Re: [APML] Manual guiding photo

[Matt BenDaniel <matt@starmatt.com>]

At 02:40 PM 5/9/2002, roland morvan wrote:
>I am presently observing from Brazil for several months instead of observing
>from France, and I enjoy southern sky for the first time.
>In France I have a Meade Lx 200 and I use to take pictures of deep sky
>objects in prime focus or in parallel with a 50 mm and a 135 mm telephoto
>lens.
>
>I came in Brazil ( isolated place with beautifull sky at 20 ° South
>latitude)  with light material, i.e. a 115/900 mm scope that a friend of my
>gave me and my 50 mm, 135 mm lenses to take pyggyback pictures of the
>southern sky. The scope has no motor, so I have to guide manually by
>following a star with a reticulated ocular.
>My 50 mm is at f/1.7 and my 135 mm is at f/2.8 and I expose 5 min with the
>50 mm and 10-15 min with the 135 mm witha  400 ASA film.

Most lenses don't have their best performance wide open. If you can manage 
to double the exposure with the lens closed one stop, it might improve the 
quality. However you have to get your guiding working better before you 
increase exposure times.


>The pictures taken with the 50 mm are ok, but ALL pictures taken with the
>135 mm have strar trails instead of points. I am not used with these kind of
>problems with my motor driven LX 200.
>
>My diagnostic is:
>
>- Polar positioning error: the field rotation can cause star trails but I
>should have the same trails with 50 mm focal length and 5 min exposure
>compared with 135 mm focal length (same angular rotation),

You should drift align until you can go five minutes (in both the north and 
east/west) with no discernable N/S movement.

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

Even a wood insert to stabilize the front of the lens could help.

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

That's probably not the problem.
In general it's good to guide on a star in the center of the camera's field 
of view.

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

If you have to make DEC corrections consistently in one direction, then the 
poor polar alignment is causing field rotation. The pictures will show the 
stars rotating around the guide star.


--
Matt BenDaniel
matt@starmatt.com
http://starmatt.com


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