer one is
to the earth's equator, while still being as far
north of it as we are talking of...

David


Robert Hoskin <r_hoskin@...>
 

Thanks, David!

All of my yahoo groups seem to have disappeared from view overnight, so I'm trying an email response...

I'll try what you've suggested (when the skies clear again...).  I hadn't seen how to go about isolating/eliminating the components of the problem, and that seems a very good start.

The 102EDT is about 15 years old, with its original JMI Crayford focuser.  I read that some folks who rotated the diagonal/eyepiece frequently had had trouble with them back in the day, but that's not me and I've been happy with it.  That said, if I grasp the focuser barrel and give it a lateral shake, I wonder if I don't feel the tiniest movement, but can't be sure. Any ideas on how to isolate and test for focuser shift?

The photo with the sine waves is the white one: DSC_0189_010.  It was supposed to be a 240 second trial, but the clouds moved in fast as it was running.  I stopped the intervalometer to try to salvage something (hence, the odd 192 second time) .  If you drill into the white, the stars in the middle are still there, but now they show as "S"'s,  looking a bit like little Safeway logos.  

- Bob 


David Pickett
 

Robert,

I think the first thing to do is to establish the
stability of the mount and camera, and I also
think you can do this at home. I have in mind
using a small flat mirror which can be attached
to various parts of the assembly. It doesnt have
to be fancy, or even terribly flat, just small so
it can be attached with double sided sticky or
Scotch tape. Then reflect the thin beam of a
laser pointer from it so it hits a distant
surface and can be seen as a dot. If the mirror
moves, the dot will move. The further the beam
travels after hitting the mirror, the more
sensitive your test is. This is a neat way of
amplifying motion [a = tan(a)], if you get me: if
you have a long yard, you can send a laser a long
way! I have no idea how to fix this if the
focusser is the problem, but you could ask a question on Cloudynights.com.

I pulled image 10 into Photoshop and fooled with
the histogram. (It is now very red!) I think the
sine waves may actually be two u-shapes: like
"nu" on its side, and speculate that something moved in between registering the n and the u, or
vice versa. Looking more carefully, it seems that
the bottom part is not a "u" but an "o". But the
shape doesnt matter and I wouldnt worry about
it's actual form. The thing to do is to eliminate
the possibility that it is a movement of the
camera with respect to the objective. If you are
certain that the mount and camera move as one,
then look at the motors/belts -- but I am betting that they are OK.

With you camera, as a camera, you could use an
artificial star. I have one of these
http://www.hubbleoptics.com/artificial-stars.html
, but you could make something good enough for
what you need at present, with a toilet roll
inner tube, a light, some kitchen foil and a small needle (or very thin wire.)

Good luck!

David

At 15:28 01-12-17, Robert Hoskin r_hoskin@... [ESPMC-Eight] wrote:


Thanks, David!

All of my yahoo groups seem to have disappeared
from view overnight, so I'm trying an email response...

I'll try what you've suggested (when the skies
clear again...). I hadn't seen how to go about
isolating/eliminating the components of the
problem, and that seems a very good start.

The 102EDT is about 15 years old, with its
original JMI Crayford focuser. I read that some
folks who rotated the diagonal/eyepiece
frequently had had trouble with them back in the
day, but that's not me and I've been happy with
it. That said, if I grasp the focuser barrel
and give it a lateral shake, I wonder if I don't
feel the tiniest movement, but can't be sure.
Any ideas on how to isolate and test for focuser shift?

The photo with the sine waves is the white one:
DSC_0189_010. It was supposed to be a 240
second trial, but the clouds moved in fast as it
was running. I stopped the intervalometer to
try to salvage something (hence, the odd 192
second time) . If you drill into the white, the
stars in the middle are still there, but now
they show as "S"'s, looking a bit like little Safeway logos.

- Bob