Re: [APML] Manual guiding photo

[Roland Roberts <roland@astrofoto.org>]

>>>>> "roland" == roland morvan <rmorvan@stein-heurtey.fr> writes:

    roland> The pictures taken with the 50 mm are ok, but ALL pictures
    roland> taken with the 135 mm have strar trails instead of
    roland> points. I am not used with these kind of problems with my
    roland> motor driven LX 200.

I don't know what the trails look like, and that could be helpful in
trying to figure out the cause.  How long are the trails?  Are they
more than about twice the width of the stars?

    roland> - Polar positioning error: the field rotation can cause
    roland>   star trails but I should have the same trails with 50 mm
    roland>   focal length and 5 min exposure compared with 135 mm
    roland>   focal length (same angular rotation),

If you are guiding on a star near the middle of the field, field
rotation should show trails around the guide star, just as if you had
done star trails with the camera pointed at the pole.

    roland> - Mechanical flexure: with heavy (for this kind of scope)
    roland>   135 mm lens , but I tried several positionments of the
    roland>   camera to avoid flexure but it is the same in the
    roland>   pictures,

Can you rig something to hold the lens to the scope?  Something as
simply as a small wooden block between the scope and the lens with a
strap around both to hold them fixed.

    roland> - Guiding error: according to what I heard, with a 135 mm,
    roland>   the guiding time has to be at least 5 seconds maximum
    roland>   before we see trails in the negative. I tried to guide
    roland>   continuously or each 3-5 seconds, so according to this
    roland>   point I think that I made no mistake.

    roland> Anyody has already seen these kind of problems ? How can I
    roland> detect the cause of this ? Can the trails direction or
    roland> anything ele give clues ?

Sometimes they can.  Trails which are arcs centered on the guide star
are usually a polar alignment problem.  I saw _usually_ because I have
one lens combination that makes small arcs out of stars which looks
like field rotation but is actually aberration.  I think Michael
Covington printed an example of the same problem in his book.  In my
case, the arcs are not quite centered in the frame in spite of my
guide star having been in the center; I suspect I have a bad lens with
an off-center element.

If in addition to moving in RA you are having to make Dec corrections
in a consistent direction, your alignment needs to be redone.



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Roland B. Roberts, PhD                             RL Enterprises
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