Re: [APML] Trailed Stars, Large Errors: Help for a Newbie

[Jerry Lodriguss <jerry@astropix.com>]

Hi Yi-Zen,

Some general points first without actually seeing the images...

Take the time to drift polar align so you eliminate field rotation as a 
potential problem (it can sometimes look like trailing).

Single lock downs can pivot and move. Always try to use two lock downs for 
everything.

Make sure the scope is balanced in all three planes, and then adjust the 
counterweight so that the scope lifts weight, depending on which side of 
the meridian the lens is pointing (i.e. if the scope is pointing to the 
east, the counterweight side of the mount should be a little bit heavy.) 
This takes up any play in the right ascension worm mesh. You do not want it 
perfectly balanced.


>First problem: I tried to autoguide with the STV and saw very large 
>errors, on the order of 2-4 arc seconds, sometimes even larger. Is this 
>normal?

I don't know about normal, but it's probably not good for long focal lengths.



>I did notice that seeing wasn't too good, which surprised me somewhat, as 
>I was observing from an elevation of 4-5K feet or higher (SE AZ).

If the seeing wasn't good, that could possibly cause the autoguider to 
chase the seeing and lead to errors, depending on how sensitive you had the 
autoguider set.  I'm not familiar with the STV, but on the ST-4 you could 
average several short exposures when the seeing was bad.

2-4 arc seconds is not incredibly terrible, but not incredibly great either.

Elevation helps you get above some of the crud in the lower 
atmosphere,  but being at 4-5K feet or higher does not necessarily mean you 
will have good seeing.



>What are the possible causes of large errors? I'd like to try to 
>investigate this more carefully but would appreciate inputs on what to 
>look out for.


It could be a mount with a drive or gear problem, perhaps large sporadic 
errors. Or the gears might not be adjusted correctly.

It could be flexure between the STV and mount, or mount and camera, or a 
permutation or combination of all three. With single mounting bolts and a 
180mm lens on a ball head, you are asking for trouble.  Personally I have 
tried the 180mm on a very large and substantial ball head, and I got 
trailing every time. That size lens really needs to be in a set of rings, 
or mounted at two points minimum.

My best guess would be flexure based on the info you gave, especially since 
the trailing was not E-W.


Try visually observing at high power through the Borg with a cross-hair 
eyepiece while the STV is guiding. That will give you a better feel for 
what is happening with the mount. Of course, it won't tell you if the ball 
head is flexing for the camera lenses.



>I noticed that the camera lens can be pulled away from the CLA-5 adaptor a 
>little (i.e. the lens does not sit on the flange tightly) - could this be 
>a source of flexure?

Very possibly. The rule in astrophotography is: If it can move, it will. <G>


Good luck,

Jerry


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